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A czmmfi'm’mz STUDY €31: THZE CROWN MfliSTURE
CGNTEN?’ {N RELAY‘EC'N TC} gimme; TEMEERATURE'
CF WE‘éTER WE'VE/W ANE' EARLS?

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MECHEQAN STATE UM ‘ERSE‘JY
33622125523 L. Davis
1962';

  
   
     

  

LIBRAR Y “

Michigan Sta tc
University

A QUANTITATIVE STUDY OF THE CROWN
MOISTURE CONTENT IN RELATION TO
KILLING TEMPERATURE OF WINTER

WHEAT AND BARLEY

BY

Daniel L. Davis

A THESIS

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

MASTER OF SCIENCE

Department of Farm Crops

1962

  

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TABLE OF CONTENTS

Page
INTRODUCTION . . . . . . . . . . . . . . . . . . 1
REVIEW OF LITERATURE . . . . . . . . . . . . . . 2
MATERIALS AND METHODS . . . . . . . . . . . . . 10
RESULTS AND DISCUSSION . . . . . . . . . . . . . 20
SUMMARY . . . . . . . . . . . . . . . . . . . . 38

BIBLIWMPI'IY O O O O O O O O O O O O O O O O O O 39

LIST OF FIGURES
FIGURE PAGE

1. The vacuum oven used for drying the tissue
for crown moisture tests . . . . . . . . . . 13

2. The chamber and apparatus used to gradually
lower the temperature for the standard
freezing test . . . . . . . . . . . . . . . 15

3. The recovery chamber used following the
freezing test . . . . . . . . . . . . . . . l6

4. The vacuum chamber used to mechanically
induce a high level of hydration in wheat
plants 0 O O O O O O O O O O O O O O O O O 0 l7

5. and 6. The killing temperature for hardened
Genesee and Redcoat wheat plotted against
percent moisture of the upper crown . . . . 24

7. A dying wheat plant which illustrates the
total destruction of tissue that occurs at
a high level of hydration when exposed to
temperatures of 25° to 28° F. . . . . . . . 25

8. A surviving wheat plant showing injury in
the lower central part of the crown which
occurs when the upper crown moisture is
between 72.1%.and 74.6% and exposed to a
temperature just above the killing point
of the tissue . . . . . . . . . . . . . . . 26

9. A dying wheat plant showing the destruction
of the entire lower crown and meristem. This
occurs when the upper crown moisture is
between 72.1% and 74.6%»and the plant is
exposed to a temperature below the killing
point of the tissue . . . . . . . . . . . . 27

10. The complete destruction of tissue found in
a dying, dehydrated wheat plant when exposed
to temperatures less than 00 F. . . . . . . 28

FIGURE PAGE

11, 12, 13. The killing temperature for Dicktoo,
Hudson, and ang barley respectively plotted
against percent moisture of the crown . . . . 30

14. The complete destruction of the crown tissue
in the variety ang at the left; whereas the
variety Dicktoo from the same pot has the
injury confined to the lower central part of
the crown . . . . . . . . . . . . . . . . . . 31

15. The variety Dicktoo at the same hydration
level as the plants shown in Figure 14 but
exposed to a lower temperature . . . . . . . 32

LIST OF TABLES

TABLES

l.

The relation of crown moisture
temperature and plant survival
Genesee wheat plants . . . . .

The relation of crown moisture
temperature and plant survival
Redcoat wheat plants . . . . .

The relation of crown moisture
temperature and plant survival

to freezing
of hardened

to freezing
of hardened

to freezing
of hardened

Dicktoo, Hudson, and ang barley plants .

PAGE

22

23

29

ACKNOWLEDGEMENTS

The author wishes to express sincere gratitude to
Dr. C. R. Olien and Dr. Everett H. Everson for their
guidance, encouragement, and constructive criticism
during the entire course of study.

Dr. William Meggitt's and Dr. Albert Ellingboe's

critical reviews of the manuscript are appreciated.

