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Effect of moisture stress on the ethephon response in tomatoes
Scientia Horticulturae, 8 (1978) 299--305 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
299
EFFECT OF M O I S T U R E S T R E S S O N T H E E T H E P H O N R E S P O N S E IN TOMATOES
L.L. MUTTON Agricultural Research Centre, N.S.W. Department of Agriculture, Yanco (Australia)
ABSTRACT Mutton, L.L., 1978. Effect of moisture stress on the ethephon response in tomatoes. Scientia Hortic., 8: 299--305. Potted plants (cultivar 'Napoli') were maintained at 3 levels of moisture (continuously moist, moderate moisture stress and severe moisture stress) and sprayed to run-off with 3 levels of ethephon (0, 250 and 500 p.p.m.) in factorial combination, in order to examine the effect of moisture status on the ethephon-accelerated ripening-response in tomatoes. The largest increases in ripening-rate occurred where plant~ were moderately stressed and sprayed with 250 p.p.m, ethephon, or where plants were kept continuously moist and sprayed with 500 p.p.m, ethephon. No substantial increase in ripening-rate occurred at higher levels of either ethephon or moisture stress. Leaf senescence was rapid in all treatments other than the moderately stressed and continuously moist controls. In some of the water-stressed treatments a partial recovery of plant water potential occurred as a result of reduced transpiration due to rapid and extensive leaf loss.
INTRODUCTION Increasingly, e t h e p h o n is b e i n g used t o p r o m o t e g r e a t e r u n i f o r m i t y o f r i p e n i n g in m e c h a n i c a l l y h a r v e s t e d p r o c e s s i n g - t o m a t o e s . H o w e v e r , p l a n t r e s p o n s e t o e t h e p h o n c a n r a r e l y b e p r e d i c t e d w i t h a high d e g r e e o f a c c u r a c y . O n e o f t h e m a j o r r e a s o n s f o r this is t h e l a c k o f d e t a i l e d studies o n t h e m a n n e r in w h i c h e n v i r o n m e n t a l c o n d i t i o n s a f f e c t t h e r e s p o n s e . W o r k b y Bussell ( 1 9 7 3 ) a n d T o m p k i n s ( 1 9 7 5 ) s u p p o r t s n u m e r o u s field o b s e r v a t i o n s t h a t m o i s t u r e stress e n h a n c e s t h e e t h e p h o n - a c c e l e r a t e d ripeningr e s p o n s e , a l t h o u g h Bussell's d a t a suggest t h a t e x t r e m e s o f m o i s t u r e stress "limit" the response. However, both workers based their conclusions on the results o f a single harvest. T h e y also gave n o i n d i c a t i o n o f t h e w a t e r p o t e n t i a l s r e a c h e d in t h e i r t r e a t m e n t s or t h e e x t e n t o f l e a f s e n e s c e n c e w h i c h o c c u r r e d . M o r e i m p o r t a n t l y , t h e y a p p l i e d stress t r e a t m e n t s s o m e t i m e p r i o r t o spraying. I t is well k n o w n (Sims, 1 9 7 3 ; K r e t c h m a n e t al., 1 9 7 4 ) t h a t a n y t y p e o f stress a t t i m e o f s p r a y i n g i n d u c e s g r e a t e r d e f o l i a t i o n t h a n t h a t w h i c h o c c u r s if p l a n t s are non-stressed. I t is p o s s i b l e also t h a t t h e r i p e n i n g - r e s p o n s e o f p l a n t s sprayed under these conditions might be different to that obtaining under n o r m a l c i r c u m s t a n c e s . F o r t h e s e r e a s o n s it is i m p o r t a n t t o d i f f e r e n t i a t e b e t w e e n c o n d i t i o n s a t t i m e o f s p r a y i n g and t h o s e d u r i n g t h e p o s t - s p r a y p e r i o d .
