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thesis entitled

FACTORS AFFECTING THE LEAF AND SHOOT MORPHOLOGY
AND PHOTOSYNTHETIC RATE OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

presented by

Carl E . Sams

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FACTORS AFFECTING THE LEAF AND SHOOT MORPHOLOGY
AND PHOTOSYNTHETIC RATE OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

By

Carl E. Sams

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Horticulture

1980

ABSTRACT

FACTORS AFFECTING THE LEAF AND SHOOT MORPHOLOGY
AND PHOTOSYNTHETIC RATE OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

By

Carl E. Sams

The effects of leaf age, leaf position on the shoot,
and environmental factors on net photosynthetic rate (Pn)
of sour cherry were determined using an infrared, differen-
tial open gas analysis system. Diurnal and seasonal pat-
terns of Pn and the effect of fruit on Pn were evaluated.
The effects of shading on leaf and shoot morphology and
leaf Pn of sour cherry were also determined. Pn was
greater for leaves which had recently completed expansion
or were at least 50% expanded than for either older. ma-
ture leaves or newly expanding leaves on the same shoot.
Pn of individual leaves reached a maximum when the leaf
was greater than 80% expanded, remained constant for 2 to
h weeks. then gradually declined.

Maximum Pn occurred at light intensities between 800—
lhOO uE m-2 s-l. As temperature decreased from 35 to 10 C
higher light intensities were required for maximum Pn.

Optimum temperature range for Pn was 15-30 C, and Pn at

Carl E. Sams

optimum temperature was greater at light intensities of
1200 and 2000 uE m‘2 s’1 than at 300 uE m“2 s‘l. In
general. Pn was greater at high (85—95%) than at low
(30-ho%) humidity. Pn increased with increased ambient
CO2 concentration between 0 and 600 ppm, the compensation
point being 82 ppm.

There was no significant diurnal change in Pn for in-
dividual leaves kept under optimum conditions. However,
there was a pronounced diurnal pattern in Pn of whole
trees measured under natural sunlight conditions from sun—
rise to sunset. Maximum Pn was reached before solar noon,
remained constant for a short time, then declined.

Seasonal patterns in Pn varied, but. in general, Pn
reached a peak early in the season as leaves expanded,
remained stable for several weeks, then gradually declined.
During the 1978 season leaves on shoots with fruit had a
higher average seasonal Pn than leaves on shoots without
fruit. However, in the 1979 season there was no signifi-
cant effect of fruit on average seasonal Pn.

The effect of shading on leaf and shoot morphology
and Pn of sour cherry was evaluated by growing one year
old potted trees to the 11-15 leaf stage in full sunlight
then transferring them to 100, 36, 21, and 9% of full sun-
light. Trees grown in full sunlight produced leaves with
greater specific leaf weights, less chlorophyll, greater
palisade. spongy mesophyll. and total leaf thickness. and

smaller average terminal leaf areas than those grown in

Carl E. Sams

shade. Trees grown in full sunlight also produced thicker
shoots and a larger number of flowers and flower buds per
tree than shade grown trees. Trees grown in less than 36%
of full sunlight produced no flowers.

At light intensities of 1200 and 2000 uE m'2 s'l,

I'V-I /

f the Pn of leaves grown in full sunlight was greater than
:Mthe Pn of leaves grown in shade. Also. the Pn of leaves
which expanded and were then shaded to 9% of full sunlight
was greater than that of leaves which completed expansion
under 9% of full sunlight. However. at low light inten-

2 3-1) the Pn was not significantly differ-

sity (320 uE m-
ent among leaves from all shade treatments. Maximum Pn

was greater for leaves grown in full sunlight than for
leaves grown in shade, and maximum Pn of leaves grown in
shade occurred at lower light intensities than that of
leaves grown in full sunlight. Pn was greater at 25 C

than at 10 or 40 C for leaves grown in both full sunlight
and 9% of full sunlight. Pn at 25 C was greater for leaves

grown in full sunlight than for leaves grown in 9% of full

sunlight.

ACKNOWLEDGMENTS

I wish to express my sincere appreciation to
Dr. J. A. Flore for his counsel and encouragement
during the course of my graduate program. I would
also like to give special thanks to Drs. M. J. Bukovac,
F. G. Dennis. D. E. Linvill, A. J. Smucker. J. R. Clark,
and C. A. Rotz for serving on my guidance committee.

I would also like to express my appreciation to
my fellow graduate students (especially all of the
Bob boys) who have provided friendship and sufficient
distractions to make graduate school an enjoyable.
as well as educational. experience.

Special gratitude to my wife, Debra, and my
daughter, Karen, who never failed to give me their
love and unqualified support and gracefully accepted
the many sacrifices that my graduate studies imposed

on our family.

ii

Guidance Committee:

The Paper-Format was adopted for this thesis in
accordance with departmental and university regulations.
The thesis body was separated into four sections. Each
section is intended for publication in The Journal of

the American Society for Horticultural Science.

iii

LIST OF TABLES .

LIST OF FIGURES . .
INTRODUCTION . . .
SECTION ONE:

TABLE OF CONTENTS

THE INFLUENCE OF LEAF AGE. LEAF POSI—

TION ON THE SHOOT. AND ENVIRONMENTAL VARIABLES ON
NET PHOTOSYNTHETIC RATE OF SOUR CHERRY (PRUNUS

CERASUS L. 'MONTMORENCY')

Abstract . .

Introduction .

Materials and Methods

Results . .
Discussion . .
Literature Cited

SECTION TWO:

FACTORS AFFECTING DIURNAL

AND SEASONAL

NET PHOTOSYNTHETIC RATE OF SOUR CHERRY (PRUNUS

CERASUS L. 'MONTMORENCY')

Abstract . .

Introduction .

Materials and Methods

Results . .
Discussion . .

Literature Cited

iv

Page
vi

ix

CDV O\U\ «I:

36

40
41
41
43
as
5n
65

SECTION THREE:
(PRUNUS CERASUS L.

 

Abstract

Introduction

Materials and Methods

Results .
Discussion .
Literature Cited
SECTION FOUR:
(PRUNUS CERASUS L.

 

Abstract .

Introduction .

Materials and Methods

Results .
Discussion .

Literature Cited

THE EFFECTS

OF ARTIFICIAL SHADE ON
THE LEAF PHOTOSYNTHETIC RATE OF SOUR CHERRY
'MONTMORENCY').

SOUR CHERRY

THE EFFECTS OF ARTIFICIAL SHADE
ON THE LEAF AND SHOOT MORPHOLOGY OF
'MONTMORENCY').

Page

70
71
72
74
79
91.
100

105
106
107
109
112
116
125

Table

LIST OF TABLES

Section One

A description of the major components of the
differential open gas analysis system . .

The effect of shoot position on net photo-

synthetic rate of 'Montmorency' cherry leaves

The effect of temperature and humidity on
maximum net photosynthetic rate and light
compensation point as predicted from
asymptotic equations of light response
curves for 'Montmorency' cherry leaves . .

The effect of light intensity and humidity
on maximum net photosynthetic rate and
optimum temperature as predicted from para-
bolic equations of temperature response
curves for 'Montmorency' cherry leaves .

Section Two

The effect of shoot excision on net photo-

synthetic rate.of 'Montmorency' cherry leaves

Diurnal change in net photosynthetic rate
of 'Montmorency' cherry leaves under
optimum conditions . . . . . . . . .

The effect of fruit on net photosynthetic
rate of 'Montmorency' cherry leaves in 1978

The effect of fruit on net photosynthetic
rate of 'Montmorency' cherry leaves in 1979

Net photosynthetic rate of terminal and spur
leaves of 'Montmorency' cherry in 1979 . .

vi

Page

11

19

2h

28

“7

48

52

53

57

Table

Page
Section Three
The effect of different light intensities on
the diameter of shoots from 'Montmorency'
cherry trees grown under artificial shade . . 80

The effect of different light intensities on
shoot development of 'Montmorency' cherry

_trees grown under artificial shade . . . . 82

The effect of different light intensities
on leaf development of 'Montmorency' cherry
trees grown under artificial shade . . . . 83

The effect of different light intensities

on the specific leaf weight of leaves from
'Montmorency' cherry trees grown under

artificial shade . . . . . . . . . . 8A

The effect of different light intensities

on chlorophyll A content of leaves from
'Montmorency' cherry trees grown under

artificial shade expressed as amount per

unit leaf area. per unit leaf dry weight,

and per unit leaf volume . . . . . . . . 86

The effect of different light intensities

on chlorophyll B content of leaves from
'Montmorency' cherry trees grown under

artificial shade expressed as amount per

unit leaf area, per unit leaf dry weight.

and per unit leaf volume . . . . . . . . 87

The effect of different light intensities

on total chlorophyll content of leaves from
'Montmorency' cherry trees grown under

artificial shade expressed as amount per

unit leaf area, per unit leaf dry weight,

and per unit leaf volume . . . . . . . . 88

The effect of different light intensities

on the ratio of chlorophyll A to B for

leaves from 'Montmorency' cherry trees

grown under artificial shade . . . . . . 89

The effect of different light intensities

on the cross section thickness of leaves

from 'Montmorency' cherry trees grown under
artificial shade . . . . . . . . . . 90

vii

Table

10.

11.

The effect of different light intensities
on the leaf volume of leaves from 'Mont-
morency' cherry trees grown under artifi-
cial shade . . . . . . . . . .

The effect of different light intensities
on flower development and fruit set of
'Montmorency' cherry trees grown under
artificial shade . . . . . . . . .

Section Four

Gross photosynthetic rate and stomatal
resistance of leaves from 'Montmorency'
cherry trees grown under artificial shade .

Gross photosynthetic rate and stomatal
resistance of leaves from 'Montmorency'
cherry trees grown under artificial shade .

Net photosynthetic rate of leaves from
'Montmorency' cherry trees grown under
artificial shade . . . . . . . . .

The effect of temperature on net photosyn—
thetic rate of leaves from 'Montmorency'
cherry trees grown under artificial shade .

The effect of different light intensities
on maximum net photosynthetic rate as
predicted from asymptotic equations of
light response curves for 'Montmorency'
cherry trees grown under artificial shade .

viii

Page

93

95

113

114

115

117

120

Figure

LIST OF FIGURES

Page
Section One
Flow diagram of the differential open gas
analysis system used for net photosynthetic
rate determinations. Solid lines represent
gas flow through the system, and dashed lines
represent equipment connections . . . . . 9
Spectral distribution of GE 400 W multi-
vapor (metal halide) lamp and GE #00 W
multivapor lamp through neutral density
plastic filter . . . . . . . . . . . 14

The effect Of leaf age on net photo-

synthetic rate of sour cherry. Closed

circles represent percent maximum net
photosynthetic rate, and open circles

represent percent full leaf expansion.

Each point is the mean of eight leaves i SE . 20

Light response curves for 'Montmorency'
cherry leaves measured under the following
conditions: A. 30—40% relative humidity
and 10 C. B. 85-95% relative humidity and
10 C. C. 30-40% relative humidity and

25 C. D. 85-95% relative humidity and 25 C.
E. 30-40% relative humidity and 35 C.
F. 85-95% relative humidity and 35 C.
Different symbols on the curve represent
replications. and each replication is
the average Pn of two leaves .

ix

Figure
50

Page

Temperature response curves for 'Montmorency'
cherry leaves measured under the following
conditions: A. 30-40% relative humidity and
300 uE m"2 8‘1 light intensity. B. 85-95%
relative humidity and 300 uE m‘2 3-1 light
intensity. C. 30-h0% relative humidity and
1200 uE m-2 3'1 light intensity. D. 85-95%
relative humidity and 1200 uE m'2 3’1 light
intensity. E. 30-40% relative humidity and
2000 uE m'2 s-1 light intensity. F. 85-95%
relative humidity and 2000 uE m-2 s-1 light
intensity. Different symbols on the graph
represent replications. and each replication
is the average Pn of two leaves. . . . . 26

The effect of ambient CO2 concentration on

the net photosynthetic rate of sour Cherry.
Different symbols represent replications,

and each replication is the average Pn of

two leaves. . . . . . . . . . . . 30

Section Two

Diurnal pattern of net photosynthetic rate

for a sour cherry tree. Closed circles

are the photosynthetic rate. and open circles

are the PAR levels. Each point represents

the average of three replications. and each
replication is one tree monitored for one day 50

Seasonal pattern of net photosynthetic rate

of the first mature leaf from the a ex of

terminal shoots (with fruit present of

sour cherry for two consecutive years. Closed
squares represent the 1978 season. and open
squares represent the 1979 season. Each point

is the average of four replications . . . 55

Seasonal pattern of net photosynthetic rate.
number of leaves expanded, and fruit growth

for sour cherry in 1979. Open circles are

net photosynthetic rates of the first mature

leaf from the apex of terminal shoots. open
squares are average fresh weight of 100 fruit,

and open triangles are number of leaves ex-
13311de O O O O O O O O O O I O O 61

Figure

Page
Section Three

Spectral distribution of sunlight and
sunlight through two densities of black
polypropylene shade fabric . . . . . . . 76

Cross sections of 'Montmorency' cherry
leaves which expanded prior to (pre-shade)
or after (post-shade) being placed under
artificial shade. A. Leaf expanded
pre-shade, 100% full sunlight treatment.
B. Leaf expanded post-shade, 100% full
sunlight treatment. 0. Leaf expanded
pre-shade, 36% full sunlight treatment.

