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Osmoconditioning of tomato and onion seeds
Scientia Horticu[turae, 43 (1990) 213-224 Elsevier Science Publishers B.V., Amsterdam
213
Osmoconditioning of tomato and onion seeds A. Ali 1, V. Souza M a c h a d o 1 a n d A.S. H a m i l l 2 aDepartment of Horticultural Science, University of Guelph, Guelph, Ont., N I G 2 W1 (Canada) 2Canada Department of Agriculture, Harrow, Ont. (Canada) ( Accepted for publication 8 December 1989)
ABSTRACT Ali, A., Souza Machado, V. and Hamill, A.S., 1990. Osmoconditioning of tomato and onion seeds. Scientia Hortic., 43:213-224. Seeds of tomato and onion were osmoconditioned for periods of 1 to 4 weeks in polyethylene glycol, molecular weight about 8000 (PEG-8000), solutions at water potentials between - 5 . 8 and - 14.9 bars. Subsequently the seeds were germinated at 10, 18 and 26°C. Most of the treatments led to a rapid germination response and this enhancement was greater at cooler temperatures. Minimum 7day o smotreatments of tomato and onion seeds at - 8.6 and - 11.9 bars, respectively, produced rapid germination responses at 15 ° C. Osmoconditioning of tomato seeds, followed by a moist osmoticum storage at 5 + 1 ° C, resulted in reduced germination and seedling emergence, but with onions the enhanced germination and emergence response of the osmoconditioned seeds remained unaffected. Field testing of osmoconditioned seeds of several tomato cultivars at two sites in Ontario showed enhanced seedling emergence, plant development and more breaker-red tomato fruits at harvest, indicating earlier maturity. However, no total yield differences between the osmoconditioned and non-osmoconditioned plants were observed. Keywords: onion; polyethylene glycol; seed germination; seed priming; tomato. Abbreviations: non-osmo=non-osmoconditioned; osmo=osmoconditioned; PEG-8000 =polyethylene glycol molecular weight ~ 8000; Tso= days to 50% seed germination.
INTRODUCTION C o l d w e t soils, i n t o w h i c h c r o p s e e d s a r e s o w n i n t h e s p r i n g , a r e n o t f a v o r able for rapid germination and emergence. Often the seedlings grow slowly and are susceptible to damage by seed and root-rot organisms, resulting in an uneven plant stand of low vigor and poor yield potential. A wide variety of seed treatments has been proposed (Heydecker and Coolbear, 1977) and attempts have been made to improve germination rate, stand uniformity and y i e l d ( B o d s w o r t h a n d B e w l e y , 1 9 8 1 ; W o l f e a n d S i m s , 1 9 8 2 ). Seed treatment with osmotic solutions, known as "priming" (Heydecker
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A. ALI ET AL.
and Coolbear, 1977 ) or "osmoconditioning" (Khan et al., 1978), is one of several physiological methods designed to improve performance and appears to be useful for inducing early and synchronous seed germination. Because of its large molecular size and chemically inert properties, PEG-8000 is considered to be a more desirable osmoticum than salts, which readily penetrate the cell membranes and could affect the seed physiology. In our study, the germination response at several temperatures in the laboratory as well as field responses were examined in relation to various osmoconditioning treatments of tomato and onion seeds. Tomato is a warm season crop and, because of a short growing season in Ontario, growers rely on transplants imported from the Southern U.S.A., which means higher input costs compared to direct seeding and the possibility of pathogen introduction. To assess the potential of osmoconditioning, tomato seeds were direct seeded in the field and tested for early maturity and yield responses at Cambridge and Harrow, Ontario. Onions are considered a cool season crop and were included for comparison. MATERIALS AND METHODS
