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LETTUCE TIPBURN AS RELATED TO NUTRIENT IMBALANCE
AND
NITROGEN COMPOSITION
Saleh A. Ashkar

Michigan State University, East Lansing, Michigan

Abstract. Preliminary analyses of lettuce revealed
that leaves with tipburn necrosis contained less calcium,
and more organic nitrogen, particularly free amino acids,
than normal leaves. A susceptible head lettuce, 'Great

Lakes 659,‘ was grown under varying NO Ca, Mg, and light

3,
intensities in the greenhouse and growth chambers. Tipburn
was easily induces by a high NO3 and low Ca nutritional
regime. Five mM or more Ca in the nutrient solution prevented
tipburn. Necrosis was aggravated by high Mg and light inten-
sities. This disorder was accompanied by an accumulation

of free amino acids, particularly aspartic and glutamic acids,
and their amides. It is postulated that with Ca under stress,
high N levels and conditions favoring transpiration, high
temperature and light intensity, resulted in rapid nitrogen
uptake. Under these environmental conditions protein
synthesis probably is limited but protein hydrolysis continues

at a rapid rate resulting in the accumulation of free amino

acids, which may be the toxic moiety causing tipburn necrosis.

LETTUCE TIPBURN AS RELATED TO NUTRIENT IMBALANCE
AND
NITROGEN COMPOSITION
By

Saleh A. Ashkar

A THESIS

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

MASTER OF SCIENCE

Department of Horticulture

1970

ACKNOWLEDGMENTS

The author expresses his appreciation for the guidance
received from his advisor Dr. S. K. Ries. Thanks are also
extended to Drs. L. Baker, S. Honma, R. L. Carolus and
I. w. Knobloch for serving on the guidance committee and
for their conscientious review of the final manuscript.

Gratitude is also expressed to Betty Huemoeller and
Violet Wert for assisting in analysis of amino acids and

total N.

ii

NOTE TO THE GUIDANCE COMMITTEE

The body of this thesis is a condensed version, intended

for publication in the Journal of the American Society for

 

Horticultural Science.

 

iii

TABLE OF CONTENTS

Page
ACKNOWLEDGEMENTS . . . . . . . . . . . . . ii
NOTE TO GUIDANCE COMMITTEE . . . . . . . . . . iii
LIST OF TABLES . . . . . . . . . . . . . . v
LIST OF FIGURES. . . . . . . . . . . . . . vi
INTRODUCTION. . . . . . . . . . . . . . . 1
MATERIALS AND METHODS. . . . . . . . . . . . A
RESULTS AND DISCUSSION . . . . . . . . . . . 7
LITERATURE CITED . . . . . . . . . . . . . 23

iv

Table

LIST OF TABLES

The elemental composition of different portions
of normal and tipburned 'Grand Rapids' lettuce
leaves. . . .

A comparison of N fractions in normal and
tipburned 'Grand Rapids' lettuce leaves. . . .

The effect of N and asparagine applications on
growth and nitrogen components of 'Great Lakes
659' lettuce leaves . . . . . . . . . .

The relationship between N and Ca levels in
nutrient solution with N constituents of 'Great
Lakes 659‘ leaves . . . . . . . . . . .

Amino acid composition of 'Great Lakes 659'
lettuce leaves receiving different levels of
KNO and CaCl . . . . . . . . . .

3 2
The relationship of growth and nitrogen
components of 'Great Lakes 659' lettuce leaves
with different levels of Ca and Mg when grown on
18 mM of N03.
Visual tipburn injury as related to relevant
elemental components of 'Great Lakes 659' lettuce
leaves.

The relationship between nitrogen, calcium, and
magnesium levels under different light levels
with the nitrogen constituents of 'Great Lakes
659' lettuce leaves . . . . . . . . . .

A comparison of the predominant free amino acids
in 'Great Lakes 659' lettuce leaves grown with
varying levels of N03, Ca, and Mg. . . . . .

Page

ll

l3

l6

17

19

21

LIST OF FIGURES

Figure Page

1. The amino acid and total N content of 'Great
Lakes 659' lettuce leaves grown at two KNO

and CaCl2 levels . . . . . . . . .3 . . 20

vi

 

INTRODUCTION

Tipburn of lettuce (Lactuca sativa L.) is a serious

 

problem confronting most growers. A number of investigators
have approached this problem in both the greenhouse and
field in different ways, including anatomical studies,
morphological develOpment of laticifers, and environmental
factors related to tipburn, such as light intensity and
duration, temperature, moisture, humidity and nutrient
imbalance.