INTRODUCTION

A major problem of the northern winter cereal areas
of the wheat belt is the extensive damage due to ice
formation in plants during severe winters. Different
aspects of the phenomena concerning the reaction of
plants subjected to low temperature stresses have been
studied. Investigations have resulted in a partial under—
standing of low temperature hardiness but several ques-
tions remain unanswered at the present time. Different
patterns of injury as related to moisture content of the
plant has been one of the recent aspects described. This
research has been on a qualitative basis and using barley
as plant material.

The objectives of this research problem were to estab-
lish the exact hydration range of winter wheat and barley
in which different patterns of injury occur and to quanti-
tatively determine the relationship between the killing
temperature during freezing and the crown moisture content.
Established plants in a uniformly hardened condition were

used in the experiments.

REVIEW OF LITERATURE

The literature is voluminous on the theories proposed
to explain the effect of low temperatures on plants. Luyet
and Gehenio (6), Levitt (3), Dexter (2) and Vasil'yev (19)
have compiled comprehensive reviews on the subject of
winter hardiness and no attempt will be made to review
the entire area of work.

An association between ice formation and injury of
plant tissue has been recognized for many years. Early
investigators observed plant survival following the forma-
tion of ice within the tiSsues. Luyet and Condon (5)
using potato tissue, determined the freezing curve, num-
ber of living cells after various freezing periods and
the amount of ice present in the tissue at the end of
specific intervals of time and temperature. Cells of the
potato tuber are killed when gradually increasing propor-
tions of water are withdrawn by ice formation in the
tissue.

The potato tissue could be frozen for at least 12
minutes and have approximately 35 percent of its water
congealed by exposure to a temperature 2 or 3 tenths of a

2

degree below its freezing point without death occurring.
The cells died when exposed to temperatures 0.50 to 3.50
below the freezing point of the tissue for fifteen to
twenty-five minutes. During this period of time, forty
to seventy percent of the water congealed. The small
cells of a very young sprout, within the tuber had a
higher resistance than the other cells, surviving after
exposure to -6.50. After death of all the cells in the
potato tissue, ice continued to form. A good value of
relative proportions of ice at specific time intervals was
obtained but a poor quantitative value of the total ice
formed in the tissue was determined. Lockett and Luyet
(4) found that the embryos of air-dry wheat seeds which
contained 10 percent water were not affected by freezing
in liquid nitrogen at -l95o C; however, seeds soaked until
the embryos contained fifty percent water were killed by
the treatment. As the water content of the embryos
increased, the damage by freezing also increased. The
water was absorbed faster on the root side of the embryo
and the pattern of injury followed the advancement of the
higher water content through the embryo.

Molisch (10), working with algae in 1897 was one of

the first to observe the lethal effect of intracellular

ice formation. Siminovitch and Scarth (18) stated that
intracellular ice formation is always fatal to cells
irrespective of their hardiness. Chambers and Hale (1)
found a functional tonoplast in an onion epidermis that
had been exposed to -100 C; however, the protoplasm was
disintegrated. Levitt (3) states that injury occurs as a
result of crystal formation within the protoplasm when ice
formation is intracellular. Chambers and Hale (1), working
with onion epidermis in the laboratory observed that intra—
cellular freezing of tissue occurs in sudden flashes in
one cell at a time. Luyet and Gibbs (7) describe the pat-
tern of rapid freezing in the epidermal layer of plant
cells. The cells freeze in sudden wavelike flashes and the
opacity decreases considerably during the first seconds
after freezing. The cell sap and ice separate into two
phases, which can be observed in detail. After a period of
time the ice crystals unite into a mass. It was also repor-
ted that dead cells do not possess the ability to flash.
The investigators mentioned above observed intracellular
ice formation in plants that were exposed to rapid rates
of freezing.

Levitt (3), Meryman (9) and other workers have

reported that ice formation normally occurs in the

extracellular spaces of biological cellular systems. The
location, rate and effect of extraprotoplasmic ice forma-
tion have been investigated by many workers. The forma-
tion of extracellular ice is not unexpected since Lusena
and Cook (8) have demonstrated that membranes that are
freely permeable to liquids may be impermeable to growing
ice crystals. It was stated that in a given material,
permeability to ice crystals increases with porosity. It
was also found that the composition of the membrane must
affect the permeability and that solutes in the freezing
medium may enhance the resistance of the membrane to ice
cyrstals by depressing the freezing point or retarding the
rate of crystal growth. Siminovitch and Briggs (l7)
explained that extracellular ice crystals grow from the
addition of water derived from the dehydration of cells
adjoining these spaces. It was stated that if ice crystals
are to form in the extracellular spaces, the temperature
drop must be sufficiently slow to permit diffusion proc-
esses to occur at a rate that will allow a state of equil-
ibrium to exist. If this is not the case, intracellular
freezing eventually occurs in the tissue.