Laboratory studies Experiment L - Initial investigations were designed to evaluate the effects of seed osmoconditioning techniques on germination. Tomato (Lycopersicon eseulentum Mill. cultivar E-870) and onion (Allium cepa L. cultivar 'Rocket') seeds were imbibed in aqueous PEG-8000 solutions of varying osmotic potentials of - 5.8, - 8.6, - 11.9 and - 14.9 bars at 15 °C. These osmotic potentials were obtained as described by Michel and Kaufmann ( 1973 ). Fifty seeds were placed in a 9-cm Petri plate lined with Whatman No. 1 filter paper and contained 8 ml PEG-8000 solution. The plates were sealed with parafilm-M and incubated in the dark for a period of 1, 2, 3 or 4 weeks. Subsequently, the seeds were washed with distilled water and placed in a filter paper-lined Petri plate containing 6 ml distilled water. These plates were sealed with parafilm, transferred to germination cabinets and stored in the clark at 10, 18 or 26 °C. Seeds germination was recorded daily. Seeds were considered germinated when the radicle had emerged through the seed coat. Tso, the number of days required for germination of 50% of seeds, was determined by plotting germination response against time. Some germination occurred in the osmotic solution when the seeds were treated either at sub-optimal concentration or for an extended period. Therefore the treatments which resulted in germination of > 20% seeds in the osmoticum were not included for determination of Tso values. Experiment II. - To assess the m i n i m u m osmoconditioning period required to enhance germination, tomato and onion seeds were osmotreated for periods ranging from 1 to 7 days, using 250 g 1-1 H 2 0 ( --8.6 bars) and 300 g
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
215
1- ~ H 2 0 ( - 11.9 bars) P E G - 8 0 0 0 solutions, respectively. The germination response o f the osmo seeds was recorded at 15 ° C. E x p e r i m e n t I I I . - Seedling emergence in flats was evaluated for osmo seeds
stored in moist osmoticum. T o m a t o and onion seeds were o s m o c o n d i t i o n e d for 7 days at - 8.6 and - 11.9 bars, respectively. The osmo seeds submerged in P E G - 8 0 0 0 solution were stored at 5 + 1 °C in the dark, for pe.riods ranging from 0 to 28 days. Following storage, the o s m o seeds were equilibrated at 15 ° C for 1 h and quickly washed with distilled water. The washed seeds were air dried, " coated with thiram fungicide dust, planted in plastic flats ( 175 X 135 X 50 m m ) containing a Fox sandy loam and placed in cabinets at 18 + 2 ° C day, 12 + 2 ° C night temperature and with a 14-h photoperiod. The seeds were planted in rows o f 50 seeds per row and each treatment was replicated four times. Seedling emergence was m o n i t o r e d daily. Field studies Harrow trial 1983. - T o m a t o seeds cultivars H-2653 and TH-318 were os-
m o c o n d i t i o n e d for 7 days by submerging seeds in PEG-8000 solution (250 g I- 1 H 2 0 , - - 8.6 bars). Osmoconditioning was conducted in large stainless steel trays ( 5 7 X 5 7 X 2 c m ) lined with filter paper, sealed and placed in a controlled temperature cabinet at 15 + 1 ° C in the dark. After a 7-day treatment period, the seeds were quickly rinsed with deionized water, air dried and coated with thiram fungicide dust. A Stanhay planter was used for direct seeding o f the n o n - o s m o and osmo t o m a t o seeds on 12 M a y 1983. A clump o f 4 7 seeds was d r o p p e d every 20-22.5 cm in Fox sandy loam, soil fertilized with 8 - 3 2 - 1 6 N P K at a rate o f 560 kg ha -I. The herbicide treatments involved trifluralin pre-plant incorporated at 1.0 kg h a - 1 and metribuzin applied preemergence at 0.125 kg ha-1. A post-emergence application o f metribuzin at 0.5 kg h a - 1 was applied in the 4-leaf stage o f tomatoes. Plots consisted o f two 8-m rows and the rows were 1.5 m apart. Each treatment was replicated four times in a r a n d o m i z e d block design. Cambridge trials 1984 a n d 1985. - T o m a t o seeds cultivar FM-6203 were osm o c o n d i t i o n e d in 1984 for 7 days b y submerging in PEG-8000 solution (250 g 1- i H 2 0 , - 8.6 bars) in a sealed plastic tray (250 X 174 X 45 m m ) lined with filter paper and placed in a controlled temperature cabinet at 15 + 1 oC in the dark. After a 7-day treatment period, the seeds were quickly rinsed with deionized water, air-dried and coated with thiram dust. Using a Planet Junior, two 60-m rows o f o s m o and two rows of non-osmo t o m a t o seeds plus the guard rows were planted on 28 M a y 1984. The plants within the rows were thinned to 15-cm spacing after emergence. In the 1985 trials, seeds o f t o m a t o cultivars FM-6203, H-2653, O H - 7 8 1 4 and O H - 8 2 4 3 were osmoconditioned, as in the previous year 1984, for 4- and