Tibbitts et al. (29) studying laticifers as related
to lettuce tipburn reported that the release of latex into
the surrounding parenchyma cells resulted in cell collapse
and necrosis of leaves. Rapid rates of growth increased
tipburn severity. Olson e£_al. (21) studying the morphology
of laticifers, also related the rupture of laticifers with
lettuce tipburn. Light intensity and duration studies by
Tibbitts and Rama Rao (30) showed that both factors
appreciably influenced tipburn. Tipburn was more severe
at 1800 ft—c for 2“ and 20 hr daily light periods than 800
.ft—c for periods of 16, 12 and 8 hr. Light increased
photosynthetic activity with a resultant increase in growth
which lead to rupture of laticifers and injury.

Plants, other than lettuce, with laticifers in their

Eitructures, also exhibit tipburn symptoms, i.e. chicory

(Cichorium in§ybus (33) and escarole (Cichorium endivia) (l7).

 

However, tipburn phenomena are not restricted to plants with
laticifers. Cabbage (Brassica oleracea) (32), celery (Apium

graveolens) (7) and potatoes (Solanum tuberosum) (15) are

 

among the horticultural crops susceptible to tipburn or
tipburn—like disorder, despite the absence of laticifers
in their structure.

Calcium is an essential element in the structure of
pectinacious substances. A deficiency of this element
causes a weakness in the cementing between cells, particularly
between cells in the rapidly growing tissues. Studies of
the distribution and redistribution of uBCa in bean

(Phaseolus vulgaris L.) showed that Ca movement involved

 

continued distribution and redistribution to newly formed
tissue (9). Ca is a membrane stabilizer, and has a signi-
ficant effect on membrane permeability (31). Availability
of the relatively immobile Ca is reported to be of utmost
importance to newly developed tissues (1, 16, 18). An
antagonistic interrelationship between Ca and other cations
.has been reported to decrease the content of this element
in the tissue of soybean (Glycine soia). Calcium in these
[Blants was primarily associated with pectin of the middle
Ilamella and/or the protoplasm (10).

Enzymes may proteolyze to amino acids when not used
if) the synthesis of other enzymes or proteins. There is an

aPpreciable accumulation of free amino acids during and after

plant senescence. Changes in amino acid levels in plants

are found to be due to factors such as; water stress, patho-
logical diseases and mineral deficiencies (3, 25, 27). Water
stress may alter amino acid composition, increase proteolysis,
interrupt protein synthesis and consistently increase proline
and sometimes increases the amide compounds in plants. Patho—
logical diseases have been associated with the accumulation

of certain amino acids. Verticillium alboatrum in Chrysan-

 

themum caused an increase in proline, and wheat rust caused
an accumulation of aromatic amino acids. Mineral deficiencies
in tobacco resulted in an accumulation of leucine.

The control mechanism for the synthesis of several
amino acids in E. 391i, is well established. When specific
amino acid levels increase, the synthesis of others is
inhibited (8, 19). Unfortunately these studies have not
been done with intact plants. The interruption of protein
synthesis or proteolysis results in an accumulation of high
levels of amino acids, many of which have been found to
be toxic (2, 26, 27).

Retardation of leaf senescence by N6—benzyladenine in
intact bean plants has resulted in an increase in chlorophyll
and protein with concurrent increase in DNA and RNase activity
at all stages of develOpment (6).

Although studies of lettuce tipburn by others have
shown correlations between environmental factors and the

level of injury, the precise cause has not been elucidated.

 

This study was undertaken in an effort to determine the causes
and the nature of tipburn. This information could be
beneficial to the plant breeder in selecting efficiently

for tipburn resistant varieties.

MATERIALS AND METHODS

Culture and Nutrition. Preliminary analysis of N, and

 

 

r.
mineral elements of lettuce leaves, 'Grand Rapids,‘ exhibiting E.
tipburn as described by Tibbitts (29) were compared with E
those free of the necrosis. These plants were obtained from

the same commercial greenhouse. Uniform head—lettuce seed- i

lings, 'Great Lakes 659,‘ at the 2—leaf stage were trans-
planted from sand to 6-inch plastic containers filled with
60 g of vermiculite. Plants were grown in a greenhouse with
a night temperature of 22°C. The temperature of growth
chambers was maintained at 25°C during the day and 20°C at
night. The plants were watered with 1/2 Hoagland's solution
(11) with N supplied as KNO3, Ca as CaCl2 and Mg as MgSOu
throughout the remaining period of growth.