300
The experiment described here aimed to examine more fully the relationship b e t w e e n moisture status and the ethephon-accelerated ripening-response. MATERIALS AND METHODS Thirty six plants (cultivar 'Napoli') were grown in 60 cm pots under similar conditions in a glasshouse and pruned to a single leader. A sandy loam was used as the potting-medium. Flowers were tagged at anthesis over a period of 20 days to ensure a wide range of fruit age on each plant. When the oldest fruit on the majority of plants were showing signs of ripening (about 42 days from anthesis), pots were randomly allocated to the following treatments: 1. Continuously moist ; 0 p.p.m, ethephon 2. Continuously moist ; 250 p.p.m, ethephon 3. Continuously moist ; 500 p.p.m, ethephon 4. Moderate water stress; 0 p.p.m, ethephon 5. Moderate water stress; 250 p.p.m, ethephon 6. Moderate water stress; 500 p.p.m, ethephon 7. Severe water stress ; 0 p.p.m, ethephon 8. Severe water stress ; 250 p.p.m, e t h e p h o n 9. Severe water stress ; 500 p.p.m, ethephon Each treatment consisted of 4 pots as replicates. Plants receiving various ethephon treatments (expressed as p.p.m, active ingredient) were sprayed to run-off. Control plants were sprayed with water only. No surfactant was added to the spray material. The glasshouse temperature was held at 20°C (day) and 15°C (night). Continuously moist treatments were watered daily. Moderate levels of stress were obtained b y replacing half the moisture lost daily as determined b y weighing. Severe stress levels were obtained b y a complete absence of watering. The above watering-regimes were c o m m e n c e d immediately after spraying. The moisture status of plants was determined at regular intervals using a Scholander pressure bomb. B o m b readings were taken at 10 am (Eastern Standard Time) using the youngest fully-expanded leaf. Ripening was recorded as the time at which individual fruit achieved a redripe condition, the fruit being inspected daily. Whole plants were subjectively categorised for leaf senescence as follows: 1. Leaves completely green 2. Leaves mainly green b u t showing some signs of yellowing 3. Leaves mainly yellow b u t showing some traces of green 4. Leaves completely yellow 5. Leaves withered and brown. In recording leaf senescence, juvenile growth at the t o p of the plant (i.e. leaves still expanding) was ignored. Juvenile growth is typically unaffected b y ethephon. On completion of the experiment, fruit from the extreme treatments 1, 3, 7 and 9 were tested for titratable acidity and percent solids.
301
RESULTS E t h e p h o n and moisture stress b o t h accelerated the ripening o f y o u n g fruit, the time to ripen being progressively reduced the y o u n g e r the fruit at the time t r e a tmen ts were imposed. Hence, the slope of the regression (b) o f time from anthesis to m a t u r i t y on age o f fruit at time o f spraying (an example o f which will be shown in Fig. 1), provided a good index of a t r e a t m e n t ' s effect on the ripening-response. Differences am ong t r e a t m ent s obtained in this w a y w e r e tested b y analysis o f variance o f the regression coefficients. Increasing levels o f e t h e p h o n p r o m o t e d a greater acceleration o f ripening than increasing levels o f moisture stress. T h e greater increases occurred where plants were m o d e r a t e l y stressed a n d sprayed w i t h 2 5 0 p.p.m, et hephon, or where plants were k e p t c o n t i n u o u s l y moist and sprayed with 500 p.p.m. ethephon. B e y o n d these points no substantial acceleration of ripening occurred (Table 1). T he intrinsic variation in ripening-time f or fruit o f similar age was also progressively reduced by the action o f b o t h e t h e p h o n and moisture stress. This can clearly be seen in Fig. I and is quantified b y the coefficients of determination (R 2) listed in Table 1. However, the low R 2 values for the continuously moist and m o d e r a t e l y stressed controls are m o r e likely to be a f u n c t i o n o f the low b-values than inherent variation a b o u t t he regression line. T h a t substantial differences in moisture stress occurred is clearly shown in Fig. 2. In th e m o d e r a t e l y stressed treatments, water potentials decrease and TABLE 1 Analysis of variance of the coefficients for the regression of time from anthesis to ripening on age of fruit at time of spraying. The symbols *** and ** represent differences which are significant at p < 0.001 and p < 0.01 respectively. Ethephon level
Moisture stress Continuously moist
Control
250 p.p.m. 500 p.p.m.
b
R2
Moderate moisture stress b Re
Severe moisture stress b R2
0.06 0.31 0.54
0.06 0.67 0.92
0.14 0.50 0.61
0.22 0.55 0.58
0.13 0.86 0.94
L.S.D. (b values) = 0.043 (p < 0.05) Source
df.