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7-day periods. Using a Planet Junior, 5-m rows of osmo and non-osmo seeds were planted as a randomized block on 22 May 1985. The rows were 1.5 m apart and thinned to 17.5-cm spacing within the row after emergence. Trifluralin at 1.5 kg h a - 1 was shallow pre-plant incorporated for weed control. Weed escapes in 1984 and 1985 were controlled during the growing season by a combination of inter-row cultivation and hand-weeding within the rows. Laboratory as well as field data were analyzed using SAS ( 1985 )procedure GLM and the treatment means separation involved LSD with P = 0.05. RESULTS
Laboratory studies Germination during seed osmoconditioning. - Seed germination in the osmoticum occurred during osmoconditioning and varied with the species, PEG concentration and the duration of osmotreatment (Table 1 ). Osmotreatment with 300 g 1- 1 H20 or a higher PEG for 1 week resulted in minimal, i.e. < 1% germination of onion seeds in the osmoticum. Tomato seeds exhibited minimal or no germination in the osmoticum at all concentrations tested during 1 week of osmoconditioning. A sub-optimal concentration of PEG-8000 solution as well as osmotreatment for an extended period resulted in considerable germination of both onion and tomato seeds in the osmoticum (Table 1 ). Germination following osmoconditioning. - Germination responses of the tomato and onion seeds were temperature-dependent. The cultivar E-870 of tomato seeds used was very sensitive to the low temperature and resulted in a very low germination response at 10°C (Fig. 1 ), whereas the onion seeds, osmo as well as non-osmo, germinated well at all temperatures. Compared to non-osmo, the onion seeds which were osmotreated with PEG solution for 1 week showed accelerated germination and reduced Tso values (Fig. 1; Table TABLE 1 G e r m i n a t i o n (%) o f t o m a t o a n d o n i o n in the o s m o t i c u m PEG-8000 solutions. Values are m e a n s o f 12 replications o f 50 seeds each. M e a n in the s a m e c o l u m n followed by the same letter are not significantly different (LSD, P = 0.05 ) PEG-8000
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217
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
2). At 10 ° C, the 7"50 value for osmo onion seeds was reduced, indicating a gain in germination time. At 18 ° C, the Tso values for the osmo onion and tomato seeds were much lower than for non-osmo seeds (Table 2). At 10°C, the non-osmo and osmo tomato seeds resulted in < 5 and 20% germination, respectively, over a 2-week period. However, transferring these seeds to 18 ° C led to germination of > 75% osmo seeds within 2 days compared to < 10% of the non-osmo seeds, indicating that enhanced germination potential was sustained. Osmotreating tomato and onion seeds at 1-day intervals from 1 to 7 days resulted in a progressively faster germination response at 15 °C (Fig. 2 ). The Tomato 100
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Fig. 1. G e r m i n a t i o n responses o f the o s m o ( ) and non-osmo ( - - - ) tomato and onion seeds at 10, 18 a n d 26°C. The t o m a t o a n d o n i o n seeds were o s m o c o n d i t i o n e d for 1 week at 15 ° C with PEG-8000 solution o f 250 g 1-1 H 2 0 ( - 8.6 b a r s ) a n d 300 g 1-1 H 2 0 ( - 11.9 bars), respectively. TABLE 2 Tso (days required for 50% seed g e r m i n a t i o n ) values o f the o s m o a n d n o n - o s m o t o m a t o a n d o n i o n seeds. Values are m e a n s o f four replications. M e a n s in the same column followed by the same letter are not significantly different (LSD, P = 0.05 ) PEG-8000 g 1- I
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Days to germination Fig. 2. G e r m i n a t i o n responses (15 ° C ) o f t o m a t o a n d o n i o n seeds after they were osmocondit i o n e d for 1-7 days using P E G - 8 0 0 0 solutions o f 250 g 1-1 H 2 0 ( - 8.6 b a r s ) a n d 300 g 1- ~ H 2 0 ( - 11.9 b a r s ) , respectively. C o m p a r e d to n o n - o s m o seeds, the g e r m i n a t i o n o f o s m o seeds which were o s m o t r e a t e d for ~>2 days was significantly ( P = 0.05 ) higher over a 6-day period. N u m b e r s along the g e r m i n a t i o n curves indicate days of o s m o t r e a t m e n t .