D. Leaf expanded post-shade, 36% full
sunlight treatment. E. Leaf expanded
pre-shade, 21% full sunlight treatment.

F. Leaf expanded post-shade, 21% full
sunlight treatment. G. Leaf expanded
pre-shade, 9% full sunlight treatment.

H. Leaf expanded post-shade, 9% full
sunlight treatment. . . . . . . . . . 91

Section Four

Light response curves from 'Montmorency'

cherry trees grown at different light

intensities under artificial shade.

Each symbol represents the average of

three replications - - - - - - - . - 118

xi

INTRODUCTION

Approximately 60-70% of the national sour Cherry crop
is produced in Michigan. Almost all of the crop is of one
cultivar, 'Montmorency'. There has been a trend toward
higher density cropping systems for sour cherry in Michi-
gan which may require more intensive cultural practices
due to increased competition for natural resources. An
accurate assessment of the effects of environmental fac-
tors and current cultural practices on the physiological
processes which determine plant productivity is essential
for the development of improved cultural practices and for
more efficient use of natural resources (energy, water,
land).

Photosynthetic efficiency may or may not be the main
factor limiting yield, but it certainly has a direct effect
on yield. Photosynthetic rate might be influenced by the
environment in which photosynthesis measurements are made
and by the environment in which a plant develops. The
effects of environmental variables on the photosynthetic
rate of sour cherry have not been evaluated.

Sour cherry fruit mature approximately 60 days after
full bloom, and canopy development is generally completed

by fruit harvest. Flower initiation for next year's crOp

1

2

occurs during this same time period (5-6 weeks after full
bloom). Thus. vegetative and reproductive growth are com-
petitive sinks for photosynthate, with both having rapid
but short term annual growth. Summer hedging (remOving
part of the foliage during the growing season) is prac-
ticed commercially, yet the effect of this and other cul-
tural practices on photosynthetic potential, flowering,
vegetative growth, and translocation pattern (photosyn-
thate, water, nutrients) of sour cherry have not been well
documented.

Diurnal and seasonal changes in photosynthetic rate
and other physiological processes should be considered
when making decisions regarding summer hedging and other
cultural practices. Knowledge of how environmental fac-
tors and cultural practices affect the growth and develop-
ment patterns of sour cherry would provide a scientific
basis for making decisions concerning orchard management.
Therefore. experiments were designed to study the factors
affecting the morphology and photosynthetic rate of sour
cherry.

The Objectives of this research were (a) to charac-
terize the effects of environmental factors on the photo-
synthetic rate of sour cherry and determine the optimum
environmental conditions for photosynthesis, (b) to deter-
mine the diurnal and seasonal patterns of photosynthetic
rate and to evaluate the effect of fruit on the photosyn-

thetic rate of sour Cherry, and (c) to determine the effect

3

of various levels of shade on leaf and shoot morphology

and leaf photosynthetic rate of sour cherry.

SECTION I

THE INFLUENCE OF LEAF AGE, LEAF POSITION ON THE SHOOT, AND
ENVIRONMENTAL VARIABLES ON NET PHOTOSYNTHETIC RATE
OF SOUR CHERRY (PRUNUS CERASUS L. 'MONTMORENCY')

 

Abstract. An infrared, differential open gas
analysis system was utilized in experiments to de-
termine the effects Of leaf age, leaf position on
the shoot, light intensity, temperature, humid-
ity, and ambient CO2 concentration on leaf net
photosynthetic rate (Pn) of sour cherry. Pn was
greater for leaves which had recently completed
expansion or were at least 50% expanded than for
either older, mature leaves or newly expanding
leaves on the same shoot. For individual leaves.
Pn reached its maximum when the leaf was greater
than 80% expanded, remained constant for 2 to a
weeks, then gradually declined. Maximum Pn oc-
curred at light intensities between 800-1000
uE m-2 s-l. As temperature decreased from 35 to
25 to 10 C maximum Pn occurred at higher light
intensities. At all light intensities Pn was
greater (10-40%) between 15-30 C than at lower
or higher temperatures. Pn at optimum tempera-
ture was greater (15-60%) at intermediate and high

2

(1200 and 2000 uE m‘ s’l) than at low (300

uE m-2

s'l) light intensity. In general, Pn was
greater at high (85-95%) than at low (30-#0%)

humidity, the effect being most pronounced at

high temperature (35 C) and high light intensity
(2000 uE m.2 s-l). Pn increased with increased
ambient CO2 concentration between 0 and 600 ppm,
the CO2 compensation point being approximately 82
ppm. Optimum conditions for maximum Pn of sour
cherry occur when recently expanded leaves are
exposed to light intensities between 1000-1200

uE m-2 s-l, temperatures of 20-30 C, high (85-95%)
humidity, and CO2 concentrations greater than 300

ppm (normal ambient).

There is a trend toward higher density cropping sys-
tems for sour cherry which may require more intensive cul-
tural practices due to increased competition for natural
resources (9). Accurate assessment of the effect of en-
vironmental factors on the physiological processes that
determine plant productivity is essential for the develop-
ment of improved cultural practices and more efficient
use of natural resources (energy, water, land).

The possibility Of improving yield by increasing pho-
tosynthetic efficiency has Often been suggested (15, 19,
29). Though photosynthetic efficiency may or may not be
the main factor limiting yield, it certainly has a direct
effect on yield. Photosynthetic rate is influenced by the
environment in which a plant develops and by the environ-
ment in which photosynthetic measurements are made. The

influence of environmental variables on the leaf

7

photosynthetic rate of apple (3, 5, 8, 13, 17, 18), pear
(12), citrus (10, 11), and peach (7) has been investigated.
Apple trees grown under shade have lower leaf photosyn-
thetic rates at light saturation than trees grown in full
sunlight (4, 13). Photosynthetic rates of apple, peach,
and citrus increase with increasing light intensity in a
hyperbolic pattern typical of most C3 plants (7, 11. 13).
The photosynthetic rate of apple, in general, reaches a
maximum between 20 and 30 C then declines at higher tem-
peratures, and the temperature response curve is of a
parabolic shape (13). The effects of environmental vari-
ables on the photosynthetic rate of sour cherry have not
been evaluated.

Therefore, this study was designed to Characterize
the effects of temperature. light intensity, humidity, and
ambient CO2 concentration on the net photosynthetic rate
of sour cherry. Effects of leaf position on the shoot
and leaf age were also examined. Information obtained
from these experiments will be used to study the influ-
ence of cropping systems, nutrition, growth regulators,
fruit load, and other factors on the photosynthetic effi-

ciency and carbon utilization of sour cherry.

Materials and Methods

Tree culture. One year old sour cherry trees (Prunus

cerasus L. 'Montmorency') on 'Mahaleb' rootstock were

8

grown in 20 1 plastic pots in a mixture of peat, loam, and
sand (1:2:1). Fertilizer, pesticides (Cyprex, Guthion,
Captan, and Plictran), and water were added as needed. The
trees were grown outside under natural conditions and were
moved into the laboratory for CO2 exchange measurements.
Unless otherwise indicated, photosynthetic measurements
were made using attached leaves on 6-8 week old shoots.
Photosynthetic measurements. A Beckman 865 Infrared
Gas Analyzer (Beckman Instruments Inc., Fullerton, CA) was
used to measure differential CO2 concentrations in an open
gas analysis system similar to that described by Augustine
.623 el- (1) as modified by Sams and Flore (21). A flow
diagram Of the system used is shown in Figure 1, and a list
of the system components is presented in Table 1.
Individual leaves were placed in controlled environ-
ment chambers to measure the steady state exchange of car-
bon dioxide. Gaseous fluxes were calculated per unit leaf
area. Leaf area was measured with a LI-COR Model LI 300
leaf area meter (LI-COR Inc., Lincoln, NE), and photosyn-
thetic rate was expressed as mg CO2 din"2 hr-l.
Series 500 plexiglass leaf chambers (15.3 x 10 x 10 cm)
(Paige Instruments, Davis, CA) were used. The chamber bot-
tom was constructed Of finned aluminum heat sink material
beneath which water from a refrigerated water bath was cir-
culated for temperature control. A variable speed fan

(Pamoter Model 900, Pamoter Co., Burlingame, CA) in the

bottom of the chamber provided air circulation. Boundary

Figure 1.

Flow diagram of the differential open gas
analysis system used for net photosynthetic
rate determinations. Solid lines represent
gas flow through the system, and dashed

lines represent equipment connections.

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Table 1. A description of the major components of the
differential open gas analysis system.
Component Description
A. Air Speedaire Model 22870 oilless
compressor air compressor (4.1 cfm) (W. W.
Grainger, Inc., Lansing, MI)
B. Air storage 189.3 1 reservoir with .69 kg/cm3

unit

CO2 control

Humidity
control

Air
manifold

Flowmeters

Assimilation
chambers

Automatic
switching
system

Sample
manifold

Dew point
hygrometer

IRGA
flowmeters

pressure

Aalborg proportioner with FM102-
05 flow meters (Aalborg Instru-

ments and Controls Inc., Monsey,
NY

Air saturation at dew point
temperature in a refrigerated
water bath

Manifold splits air stream to
assimilation chambers and IRGA

Aalborg Model FM102—O5
(Aalborg Instruments Inc.,
Monsey, NY)

Paige Instruments (Davis, CA)
series 500 leaf chambers;
custom built tree chamber

Versa—valve Type 31 solenoid
valves (Herbach and Rademan Inc.,
Philadelphia. PA) connected to
Dataplex 10 automatic signal
scanner (Hampshire Control Corp.,
Exeter, NH)

Transfers sample gas to IRGA
from solenoid

General Eastern System 1100 AP
dew point hygrometer (General
Eastern Equipment Corp..
Watertown, MA)

Aalborg Instruments Model
FMO92-04G (Aalborg Instru-
ments and Controls Inc.,
Monsey, NY)

Table 1 (cont'd.).

12

 

IRGA

Recorder

Standard
gases

Manual
switching
system

Reference
manifold

Temperature
control

Light
control

Light
sensor

Temperature
monitor

Beckman Model 865 Infrared Gas
Analyzer with water vapor filter

(Beckman Instruments Inc.,
Fullerton, CA)

Linear series 300 three pen
(Linear Instruments Corp.,
Irvine, CA)

Matheson : 1%; high 345-365 ppm
002; low 300-325 ppm C02
(Matheson Gas Products,
Lyndhurst, NJ)

Versa-valve Type 31 solenoid
valves (Herbach and Rademan Inc.,
Philadelphia, PA) connected to

a push button electrical switch

Transfers reference gas to IRGA
from solenoid

YSI Model 7# temperature con-
troller (Yellow Springs Inst.
Co., Yellow Springs, OH) wired
to a Blue M Model MR3210A-1
water bath (Blue M Electric Co.,
Blue Island, IL) circulating
water through assimilation
chamber heat sink

Stands with movable 400 W GE
multivapor lamps (General Elec-
tric Co., Cleveland, OH) and/or
neutral density filters (Herbach
and Rademan Inc., Philadelphia)

LI-COR Model LI-19OS quantum
sensors connected to a LI-COR LI
185 Quantum/Radiometer/Photometer
(LI-COR Inc., Lincoln, NB)

Omega Model 250 EQ 10 channel
digital temperature indicator
(Omega Engineering Inc.,
Stamford, CT) connected to chro-
mel constantan thermocou les
(.003") and a YSI Model 7
scanning telethermometer

13

layer resistance was determined to be less than .2 s cm-l.

GE #00 W multivapor (metal halide) lamps were used as
a light source. Control of light intensity was accom-
plished either by adjusting the distance between the light
source and the chamber or by using neutral density plastic
filters. The effect of the neutral density plastic filter
on the spectral distribution of light was determined with
an ISCO Model SR portable spectroradiometer. Spectral
measurements of the 400 W lamp through the neutral density
plastic filter revealed no apparent change in the spectral
distribution within the range of wavelengths tested (Fig—
ure 2). Light intensity in all chambers was monitored
with LI—COR Model LI 1908 quantum sensors connected to a
LI-COR Model LI 188 Integrating Quantum/Radiometer/Photo-
meter.