The first controlled experiment consisted of application
of asparagine and three levels of N (2, A and 16 mM). K concen-
tration was 6, 8 and 20 mM respectively with the three N
levels. Plants in the second experiment received a combina-
tion of two levels of N (6 and 2A mM) and three levels of

Ca (0, 5 and 20 mM). K concentration was 10 mM with the first

N level and 28 mM with the second N level. Supplemental

5 I;

light was furnished with an intensity of l7, l2 and 5
microwatts/cm2 per nanometer for blue, red and far-red
respectively (approx. 2000-2200 ft-c). Plants in the third
test received 18 mM of N with 22 mM of K and all combina-
tions of O, and 20 mM Ca, with O, 5, and 20 mM Mg and were
maintained at the same light intensity. The fourth experiment
was split between two growth chambers, one of which had P?
the same light intensity as in the previous experiment, and

the other received 6, 5.5 and 3.7 microwatts/cm2 per nanometer

 

for the three light bands (approx. 800-1000 ft-c). Treatments i“
within a chamber consisted of all combinations of 6 and 18 t
mMN, 10 mM of K with the first N level and 22 mM with the
second N level, 0 and 5 mM Ca, and O and 10 mM Mg. The fifth
test was maintained under the same light regime as experiment
two. Plants received nutrient solution with 18 mMN with

22 mM of K, 10 mM Mg and 0 Ca. After four weeks they received
a spray of either N6—benzyladenine or 6—furfury-laminopurine
at the rate of O, 5, 10, 20 and A0 ppm. A second spray at

the same rate was applied after 10 days.

Analytical Procedures. Elemental content of the margin,

 

midrib and the remainder of the leaf blade was analyzed spectro-
graphically (12). Total N was determined by the micro-Kjeldahl
and an automated Kjeldahl procedure. Samples for the automatic
analyzer were predigested in sulfuric acid. Perchloric acid
and selenium were added to the mixture prior to digestion and

distillation. Ammonia was determined by a color reaction with

alkaline phenol and sodium hypochlorite. Total nitrogen was
estimated by Optical density standardized by micro-Kjeldahl
analysis. Total free amino acids were assayed spectrophoto—
metrically as described by Rosen (2A). Individual amino
acids were quantitatively determined in the preliminary

analyses and the first experiment by the thin—layer chromato-

 

graphy techniques of Pataki (22). To obtain more accuracy, F
automatic amino acid analysis (23) procedures were utilized N
with citrate buffers for the next two tests. This system

did not adequately separate the amides, asparagine, and A
glutamine. A lithium buffer system was used in later tests I

to accomplish this separation. The Lowry method (1A) was used
to measure water extractable protein. Nitrate was determined
by the Lowe—Hamilton technique (13) utilizing soybean nodule
bacteroids for reduction of nitrate to nitrite.

All experiments were terminated when plants were 50 to
60 days old (3 to A days after exhibiting visual signs of
tipburn). Ratings were made of tipburn injury of inner
leaves (1 = no injury, 9 = internal leaves all necrotic),
after which, plants were weighed and freeze dried for later
analysis. All values were expressed on a dry wt basis.

Statistical Procedures. A statistical analysis was not

 

conducted on the preliminary analyses of lettuce from commercial
greenhouses, because there were no replicates. All experiments
were arranged in randomized block designs with three or more

replications. The data were submitted to analysis of variance

and the means compared with Duncan's Multiple Range Test.

H tests were applied where relevant.
RESULTS AND DISCUSSION

Leaf—lettuce plants, 'Grand Rapids,‘ from a commercial
greenhouse exhibiting tipburn symptoms were low in Ca, Mg,
Mn and B for all leaf parts compared to normal plants (Table
1). In particular, the Ca content of the 1 cm leaf margin
was low compared to the midrib and the remainder of the
blade. The difference in Ca levels between tipburned and
normal plants was relatively greater than that for the other
elements evaluated.

The total N and, particularly the free amino acid content
was considerably higher in tipburned plants (Table 2). Conse—
quently the ratio of free amino acids to total N was higher
in tipburned plants. This difference in free amino acids
could not be accounted for by more dry matter per g fresh
wt since the percent dry wt did not vary greatly. Of the
individual amino acids that were analyzed by TLC; arginine,
asparagine, aspartic acid and glutamine were appreciably
higher in tipburned plants. A large portion, approximately
30% of the total N difference between tipburned and normal
plants may be accounted for by the increase in free amino
acids. (In contrast, the nitrate content was highest in the
margin of normal plants.