Moisture stress
2
Ethephon levels Interaction
2
Error
4 27
Total
35
M.S.
F ratio
0.06775 0.62140 0.01245 0.00264
25.7*** 235.4*** 4.7**
0.56 0.85 0.76
302
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oo
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~' 2'o
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~b
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"~ 5 0
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;
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~
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~ w
250 p.pm.
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•
250 p.p.m.
~ 35
26
25
36
3;
4;
50
~"
-%
;
1;
1;
0
45
40 ~ 35
• ~-20
I
25
I
30
I
35
Age of fruit at time of spr,aying (days from anthesis)
I
40
-%
;
•
500 p.p.m.
lb
&
Days after spraying
Fig. 1. Relationship between the time taken for fruit to ripen and their age at time of spraying for the continuously moist treatments. Fig. 2. Plant water potential during the perio d following ethephon application. (A, severe moisture stress; m, moderate moisture stress;., continuously moist.)
then increase. Also, in the moderate and severe stress treatments there is an inverse relationship between the minimum water potentials reached and the e t h e p h o n level, as well as a delay in the time taken to reach minimum water potentials as the e t h e p h o n level increases. The above features can be explained by the rate of leaf senescence which occurred in the treatments concerned. In addition, rapid leaf senescence in many treatments meant that bomb readings had to be curtailed prematurely (Fig. 3). E t h e p h o n and moisture stress appear to have had a greater impact o n leaf senescence than o n ripening (Fig. 3) since rapid leaf senescence occurred in all treatments other than the c o n t i n u o u s l y moist and moderately stressed controls.
303
•
AjA
2 1:
A / • "-"~'~o
p.p.m.
i f O ..o
•
•
5
~ 4 8 $ ._/ 5
6'
•
•
,4 3 2 1
Days after spraymg Fig. 3. Ratings for leaf senescence (1 ffi green, 5 ffi completely senesced) during the period following ethephon application. (A, severe moisture stress; =, moderate moisture stress; o, continuously moist.)
TABLE 2 Titratable acidity and percent solids for selected treatments Treatment
Titratable acidity (ml 0.1 N, NaOH/5 g pulp)
Percent solids
1. 3. 7. 9.
4.5 4.3 6.2 4.9
5.1 5.2 6.1 5.6
Continuously moist, 0 p.p.m, ethephon Continuously moist, 500 p.p.m, ethephon Severe moisture stress, 0 p.p.m, ethephon Severe moisture stress, 500 p.p.m, ethephon
304
Titratable acidity and percent solids were inversely related to the maximum leaf water potentials in the treatments chosen for testing (Table 2). Such results indicate that fruit from stressed plants were also measurably dehydrated. DISCUSSION
The results show that moisture status is an important consideration where ethephon is used to accelerate ripening. The substantial differences that occurred among treatments suggest that differences in moisture status could well account for some of the variation in response observed in field situations, although it seems unlikely that the very high levels of moisture stress recorded in this experiment would normally be attained in field-grown crops unless they were already partly stressed prior to ethephon application. The results of this experiment are similar to those of Bussell (1973) except that at the highest level of moisture stress there was no significant reduction in ethephon-accelerated ripening as occurred in Bussell's experiment. Bussell interpreted his result as being due to an interference of the ethephon response, whereas these results simply suggest that the combined effects of ethephon and moisture stress had approached a m a x i m u m where plants had been moderately stressed and sprayed with 250 p.p.m, ethephon, or where plants kept continuously moist had been sprayed with 500 p.p.m, ethephon (Table 1). However, because the 2 experiments were conducted under different circumstances, it is probably n o t valid to make direct comparisons between them. Considerable caution needs to be exercised in interpreting experiments of this kind. Because rapid leaf senescence and defoliation leads to reduced transpiration, high levels of stress m a y n o t occur in ethephon-treated plants even though they m a y be denied water. The inverse relationship between minimum water potentials and ethephon level in the moderate and severely stressed treatments m a y thus explain w h y the severely stressed plants sprayed with 500 p.p.m, ethephon ripened more slowly than would be expected from the trends in other treatments. Early leaf senescence m a y also lead to titratable acidity and percent solids which are lower than expected on the basis of irrigation management alone (Table 2). The susceptibility o f leaves to the combined effects of moisture stress and ethephon is clearly shown in Fig. 3. Leaf loss in itself is an important consideration in most areas where field tomatoes are grown, particularly during early- and mid-season harvests when the risk of sunburn is high. Tomatoes are generally able to withstand exposure to the sun provided t h e exposure is n o t sudden (Kedar et al, 1975). Figure 3 shows the importance o f having plants well watered at the time of spraying if the rate of leaf loss is to be minimised, although under these circumstances the rate of ripening would be less than optimal. Later in the season, when leaf loss is often desirable (and the risk of rain at harvest is greater), watering could cease some time before spraying so that some measure of stress m a y be more readily achieved during the post~spray ripening-period.