Humidity was controlled by saturating the chamber air
stream with water at a temperature lower than or equal to
the chamber temperature. The air stream was then warmed
to chamber temperature before it entered the chamber.
Humidity was monitored by measuring the dew point of the
chamber air stream with a flow-through dew point hygrometer
(General Eastern System 1100 AP).

CO2 concentration in the air stream was regulated by
mixing ambient air from which the CO2 had been scrubbed
(using soda lime) with air from a compressed air tank
which contained 800 ppm 002. A mixing pump (FMI Model
RRP-D, Fluid Metering Inc., Oyster Bay, NY) was used to

’11

(m

Spectral distribution of GE #00 W
multivapor (metal halide) lamp and
GB 400 W multivapor lamp through

neutral density plastic filter.

15

2::—

 

 

 

 

 

 

 

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Damn OO¢— OON— DOD“ DOD Dom Dov DON
1 1 I .J 3 am... 2
_V. \m .l. !. J. l .J J .I. .I. n] .V. 3 .J .c. n.-
\ .t. 3
3 .t. .\ .J .J a)
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3 .1 LI
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Figure 2

16

regulate the proportions of each air supply. The C02
concentration of the air stream was continuously moni-
tored using a Beckman 865 analyzer with nitrogen flowing
through the reference cell and the chamber air stream
flowing through the sample cell.

Experimental procedure and design. Trees with shoots
on which the terminal bud had set but on which the ter-
minal leaf had not unfolded were selected for experiments
to determine the effects of leaf position on the shoot and
leaf age on the Pn of individual leaves. The Pn of alter-
nate leaves, from the terminal leaf to the second leaf
from the base of the shoot, was measured. The Pn of the
terminal leaf was also monitored periodically from the day
it unfolded. A completely randomized design with eight
replications (each rep was one tree) was utilized for
these experiments.

Light, temperature, and 002 response curves were
determined using the first new, fully expanded leaf from
the apex of each shoot. Pn was measured at light inten—
sities within the range of O-ZuOO uE m-2 s-1 of photosyn-
thetically active radiation (PAR, radiation between #00-
700 nm wavelength). General asymptotic curves of the form

y = a + bdx
as described by Peat (16) were fitted to the light response
curves by computer (SPSS Nonlinear Program) using the

Gaussian method of successive approximations as described

by Snedecor and Cochran (25). Estimates of maximum Pn

17
and light compensation point were obtained from the fitted
equations. Different symbols on the light response curve
represent replications, and each replication is the average
Pn of two leaves. Light response curves were determined
at both low (30—40%) and high (85-95%) relative humidi-
ties for low (10 C), intermediate (25 C), and high (35 C)
temperatures.

The effect of temperature on Pn was determined by
varying the chamber temperature within the range of 5-40 C
while monitoring the steady state C02 exchange. Parabolic
equations were fitted to the data by computer (SPSS Regres-
sion Subprogram). Estimates of optimum temperature and Pn
at optimum temperature were obtained from the fitted equa—
tions. Different symbols on the temperature response
curve represent replications, and each replication is the
average Pn of two leaves. Temperature response curves

were determined at both low (30-40%) and high (BS-95%)

2

relative humidities for low (300 uE m' s'l), intermediate

2 2

(1200 uE m‘ s'1), and high (2000 uE m- 3'1) levels of

PAR.

C02 compensation point and the effect of CO2 concen-
tration on Pn were determined by decreasing the CO2 con-
centration in the air stream from 600 to 0 ppm and by
increasing the CO2 concentration from 0 to 600 ppm while
monitoring the steady state flux of 002. A logarithmic
curve was fitted to the data, and CO2 compensation point

was estimated from the equation. Different symbols on the

18

curve represent replications, and each replication is the
average Pn of two leaves. 002 response curves were deter-
mined at high (85-95%) relative humidity, 25 C, and 1200

2

uE m- s-1 PAR.

Results

Leaf positign on the shoot and leaf agp. Leaves on
the same shoot which were at least 50% expanded or had
recently attained 100% eXpansion had the greatest Pn per
unit area (Table 2). Older, fully expanded leaves at the
base of the shoot and younger, less than 50% expanded
leaves at the apex had lower Pn. Pn of individual leaves
was monitored from the first day the leaf unfolded until
several weeks after full leaf expansion (Figure 3). The
Pn of individual leaves increased until the leaf reached
greater than 80% full expansion, remained constant for 2-4
weeks, then began to decline.

Light response curves. Leaf Pn response to light was
determined at both high (85-95%) and low (30-u0%) relative
humidities for high (35 C), intermediate (25 C), and low
(10 C) temperatures (Figure h). The best fit asymptotic
equation was determined for each light response curve.
These equations and the predicted values for maximum Pn
and light compensation point are shown in Table 3.

The maximum Pn was higher for 25 C than for either 10

or 35 C at both high and low humidities. At low humidity

19

Table 2. The effect of shoot position on net photosyn-

thetic rate of 'Montmorency' cherry leaves.

 

 

 

Number of nodes from Leaf area Pnz
base of shoot (cm2) (mg CO2 dm-2 hr—l)
2 14.1 19.3 by
4 22.3 20.7 b
7 24.7 20.9 b
9 25.5 24.3 ab
11 20.3 30.4 a
12 19.8 30.9 a
13 14.8 27.5 a
14 13.1 20.6 b
15 (terminal) 10.2 8.2 c

 

zDetermined by differential infrared gas analysis under

constant conditions (1200 uE m-

light intensity,
temperature 25 C, and 85—95% relative humidity).

yMean separation by Duncan's multiple range test. 5%

level.

Figure 3.

20

The effect of leaf age on net photosynthetic
rate of sour cherry. Closed circles repre-

sent percent maximum net photosynthetic rate,
and open circles represent percent full leaf

expansion. Each point is the mean of eight

leaves : SE.

21

uogsuedxa JBO'I und %

ON

O?

cm

on

Oh

On

O 96 .00.. :3“. X

322.5 .2... .22 :8

cm

as .8: x

0?

On

ON

Ow

ON

9
Nd wmugxew 95

00

on

GOP

Figure 3

Figure 4.

22

Light response curves for 'Montmorency'

cherry leaves measured under the following

conditions:
A. 30-40%
B. 85-95%
C. 30-40%
D. 85-95%
E. 30-40%
F. 85-95%

relative
relative
relative
relative
relative

relative

humidity
humidity
humidity
humidity
humidity

humidity

and 10 C.
and 10 C.
and 25 C.
and 25 C.
and 35 C.
and 35 C.

Different symbols on the curve represent

replications, and each replication is the

average Pn of two leaves.

23

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PN lmg C02 0111'2 In”!

 

 

 

 

 

 

 

351 A 3 B
o O O
A 2 ‘
.01
a 0 0° ‘
D .0 0 {1-
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9 A
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01»
0 200 000 Y 1000 ' 1500 #1000 ' zinc 6 £00 ' 0'00 ' 1070 1470 Y 1000 T 2i00
C o 0 J D ' -
0 o o A o o
o I ‘ o p O o a A A
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A I“ o
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8
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0 £00 - 000 ' 1000 r 1100 1000 2200 i 500 T 0'00 1 1000 V 1100 ' "'00 0:00
9 01
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a U I! 0 A
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a. t "
A .1 A
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0 200 ' 000 1000 ' 1100 107007 2700 0 {00 ficoo 1000 1700 1000 ' 2:00

Figure 4

21.

 

 

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25

the maximum Pn for 25 C was approximately 30% higher than
for 35 C and 10% higher than for 10 C, while at high
humidity the Pn for 25 C was approximately 10% higher
than for either 10 or 35 C.

At temperatures of 10 or 25 C the maximum Pn was
similar for both high and low humidities. However, at
35 C the maximum Pn was greater for high than for low
humidity.

2 s.1 PAR

Maximum Pn occurred between 800-1400 uE m-
for both humidities at all three temperatures. However,
slightly higher light intensities were required for maxi-
mum Pn at 10 C than at 25 C, and at 25 C higher light
intensities were required than at 35 C. Light compensa-

2 s-1 PAR for both humid-

tion occurred between 20-80 uE m-
ities at all three temperatures. The light compensation
point was higher at 35 C than at either 25 or 10 C for

both humidities.

Temperature response curves. The effect of tempera-

2 8.1), inter-

2 s-l)

ture on Pn was determined for low (300 uE m-

2 8'1), and high (2000 uE m—

mediate (1200 uE m-
levels of PAR at both high (BS-95%) and low (30-40%) rela—
tive humidities (Figure 5). The best fit parabolic equa-
tion was determined for each temperature response curve.
These equations and the predicted optimum temperature and
maximum Pn for each curve are given in Table 4.

Maximum Pn occurred between 15—30 C and was 15-60%

greater than at either higher or lower temperatures for

Figure 5.

26

Temperature response curves for

’Montmorency'

cherry leaves measured

under the following conditions:

A.

30-40% relative

300 uE m‘2 s"1

85-95% relative

300 uE m"2 s-1

30-40% relative

1200 uE m'2 s‘1

85—95% relative

1200 uE m—2 s-1

30-40% relative

2000 uE m’2 s'2

85-95% relative

2000 uE m“2 s’1

humidity and
light intensity.
humidity and
light intensity.
humidity and
light intensity.
humidity and
light intensity.
humidity and
light intensity.
humidity and

light intensity.

Different symbols on the graph represent

replications, and each replication is the

average Pn of two leaves.

Pu lmg C02 (1111"2 hr'Il

27

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1’ #-
27 20..
e 0
1b a o. {D . o o o
J 0 C
M . .
[OT Q . a ‘4 .0 a
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, ° 010 “’0
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0 10 20 n ‘0 o to 20 30 60
3“ 3
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O. a 0 Cu .
’ a 13° ..
a a 0 0°
0% a 9
10» 109
0 '10 ' Yzo ' '30 Y 60 0 '10 V '20 ' ‘30 ' '10

Temperature (C!

Figure 5

 

 

 

28

 

 

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29

all three levels of Par at both relative humidities.

At optimum temperature the Pn was greater at high
and intermediate levels of PAR than at low levels of PAR
for both high and low humidities. For intermediate and
low levels of PAR the Pn at optimum temperature was only
slightly higher at high humidity than at low humidity.
However, for the high level of PAR, Pn at optimum tempera-
ture was much greater at high humidity than at low
humidity.

CO2 effect. The effect of ambient C02 concentration

on Pn was evaluated at 1200 uE m-2 s.1 light intensity,

 

25 C, and 90% relative humidity (Figure 6). Pn increased
with increased CO2 concentration between 0 and 600 ppm.
The CO2 compensation point predicted from the best fit

logarithmic equation was 82 ppm.

Discussion

Leaves which had recently attained 100% expansion or
were at least 50% expanded had greater Pn than younger or
older leaves on the same shoot (Table 2). Similar findings
have been reported for mulberry (22). The Pn of individual
leaves increased from the time the leaf unfolded until it
reached full expansion, remained constant for a time, then
declined (Figure 3). This finding is in general agreement
with reports for other species that Pn increases as the

leaf expands (14). From these results it appears that leaf

30

Figure 6. The effect of ambient CO2 concentration on
the net photosynthetic rate of sour cherry.
Different symbols represent replications,
and each replication is the average Pn

of two leaves.

31

.59: ~00

OOO OOm OOQ OOn OON OOP

OO. 0 .—
BTNoo 5 0.9.1.5.

Ow

9.

ON

mN

1,.m z.uulb :03 5w] Nd

Figure 6

32

age is more important in determining photosynthetic poten-
tial than position on the shoot. The lower Pn of young
leaves might be due to the presence of immature stomates
as has been reported for apple leaves (24), or the photo-
synthetic apparatus in the leaf may not be completely
developed. The lower Pn of older, fully expanded leaves
is probably due to normal senescence of the leaves.
Maximum Pn was higher for 25 C than for either 10 or
35 C at both humidities tested (Figure 4). At temperatures
of 10 or 25 C the maximum Pn was similar for both high and
low humidities. Thus, the differences in vapor pressure
deficit did not affect the Pn. This occurrence is in
general agreement with reports for other species that pho-
tosynthesis and diffusion resistances of individual leaves
are not affected by vapor pressure deficits (20, 27).
However, at 35 C maximum Pn was greater at high humidity
than at low humidity. Perhaps at this high temperature
the plant's ability to supply water to the actively tran-
spiring leaves (at high vapor pressure deficit) has been
exceeded, resulting in partial closure of the stomates.
Higher light intensities were required for maximum
Pn as the temperature decreased from 35 to 10 C. It is
generally accepted that no response to increasing light
intensity occurs when C02 concentration becomes limiting
(14). Thus, the higher light requirement at low tempera-
ture could mean that CO2 is not as limiting at the low

temperature as it is at higher temperatures (perhaps due

33

to a reduced rate of the dark reactions). The general
shape of the light response curve is hyperbolic, which is
typical of other fruit trees and 03 plants in general
(7, 14, 17, 23). However, we found that an asymptotic
curve of the form
y = a + bdx

gave a better fit for the data points in most cases. This
finding is in agreement with others who have shown that
the asymptotic relationship gave a better fit than the
hyperbolic relationship (which tends to over-estimate
maximum photosynthesis) for light response curves of other
species (6, 16).