Tso value of 5 days for non-osmo seeds was reduced to 1 day for seeds which were osmoconditioned for 7 days. These results indicate that a m i n i m u m of 7 days osmotreatment was necessary for tomato and onion seeds to gain a significant advantage in germination time over the non-osmo seeds. G e r m i n a t i o n a n d e m e r g e n c e o f s e e d s stored in o s m o t i c u m . - To allow for convenience for the grower, the osmo tomato and onion seeds were moist-stored in the osmoticum at 5 _+ 1 °C for a period of 3-28 days. The enhanced germination response in Petri plates of the osmo onion seeds remained unaffected even after storage for 28 days, whereas germination of the stored osmo tomato seeds was much reduced (Fig. 3a). Osmoticum-stored osmo onion seeds emerged earlier than the non-osmo seeds when seeded in sandy loam in flats, however when moist storage was extended for ~>14 days the magnitude of the enhancement effect was reduced (Fig. 3b). The emergence of the osmo tomato seeds was adversely affected by storage in the osmoticum at 5 _+ 1 °C for as little as 3 days (Fig. 3b). F i e l d studies H a r r o w trial 1983. - During early crop establishment in the field, seedlings
from non-osmo seeds ofTH-318 emerged earlier and were sturdier than those of H-2653, indicating seedling vigor differences between the two cultivars. Regardless of the method of weed control, i.e. hand weeded or chemical, the osmo seeds of H-2653 and TH-318 showed early emergence. Non-osmo seeds showed no sign of emergence 12 days after seeding, whereas a 20 or 40%
219
OSMOCONDITION1NG OF TOMATO A N D O N I O N SEEDS a
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Fig. 3. Germination and emergence of the osmoconditioned tomato and onion seeds after storage ( 5 + 1 °C) in the PEG-8000 solution for 0, 3, 14 or 28 days. The response curves are indicated by So, $3, S~4 and Szs. The tomato and onion seeds were osmoconditioned for 1 week with PEG-8000 solution of 250 g 1-1 H20 ( - 8 . 6 bars) and 300 g 1-1 H20 ( - 11.9 bars), respectively.
emergence o f the osmo seeds was already observed in both cultivars at this time (Fig. 4 ). Fifteen to 16 days after seeding, emergence o f the H-2653 osmo seeds remained considerably higher than that o f the non-osmo seeds, but the non-osmo seeds ofTH-318 soon caught up and at 15 days there was no difference in percent emergence between the non-osmo and the osmoconditioned seeds. Evaluation at 6 weeks after seeding revealed that H-2653 osmo seeds had considerably higher ( > 80%) emergence and more plants at the 4-leaf stage than the non-osmo seeds (Table 3 ). Enhanced emergence and developmental responses were eventually reflected in increased yield and fruit maturity. However with TH-318, no differences with respect to total emergence, seedling development, fruit maturity and yield were observed between the non-osmo and osmo seeds (Table 3).
220
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Fig. 4. Tomato seedling emergence in the field. Seeds of the cultivars TH-318 and H-2653 were osmoconditioned for 1 week with PEG-8000 solution of 250 g I-~ H20 ( - 8 . 6 bars) and were planted in the field along with the non-osmoconditioned seeds. The values are means of four replications.