305
REFERENCES Bussell, W.T., 1973. Effects of Soil moisture on ripening in dwarf tomatoes after ethephon treatment. N.Z.J. Exp. Agric., 1: 381--382. Kedar, N., Rabinowitch~ H.D. and Budowski, P., 1975. Conditioning of tomato fruit against sunscald. Scientia Hortic., 3: 83--87. Kretchman, S.W., Wittmeyer, E.C. and Short, T.H., 1974. Considerations for using ethephon on tomatoes for processing in 1974. Dept. Hortic., Series No. 408, Ohio Agric. Res. Dev. Centre, Wooster, Ohio. Sims, W.L., 1973. Ripin 'em early with Ethrel. Am. Veg. Grower, March 1973, 32--34. Tompkins, D.R., 1975. Ethephon promotes ripening of tomatoes for once over harvest. Arkansas Farm Res., 24: 4.
299
EFFECT OF M O I S T U R E S T R E S S O N T H E E T H E P H O N R E S P O N S E IN TOMATOES
L.L. MUTTON Agricultural Research Centre, N.S.W. Department of Agriculture, Yanco (Australia)
ABSTRACT Mutton, L.L., 1978. Effect of moisture stress on the ethephon response in tomatoes. Scientia Hortic., 8: 299--305. Potted plants (cultivar 'Napoli') were maintained at 3 levels of moisture (continuously moist, moderate moisture stress and severe moisture stress) and sprayed to run-off with 3 levels of ethephon (0, 250 and 500 p.p.m.) in factorial combination, in order to examine the effect of moisture status on the ethephon-accelerated ripening-response in tomatoes. The largest increases in ripening-rate occurred where plant~ were moderately stressed and sprayed with 250 p.p.m, ethephon, or where plants were kept continuously moist and sprayed with 500 p.p.m, ethephon. No substantial increase in ripening-rate occurred at higher levels of either ethephon or moisture stress. Leaf senescence was rapid in all treatments other than the moderately stressed and continuously moist controls. In some of the water-stressed treatments a partial recovery of plant water potential occurred as a result of reduced transpiration due to rapid and extensive leaf loss.