Optimum Pn occurred between 15 and 30 C for all three
PAR levels at both humidities. This finding is similar
to that for apple, peach, and citrus, which had optimum
temperatures of 20-30 C, 30 C, and 15-30 C respectively
(7. 1o, 11, 13). At high light intensity the Pn at opti-
mum temperature was greater for high than for low humidity.
A similar decrease in Pn at low humidity has been reported
for citrus (11). This finding again indicates that under
conditions of high temperature and light intensity the
plant may not be capable of maintaining the high rate of
transpiration which is present under conditions of
greater vapor pressure deficit, thus resulting in par-
tial stomatal closure. The general shape of the tempera-
ture response curve is parabolic. This finding is in

agreement with reports of temperature response curves for

34

other fruit trees (7, 111 13).

Pn increased with increased CO2 concentration be—
tween 0 and 600 ppm. This is a typical response of many
other plants to increased CO2 concentrations (2, 11, 14,
26). The CO2 compensation point was 82 ppm. This value
is higher than has been reported for some other fruit
trees (11, 26).

Sour cherry leaves exhibit a positive response to
increased C02 concentration under optimum temperature and
light conditions. An increase in CO2 concentration from
300 to 400 ppm resulted in a 10% increase in Pn. Further
increasing the ambient C02 concentration to 600 ppm re-
sulted in a 40-50% increase in Pn. It has been stated
that the C02 concentration of the atmosphere could exceed
600 ppm by the year 2020 if current trends in CO2 increases
continue (28). If this projection is true, an increase in
Pn of sour cherry should result, assuming other environ—
mental factors can be optimized.

Leaves which have recently completed expansion have
the highest photosynthetic potential under optimum con-
ditions. Optimum conditions for photosynthesis of sour
cherry were found to be 1000-1200 uE m'2 s’1 light inten-
sity, temperature 20—30 C, 85-95% relative humidity, and
CO2 concentrations greater than ambient. Any cultural
practice which will improve environmental conditions
within the tree canopy might lead to increased produc-

tivity if Pn is limiting yield. However, more information

35

is needed about the effects of environmental factors on
photosynthesis and the translocation of photosynthates if
better decisions are to be made concerning canopy design
and orchard management to optimize Pn. Factors such as
the effect of pre-exposure to shading or temperature and
humidity stress, the effect of fruit load, and the par-
titioning and translocation of photosynthates require con-

tinued study.

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York.

SECTION II

FACTORS AFFECTING DIURNAL AND SEASONAL NET
PHOTOSYNTHETIC RATE OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

 

4O

Abstract. Diurnal and seasonal net photosyn-
thetic rates (Pn) of sour cherry were determined.
Leaf Pn was not affected by shoot excision for 24
hours after the shoot was excised. Under constant
light intensity (1200 uE m-2 s-l), temperature
(25 C), and relative humidity (80-90%) there was
no significant diurnal change in Pn for indivi-
dual sour cherry leaves. However, there was a
pronounced diurnal pattern in Pn for whole trees
measured under constant temperature and natural
variation in sunlight from sunrise to sunset.
Maximum Pn occurred before solar noon, remained
constant for a short time, then declined. Sea-
sonal patterns in leaf Pn varied, but, in general,
Pn increased in the spring as leaves eXpanded,
reached a peak, remained stable for several weeks,
then gradually declined. The Pn of leaves on ter-
minal shoots was not significantly different from
the Pn of leaves on spurs, and the presence of
fruit did not have a consistent effect on the Pn

of sour cherry leaves.

Diurnal and seasonal changes in photosynthetic rate

have been demonstrated for several species (31 71 9. 12).

L11

42

In apple, diurnal fluctuations in photosynthetic rate have
been reported, with greater rates occurring in the morning
than in the afternoon (16, 25). Other reports have in-
dicated that there is no consistent diurnal pattern of
photosynthesis for individual leaves if kept under con-
stant conditions (1. 31 5). In peach, higher rates of
photosynthesis have been observed early in the day, and
photosynthetic rates ranged from 3.6 to 12.5 mg CO2

dm-2 -1

hr during the growing season (7). Photosynthetic
rate of apple increases as the leaf expands, reaches a
maximum just after expansion is completed, remains high
for several weeks, than gradually declines during the rest
of the growing season (4, 9).

Changes in photosynthetic rate have been associated
with flowering, fruiting, and vegetative growth (12, 22).
The presence of fruit has been reported to increase the
photosynthetic rate of leaves of some species (2, 4, 7, 14,
18, 21). while in others lower gaseous diffusive resis-
tances and higher transpiration rates were found when fruit
were present (14, 23, 24, 29).

Fruit have been shown to be stronger sinks for
photosynthate than vegetative growth in apricot and peach
(20). In sour cherry, fruit growth occurs in a double
sigmoidal pattern, and fruit mature in about 60 days after
full bloom (27). Canopy development is generally completed

in sour cherry by fruit harvest, with spur leaf develop-

ment completed approximately 20-25 days after full bloom

43

and terminal leaf development completed 60 days after full
bloom (8). Flower initiation also occurs during this
period (5-6 weeks after full bloom) (11). Thus, vegeta-
tive and reproductive growth are competitive sinks for
photosynthate, with rapid but short term annual growth.
Current trends toward higher density cultural prac-
tices for sour cherry and the increasing use of summer
hedging (removing part of the foliage during the growing
season) (19) make management decisions regarding when and
how to prune more difficult. Diurnal and seasonal changes
in Pn and other physiological processes should be con-
sidered when making decisions regarding summer hedging and
other cultural practices. Knowledge of how environmental
factors and cultural practices affect the growth and de-
velopment patterns of sour cherry would provide a scien-
tific basis for making decisions concerning orchard man-
agement. Therefore, experiments were designed to deter-
mine the diurnal and seasonal patterns of Pn and to evalu-

ate the effect of fruit on Pn of sour cherry.

Materials and Methods

Tree culture. One year old sour cherry trees (Prunus
cerasus L. 'Montmorency') on 'Mahaleb' rootstock were
grown in 20 1 plastic pots in a mixture of peat, loam, and
sand (1:211). Fertilizer, pesticides (Cyprex, Captan,

Guthion, and Plictran), and water were added as needed.

44

Potted trees were used in experiments to determine the
diurnal patterns of Pn and the effect of shoot excision
on leaf Pn. Mature, six year old 'Montmorency' sour
cherry trees on 'Mahaleb' rootstock (1.8 m x 4.3 m, Horti-
culture Research Center, East Lansing, MI) were used in
experiments to determine the effect of fruit load on Pn
and the seasonal patterns of Pn.

Photosynthetic measurements. Pn was determined
utilizing an open gas analysis system as previously de-
scribed (26). For all measurements except whole tree Pn,
CO2 exchange rates were determined for intact leaves
placed in environmentally controlled chambers. The cham-
ber temperature was maintained at 25‘: .5 C, PAR (photo-
synthetically active radiation, radiation in the 400-700 nm
range) at 1200 uE m-2 s'z, and relative humidity between
85-95% for optimum Pn (26). Pn was eXpressed as mg 002

cm"2 hr'l.

Pn of whole trees was determined for small potted
trees in a .9 m x .9 m x 1.2 m clear plexiglass chamber
maintained at 25 i 3 C, and in which relative humidity
was monitored and found to be 80-90%. The chamber was
placed outside in full sunlight, and Pn was measured from
sunrise to sunset. PAR was recorded at the level of the
tOp of the tree canopy within the chamber. Soil respira-
tion was eliminated by enclosing the pot in a plastic bag.

2

The average Pn was calculated as mg CO2 fixed per dm leaf

area per hour.

45

Shoot excision. Potted trees were selected which had
two uniform shoots. An initial determination of Pn was
made on the first fully expanded leaf from the terminal
on each of the two shoots, then one of the shoots was ex-
cised from the tree (15—23 cm below the leaf to be mea-
sured) and placed immediately in a beaker of distilled
water. The end of the shoot was recut under water, and
Pn was determined 1, 2, 4, 5, and 24 hours after excision
for leaves on both excised and non-excised shoots. The
experimental design was completely randomized with eight
replications.

Measurement of diurnal Pn. Pn of leaves from potted
trees was monitored from 9:00 a.m. until 7:00 p.m. in
individual leaf chambers under constant conditions (tem—
perature 25 i .5 C, PAR 1200 uE 111‘2 s‘l. and 85-95% rela-
tive humidity). Diurnal Pn of a whole tree was measured
by placing small potted trees inside the whole plant
chamber where 002 exchange and PAR were monitored under
natural sunlight on clear days from sunrise to sunset.

Seasonal trends of and fruit effects on Pn. Two
uniform scaffolds were selected on both the east and west
sides of individual trees planted in a north-south row
orientation. Just prior to bloom and before leaves
emerged, the flower buds were removed from one scaffold
on each side of the tree. The other scaffold was allowed
to flower and set fruit. Fresh weight of 100 fruit, the

average number of leaves on terminal shoots and spurs.

46

and the average leaf area of 50 shoots and spurs were
monitored to determine the stage of fruit growth and
foliage development.

All Pn measurements were initiated between 9:00 and
10:00 a.m.. and standard leaf chamber conditions were
used. The Pn of both the first leaf which expanded at the
base of each shoot and the first mature leaf from the apex
of terminal shoots were monitored by excising shoots
periodically throughout the 1978 and 1979 growing seasons.
Seasonal trends in Pn were determined for the first mature
leaf from the apex of shoots with fruit. The Pn of leaves
on terminal shoots was also compared to the Pn of leaves

on spurs (both had fruit present).

Results

Shoot excision. The Pn of leaves on excised shoots
was not significantly different from the Pn_of leaves on
non-excised shoots at 1, 2, 4, 5, or 24 hours after shoot
excision (Table 1).

Diurnal trends. For individual leaves under con-

2 s‘1 PAR, 25 C, and 85—95%

stant conditions (1200 uE m-
relative humidity) there was no significant change in
diurnal Pn over a ten hour period (Table 2). The ex-
periment was repeated, and similar results were found.

The diurnal trend of a whole tree was determined

using small potted trees (without fruit) in a whole plant

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48

Table 2. Diurnal change in net photosynthetic rate of

'Montmorency' cherry leaves under optimum

conditions.

 

 

Hours after first
measurement

OOCDVOUI-PUNHO

H

(mg CO

PnZ
2

16.7
17.2
17.5
18.0
17.8
17.1
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17.9
16.9
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under constant conditions (1200 uE mm2 s

‘1 light

intensity, temperature 25 C, and 85-95% relative

humidity.

yMean separation by Duncan's multiple range test. 5%

level.

49

chamber placed outside from sunrise to sunset (Figure 1).

2 hr-1

Pn increased to 18 mg CO2 dm' four hours after sun-
rise, remained constant for two to three hours, then grad-
ually declined toward sunset. Pn reached the maximum
level three to four hours before PAR reached its peak in-
tensity, and Pn began to decline before PAR reached maxi-
mum intensity.

Fruit effect on Pn. The effect of fruit on Pn was
monitored in 1978 and 1979. Measurements of the Pn of
leaves on shoots with and without fruit were made at
several stages of fruit development and later in the sea-
son after harvest. Leaves on shoots with fruit had a
higher average seasonal Pn than leaves on shoots without
fruit in 1978 (Table 3). The Pn was higher for leaves on
shoots with fruit present when measured in stages II and
III of fruit growth. However, when measured after har-
vest no difference was noted.

During the 1979 season there was no significant
difference in Pn between leaves on shoots with and without
fruit (Table 4). In stages I and III of fruit develop-
ment the leaves on shoots with fruit tended to have higher
Pn. However, in stage II of fruit development and after
harvest Pn tended to be higher on shoots without fruit.

Seasonal trends. Pn during the 1978 season was high-
est at the beginning of the season (41.2 mg 002 dm-2 hr'1)1

2

declined (to about 18-20 mg CO dm- hr'l) during stage II

2
of fruit development, remained constant for several weeks,

Figure 1.

50

Diurnal pattern of net photosynthetic
rate for a sour cherry tree. Closed
circles are the photosynthetic rate,
and open circles are the PAR levels.
Each point represents the average of
three replications, and each repli-
cation is one tree monitored for one

day.