TABLE 3 Emergence, plant maturity, fruit maturity and yield in tomato (cultivars H-2653 and TH-318) plants grown in the field from non-osmo and osmo seeds. The seeds were osmoconditioned at 15 °C for 1 week, using PEG-8000 solution of 250 g 1-1 H20 ( - 8 . 6 bars). Values are means of four replications. Means in the same column followed by the same letter are not significantly different within the cultivars (LSD, P = 0.05 ). Emergence and plant maturity were assessed at 6 weeks, fruit maturity and yield were evaluated 16 weeks after seeding (Harrow trial 1983) Cultivar
Seed treatment
Emergence (%) 1
Plants at four leaf stage (%) ~
Fruit yield per plot Breakers (kg)
Green (kg)
Total (kg)
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100b 186a
100b 184a
25.0b 48.5a
18.9b 24.2a
43.9b 72.7a
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Non-osmo Osmo
100a 102a
100a 106a
37.8a 38.2a
18.2a 14.0a
56.0a 52.2a
1Based on non-osmo seedlings.
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
221
Cambridge trials 1984 a n d 1985. - Sixteen weeks after seeding, the breakerred fruit count in the two 60-m rows sown with osmo tomato seeds had 83% more breakers than the non-osmo rows, indicating earlier fruit maturity. Subsequent harvesting and evaluation of the trial 17 weeks after seeding revealed that the plant count, the total fruit yield as well as the green fruit yield, showed little difference between osmo and non-osmo treatments. The breakers and the total fruit yield of osmo seed plots were 56 and 18% higher than for the non-osmo seeds. The total yield differences between the non-osmo and osmo seed plots were statistically not significant. In the 1985 trial, the total fruit yield (breakers and green fruits) of the four tomato cultivars FM-6203, H-2653, OH-7814 and OH-8243 showed little difference between the non-osmo and osmo seed plants (data not presented). For FM-6203, OH-7814 and OH-8243, the osmo seed plants produced a greater number and weight of breaker-red fruits than the non-osmo seed plants. These cultivars showed a 1-week delay in fruit ripening as compared to H2653 (data not presented). DISCUSSION
Laboratory studies
Seed osmoconditioning has been claimed to be effective in accelerating germination, seedling emergence and performance of the crop in the field (Heydecker and Coolbear, 1977; Brocklehurst and Dearman, 1983, a,b; Alvarado et al., 1987 ). Seeds of tomato and onion, investigated in our study, have shown that the m i n i m u m period for osmoconditioning treatment is ~ 7 days and enhanced germination of the osmo seeds is dependent on temperature. Following osmoconditioning, the emergence response with tomato and onion was less pronounced than that noted with the germination studies (Figs. 1 and 3 ), which indicates that osmoconditioning is more directly targeted towards the germination process. These observations are partially in agreement with previous reports where osmoconditioning of vegetable and flower seeds resulted in improved germination and seedling emergence (Heydecker and Wainwright, 1976; Simmonds, 1980; Bodsworth and Bewley, 1981; Khan et al., 1983). Previous reports indicated that storage of air-dried osmo seeds, depending on the length of storage, tends to reduce the enhancement of germination potential (Heydecker and Coolbear, 1977; Brocklehurst and Dearman, 1983 a,b). However, Haig et al. (1986) noted that enhanced germination and emergence characteristics of salt-primed tomato and carrot seeds were maintained after air drying and storage for up to 28 days, although air drying of the onion seeds led to a reduced percentage of emergence. In our study, the osmo tomato and onion seeds, stored moist in the PEG-8000 solution at
222
A. ALl ET AL.
5+ I°C, revealed that the enhanced germination potential and seedling emergence of the osmo onion seeds was sustained after storage for up to 28 days (Fig. 3a and b). In contrast, storage of the osmo tomato seeds under the same conditions for as little as 3 days adversely affected the enhanced germination and emergence potential, possibly due to chilling injury to the embryo (Fig. 3a and b ). Also germination of the osmo tomato seeds, which was practically suspended at 10 ° C (Fig. 1 ), resulted in > 75% seed germination within 2 days when the seeds, after 2 weeks at 10 ° C, were transferred to 18 ° C (data not presented). It is evident that, depending on species, the proper conditions for storage of osmo seeds will vary. To sustain the enhanced germination potential, consideration must be given to the temperature sensitivity of the osmo seeds.