Steinburg gt al. (27) associated frenching of tobacco

seedlings with L-isoleucine. Tyrosine (25) and L—leucine (20)

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Table 2. A comparison of N fractions in normal and tipburned

'Grand Rapids' lettuce leaves.a

 

 

 

Observations Normal Tipburned
(Amount/g dry wt) Margin Remainder Margin Remainder
Per cent dry wt 8.5 7.8 10.9 7.9
Total N (mg) 16.A 13.3 19.3 16.6
N03 (umoles) 327 33 19 59
Total free amino acids

(pmoles) 209 3A2 5A2 893
Individual amino acids
(umoles)
Aspartic acid 37 AA A0 78
Asparagine 15 20 23 3A
Glutamic acid 36 30 A0 32
Glutamine - 22 20 37 21
Plenylalanine 18 21 ll 13
Arginine l5 15 26 25

 

aEach observation is the mean of three plants consisting of a

composite of six leaves from each plant.

10

are reported to be toxic to wheat. Audus et_al, (2) showed
that L—tryptophane, glycine, L-aspartic acid and D—arginine
were toxic to cress seedlings. These reports along with the
preliminary results in this report indicated that certain
amino acids might cause tipburn in lettuce. To test this
hypothesis, an experiment was conducted to determine the
effect of asparagine on tipburn, since asparagine was one
of the predominant amino acids in the tipburned lettuce.
Plants treated with 16 mM of N, with and without
asparagine did not develop severe tipburn. However, necrosis

was apparent with 16 mM of N plus three mM asparagine (Table

W&{'

3). At this higher N level, the plants grew poorly and the

total N, NO applications of asparagine increased total N,

3)

but did not increase either NO total free amino acids, or

3,
the individual amino acids analyzed including asparagine.
This test failed to relate any relationship of analytical
data with the occurrence of tipburn other than total N.
Nutrient imbalance has been reported by several workers
to cause physiological disorders in many economic plants
(5, 7, 32, 33). Hashimoto (10) reported that lack of Ca in
soybean plants caused collapse of plant tissue, meanwhile
K deficiency caused brittleness and buckling. Struckmeyer
and Tibbitts (27), reported that Ca deficient lettuce had
collapsed cells and necrosis of certain leaf areas. Boron

deficiency on the other hand, caused a swelling of laticifers

and extrusion of latex. However, they reported that neither

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12

Ca nor B deficiency symptoms truly resembled the lettuce
tipburn described by Tibbitts gt gt. (29).

The previous observations lead to further experiments
in an attempt to induce tipburn. Ca was eliminated from the
growing media, leaving all other environmental factors
favoring rapid growth and development unchanged. Typical
lettuce tipburn developed in plants that received 2A mM N E»
and 0 Ca. The dry wt of plants was not altered by treatment.
The N03 content and the free amino acids were considerably
higher in tipburned plants (Table A). In general, both NO3
and free amino acids decreased with increasing levels of Ca :1
at both N levles. In this test, neither total N nor water
extractable protein changed appreciably with different levels
of Ca.

Mineral analysis indicated, as expected, Ca increased
in lettuce leaves, with increasing Ca levels in the nutrient
solution. The occurrence of Ca at the zero level may be
explained by endogenous Ca from the seed and the presence
of 0.5% Ca in vermiculite. Results of free amino acid by
automatic analysis showed that serine and/or asparagine, and
glutamic and/or glutamine were predominant (Table 5). Unfor-
tunately the citrate buffer used for separation of these amino
acids did not separate serine from asparagine, and glutamic
acid from glutamine. Asparagine and/or serine accounted for

more than 1/3 of the free amino acids where Ca was not in

the nutrient solution.

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Table 5. Amino acid composition of 'Great Lakes 659' lettuce

leaves receiving different levels of KNO and CaCl .a

 

 

 

3 2
Nutrient level (mM)
Amino Acids NO3 6 6 6 2n 2a 2a ?
Ca 0 5 2O 0 5 2O
(pmoles Amino Acid/g dry wt) ;
Aspartic acid 7 16 11 26 11 27 A
Threonine 25 21 21 52 35 18
Serine and/or
Asparagine 151 75 5A 356 196 169
Glutamic acid
and/or glutamine l5 19 21 5A 66 3O
Alanine 20 1A 1A 31 22 20
Valine 11 5 3 16 10 10
Isoleucine 12 A 3 1A 8 8
Phenylalanine A 3 2 6 A A
Arginine 8 10 5 A7 32 A8

 

Total of all amino

acids 307 216 228 8A8 557 52A

 

aEach observation is the mean of two replications.