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54

then declined late in the season (Figure 2). However,
the seasonal pattern of Pn was quite different during
the 1979 season (Figure 2). The Pn was 27-30 mg CO

dm-2 hr-1

2
early in the season, remained constant for 8-10

weeks, then declined.

The Pn of the first leaf at the base of each terminal
shoot was compared to that of the youngest fully expanded
leaf on spurs several times during the 1979 season (Table
5). These leaves completed expansion at about the same
time and were the same age. The Pn of leaves at the base
of the terminal shoots was not significantly different
from that of leaves of a similar age on spurs at any stage

of development measured during the season.

Discussion

Twenty-four hours after shoot excision there was no
significant difference in the Pn of 'Montmorency' sour
cherry between leaves on excised shoots (placed in dis-
tilled water) and leaves on shoots remaining on the tree
(Table 1). Therefore, we concluded that shoots could be
excised from mature trees in the field, placed in water,
and taken to the laboratory for measurement of Pn. This
procedure allowed measurement of the Pn of mature trees
treated in the field without the use of mobile equipment.

Leaves on shoots with fruit had a significantly

greater Pn than leaves on shoots without fruit during

Figure 2.

55

Seasonal pattern of net photosynthetic
rate of the first mature leaf from the
apex of terminal shoots (with fruit
present) of sour cherry for two consecu-
tive years. Closed squares represent the
1978 season, and open squares represent
the 1979 season. Each point is the

average of four replications.

56

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58

the first year of the experiment (Table 3). But in the
second year of the experiment, no significant difference
in Pn was found between leaves on shoots with or without
fruit. Increased Pn of leaves due to the presence of
fruit has been reported for peach (6, 7), apple (2, 13,
18), and citrus (21). However, for sour cherry the pre-
sence of fruit does not appear to have a consistent effect
on the Pn of leaves. Increases in the photosynthetic rate
of leaves caused by the presence of fruit has been attri-
buted to many factors, including the hormonal content of
the fruit (22) and lower assimilate concentrations in the
leaves with fruit present (thus, preventing a decline in
the photosynthetic rate due to end product inhibition of
enzyme activity) (22). Several hormones have been asso-
ciated with increased photosynthetic rates of leaves (22).
In grape, changes in hormonal levels have been shown to
occur when sink strength changes, and the photosynthetic
rate changes with these changes in hormonal levels (15).
Cherry is a much smaller fruit than apple, peach, or
citrus, and the vegetative growth rate is extremely rapid.
Perhaps the apparent inconsistency in the effect of fruit
on Pn is due to the rapidly growing shoots and leaves
which may be more powerful sinks in some situations.
Kriedemann (20) has shown that developing peach and apri-
cot fruits are strong sinks for photosynthetic assimi-
lates, but that in citrus the young vegetative growth is

a stronger sink than the fruit.

59

Differences in fruit load, rate of growth, and envi-
ronmental conditions between years might cause changes in
the sink strength of the fruit and/or vegetation. Tem-
perature has been shown to affect the sink activity of
wheat grain, thus influencing assimilate movement (28).
Yield data were not taken on the trees used in our study,
so differences in fruit load could not be documented.

The Pn of leaves on terminal shoots was not signifi-
cantly different from the Pn of leaves of similar ages
on spurs (Table 5). In apple, spur leaves have been re-
ported to have lower photosynthetic rates than terminal
leaves (10). Barden (4) attributed the difference in pho-
tosynthetic rate between spur and terminal leaves to dif-
ferences in light exposure, because spur leaves were in-
side the canopy growing under heavy shade conditions.

There was no significant change in diurnal Pn for
sour cherry leaves when maintained under optimum condi-
tions (Table 2). This finding is in agreement with some
reports for other species (1, 3, 17). We therefore con-
cluded that leaves could be used at least for short
periods of time in experiments without adjusting for di-
urnal changes in Pn.

There was a pronounced diurnal pattern of Pn for
whole trees when measured under conditions of natural sun-
light from sunrise to sunset (Figure 1). Maximum Pn was
reached before solar noon, remained relatively constant

for a short time, then declined toward sunset. Pn

6O
declined before PAR began to decrease. A similar diur-
nal pattern of photosynthesis has been reported for apple
by Mika (25), who suggested that the plant may experience
a progressive water deficit which lowers the photosynthetic
rate because of stomatal closure. Since Pn reached a maxi-
mum then began to decrease before PAR reached its maximum
and decreased, it is possible that PAR levels needed for
maximum Pn were reached before solar noon, and that a
buildup of photosynthate in the leaves could have resulted
in feedback inhibition of photosynthesis. Optimum light
intensity for sour cherry has been determined to be ap-
proximately 1000—1200 uE m'2 s’1 (26). This level of PAR
coincides with the maximum Pn in Figure 1.

The seasonal trend in Pn was monitored for two con-
secutive years (Figure 2). In the 1978 season the Pn was
very high at the beginning of the year, declined rapidly,
leveled off and remained constant for several weeks, then
declined in late fall. However, in the 1979 season the Pn
was not as high at the beginning of the season. Pn re-'
mained constant from the beginning of the season until
late in the season when there was a rapid decline. Pn
during the season may be affected by many factors including
environment (light, temperature, humidity), fruit load,
and leaf age. The rapid decline in photosynthetic rate
at the end of the season is associated with leaf senes-
cence and may be accelerated by lower temperatures.

Figure 3 compares the Pn. number of leaves expanded, and

Figure 3.

61

Seasonal pattern of net photosynthetic
rate, number of leaves expanded, and
fruit growth for sour cherry in 1979.
Open circles are net photosynthetic rates
of the first mature leaf from the apex of
terminal shoots, open squares are average
fresh weight of 100 fruit, and open tri-

angles are number of leaves expanded.

62

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63

fruit growth for part of the 1979 season. It is apparent
that leaf and fruit growth are competitive sinks for pho-
tosynthates during this part of the growing season.

Flower initiation also occurs during this period (5-6
weeks after full bloom) (11), and may also compete for
photosynthate during this period. Thus, decisions re-
garding cultural practices are quite critical during this
period because they may affect not only the present, but
also next year's crop. For example, summer hedging is be-
coming a commercially accepted practice and is normally
done 6-7 weeks after full bloom (19), which would occur
near the time of flower bud initiation, maximum leaf growth,
and just prior to early stage III of fruit growth.

The seasonal pattern of Pn for sour cherry is not
consistent from year to year (Figure 2). In a given year,
environmental factors (light, temperature, water) may be
limiting, especially during critical stages of develop-
ment (such as stage III of fruit development when most of
the fruit weight is attained). Optimum conditions for Pn
of sour cherry leaves occur at 1000-1200 uE m-2 8'1 light
intensity, 25 C, and high humidity (26). If Pn is lim-
iting yield at certain stages of development, then any
cultural practice that would optimize conditions for Pn
would be desirable. Further work is needed to determine
if Pn is limiting yield (and at which stage of development
it is most limiting), and to determine the effects of en-

vironmental factors on Pn, vegetative growth, reproductive

64

growth. and partitioning of photosynthate. The knowledge
obtained from such work could be used to develop cultural

practices and orchard designs that would optimize yield.

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in the apple tree. Photosynthetica 6:381—386.

Sams, C. E., and J. A. Flore. 1980. The influence
of leaf age, leaf position on the shoot, and en-
vironmental variables on net photosynthetic rate of
sour cherry (Prunus cerasus L. 'Montmorency'). J;
Amer. Soc. Hort. Sci. (manuscript concurrently sub-
mitted).

Tukey, H. B. 1934. Growth of the embryo, seed, and

pericarp of the sour cherry (Prunus cerasus) in re-

 

lation to season of fruit ripening. Proc. Amer. Soc.

 

Hort. Sci. 31:125-144.

Wardlow, I. F. 1974. Temperature control of trans-
location. p. 533-538. In R. L. Bieloski, A. R.
Ferguson, M. Cresswell (eds.) Bulletin 12, The Royal
Society of New Zealand, Wellington.

29.

69

Woodward, R. G., and H. M. Rawson. 1976. Photo-
synthesis and transpiration in dicotyledonous plants

II. EXpanding and senescing leaves of soybean.

Aust. J. Plant Physiol. 3:257—267.

SECTION III

THE EFFECTS OF ARTIFICIAL SHADE ON THE LEAF
AND SHOOT MORPHOLOGY OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

 

7O

Abstract. One year old potted sour cherry
trees were grown in full sunlight to the 11-15
leaf stage then shaded to establish 100, 36, 21,
or 9% of full sunlight treatments. At the end of
the growing season trees grown in full sunlight
had greater terminal (57%) and lateral (46%) shoot
diameters than trees grown in 9% of full sunlight.
Average internode length of lateral shoots and
average leaf area on terminal shoots was greater
for trees grown in 21% of full sunlight than for
trees grown in full sunlight. Specific leaf
weight was greater (38-125%) for leaves on trees
grown in full sunlight than for trees grown under
shade. There was no difference in leaf chloro-
phyll content on an area basis. However, leaves
on trees grown under 9% of full sunlight had more
(48-92%) chlorophyll than those grown in full sun-
light when expressed in mg/g leaf dry weight and
mg/cm3 leaf volume. Palisade and spongy mesophyll
layers of cells and total leaf thickness were
greater for leaves grown in full sunlight than
for leaves grown under shade. Trees grown in full
sunlight had a greater number of flower buds and

flowers per tree the following spring than trees

71

72

grown in 36% of full sunlight. Trees grown in
21 and 9% of full sunlight had no flowers pre-
sent. The number of flowers per bud and the
percent fruit set were not significantly dif—

ferent between treatments which had flowers.

There have been many reports concerning effects of
shade on the morphology of fruit trees (2, 3, 4, 14, 15,
17). In general, plants grown in shade have greater leaf
areas, decreased leaf weights and thickness, and modified
leaf structures (5). Many species have a more developed
palisade and spongy mesophyll region, resulting in thicker
leaves when grown under high light intensity (6, 8, 10.
25). Leaves of Atriplex patula grown under low light in-
tensity have smaller cells, fewer vascular strands, and
fewer cell layers across a leaf section than those grown
under high light intensity (7). Mesophyll resistance is
also higher in plants grown under low light intensities
(10, 16, 25). Leaves grown under low light intensities
also have more chlorophyll per unit weight or unit volume
of leaf, but less chlorophyll per unit area than leaves
grown under high light intensities (7, 8). There is often
a lower ratio of chlorophyll A to B in leaves grown under
low light intensities (7, 8, 12, 22).

Average specific leaf weight of apple has been re-
ported to decrease with increasing shade (3, 17, 27).

Shading has also been reported to result in reduced number

73

and weight of new shoots, reduced increase of shoot girth,
and reduced leaf thickness of apple (17). A decrease in
flowering of apple due to shading has been reported (9),
and Jackson gt al. (18, 19) reported that shading also
resulted in reduced flower bud formation, reduced fruit
set, reduced fruit size, and lower fruit quality of apple.
Heinicke (14) suggested that in apple the increased
shading with tree size resulted in an increase in leaf
area per tree.

Within a canopy, the light spectra resulting from
sunflecks (intermittent flashes of light which penetrate
the canopy due to wind movement of outer canopy leaves)
is similar to that of full sun (23). However, in natural
shade (caused by the foliage of the tree) there is a
greater ratio of near infrared to photosynthetically
active radiation (PAR, light in the 400—700 nm region)
(23). Proctor (28) observed more infrared and less visible
light within an apple canopy as penetration increased from
the top to the bottom of the canopy.

Light studies with different types of sour cherry
tree canopies have shown that the degree of shading within
the canopy differs significantly among the canopy types
tested, the 660/730 nm ratio of light decreased with in—
creased shading, and summer hedging caused a pronounced
decrease in inner canopy light intensity (11). There is
a current trend in the industry toward higher density

plantings of sour cherry, where size control is

74
accomplished by summer hedging (21).

A better understanding of the effects of shade on
tree morphology is essential if a scientific basis for de-
signing more efficient tree canopies is to be developed.
Therefore, a study was undertaken to evaluate the effects
of shade on sour cherry. The objectives were to determine
the effects of various levels of shade on leaf and shoot
morphology and on the photosynthetic rate of sour cherry.
Herein, we report on the effects of shade on the leaf and
shoot morphology of sour cherry and relate these effects

to current orchard practices.