Olien (14) has reported the type of injury resulting

from extraprotoplasmic ice formation in winter cereals is

related to the level of hydration of critical tissues.
The killing temperature is dependent on the moisture con-
tent of the crown. Prior to the work of Olien, Shutt
(16) found a correlation between frost hardiness and the
water content of apples. Newton (11), working with wheat
stated that no relation exists between the moisture con-
tent of the leaf tissue removed from hardened plants in
the field and order of hardiness, but Newton (12) reversed
his previous statement. Vasil'yev (18) determined the
water content of the tillering nodes of wheat and rye
varieties varying in order of winter hardiness. It is
stated that in all cases the more resistant plants con-
tained less water.

Levitt (3) has reported a pattern of injury due to
cell contraction resulting from frost dehydration. This
injury occurs when the plant is in the lowest level of
hydration and at a temperature of -40 F. Levitt (3)
states that plasmolysis dehydrates the protoplasm and per-
mits it to separate from the cell wall. This reduces the
tensions that arise when the cell as a whole contracts.
Scarth (15) reports that the maximum plasmolysis that
cells can withstand is at the point at which stiffness of

the ectoplasm occurs. The immediate cause of death is

usually the rupture of the rigid ectoplasm on rehydration.
Olien (14) reports that a second type of injury associated
with extraprotoplasmic ice formation occurs when ice
masses initially destroy tissues in the base of the crown
and secondary destruction extends from the base into
acritical meristematic regions as the temperature is low-
ered. This pattern of injury occurs in plants that are in
a low to moderate level of hydration and over a tempera-
ture range of 50 to 100 F. The stress patterns occurring
in tissues easily injured by freezing indicate that ice
masses develop abruptly rather than as a process which is
in equilibrium with contraction of protoplasts as occurs
in the hardier tissues.

A third pattern of injury occurs in plants with moder-
ate levels of hydration and in a temperature range of 100
to 150 F. This injury is caused by rapid crystallization
of water between prot0plasts and is associated with non-
equilibrium freezing processes in the peripheral regions
of the crown. Destruction of these tissues result in
death since they contain the meristematic regions from
which new roots should have formed. A fourth pattern
occurs in plants that are in the highest level of hydra-

tion and at a temperature of approximately 280 F.

NOn-equilibrium freezing processes cause destruction of
all meristematic regions of the crown near the freezing
point when the hydration level is extremely high. A
fifth pattern of injury resulting from extraprotoplasmic
ice occurs when ice masses originate between the tillers
of plants. This also injures meristematic tissues in the
crown.

It was reported by Olien (14) that at least six
patterns of freezing injury occur in winter cereals.
Each pattern results from a different type of stress
occurring in critical regions of a plant. The type of
stress that is of primary importance depends on the condi-
tion of the plant at the time of freezing. The type of
stress which occurs in a particular region of a plant is
determined by the manner in which the water is redistri-
buted during freezing with respect to location and state.
The stress patterns were obtained by an electrophoretic
technique whereby the relative content of extraproto-
plasmic liquid was evaluated continually during freezing
in each region of a plant. The lower portion of the crown
exhibits stress patterns similar to those of root tissue
while the pattern in the upper portion is similar to leaf

tissue. The peripheral region of the crown has a stress

pattern that may be similar to either depending upon the
level of hydration on the plant. Since the critical
meristematic tissues are located in the peripheral
region, the relative hardiness of a variety in a moderate
hydration state depends to a large extent on the type of

stress occurring in this region.

MATERIALS AND METHODS

Two winter wheat varieties; Genesee and Redcoat and
three winter barley varieties; Dicktoo, Hudson, and ang
were used to study the relation of crown moisture to
freezing temperature and plant survival. At all times
during this study controlled environment chambers were
used to give uniform growing and test conditions.