15

To determine if there was an interrelationship between
Mn and Ca, those two elements were varied under a constant
NO3 regime of 18 mM. When the Mg level was increased in
a Ca—deficient growth media, tipburn developed more rapidly
and necrosis was more severe than in the previous experiment
(Table 6). Calcium deficient plants were the first to show
tipburn, particularly when the Mg level was at 20 mM. Appar—
ently Mg aggravated the occurrence of tipburn. This type of
elemental antagonism agrees with the findings of several
workers (10, 31). None of the plants that received Ca
exhibited tipburn symptoms. The total N, and N03 and the
level of free amino acids were all higher in tipburned plants.

In the experiment to compare the incidence of tipburn
at different light and nutrient levels, plants grown under
high light intensity developed tipburn one week earlier than
plants grown under low light intensity (Table 7). The first
lettuce plants that manifested tipburn received high N,
10 mM Mg and no Ca, the second group of plants where tipburn
appeared were on a regime of high N and no Ca and no Mg.
Mineral analyses of lettuce leaves suggested that Mg in the
nutrient media competed with Ca, reducing the Ca content of
the plant tissue, which resulted in tipburn necrosis. The
Ca concentration, was greater in the high light regime
compared to low light. As the Ca content increased, Na
content decreased. This negative correlation between Ca_

and Na content indicated that calcium limited Na and N

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Table 7. Visual tipburn injury as relatvd to relevant elemental components
of 'Great Lakes 659' lettuce leaves.
Light Nutrient Tipburn ratingsa ObservationsC

Intensity level (mt) (Ratings of (Lean of P Ca Mg Na
NO3 Ca Ag all treatments) high and low) (A) A) (p) (ppm)

_H_i_.»_.}_1 6 0 O 24.5 b 3.8 b .145 .68 .25 721
6 0 10 5.3 A.6 c .5A .6A .53 658

6 5 0 1.0 1.0 a .30 56 .31 551

b 5 10 1.0 1.0 a .31 .A0 .37 381

18 0 0 6.7 5.5 d .38 .73 .27 685

18 0 10 8 0 7.1 8 .5A 70 .60 676

18 5 3 1.2 1.1 a .33 .32 .36 537

18 5 10 1.0 1.0 a .33 1A .35 A88

L_O_‘.i 6 0 O 3.0 .58 .uu .25 7A0
6 O 10 3.8 5A .37 .A8 751
6 5 0 1.0 .3A .7A .30 60A.

6 5 13 1.0 .32 .70 .142 71195

18 0 0 A.3 .A6 .6A .28 968

18 0 10 6.2 .A8 .A6 .A6 855

18 5 0 1.0 .29 .77 .36 558

18 5 10 1.0 .59 .71 .51 866

3Rating scale; 1 = no injury, internal leaves all necrotic.

bThe R value for interaction of nutrient level and light intensity is

significant at .01 level.

C

Correlation between Ca and Na significant at

ZMeans followed by unlike letters are significant at .01 level.

.01 level, r = -0.68.

18

uptake. Again, the total N and free amino acid content of
the injured plants was highest (Table 8). The nitrate level
as in other tests was highest in the plants having tipburn.
This may be the result of the lack of synthesis of nitrate
reductase, to reduce the N03 to N02.

High Ca compared to low Ca levels in the nutrient
solution, resulted in less accumulation of total N and free
amino acids at the higher N level as compared to the lower
level (Figure 1). This suggested that Ca directly or
indirectly might have decreased N uptake. The decrease in
N uptake may also have been due to Cl since the Ca was
added as CaClz. This may account for the reduced tipburn
with the addition of Ca and the interrelationships with other
ions available in the nutrient media.1

The predominant amino acids were asparagine, glutamine,
aspartic acid, glutamic acid and arginine (Table 9). The
growth of tipburned leaves was completely retarded, during
the last two weeks. This suggested that glucose may have
become limiting which accounted for the accumulation of
asparagine and glutamine, particularly under the growing
conditions with high levels of N03.
El—Mansy gt gt. (A) reported that the shelf life of

lettuce (cv. Great Lakes) treated with 6-furfurylaminopurine

 

lConcurrently with this research, it was demonstrated
that foliar applications of Ca prevented tipburn. Thibodeau,
P. O. and P. L. Minotti. 1969. The influence of calcium on
the development of lettuce tipburn. Proc. Amer. Cos. Hort.