Materials and Methods

Tree culture. One year old sour cherry trees (Prunus
cerasus L. 'Montmorency') on 'Mahaleb' rootstock were
grown in 20 1 plastic pots in a mixture of peat, loam, and
sand (1:2:1). Fertilizer, pesticides (Captan, Plictran,
Guthion, Cyprex, and Benlate), and water were added as
needed. Trees were grown to the 11-15 leaf stage in full
sunlight then transferred to artificial shade treatments
for the remainder of the growing season. The leaf just
below the terminal bud was tagged to distinguish pre- and
post-shade grown leaves. Six weeks after all shoot and
leaf growth had ceased (Sept. 1) the plants were evaluated
to determine the effects of shading on leaf and shoot mor-

phology. These experiments were conducted in 1978 and

75

repeated in 1979. Since results were similar between
years, only the 1979 data are reported.

Shade treatments and experimental design. Solar radi-
ation was reduced with pipe frame structures (3.7 m x 2.4 m
x 1.8 m) covered with black polypropylene shade fabric
(A. H. Hummert Co., St. Louis, MO) which transmitted an
average of 36, 21, or 9% of PAR (photosynthetically active
radiation measured with a LI-COR Model LI 188 Quantum/
Radiometer/Photometer). Full solar radiation was obtained
by growing trees outside without shading. Structure ven-
tilation prevented temperature differences of greater than
i 3 C and relative humidity differences greater than i 5%.
The effect of the shade cloth on the spectral distribution
of light was determined with an ISCO Model SR portable
spectroradiometer (ISCO, Lincoln, NB). Spectral measure-
ments of full solar radiation through the lightest (36%
of full sunlight) and heaviest (9% of full sunlight) shade
cloths revealed no apparent changes in spectral distribu-
tion within the range of wavelengths tested (Figure 1).

For chlorophyll, specific leaf weight, and leaf thick-
ness evaluations a completely randomized factorial design
with four replications was utilized. There were two leaf
ages (a leaf expanded pre-shade vs. a leaf expanded post-
shade) and four light intensities (100, 36, 21, and 9% of
full solar radiation). For all other evaluations a com-
pletely randomized design with four replications of each

treatment was used.

76

Figure 1. Spectral distribution of sunlight
and sunlight through two densities

of black polypropylene shade fabric.

77

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78

Shoot morphology. Following shade treatment, the
length (cm) and diameter (cm) of each shoot were measured.
Shoot diameter was measured both at the base and at the
point between pre- and post-shade treatments. No lateral
shoots were present at the time of shade treatment. The
number, length (cm), and base diameter (cm) of lateral
shoots were determined for each plant after shade treat-
ment.

Leaf morphology. The number of leaves, average area

 

per leaf, and total leaf area developed both pre- and post-
shade were determined for terminal and lateral shoots.

Leaf area was determined with a LI—COR Model LI 3000 leaf
area meter.

Specific leaf weight, chlorophyll, and leaf anatomy
measurements were made for the third leaf on the main
shoot above (post-shade) and below (pre-shade) the point
where shading was applied. Discs (8.5 mm and 4.0 mm) were
cut from the interveinal area of each leaf for chlorophyll
measurement and leaf cross sections. After measurement
of the remaining leaf area, the leaves were placed in plas—
tic bags, frozen on dry ice, lypholized, and the dry
weights were measured. Specific leaf weight (SLW) was
calculated as mg leaf dry weight per cm2 leaf area.

Two leaf discs (8.5 mm diameter) were used for chloro-
phyll determinations according to the method described by
MacKinney (26) as modified by Arnon (1). Smaller leaf

discs (4.0 mm diameter) were fixed in FAA (50% ethyl

79

alcohol, 10% formaldehyde, 5% glacial acetic acid, and 35%
water), dehydrated with tertiary butyl alcohol, and infil-
trated with paraffin (20). Sections (10 um) were cut with
a rotary microtome, fixed on slides with Weaver's fixer,
and stained with safranin—fast green. The number of pali-
sade layers, thickness of the palisade and spongy meso-
phyll, and total leaf thickness (um) were estimated by
examining five sample sections from each of four replica-
tions. Photographs were taken using a Wild M20 research
microscope equipped with a 35 mm film carrier and a photo-
automat exposure control unit. The average leaf volume
(cm3) was calculated by multiplying the leaf area by the
leaf thickness.

Flowering data. At the end of the growing season the
plants were removed from the shade structures and placed
in a 4 C cooler to fulfill the chilling requirement. The
following spring the trees were placed in full sunlight,
and the number of flower buds per tree, number of flowers
per tree, number of flowers per bud, and the percent fruit

set at fruit maturity were recorded.

Results

Shoot growth. Trees grown in full sunlight had
greater terminal and lateral shoot diameters than the trees
grown in 9% of full sunlight (Table 1). Average lateral

internode length was significantly greater for trees grown

80

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81
under 21% of full sunlight (Table 2). There were no

statistically significant differences among treatments

for length of terminal of lateral shoots, number of lateral
shoots, internode length of terminal shoots, or total shoot
growth per tree. However, there was a general trend for an
increase in these growth parameters as percent sunlight
decreased to 21%, followed by a slight decrease as light
was further reduced to 9% of full sunlight.

Leaf number and area. There were no significant dif-
ferences among treatments for number of leaves per terminal
shoot, number of leaves on lateral shoots, total leaf area
on terminal shoots, total leaf area on lateral shoots, or
total leaf area per tree. However, average leaf area on
terminal shoots for trees grown in 21% of full sunlight
was significantly greater than for trees grown under full
sunlight (Table 3).

Specific leaf weight. Specific leaf weight was

 

greater for leaves from trees grown in full sunlight than
for leaves from trees grown under all shade treatments,
regardless of pre- or post-shade expansion (Table 4).
Specific leaf weight of leaves expanded pre—shade was not
significantly different from the specific leaf weight of
leaves which expanded post-shade. There was no signifi-
cant interaction between shade level and time of leaf ex-
pansion.

Chlorophyll determination. Chlorophyll A, chloro—

phyll B, and total chlorophyll contents were determined

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84

 

 

 

 

Table 4. The effect of different light intensities on
the specific leaf weight of leaves from 'Mont-
morency' cherry trees grown under artificial
shade.

Specific leaf weight

Sunlight (mg/cmz)

(%) Pre-shadeZ Post-shade
100 13.7 ay 12.6 a
36 9.9 b 8.8 b
21 9.3 b 8.1 b

9 8.0 b 5.6 c

 

zTrees placed under artificial shade when in the 11-15

leaf stage, and measurements determined 6 weeks after

terminal set.

yMean separation within columns by Duncan's multiple

range test, 5% level.

85

and expressed as mg/dm2 leaf area, mg/g leaf dry weight,
and mg/cm3 leaf volume (Tables 5-7). There was no signi-
ficant difference between pre— or post-shade leaves or
among shade treatments for chlorophyll A, chlorophyll B,

2 leaf area.

or total chlorophyll when expressed as mg/dm
However, all chlorophyll measurements were significantly
greater for plants grown under 9% of full sunlight when

3

expressed as mg/g leaf dry weight or mg/cm leaf volume.
The ratio of chlorophyll A to chlorophyll B in post-shade
leaves was similar for all shade levels (Table 8). Pre-
shade leaves grown in full sunlight had a higher A to B
ratio than leaves grown in 36 or 21% of full sunlight.

Leaf anatomy. The thickness of the palisade and
spongy mesophyll cell layers and total leaf thickness was
greater for leaves grown in full sunlight than for leaves
grown in 36, 21, or 9% of full sunlight regardless of
pre- or post-shade eXpansion (Table 9). The spongy meso-
phyll of leaves grown under shade was less dense, and there
were fewer palisade layers in these leaves (Figure 2).

The leaves grown under full sunlight had at least three
layers of palisade cells, while leaves grown in 9% of full
sunlight had as few as one layer. Leaf volume was not
significantly different among shade treatments for either
pre- or post-shade leaves (Table 10).

Floweringgdata. Plants grown in full sunlight had a
greater number of flowers per tree and a greater number

of flower buds per tree than plants grown in 36, 21, or

86

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89

Table 8. The effect of different light intensities on
the ratio of chlorophyll A to B for leaves from
'Montmorency' cherry trees grown under artifi—

cial shade.

 

 

 

Sunlight Pre-shadez Post-shade
(%)
100 1.99 ay 1.38 a
36 1.08 b 1.39 a
21 0.81 b 1.34 a
9 1.58 ab 1.96 a

 

zTrees placed under artificial shade when in the 11-15
leaf stage, and measurements determined 6 weeks after
terminal set.

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range test, 5% level.

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Figure 2.

91

Cross sections of 'Montmorency' cherry
leaves which expanded prior to (pre-shade)
or after (post-shade) being placed under
artificial shade.
A. Leaf expanded pre-shade,
100% full sunlight treatment.
B. Leaf expanded post-shade,
100% full sunlight treatment.
C. Leaf expanded pre-shade,
36% full sunlight treatment.
D. Leaf expanded post-shade,
36% full sunlight treatment.
E. Leaf expanded pre-shade,
21% full sunlight treatment.
F. Leaf expanded post-shade,
21% full sunlight treatment.
G. Leaf expanded pre-shade,
9% full sunlight treatment.
H. Leaf expanded post-shade,

9% full sunlight treatment.

92

 

93

Table 10. The effect of different light intensities on
the leaf volume of leaves from 'Montmorency'

cherry trees grown under artificial shade.

 

 

Leaf volume

 

 

Sunlight (cm3)
(%) Pre—shadez Post-shade
100 1.3 ay 0.6 a
36 0.8 a 0.6 a
21 0.8 a 0.6 a
9 0.8 a 0.4 a

 

zTrees placed under artificial shade when in the 11-15
leaf stage, and measurements determined 6 weeks after
terminal set.

yMean separation within columns by Duncan's multiple
range test, 5% level.

94

9% of full sunlight (Table 11). The number of flowers per
bud was not significantly different between the 100 and 36%
of full sunlight treatments. Plants in the 21 and 9% of
full sunlight treatments had no flowers present. The
percent fruit set was not significantly different between

the 100 and 36% of full sunlight treatments.

Discussion

Levels of shading (36, 21, and 9% of full sunlight)
were selected to simulate light intensities found inside
cherry tree canopies when grown under commercial condi—
tions. Light intensity within the fruit bearing surface
of a cherry tree may be as low as 10—25% of full sunlight,
depending on the canopy structure (11). Canopy closure
has been shown to be rapid for sour cherry (11). There-
fore, trees were grown in full sunlight for part of the
growing season and transferred to shade for the remainder
of the season to simulate leaves at the base of the shoot
(which expand in full sunlight) being shaded by leaves
developing later in the season.

Previous reports for cherry and apple trees have in-
dicated that shading either did not affect shoot length
or resulted in smaller increases in length (3, 29).
Jackson and Palmer (17), however, reported that shading
to 37, 25, and 11% of full sunlight resulted in longer

shoot length for apple. Our data show an increasing trend

95

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96
with decreasing light intensity for all growth parameters.
Further, for each parameter the value at 9% of full sun-
light was less than the value at either 36 or 21%. These
findings indicate a tendency for an increase in shoot
length under moderate to heavy shade then a decrease under
severe shade. Average internode length of lateral shoots
displayed the same tendency, with the length for trees
grown under 21% of full sunlight being significantly
longer than the length for trees grown in full sunlight
(Table 2).

Smaller shoot diameters have been reported for apple
trees grown under shade (17). We found that the terminal
shoot diameter at the point of shading and average lateral
shoot diameter at the base were significantly greater for
trees grown in full sunlight than for trees grown in 9%
of full sunlight (Table 1). The trend appears to be
toward the development of longer and smaller diameter
shoots under shade than in full sunlight. Perhaps this
occurrence could be attributed to less carbohydrate being
produced under shade conditions, resulting in a higher
ratio of nitrogen to carbohydrate and increased cell
elongation. A reduction in total sugars and dry matter
accumulation in cherry trees grown under heavy shade has
been reported (13). Maximum terminal growth of cherry
has been shown to occur at higher levels of nitrogen than
maximum dry weight increases (29). The decreasing trend

in shoot growth under severe shade (9%) could result when

97

the photosynthetic rate is so low that not enough energy
is produced to maintain growth (even cell elongation).

Heavy shade has been reported to decrease leaf num-
ber and total leaf area (27), to have no effect on leaf
number and total leaf area (3), and to increase the total
leaf area (14) of apple. We found that the average leaf
area on the terminal shoot was significantly greater for
trees grown under 21% of full sunlight than for those
grown in full sunlight (Table 3). Average leaf area on
terminals and laterals, number of leaves on laterals.
total leaf area on terminals and laterals, and total leaf
area per tree showed an increasing tendency with decreasing
light intensity similar to the increase in shoot growth.