INTRODUCTION Increasingly, e t h e p h o n is b e i n g used t o p r o m o t e g r e a t e r u n i f o r m i t y o f r i p e n i n g in m e c h a n i c a l l y h a r v e s t e d p r o c e s s i n g - t o m a t o e s . H o w e v e r , p l a n t r e s p o n s e t o e t h e p h o n c a n r a r e l y b e p r e d i c t e d w i t h a high d e g r e e o f a c c u r a c y . O n e o f t h e m a j o r r e a s o n s f o r this is t h e l a c k o f d e t a i l e d studies o n t h e m a n n e r in w h i c h e n v i r o n m e n t a l c o n d i t i o n s a f f e c t t h e r e s p o n s e . W o r k b y Bussell ( 1 9 7 3 ) a n d T o m p k i n s ( 1 9 7 5 ) s u p p o r t s n u m e r o u s field o b s e r v a t i o n s t h a t m o i s t u r e stress e n h a n c e s t h e e t h e p h o n - a c c e l e r a t e d ripeningr e s p o n s e , a l t h o u g h Bussell's d a t a suggest t h a t e x t r e m e s o f m o i s t u r e stress "limit" the response. However, both workers based their conclusions on the results o f a single harvest. T h e y also gave n o i n d i c a t i o n o f t h e w a t e r p o t e n t i a l s r e a c h e d in t h e i r t r e a t m e n t s or t h e e x t e n t o f l e a f s e n e s c e n c e w h i c h o c c u r r e d . M o r e i m p o r t a n t l y , t h e y a p p l i e d stress t r e a t m e n t s s o m e t i m e p r i o r t o spraying. I t is well k n o w n (Sims, 1 9 7 3 ; K r e t c h m a n e t al., 1 9 7 4 ) t h a t a n y t y p e o f stress a t t i m e o f s p r a y i n g i n d u c e s g r e a t e r d e f o l i a t i o n t h a n t h a t w h i c h o c c u r s if p l a n t s are non-stressed. I t is p o s s i b l e also t h a t t h e r i p e n i n g - r e s p o n s e o f p l a n t s sprayed under these conditions might be different to that obtaining under n o r m a l c i r c u m s t a n c e s . F o r t h e s e r e a s o n s it is i m p o r t a n t t o d i f f e r e n t i a t e b e t w e e n c o n d i t i o n s a t t i m e o f s p r a y i n g and t h o s e d u r i n g t h e p o s t - s p r a y p e r i o d .
300
The experiment described here aimed to examine more fully the relationship b e t w e e n moisture status and the ethephon-accelerated ripening-response. MATERIALS AND METHODS Thirty six plants (cultivar 'Napoli') were grown in 60 cm pots under similar conditions in a glasshouse and pruned to a single leader. A sandy loam was used as the potting-medium. Flowers were tagged at anthesis over a period of 20 days to ensure a wide range of fruit age on each plant. When the oldest fruit on the majority of plants were showing signs of ripening (about 42 days from anthesis), pots were randomly allocated to the following treatments: 1. Continuously moist ; 0 p.p.m, ethephon 2. Continuously moist ; 250 p.p.m, ethephon 3. Continuously moist ; 500 p.p.m, ethephon 4. Moderate water stress; 0 p.p.m, ethephon 5. Moderate water stress; 250 p.p.m, ethephon 6. Moderate water stress; 500 p.p.m, ethephon 7. Severe water stress ; 0 p.p.m, ethephon 8. Severe water stress ; 250 p.p.m, e t h e p h o n 9. Severe water stress ; 500 p.p.m, ethephon Each treatment consisted of 4 pots as replicates. Plants receiving various ethephon treatments (expressed as p.p.m, active ingredient) were sprayed to run-off. Control plants were sprayed with water only. No surfactant was added to the spray material. The glasshouse temperature was held at 20°C (day) and 15°C (night). Continuously moist treatments were watered daily. Moderate levels of stress were obtained b y replacing half the moisture lost daily as determined b y weighing. Severe stress levels were obtained b y a complete absence of watering. The above watering-regimes were c o m m e n c e d immediately after spraying. The moisture status of plants was determined at regular intervals using a Scholander pressure bomb. B o m b readings were taken at 10 am (Eastern Standard Time) using the youngest fully-expanded leaf. Ripening was recorded as the time at which individual fruit achieved a redripe condition, the fruit being inspected daily. Whole plants were subjectively categorised for leaf senescence as follows: 1. Leaves completely green 2. Leaves mainly green b u t showing some signs of yellowing 3. Leaves mainly yellow b u t showing some traces of green 4. Leaves completely yellow 5. Leaves withered and brown. In recording leaf senescence, juvenile growth at the t o p of the plant (i.e. leaves still expanding) was ignored. Juvenile growth is typically unaffected b y ethephon. On completion of the experiment, fruit from the extreme treatments 1, 3, 7 and 9 were tested for titratable acidity and percent solids.