FieM studies Except for a few reports which involved field testing of osmo seeds, most of the published research was conducted under controlled conditions. In several crop species tested under controlled conditions, seed osmoconditioning resulted in earlier seedling emergence (Heydecker and Coolbear, 1977; Wolfe and Sims, 1982; Brocklehurst and Dearman, 1983b; Khan et al., 1983; Haig et al., 1986). Furthermore, enhanced plant development and fruit maturity (Wolfe and Sims, 1982), increased plant weight (Brocklehurst and Dearman, 1983b; Alvarado et al., 1987 ) and harvest yields (Szafirowska et al., 1981 ) were observed with osmoconditioned seeds. Wolfe and Sims (1982) noted enhanced seedling emergence, plant develo p m e n t and fruit maturity which was similar to our resuRs with osmo tomato seeds of the FM-6203 and H-2653 ( 1983 and 1984 trials). Early maturity and increased yield due to rapid emergence of the osmoconditioned seeds has been reported for carrots (Szafirowska et al., 1981 ). However, yield increases were not always associated with osmoconditioned seeds (Wolfe and Sims, 1982; Zbitnew, 1984; Alvarado et al., 1987 ). The increased fruit yield observed in our study with osmo seeds of H-2653 was more a reflection of sustained differences in seedling emergence and development between osmo and non-osmo seeds. It should be noted that the fast emergence of TH-318, except for an initial difference in seedling emergence, showed no developmental or maturity differences and the fruit yield of the osmo and non-osmo plants was similar (Fig. 4, Table 3 ). It therefore appears that the field performance of the osmo seeds could be related to genetic cultivar differences, and the potential benefits of seed osmoconditioning for early crop establishment and maturity are more evident with low-vigor, slow-emerging cultivars. Early crop establishment and fruit maturity in these cultivars is of economic significance, as a reduction in the growing season of only a few days is considered economically important for commercial growers in the tomato processing industry
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
223
(Wolfe and Sims, 1982 ). Rapid seedling establishment also reduces the risks associated with early crop development. Contrary to the 1983 and 1984 field trials in which FM-6203 and H-2653 showed developmental and maturity differences between osmo and non-osmo seeds, no such differences were evident during the 1985 growing season. Therefore, in addition to the cultivar differences, the climatic variation during crop establishment could influence field performance of the osmoconditioned seeds. Seed osmoconditioning can therefore be regarded as an insurance measure with potential benefits o f rapid and uniform crop emergence, development and maturity. Zbitnew (1984), working with the onion hybrid cultivar 'Aries' in Ontario, noted that differences between osmo and non-osmo seeds in emergence and subsequent growth, as indicated by Ts0 values at the loop, crook, flag and first leaf stage, decreased as seedlings developed and as growth cabinet temperature regimes increased from day/night levels of 10/6°C to 15/11 °C. The greatest response to osmoconditioning was noted at the loop stage and at the lower temperature regime. In field trials involving three planting dates, initial differences in seedling development were noted at the early planting date, but not later, with eventually no differences in the final yield or time to maturation. Seed germination is a temperature-sensitive process with its high and low temperature limits, beyond which the rate of germination is adversely affected. The high and low temperature limits of the cultivars used in our study appear to be 26 and < 15°C for tomato, and 18 and < 10°C for onion seeds, respectively. Between these limits, the potential gain in germination time of the osmo seeds was maximal toward the lower temperature limits (Fig. 1, Table 2). Furthermore, the sustained gains due to osmoconditioning expressed at the germination stage can be maintained up to harvest only if the cultivar has a relatively low seedling vigor due to its inherent genetic background. Results from our study indicate that with the tomato and onion cultivars tested, seed osmoconditioning can lead to rapid and synchronous germination, more uniform seedling emergence and, depending on the species and cultivars, enhanced plant development and earlier fruit maturity. Since the enhanced germination and emergence response ofosmo seeds were more pronounced at sub-optimal cooler temperatures, this technique of seed osmoconditioning could be useful with slow-germinating cultivars in areas experiencing cooler spring conditions. REFERENCES Alvarado, A.D., Bradford, K.J. and Hewitt, J.D., 1987. Osmotic priming of tomato seeds: effects on germination, field emergence, seedling growth and fruit yield. J. Am. Soc. Hortic., Sci., 112: 427-432.