Sci. 9A: 372-375.

19

Table 8. The relationship between nitrogen, calcium and magnesium levels
under different light levels with the nitrogen constituents of 'Great

Lakes 659' lettuce leaves.

 

 

ASIA AA‘AEAOAAAA (.112 2..
N03 Ca Mg (umoles)

Htgg 6 O 0 3.3 27.: 7A7 92 a
6 O 10 3.0 32 0 82 1A1 a

6 5 0 3.2 28.2 693 116 a

6 5 10 3 2 32 - 70A 213 ac
18 O O A.A A0.2 1A31 526 be

18 0 10 3.3 A8.8 15A0 56A b

18 5 O 3.1 38.9 1098 A63 b

18 5 10 3.1 33.6 720 A90 b

tgg 6 0 0 3.3 3. 2 A12 A9 a
6 0 10 3.5 33 1 558 119 a

6 5 0 2.9 33.3 5A0 90 a

6 5 10 3.1 3A.6 531 202 ac
18 0 0 3.8 A3.7 68A 5A2 bc
18 0 10 A.A AA.3 702 535 be
18 5 0 2.8 37.3 621 510 be

18 5 10 2.9 38.2 A80 609 b

 

aThe F value for Ca x N0 interaction is significant at the .01 level.

3
DThe F value for difference between N03 levels is significant at the .01

level.

 

*
.s
Q 1000-
i
>~ 900-
B
O
\0
.0 soo—
0
‘<
O
,5 700-
E
'¢
a
0
o 600-
E
a,

Figure 1. The amino acid and total N content of

659' lettuce leaves grown at two KNO

levels.

A

V----

 

20

AMINO ACIDS

TOTAL N

 

CaC|2 (mM)

3

-44

-42

-40

-36

-32

-30

 

and CaCl

-38‘

-34.

m9 N/aory Weight

'Great Lakes

21

Table 9. A comparison of the predominant free amino acids in

'Great Lakes 659' lettuce leaves grown with varying levels of

NO
33

Ca, and Mg.a

 

Nutrient level (mM)

 

 

 

Amino acids N03 6 6 6 6 18 l8 l8 l8
(umoles/g dry wt) Ca 0 0 5 5 0 0 5 5
Mg 0 5 O 5 0 5 0 5
Aspartic acid 22 38 22 A0 68 35 32 16
Threonine 22 28 27 2A 51 98 A5 28
Serine 27 30 A7 A0 A6 53 62 A2
Asparagine A0 50 18 11 157 25A 80 AA
Glutamic acid 9A 10A 12A 170 178 225 1A1 116
Glutamine 59 68 A6 22 139 A20 90 39
Alanine 5A A7 50 32 87 65 5A 22
Valine 19 21 17 19 27 19 17 1A
Isoleucine 9 10 9 10 1A 16 1A 8
Phenylalanine 7 7 7 7 21 15 10 6
Arginine 13 11 16 16 50 3A 33 15
Total of all
amino acids 713 832 731 7A2 12A8 1615 912 707

 

aMean of two light intensities.

22

and NC—benzyladenine was extended. They suggested that these
chemicals acted by delaying senescence. Although the high
nitrate level may be responsible for more amino acids
synthesis, these levels could also result from either
proteolysis or interruption of protein synthesis. These
studies and the nutrient imbalance and light effects suggested
an experiment to test the effect of kinetin on tipburn. Under
these tipburn inducing conditions, spraying twice with
N6—benzyladenine or 6—furfurylaminopurine (0, 5, 10, 20 and
A0 ppm) did not have any effect on tipburn occurrence. All
plants regardless of chemical or concentration showed tipburn
symptoms and the dry wt, N03, water extractable protein and
total N were not significantly altered. This did not agree
with Fletcher's (6) findings reporting protein increases
in intact bean leaf treated with N6-benzyladenine.

Results of controlled experiments with 'Great Lakes
659' lettuce agree with the results of the preliminary test
with 'Grand Rapids' lettuce except for the N03 content.
These results suggest that a nutrient imbalance may cause
tipburn, particularly under the high light intensity values
in this experiment and optimum growing conditions. All
these variables had an interacting effect upon the uptake
and metabolism of N. The accumulation of free amino acids
which are a result rather than a cause of tipburn may contribute

to the actual necrosis.

10.

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