301
RESULTS E t h e p h o n and moisture stress b o t h accelerated the ripening o f y o u n g fruit, the time to ripen being progressively reduced the y o u n g e r the fruit at the time t r e a tmen ts were imposed. Hence, the slope of the regression (b) o f time from anthesis to m a t u r i t y on age o f fruit at time o f spraying (an example o f which will be shown in Fig. 1), provided a good index of a t r e a t m e n t ' s effect on the ripening-response. Differences am ong t r e a t m ent s obtained in this w a y w e r e tested b y analysis o f variance o f the regression coefficients. Increasing levels o f e t h e p h o n p r o m o t e d a greater acceleration o f ripening than increasing levels o f moisture stress. T h e greater increases occurred where plants were m o d e r a t e l y stressed a n d sprayed w i t h 2 5 0 p.p.m, et hephon, or where plants were k e p t c o n t i n u o u s l y moist and sprayed with 500 p.p.m. ethephon. B e y o n d these points no substantial acceleration of ripening occurred (Table 1). T he intrinsic variation in ripening-time f or fruit o f similar age was also progressively reduced by the action o f b o t h e t h e p h o n and moisture stress. This can clearly be seen in Fig. I and is quantified b y the coefficients of determination (R 2) listed in Table 1. However, the low R 2 values for the continuously moist and m o d e r a t e l y stressed controls are m o r e likely to be a f u n c t i o n o f the low b-values than inherent variation a b o u t t he regression line. T h a t substantial differences in moisture stress occurred is clearly shown in Fig. 2. In th e m o d e r a t e l y stressed treatments, water potentials decrease and TABLE 1 Analysis of variance of the coefficients for the regression of time from anthesis to ripening on age of fruit at time of spraying. The symbols *** and ** represent differences which are significant at p < 0.001 and p < 0.01 respectively. Ethephon level
Moisture stress Continuously moist
Control
250 p.p.m. 500 p.p.m.
b
R2
Moderate moisture stress b Re
Severe moisture stress b R2
0.06 0.31 0.54
0.06 0.67 0.92
0.14 0.50 0.61
0.22 0.55 0.58
0.13 0.86 0.94
L.S.D. (b values) = 0.043 (p < 0.05) Source
df.
Moisture stress
2
Ethephon levels Interaction
2
Error
4 27
Total
35
M.S.
F ratio
0.06775 0.62140 0.01245 0.00264
25.7*** 235.4*** 4.7**
0.56 0.85 0.76
302
50 • b
oo
• ~jD ?__oo__
• • oo edD ~
• •
• oo o ~
• • o
•
lid
o
go oo
0
~
45 •
•
•
•
O0
•
. . . . . .
•
\._\. ./
40
0 p.p.m. 35.
~' 2'o
2~
3b
3~
~b
•
-1%
"~ 5 0
0 p.p.m.
;
~b
1;
o o "~ 4 5 ._~
~
4o
E
~ w
250 p.pm.
•
cL ~ -12 ~
•
250 p.p.m.
~ 35
26
25
36
3;
4;
50
~"
-%
;
1;
1;
0
45
40 ~ 35
• ~-20
I
25
I
30
I
35
Age of fruit at time of spr,aying (days from anthesis)
I
40
-%
;
•
500 p.p.m.
lb
&
Days after spraying
Fig. 1. Relationship between the time taken for fruit to ripen and their age at time of spraying for the continuously moist treatments. Fig. 2. Plant water potential during the perio d following ethephon application. (A, severe moisture stress; m, moderate moisture stress;., continuously moist.)
then increase. Also, in the moderate and severe stress treatments there is an inverse relationship between the minimum water potentials reached and the e t h e p h o n level, as well as a delay in the time taken to reach minimum water potentials as the e t h e p h o n level increases. The above features can be explained by the rate of leaf senescence which occurred in the treatments concerned. In addition, rapid leaf senescence in many treatments meant that bomb readings had to be curtailed prematurely (Fig. 3). E t h e p h o n and moisture stress appear to have had a greater impact o n leaf senescence than o n ripening (Fig. 3) since rapid leaf senescence occurred in all treatments other than the c o n t i n u o u s l y moist and moderately stressed controls.