Each group of plants used in the various experiments
were grown under the same environmental conditions for
the first eight weeks following germination. Uniformly
sized caryopses were placed one inch deep in four-inch
clay pots containing sand. This depth permitted elonga-
tion of the mesocotyl so that the crown developed away
from the caryopsis. The pots were placed in a controlled
environment chamber at a 700 F. day and a 550 F. night
temperature with the 16-hour light period having a light
intensity of 2800 footcandles. A.moisture level of near
field capacity was maintained during this period. The
resulting plants had complete nutrient solution applied
twice each week.

During the first four weeks after planting an average

of 3 to 4 leaves, five seminal roots and a primary root

system developed. 10

ll

HARDENING PERIOD

The plants were placed in a second controlled envi-
ronment chamber under a continuous light intensity of
about 1000 footcandles and a day - night temperature of
36:10 F. to permit the development of the physiological
processes necessary for cold hardiness. Both fluores-
cent and incandescent light sources were provided. The
average relative humidity was 70 percent.

The plants are tender at the beginning of the
hardening period. Ice crystals form within the proto-
plasts even when frozen gradually resulting in the typi-
cal electrophoretic stress patterns of tender plants (13).
Physiological changes occur during the first few weeks
at 360 F. which tend to prevent crystallization within
protoplasts and increase the amount of stress the proto-
plasts can withstand. The physiological activity is
reflected by corresponding changes in stress patterns
obtained by freezing. The types of stress patterns and
the killing temperature for each hydration state stabil-
ized after barley plants had been kept at 360 F. for 3
weeks by Olien under conditions similar to those used in
this research problem. The physiological condition did
not change appreciably during a subsequent period of 3 to

6 weeks at 360 F.

12

Preliminary experiments indicated that a similar
requirement for hardening and period of stability existed
for wheat. The physiological level of hardiness of each
variety was kept as high and uniform as possible in all
experiments with crown moisture. Possible changes in
the level of physiological hardiness during periods of
adjustment of crown moisture were checked by readjusting
a portion of the plants in each experiment to a standard
level of approximately 70% and evaluating the killing

temperature and pattern of injury.

CROWN MOISTURE ANALYSIS

The crown has certain critical meristematic regions
that are essential for the formation of new tissue. The
plant may not survive when these critical regions are
destroyed. Since these regions are of such importance
and it is thought that the amount of moisture in the crown
at the time of exposure to low temperatures is related to
the tissue damage in the crown, it is logical to determine
the exact moisture content of these regions. In each
treatment the level of hydration, which refers to the
amount of water contained in the critical regions of the
seedling, was determined by a quantitative analysis imme-

diately prior to the standard freezing test. For each

l3

analysis six plants were removed from the sand, the roots
clipped at the base of the crown, the sand brushed from
the tissue, the outer leaf sheaths removed, and a section
of the crowns 1/4 inch in length removed for the test.

The vials containing the tissue were weighed and then
placed in a vacuum oven (Figure l) at a temperature of

900 C. and 15 inches of vacuum for one and one-half hours.
The vials were weighed and the percent moisture calculated.
Preliminary results showed this procedure did not induce

charring and that all the water was removed.

 

 

 

L

Figure l. The vacuum oven used for drying the tissue for
the crown moisture tests.

14

STANDARD FREEZING TEST

Environmental conditions at most locations are not
sufficiently uniform to permit low temperature freezing
tests to be conducted under natural conditions. A freez-
ing chamber (Figure 2) was used in order that the plants
be exposed to the same conditions during each test and
over extended periods of time. The plants were placed in
the chamber and thermocouples inserted in the sand of the
pots at the depth of the crown. The refrigeration unit
operated continuously during the test. The temperature
was controlled by an electric heater connected to a relay
which was activated by a modified pattern controlled
thermograph. Air was circulated in the chamber by an
electric fan which operated only while the heater was
operating. To prevent the loss of moisture from the
plants that were in a high level of hydration prior to the
test, a humidifier was constructed in the chamber. Prelim-
inary experiments indicated that the level of moisture
remained constant during the pre-freeze period.