Specific leaf weight tended to decrease as the per-
cent shade increased. Similar decreases in specific leaf
weight for leaves developed in shade have been reported
for apple (3). Leaves developed under full sunlight had
thicker palisade and spongy mesophyll layers and greater
total leaf thickness than leaves developed in shade. This
decrease in leaf thickness and change in leaf structure
is characteristic of plants grown in heavy shade (5, 8,
17, 24). Cherry leaves grown in shade had fewer palisade
layers, and the spongy mesophyll appeared to be less dense
with smaller cells (Figure 2). Shade leaves are typi-
cally thinner and have smaller spongy mesophyll cells and
less mesophyll surface area, resulting in reduced meso-

phyll conductance when compared to sun leaves (5).

98
Although leaf volume was not statistically different among
treatments, leaves grown in full sun tended to have greater
leaf volumes than those grown in shade. Thus, the leaves
from plants grown in shade appear to be larger due to
greater surface area, but the actual leaf volume is
smaller.

Leaves from trees grown in 9% of full sunlight were
found to have a significantly greater content of chloro—
phyll A, chlorophyll B, and total chlorophyll than those
grown in full sunlight when the chlorophyll content is
expressed as mg/g leaf dry weight or mg/cm3 leaf volume
(Tables 5-7). When expressed as mg/dm2 leaf area, there
was no significant difference in chlorophyll content of
leaves grown at different light intensities. Light in-
tensity has been reported to have similar effects on the
chlorophyll content of other species (5. 7. 8). These
findings are consistent with the fact that shade leaves
were thinner, had lower specific leaf weights, and had
smaller leaf volumes than leaves grown in full sunlight.

Leaves grown in shade have been reported to have
fewer chloroplasts, but the chloroplasts are usually lar-
ger and contain more chlorophyll (5). Boardman (5) has
reported that the increase in size of the chloroplast and
amount of chlorophyll per chloroplast is are offset by a
decrease in number of chloroplasts per unit of leaf
surface.

The ratio of chlorophyll A to chlorophyll B in

99
post-shade leaves tended to be greater for leaves grown
under severe shade (9% of full sunlight) than for leaves
grown in full sunlight. This finding does not agree with
reports for some species in which the ratio of chlorophyll
A to B was lower at lower light intensities than at high
light intensities (7, 8, 12, 25). The chlorophyll content
of pre-shade leaves was greater for leaves grown under
heavy shade than for those grown in full sunlight. This
occurrence indicates that cherry leaves can adapt to light
intensity changes after they have expanded as has been sug-
gested for other species (5).

Trees grown in full sunlight had more flowers per
tree and more total flower buds per tree the following
spring than trees grown in shade (Table 10). The number
of flowers per bud and the percent fruit set was not dif-
ferent between treatments with flowers present. Trees
grown in light intensities less than 36% of sull sunlight
had no flowering the following year. Decreases in the
number of flower buds by shading during the previous year
are well documented (18). Although it is obvious that
shading has a pronounced effect on flower formation in
cherry, additional studies are needed to determine the
critical levels of light required for flowering in mature
cherry trees grown under commercial conditions and to
determine the critical periods during the growing season

when light is required.

Literature Cited

Arnon, D. T. 1949. Copper enzymes in isolated

chloroplasts polyphenoloxidase in Beta vulgaris.

 

Plant Physiol. 24:1-15.

Auchter, E. C., A. L. Schrader, F. S. Lagasse, and

W. W. Aldrich. 1926. The effect of shade on the
growth, fruit bud formation, and chemical composi-

tion of apple trees. Proc. Amer. Soc. Hort. Sci.
23:368-382.

Barden, J. A. 1974. Net photosynthesis, dark respira-
tion, specific leaf weight, and growth of young apple

leaves as influenced by light regime. J. Amer. Soc.

 

Hort. Sci. 99:547-551.
. 1977. Apple tree growth, net photo-

 

synthesis, dark respiration, and specific leaf weight
as affected by continuous and intermittent shade.

J. Amer. Soc. Hort. Sci. 102:391-394.

Boardman, N. K. 1977. Comparative photosynthesis of
sun and shade plants. Ann. Rev. Plant Physiol.
28:355-357.

Bowes, G., W. L. Ogren, and R. H. Hageman. 1972.

Light saturation, photosynthesis rate, RUDP carboxylase
activity, and specific leaf weight in soybeans grown

under different light intensities. Crop Sc'. 12:77-79.

100

10.

11.

12.

101

Bjorkman, 0., N. K. Boardman, J. M. Anderson, S. W.
Thorne, D. J. Goodchild, and N. A. Pyliotis. 1972.
Effect of light intensity during growth of Atriplex
patula on the capacity of photosynthetic reactions.
chloroplast components, and structure. Carnegie
Inst. Washington Yearb. 71:115-135.

, and P. Holmgren. 1963. Adaptability

 

of the photosynthetic apparatus to light intensity
in ecotypes from exposed and shaded habitats.
Physiol. Plant. 16:889-914.

Cain, J. C. 1971. Effects of mechanical pruning
of apple hedgerows with a slotting saw on light
penetration and fruiting. J. Amer. Soc. Hort. Sci.
96:664—667.

Crookston, R. K., K. J. Treharne, P. Ludford, and
J. L. Ozbun. 1975. Response of beans to shading.
Crop Sci. 15:412-416.

Flore, J. A. 1979. Light intensity, quality, and
distribution in relation to canopy development in
sour cherry (Prunus cerasus L. 'Montmorency').

HortScience 14:407.

 

Friend, D. J. C. 1960. The control of chlorophyll
accumulation in leaves of Marquis wheat by tempera-
ture and light intensity I. The rate of chlorophyll
accumulation and maximum absolute chlorophyll con-

tents. Physiol. Plant. 13:776-785.

13.

14.

15.

16.

17.

18.

102

Gray, G. F. 1934. Relation of light intensity to

fruit setting in the sour cherry. Mich. Tech. Bull.

 

No. 135. Michigan State College.

Heinicke, D. R. 1964. The micro-climate of fruit
trees III. The effect of tree size on light pene-
tration and leaf area in 'Red Delicious' apple trees.
Proc. Amer. Soc. Hort. Sci. 85:33—41.

1966. The effect of natural shade

 

on photosynthesis and light intensity in 'Red Deli-
cious' apple trees. Proc. Amer. Soc. Hort. Sci.
88:1-8.

Holmgren, P. 1968. Leaf factors affecting light—
saturated photosynthesis in ecotypes of Solidago
virgaurea from exposed and shaded habitats.

Physiol. Plant. 21:676-698.

Jackson, J. E., and J. W. Palmer. 1977. Effects of
shade on the growth and cropping of apple trees

I. Experimental details and effects on vegetative

growth. J. Hort. Sci. 52:245—252.

 

. 1977. Effects of

 

shade on the growth and cropping of apple trees
II. Effects on components of yield. J. Hort. Sci.
52: 253-266 0

19.

20.

21.

22.

23.

24.

25.

26.

103

. 1977. Effects

 

of shade on the growth and cropping of apple trees
III. Effects on fruit growth, chemical composition,
and quality at harvest and after storage. J. Hort.
§g;. 52:267-282.

Johansen, D. A. 1940. Plant microtechnique.
McGraw-Hill, New York.

Kesner, C. 1978. Management techniques for high

density cherry plantings. Ann. Rpt. Mich. Hort.

 

§gg. 108:108-111.

Lewandowska, M., J. W. Hart, and P. G. Jarvis. 1976.
Photosynthetic electron transport in plants of Sitka
spruce subjected to different light environments

during growth. Physiol. Plant. 37:269-274.

 

Looney, N. E. 1968. Light regimes within standard
size apple trees as determined spectrophotometrically.
Proc. Amer. Soc. Hort. Sci. 93:1-6.

Louwerse, W., and W. v. d. Zweerde. 1977. Photosyn-
thesis, transpiration, and leaf morphology of
Phaseolus vulgaris and.§§§ gays grown at different

irradiances in artificial and sunlight. Photosynthe-

 

tica 11:11—21.
Ludlow, M. M., and G. L. Wilson. 1971. Photosynthe-

sis and illuminance history. Aust. J. Biol. Sci.

 

24:1065—1075.
MacKinney, G. 1941. Absorption of light by chloro-
phyll solutions. J. Biol. Chem. 140:315—322.

 

27.

28.

29.

104

Maggs, D. H. 1960. The stability of the growth pat-
tern of young apple trees under four levels of
illumination. Ann. Bot. (NS) 24:434-450.

Proctor, J. T. A. 1978. Apple photosynthesis:
Micro-climate of the tree and orchard. HortScience

13: 61+].-6Ll'30

 

Proebstring, E. L., and A. L. Kenworthy. 1954.
Growth and leaf analysis of 'Montmorency' cherry
trees as influenced by solar radiation and inten-

sity of nutrition. Proc. Amer. Soc. Hort. Sci.

63:41-48.

SECTION IV
THE EFFECTS OF ARTIFICIAL SHADE ON THE LEAF

PHOTOSYNTHETIC RATE OF SOUR CHERRY
(PRUNUS CERASUS L. 'MONTMORENCY')

105

Abstract. Sour cherry trees were grown in
full sunlight to the 11-15 leaf stage then shaded
to establish 100, 36, 21. and 9% of full sunlight
treatments. The effects of shade on leaves which
expanded before and after shade application were

2 8"1 light

determined. At 1200 and 2000 uE m’
intensities the photosynthetic rates of leaves
on trees grown in full sunlight were greater
(50—150%) than those of leaves grown in 9% of
full sunlight. Also, photosynthetic rates of
leaves which expanded before shading were higher
(70%) than those of leaves which expanded after
shading when grown in 9% of full sunlight. How-
ever, at low light intensity (320 uE m-2 s-l)
photosynthetic rate was not significantly dif-
ferent among leaves from trees grown in 100, 36,
21, or 9% of full sunlight. Maximum net photo-
synthetic rate for leaves from trees grown under
9% of full sunlight occurred at lower light in-
tensities than leaves grown in full sunlight.
Net photosynthetic rate was greater (25-170%) at
25 C than at 10 or 40 C for leaves grown in full
sunlight and in 9% of full sunlight. Net photo—

synthetic rate at 25 C was greater (47%) for

106

107

leaves grown in full sunlight than for leaves

grown in 9% of full sunlight.

The effects of shading on the photosynthetic rates
of many species have been examined (1, 3, 16, 18, 23).

In general, plants grown in shade have higher net pho-
tosynthetic rates at low light intensities but lower
maximum photosynthetic rates at high light intensities
(3). However, Barden (2) has reported that although shade
grown leaves of apple had lower photosynthetic rates at
high light intensities, the photosynthetic rates at low
irradiance were similar for sun and shade grown leaves.

Photosynthetic rates of many types of fruit trees
increase with increasing light intensity in a hyperbolic
pattern characteristic of most C3 plants (5. 12, 14).
Photosynthetic light response curves for cherry have been
reported, with maximum net photosynthetic rates occurring
between 800-1400 uE m—2 s—1 of photosynthetically active
radiation (PAR, radiation in the 400-700 nm range) (21).
If shading decreased light intensity below this level, a
reduction in photosynthesis would result.

Light distribution and penetration patterns of apple
have been studied, and it has been reported that interior
leaves receive lower light intensities than leaves of the
outer canopy (7, 8, 10, 15, 17). Studies have also shown
that similar light relations exist in sour cherry

canopies (6).

108

Heinicke (9) hypothesized that daily photosynthetic
rate could be calculated if the percent full sunlight
received by a leaf and the rate of photosynthesis of a
generalized leaf at a given light level were known.

Barden (1) has suggested that this hypothesis is an over-
simplification, and that to accurately estimate the pho-
tosynthetic potential of a leaf requires knowledge of

the previous history of the leaf in regard to environ-
mental conditions. We have previously demonstrated that
the leaf morphology of sour cherry can be influenced by
the light intensity under which the leaf develops and by
the light intensity received by the leaf after it has ex-
panded (22).

Predictions of whole canopy photosynthetic potential
and the photosynthetic potential of various leaf types
within the canopy would be useful in designing more ef-
ficient and productive orchards. However, if more effi-
cient orchards are to be designed, increased knowledge
of the influence of environmental factors on photosynthe-
sis as well as methods of modifying tree canopy and orchard
design to establish desirable environmental conditions is
needed. Therefore, experiments were designed to determine
the effects of shading on the photosynthetic rate of 'Mont—

morency' cherry leaves.

109

Materials and Methods

Tree culture. One year old sour cherry trees (Prunus
cerasus L. 'Montmorency') on 'Mahaleb' rootstock were
grown in 20 1 plastic pots in a mixture of peat, loam,
and sand (1:2:1). Fertilizer, pesticides (Captan, Plic—
tran, Cyprex, Benlate, and Guthion), and water were added
as needed. Trees were grown to the 11-15 leaf stage in
full sunlight then transferred to artificial shade treat-
ments for the remainder of the growing season. Six weeks
after all shoot and leaf growth had ceased (Sept. 1), the
plants were evaluated to determine the effects of shading
on photosynthetic rate. This study was conducted in 1978
and repeated in 1979. Since results were similar. only
the 1979 data will be reported.