303
•
AjA
2 1:
A / • "-"~'~o
p.p.m.
i f O ..o
•
•
5
~ 4 8 $ ._/ 5
6'
•
•
,4 3 2 1
Days after spraymg Fig. 3. Ratings for leaf senescence (1 ffi green, 5 ffi completely senesced) during the period following ethephon application. (A, severe moisture stress; =, moderate moisture stress; o, continuously moist.)
TABLE 2 Titratable acidity and percent solids for selected treatments Treatment
Titratable acidity (ml 0.1 N, NaOH/5 g pulp)
Percent solids
1. 3. 7. 9.
4.5 4.3 6.2 4.9
5.1 5.2 6.1 5.6
Continuously moist, 0 p.p.m, ethephon Continuously moist, 500 p.p.m, ethephon Severe moisture stress, 0 p.p.m, ethephon Severe moisture stress, 500 p.p.m, ethephon
304
Titratable acidity and percent solids were inversely related to the maximum leaf water potentials in the treatments chosen for testing (Table 2). Such results indicate that fruit from stressed plants were also measurably dehydrated. DISCUSSION
The results show that moisture status is an important consideration where ethephon is used to accelerate ripening. The substantial differences that occurred among treatments suggest that differences in moisture status could well account for some of the variation in response observed in field situations, although it seems unlikely that the very high levels of moisture stress recorded in this experiment would normally be attained in field-grown crops unless they were already partly stressed prior to ethephon application. The results of this experiment are similar to those of Bussell (1973) except that at the highest level of moisture stress there was no significant reduction in ethephon-accelerated ripening as occurred in Bussell's experiment. Bussell interpreted his result as being due to an interference of the ethephon response, whereas these results simply suggest that the combined effects of ethephon and moisture stress had approached a m a x i m u m where plants had been moderately stressed and sprayed with 250 p.p.m, ethephon, or where plants kept continuously moist had been sprayed with 500 p.p.m, ethephon (Table 1). However, because the 2 experiments were conducted under different circumstances, it is probably n o t valid to make direct comparisons between them. Considerable caution needs to be exercised in interpreting experiments of this kind. Because rapid leaf senescence and defoliation leads to reduced transpiration, high levels of stress m a y n o t occur in ethephon-treated plants even though they m a y be denied water. The inverse relationship between minimum water potentials and ethephon level in the moderate and severely stressed treatments m a y thus explain w h y the severely stressed plants sprayed with 500 p.p.m, ethephon ripened more slowly than would be expected from the trends in other treatments. Early leaf senescence m a y also lead to titratable acidity and percent solids which are lower than expected on the basis of irrigation management alone (Table 2). The susceptibility o f leaves to the combined effects of moisture stress and ethephon is clearly shown in Fig. 3. Leaf loss in itself is an important consideration in most areas where field tomatoes are grown, particularly during early- and mid-season harvests when the risk of sunburn is high. Tomatoes are generally able to withstand exposure to the sun provided t h e exposure is n o t sudden (Kedar et al, 1975). Figure 3 shows the importance o f having plants well watered at the time of spraying if the rate of leaf loss is to be minimised, although under these circumstances the rate of ripening would be less than optimal. Later in the season, when leaf loss is often desirable (and the risk of rain at harvest is greater), watering could cease some time before spraying so that some measure of stress m a y be more readily achieved during the post~spray ripening-period.
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REFERENCES Bussell, W.T., 1973. Effects of Soil moisture on ripening in dwarf tomatoes after ethephon treatment. N.Z.J. Exp. Agric., 1: 381--382. Kedar, N., Rabinowitch~ H.D. and Budowski, P., 1975. Conditioning of tomato fruit against sunscald. Scientia Hortic., 3: 83--87. Kretchman, S.W., Wittmeyer, E.C. and Short, T.H., 1974. Considerations for using ethephon on tomatoes for processing in 1974. Dept. Hortic., Series No. 408, Ohio Agric. Res. Dev. Centre, Wooster, Ohio. Sims, W.L., 1973. Ripin 'em early with Ethrel. Am. Veg. Grower, March 1973, 32--34. Tompkins, D.R., 1975. Ethephon promotes ripening of tomatoes for once over harvest. Arkansas Farm Res., 24: 4.