A pre-freeze period of 280 was maintained for approx-
imately 16 hours after the plants were placed in the

chamber. This period permitted free moisture to freeze.

o
subsequently the temperature was reduced to 2 F. each

15

hour; a rate sufficiently slow to permit diffusion
processes in hardy plants to occur and allowing a state
of equilibrium to exist. Sets of plants were removed
at four predetermined temperatures, the exact tempera-
ture at the time of removal being measured by a

potentiometer.

 

 

 

Figure 2. The chamber and apparatus used to gradually
lower the temperature for the standard
freezing test.

RECOVERY PERIOD

Following the freezing test the plants were placed
in a recovery chamber (Figure 3) under continuous light
and a temperature of 400 F. to allow gradual thawing and

recovery of the living plants. After five days the

16

plants were placed in the greenhouse for observation and
histological study. At this time plants were sectioned
and examined for tissue damage and photographs taken.
Final survival data were collected four weeks after the

freezing test.

 

 

Figure 3. The recovery chamber used following the
freezing test.

CROWN MOISTURE LEVELS

Extraprotoplasmic ice in plants at a high level of
hydration, originates in the growing point and causes
severe injury to this section of the crown. This occurs

at a temperature just below the freezing point. To

l7

attain the maximum level of hydration the plants were
clipped 1/2 inch above the soil level and placed in a
vacuum chamber (Figure 4) which had been filled to a level
above the plants with cold water. A vacuum of 24“ was
applied for a period of five minutes. The pots were
plugged to prevent the loss of water during the pre-freeze
period of the freezing test. A check series of the plants

was placed in the greenhouse for observation.

 

 

 

Figure 4. The vacuum chamber used to mechanically induce
a high level of hydration in wheat plants.

18

The maximum level of hydration which could be induced
without direct injury to the plant was obtained to study
the pattern of injury due to the formation of an ice mass
originating in the growing point of the crown. This was
accomplished by placing the hardened plants in a cold
water bath inside the controlled environment chamber for
24 hours prior to the standard freezing test. The water
level was maintained at the soil line during this period.
The bottom outlets in the pots were plugged with a cork
just prior to removal from the cold water bath. This
prevented the water from draining during the pre-freeze
period of the test. Since the pattern of injury described
for highly hydrated plants emphasizes the importance of
injury to the growing point in the upper section of the
crown as well as damage to the lower section, the moisture
content of each section was determined separately for the
moderate to high hydration range.

In order to study the pattern of injury and killing
temperature of hardened plants at a moderate level of
hydration on a quantitative basis, the plants were
watered 24 hours prior to the freezing test and allowed
to drain in the controlled environment chamber. The

highly hydrated plants and a check series which were

19

watered normally at the time the others were placed in
the water bath were placed in the freezing chamber and
the standard freezing test conducted.

To study the pattern of injury and killing tempera-
ture of hardened plants at a low level of hydration,
water was withheld and a series of plants was removed
from the chamber every seven days until the lower hydra-
tion limit was reached. Each series was subjected to a

crown moisture test and standard freezing test.

RESULTS AND DISCUSSION

In this study a series of freezing experiments were
conducted using Genesee and Redcoat wheat varieties and
Dicktoo, Hudson and ang barley varieties which were in
a vegetative stage of growth and had been uniformly
hardened prior to exposure to freezing temperatures as
described in the materials and methods section. These
experiments were conducted to determine the relation of
crown moisture content to the killing temperature and
patterns of injury. The crown moisture was maintained
at approximately 70% during the hardening period. Lower
levels were obtained by subsequently reducing the soil
moisture for several weeks before freezing. The highest
level of hydration was obtained by mechanical means.

The standard freezing test is described in the
materials and methods section. In all experiments the
plants were subjected to four freezing temperatures to
relate the killing temperature to crown moisture. Finally
after a recovery period, which is described above, plant
survival and tissue damage was observed.