Shade treatment and experimental design. Solar ra-
diation was reduced with pipe frame strctures (3.7 m x
2.4 m x 1.8 m) covered with black polypropylene shade
fabric (A. H. Hummert Co., St. Louis, MO) which transmitted
an average of 36, 21, or 9% of PAR (photosynthetically
active radiation, measured with a LI-COR Model LI 188
Integrating Quantum/Radiometer/Photometer). Full solar
radiation was obtained by growing trees outside without
shading. Structure ventilation prevented temperature dif-
ferences greater than i 3 C and relative humidity differen-
ces greater than i 5%. The shade cloth decreased light

intensity without affecting light quality (22). Unless

110

otherwise indicated, all experiments were completely
randomized with four replications.

Photosynthesis determinations. Shoot terminals were
marked at the time the trees were transferred to the shade
treatments. Both the third leaf above (post-shade) and
below (pre—shade) the marked point were used for photo-
synthetic rate determinations. Where indicated, gross
photosynthetic rate and stomatal resistance of intact
leaves were determined with a ventilated diffusion poro—
meter (Model VP—l, Cayuga Development, Ithica, NY) using
the method described by Peet gt al. (19). The porometer
contained a lithium chloride humidity sensor which al-
lowed measurement of stomatal resistance while exposing
1”002 (9.7 ul/l.
330 ppm 002, 21% 02, 14 ml) for 30 3. Immediately after

the abaxial surface of the leaf (1 cm2) to

pulsing, the eXposed area was excised with a No. 11 cork
bore and placed in a scintillation vial containing 0.5 ml
of Protosol (New England Nuclear) and was allowed to digest
48 hr. Samples were bleached with 1.0 ml of benzoyl per-
oxide in toluene (5 g in 30 ml). After 24 hr, 15 ml of
scintillation fluid (5 g PPO/l of toluene) was added and
radioactivity was determined with a Beckman LS 100 Liquid
Scintillation Spectrometer. Corrections were made for
background and quenching, and gross photosynthetic rate

2 hr"1 using leaf disc area,

was calculated as mg CO2 dm'
exposure time, radioactivity, and specific activity of

002. Gross photosynthetic measurements were made outside

111

in natural sunlight at a temperature between 28-30 C

2 s-l) and high (2000 uE m"2 8.1)

and at low (320 uE m-
light intensities.

Net photosynthetic rate determination. Net photo—
synthetic rate (Pn) was determined in the laboratory
using intact leaves placed in environmentally controlled
leaf chambers. Unless otherwise indicated, environmental
conditions were maintained at 25 i .5 C, 85-95% relative
humidity, and PAR (photosynthetically active radiation.
radiation in the 400-700 nm wavelength region) at 1200
uE m-2 s-l. A differential open gas analysis system was
used as previously described (21). Pn was calculated as
the amount of C02 fixed (mg) per unit leaf area (dm2) in
one hour.

Temperature study. Terminal leaves which were fully
expanded after initiation of the shade treatment were used
for the temperature study. Net photosynthetic rates of
intact leaves on trees from the 100 and 9% of full sun-
light treatments were determined at temperatures of 10,
25, and 40 C, 85-95% relative humidity, and PAR of 1200
uE m—2 S-l.

Light response curves. Terminal leaves which ex—
panded after initiation of the shade treatment were used
for the light response curves. Pn was determined for in-
tact leaves on trees from all shade treatments (100, 36, 21.
and 9% of full sunlight) at PAR levels between 0 and 2000

uE m-Z s-l, a temperature between 25 i .5 C, and 85—95%

112

relative humidity. Asymptotic curves were fit to the

data as previously described (21).

Results

14C02 determination of gross photosynthetic rate.

 

There was no significant difference in gross photosynthetic
rate (Ps) or stomatal resistance among leaves from trees
grown under 100, 36, 21, or 9% of full sunlight or between
leaves which expanded pre— or post-shade when determined

2 s'l) (Table 1).

at low light intensities (PAR 320 uE m’
and there was no interaction between time of leaf expansion
and shade treatment. At high levels of PAR (2000 uE

m'2 s_1) there was no significant difference between leaves
which expanded pre- and post-shade, and there was no inter-
action between time of leaf expansion and shade treatment
(Table 2). However, leaves grown in full sunlight had a
higher Ps and a higher stomatal resistance than leaves
grown in 9% of full sunlight whether the leaf expanded

pre- or post-shade (Table 2).

Net photosynthetic rate determination. Both pre-

 

and post-shade leaves which were grown in full sunlight
had significantly greater net photosynthetic rates than
leaves grown in 9% of full sunlight (Table 3). At a PAR
level of 1200 uE m-2 s'1 there was a decreasing trend in

net photosynthetic rate with increasing shade for both

pre- and post-shade leaves. Pre—shade leaves did not

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115

Table 3. Net photosynthetic rate of leaves from 'Mont-

morency' cherry trees grown under artificial

 

 

 

 

shade.
z -2 -1
Sunlight Pn (mg 002 dm hr )
(%) Pre-shade Post-shade
100 22-2 3y 22.4 a
36 18.8 ab 16.7 ab
9 13.6 b 8.0 c

 

2Determined by differential infrared gas analysis under
constant conditions (1200 uE m-2 s-1 light intensity.

temperature 25 C, and 85—95% relative humidity).

yMean separation within columns by Duncan's multiple
range test, 5% level.

116
differ significantly from post-shade leaves, and there was
no interaction between time of leaf expansion and shade
treatment.

Temperature study. Leaves from trees grown in full
sunlight had greater net photosynthetic rates than leaves
from trees grown in 9% of full sunlight at 10, 25, and 40 C
(Table 4). Pn was greater at 25 C than at either 10 or
40 C for leaves from trees grown at both 100 and 9% of
full sunlight.

Light response curves. Maximum Pn occurred between
400-1000 uE m-2 s.1 for all treatments. Leaves from trees
grown under full sunlight reached maximum Pn at higher
levels of PAR than those grown under 9% of full sunlight
(Figure 1). Maximum Pn was greater for leaves from trees
grown under full sunlight than for those grown under
shade. The initial increase in Pn with increasing light
intensity was greater for leaves from trees grown in full
sunlight than for those grown in shade. The best fit
asymptotic equation was determined for each light response
curve, and predictions of maximum net photosynthesis were

obtained from these equations (Table 5).

Discussion

It is generally believed that sun grown leaves are
less efficient under low light intensity than shade grown

leaves (3, 4). However, the Ps and stomatal resistance

117

 

 

 

 

 

Table 4. The effect of temperature on net photosynthetic
rate of leaves from 'Montmorency' cherry trees
grown under artificial shade.

Pnz (mg 002 dm"2 hr-1)

Sunlight Temperature

(%) (C)
10 25 40
100 17.72 by 22.15 a 14.37 c
9 10.27 d 15.06 c 5.51 e

 

zDetermined by differential infrared gas analysis under
constant conditions (1200 uE m.2 s-1 light intensity,
temperature 25 C, and 85-95% relative humidity).

yMean separation by Duncan's multiple range test. 5%

level.

Figure 1.

118

Light response curves of leaves from
'Montmorency' cherry trees grown at
different light intensities under
artificial shade. Each symbol represents

the average of three replications.

119

 

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120

Table 5. The effect of different light intensities on
maximum net photosynthetic rate as predicted
from asymptotic equations of light response
curves for 'Montmorency' cherry leaves grown

under artificial shade.

 

 

 

Sunlight Best fit Predicted
(%) asymptotic maximum Pn
equation (mg C02 dm-Z hr-l)
100 19.6 - 24.80994)x 19.6
36 15.3 - 20.6(.996)X 15.3
21 13.9 - 16.9(.996)x 13.9
9 9.5 - 14.9(.989)x 9.5

 

121

of sour cherry were not significantly different among
leaves grown under 100, 36, 21, and 9% of full sunlight

when measured under low (320 uE m"2

s-1) light intensity
(Table 1). Similar results have been reported for other
species (2, 16).

At higher light intensities (1200 and 2000 uE m"2 8.1)
the photosynthetic rate and stomatal resistance of leaves
on trees grown in full sunlight were greater than those
of leaves on trees grown in 9% of full sunlight
(Tables 2-3). Also, the photosynthetic rates of leaves
from the 9% of full sunlight treatment were higher for
leaves which expanded pre-shade than for those which ex-
panded post-shade when measured under the higher light in-
tensities. This finding indicates that the shading did
not affect the photosynthetic capacity of leaves which
developed under full sunlight as much as those which de-
veloped under shade. However, the photosynthetic capacity
of both pre- and post-shade leaves from the 9% of full
sunlight treatment was less than that of leaves from the
full sunlight treatment. It has been suggested that ana—
tomical changes are restricted to eXpanding leaves (3).
but that leaves can adapt to light after leaf expansion
has ceased due to factors other than basic structural
changes (1, 3). The data presented here indicate that
heavy shade did decrease the photosynthetic capacity of

leaves which expanded in full sun, but that the effect

122

of the shading was more pronounced on those leaves which
expanded under heavy shade.

Sour cherry spur leaves and the lower leaves on ter-
minal shoots complete expansion early in the season and
are soon shaded by terminal leaves which develop later
.in the season (6). The degree of shading depends on the
canopy structure. We found the maximum photosynthetic
rate to be lower for leaves which expanded in shade than
for those which expanded in full sunlight (Figure 1), but
Pn increases with increasing light intensity for all the
leaves. Similar results have been reported for other
species (3. 16). More specifically, our data indicate
that the photosynthetic rate of a sour cherry leaf which
has been shaded increases as light intensity increases
to 500-800 uE m-2 s.1 then reaches a maximum (Figure 1).
Thus, the photosynthetic rates of inner canopy leaves
may be limited when light is severely reduced by shading.
Summer hedging (removal of part of the terminal shoots
which lets more light into the canopy) is becoming a
common commercial practice in sour cherry production (11).
This practice may lead to increased photosynthetic rates
of inner canopy leaves which have been shaded.

Pn was greatest at 25 C for leaves from plants grown
in full sunlight and 9% full sunlight (Table 4). Both
leaf types had lower Pn at 10 and 40 C. The Pn at op-
timum temperature was greater for leaves grown in full

sunlight than for those grown in 9% of full sunlight.

123

However, the response of leaves grown in 9% of full sun-
light to temperature changes was similar to that of leaves
grown in full sunlight. This temperature response may

be commercially important if the temperature environment
of inner canopy leaves is changed by summer hedging. Sum-
mer hedging removes the outer canopy leaves which shade
inner canopy leaves from direct sunlight and reduce air
movement inside the canopy. It has been suggested that
the leaf temperature of some species may exceed the tem-
perature of the surrounding air by as much as 21 C under
conditions of high insolation and zero wind speed (20).

It also appears that if leaves have been growing in a
favorable light environment, they can maintain higher pho-
tosynthetic rates at suboptimal temperatures than leaves
which have developed in heavy shade.

The data presented indicate that even leaves which
have been growing under heavy shade (similar to inner
canopy environment) retain the ability to utilize higher
light intensities (as could be obtained by summer hedging)
for increased photosynthesis. Kriedemann gt gl. (13) has
indicated that the photosynthetic rate of inner canopy
grape leaves can be increased by intermittent flashes of
light (sunflecks) which penetrate the canopy due to wind
movement of the outer canopy leaves. The ability of the
inner canopy leaves to benefit from sunflecks and respond
to the continual fluctuations in light intensity, tempera-

ture, CO2 concentration, and humidity within the canopy

124

are important factors which affect the photosynthetic rates
of sour cherry leaves. Accurate estimates of the photo-
synthetic potential of sour cherry leaves will require
knowledge of these factors as well as the previous his-
tory of the leaf as suggested by Barden (1). The maximum
Pn of sour cherry leaves grown in shade is lower than

the Pn of those grown in full sunlight, but even leaves
grown in severe shade do not reach maximum Pn until light

2 s'1 (Figure 1). Light inten-

intensity is 400-800 uE m"
sity inside some sour cherry canopies is not this high
(6). Sour cherry leaves which expand in full sunlight and
are then shaded have lower maximum Pn than leaves which
are not shaded after eXpansion (Tables 2-3). Thus, any
cultural practice which leads to better light penetra-
tion into cherry canopies might increase the photosynthe-
tic potential of sour cherry leaves.

Many factors remain to be evaluated before final con-
clusions about the effects of shading can be applied to
commercial situations. Is Pn limiting yield, which
leaves contribute most to fruit growth, what types of
translocation patterns exist in sour cherry, and how many
leaves are required for continued fruit and vegetative
growth are among the many questions which remain to be

answered.

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