The relation of crown moisture to freezing tempera-

ture and plant survival of hardened Genesee and Redcoat

20

21

wheat plants is given in Tables 2 and 3. The moisture
content of the upper and lower crown sections of plants
in the intermediate to high hydration range (Experi-
ments 1, 2a, 2b) as well as for total crowns of the
plants for all experiments are given. The total crown
moisture ranged from 73.0% to 51.8% for Genesee and 73.1%
to 49.4% for Redcoat. The moisture content of the lower
section of the crown varied 1.3% for Genesee and 1.4% for
Redcoat while the upper section had a variation of 6.8%
moisture for both varieties.

It should be noted that the temperature resulting in
the survival of 75-100%, 30-70%, and 0-25% of the popula—
tions in each test are related to the crown moisture level.
A more graphic presentation of this is given in Figures
5 and 6 which show the killing temperatures for hardened
Genesee and Redcoat wheat respectively plotted against
percent moisture of the upper crown. In these figures,
killing temperature (Tk) is the estimated farenheit
temperature at which 50 percent of the population did not
survive. In both figures the killing temperature was
high with high crown moisture and progressively decreased
with decreasing crown moistures. However, between 50 and

60% crown moisture the curve changed and the plants with

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Figure 7. A dying wheat plant which illustrates the
total destruction of tissue that occurs at a
high level of hydration when exposed to
temperatures of 25° to 28° F.

26

 

 

 

Figure 8. A surviving wheat plant showing injury in the
lower central part of the crown which occurs
when the upper crown moisture is between 72.1%
and 74.6%.and the plant exposed to a temperature
just above the killing point of the tissue.

27

 

 

 

Figure 9. A dying wheat plant showing the distribution of
the entire lower crown and meristem. This
occurs when the upper crown moisture is between
72.1%.and 74.6% and the plant exposed to a
temperature below the killing point of the
tissue.

28

 

 

 

 

Figure 10. The complete destruction of tissue found in
a dying, dehydrated wheat plant when exposed
to temperatures less than 0° F.

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Figure 14. The complete destruction of the crown tissue
in the variety W0ng at the left; whereas the
variety Dicktoo from the same pot has the
injury confined to the lower central part of
the crown.

32

 

 

 

 

Figure 15. The variety Dicktoo at the same hydration
level as the plants shown in Figure 14, but
exposed to a lower temperature.

33

crown moistures around 50% were killed by higher freez-
ing temperatures.

These results can be interpreted if reference is
made to Figures 7, 8, 9, and 10 which are photographs of
tissue damage in wheat caused by freezing. It should be
pointed out that with each level of crown moisture the
tissue in the various parts of the crown are at different
states of hydration. The tissue damage in the various
regions of the crown can be directly related to the level
of crown moisture and to the freezing temperature involved.
Figure 7 is a photograph of a plant which illustrates the
type of injury that occurred at the highest level of
hydration when the hardened plants were exposed to tempera-
tures of 250 to 280 F. Under these conditions there is a
total destruction of the roots, crown and the leaf
sheaths.

Injury to a crown in a surviving wheat plant which
had an intermediate level of crown moisture and fell in
the range 72.1% to 74.6%}upper crown moisture is illus-
trated in Figure 8. The killing temperatures for the
Genesee variety at these hydration levels were 70 to 80
F. Injury was confined to the lower central part of the

crown and to the original roots occurs just above the

34

killing temperature. Injury was caused by crystaliza-
tion in intracellular spaces. Approximately four days
following freezing, the original leaves turned yellow and
died back since the original root system had been des-
troyed. New leaf growth appeared and an examination of
the crown revealed that a new root system was developing
from meristematic tissue high on the crown as seen in
Figure 8.

An extension of this pattern of injury into adjacent
regions of the crown occurred when plants in this same
range of hydration were exposed to temperatures below the
killing temperature. Figure 9 illustrates a dying wheat
plant showing this type of injury. Such injury was caused
by extensive crystalization stresses which have destroyed
the meristematic tissues from which new roots arise. The
leaves at first appeared normal and then died back after
returning the plant to an environment suitable for growth.
In some cases new leaves were initiated if the meristem
on the top portion of the crown had not been destroyed but
these leaves died back when crown food reserves were
exhausted due to the absence of a root system.

At low hydration levels the killing temperature is

determined by the limit of contraction whidh cells can

35

survive. The killing temperature is very low since lit-
tle damage is caused by crystals forming between the
cells. The cells contract as a result of frost dehydra—
tion. Total destruction of the crown and adjoining tis-
sues occurs at the killing temperature. Figure 10 shows
a wheat plant killed by freezing at a low level of hydra-
tion. The killing temperature increased at the lowest
level of hydration. The loss of hardiness was associated
with injury which occurred during the final stages of
desiccation before freezing.

This study was also designed to determine the rela-
tion of crown moisture to freezing temperature and plant
survival of uniformly hardened Dicktoo, Hudson and W0ng
barley plants. Table 3 presents data on crown moisture,
temperatures resulting in survival of 75-100%, 30-70%,
and 0-25% of the varietal population, along with the
estimated killing temperature for the varieties Dicktoo,
Hudson and wong.

Total crown moisture ranged from 53.4%»t0 73.5% for
all varieties. As with wheat the temperatures resulting
in survival of the various percentages of the population
are related to the crown moisture level. This is graphi-

cally illustrated in Figures 11, 12, and 13, in which

36

killing temperature (Tk) for Dicktoo, Hudson, and W0ng
are plotted against percent moisture of the crown. It is
interesting to note that the three curves are very simi-
lar to those of wheat in Figures 5 and 6; however, it
should be observed that the point at which the direction
of the curve changes is at a higher temperature in bar-
ley. This can be explained since barley varieties in
general are injured at higher freezing temperatures than
wheat. Furthermore it should be noted that wong is much
more sensitive to low temperatures than Hudson and Dick-
too at the same crown moistures, and Hudson somewhat more
sensitive than Dicktoo. This is illustrated in Figures
14 and 15.

Figure 14 reveals the massive destruction of the
tissue in the lower and central portion of the crown in
the variety W0ng at the left; whereas with the variety
Dicktoo grown in the same pot and subjected to the same
temperature, the destruction was less extensive and con-
fined to the central portion of the crown. In the latter
the root meristem was only partially affected and new
roots are already evident, assuring survival. Figure
15 is a photograph of Dicktoo barley exposed to a lower

temperature and illustrates the massive damage which

37

extends through the crown tissues from the initial site
as the temperature is lowered. The same regions of the
crown are destroyed in both varieties at the killing
temperature but the secondary expansion of the injured
region is more noticeable in Dicktoo than in W0ng.

As already observed, the crown moisture level has
a qualitative effect on the nature of the freezing
injury and is quantitatively related to the killing
temperature. Certainly the causes of injury overlap
along the curves in Figures 5, 6, ll, 12, and 13. Future
work should attempt to establish a greater number of
points and to associate these points with tissue damage

as shown in Figures 7, 8, 9, 10, 14, and 15.

SUMMARY

Two winter wheat varieties: Genesee and Redcoat and
three winter barley varieties; Dicktoo, Hudson, and wong
were used in this study to establish the relationship of
crown moisture to freezing temperature and plant sur-
vival. The plant materials used in the study were uni-
formly hardened and maintained at a relatively constant
level of physiological hardiness.

Experimental results indicated that plant survival
was related to crown moisture when plants were subjected
to freezing temperatures. By sectioning crowns of
plants which had been frozen, the type and extent of
damage was related to crown moisture and freezing
temperature.

Both wheat and barley responded similarly in these
studies. The three barley varieties differ quite
markedly, however, in their winter survival and there-
fore differ geneotypically for this character. The shape
of the curves [killing temperature (Tk) plotted against
crown moisture] for the three varieties were similar but
the less hardy varieties killed at higher temperatures
respectively.

38

10.

ll.

BIBLIOGRAPHY

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Levitt, J. The Hardiness of Plants. Academic Press
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Lockett, M. C. and Luyet, B. J. Survival of frozen
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, and Gehenio, P. M. Life and Death at Low

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4O

12. , Colloidal properties of winter wheat plants in
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13. Olien, C. R. A method of studying stresses occurring
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15. Scarth, G. W. Dehydration injury and resistance.
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16. Shutt, F. T. On the relation of moisture content to
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17. Siminovitch, D. and Briggs, D. R. The chemistry of
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