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Bodsworth, S. and Bewley, J.D., 1981. Osmotic priming of seeds of crop species with polyethylene glycol as a means of enhancing early and synchronous germination at cool temperatures. Can. J. Bot., 59: 672-676. Brocklehurst, P.A. and Dearman, J., 1983a. Interactions between seed priming treatments and nine seed lots of carrot, celery and onion. I. Laboratory germination. Ann. Appl. Biol., 102: 577-584. Brocklehurst, P.A. and Dearman, J., 1983b. Interaction between seed priming treatments and nine seed lots of carrot, celery and onion. II. Seedling emergence and plant growth. Ann. Appl. Biol., 102: 585-593. Haig, A.M., Barlow, E.W.R. and Milthorpe, F.L., 1986. Field emergence of tomato, carrot, and onion seeds primed in an aerated salt solution. J. Am. Soc. Hortic. Sci., 111: 660-665. Heydecker, W. and Coolbear, P., 1977. Seed treatments for improved performance - survey and attempted prognosis. Seed Sci. Technol., 5: 353-425. Heydecker, W. and Wainwright, H., 1976. More rapid and uniform germination of Cyclamen persicum. L. Scientia Hortic., 5: 183-189. Khan, A.A., Kar-ling, T., Knypl, J.S., Borkowska, B. and Powell, L.E., 1978. Osmotic coconditioning of seeds: physiological and biochemical changes. Acta Hortic., 83: 267-278. Khan, A.A., Peck, N.H,, Taylor, A.G. and Samimy, C., 1983. Osmoconditioning of beet seeds to improve emergence and yield in cold soil. Agron. J., 75: 788-794. Michel, B.E. and Kaufmann, M.R., 1973. The osmotic potential of polyethylene glycol 6000. Plant Physiol., 51: 914-916. SAS Institute, 1985. SAS User's Guide: Statistics, 5th. edn., SAS Institute, Cary, NC, 956 pp. Simmonds, J., 1980. Increased seedling establishment of Impatiens wallerana in response to low temperature or polyethylene glycol seed treatments. Can. J. Plant Sci., 60: 561-569. Szafirowska, A., Khan, A.A. and Peck, N.H., 1981. Osmoconditioning of carrot seeds to improve seedling establishment and yield in cold soil. Agron. J., 73: 845-848. Wolfe, D.W. and Sims, W.L., 1982. Effects of osmoconditioning and fluid drilling of tomato seed on emergence rate and final yield. HortScience, 17: 936-937. Zbitnew, K.D.W., 1984. The effect of osmoconditioning on the germination and development of onion (Allium cepa L. ). M. Sc. Thesis, University of Guelph, Ontario (unpublished).
213
Osmoconditioning of tomato and onion seeds A. Ali 1, V. Souza M a c h a d o 1 a n d A.S. H a m i l l 2 aDepartment of Horticultural Science, University of Guelph, Guelph, Ont., N I G 2 W1 (Canada) 2Canada Department of Agriculture, Harrow, Ont. (Canada) ( Accepted for publication 8 December 1989)
ABSTRACT Ali, A., Souza Machado, V. and Hamill, A.S., 1990. Osmoconditioning of tomato and onion seeds. Scientia Hortic., 43:213-224. Seeds of tomato and onion were osmoconditioned for periods of 1 to 4 weeks in polyethylene glycol, molecular weight about 8000 (PEG-8000), solutions at water potentials between - 5 . 8 and - 14.9 bars. Subsequently the seeds were germinated at 10, 18 and 26°C. Most of the treatments led to a rapid germination response and this enhancement was greater at cooler temperatures. Minimum 7day o smotreatments of tomato and onion seeds at - 8.6 and - 11.9 bars, respectively, produced rapid germination responses at 15 ° C. Osmoconditioning of tomato seeds, followed by a moist osmoticum storage at 5 + 1 ° C, resulted in reduced germination and seedling emergence, but with onions the enhanced germination and emergence response of the osmoconditioned seeds remained unaffected. Field testing of osmoconditioned seeds of several tomato cultivars at two sites in Ontario showed enhanced seedling emergence, plant development and more breaker-red tomato fruits at harvest, indicating earlier maturity. However, no total yield differences between the osmoconditioned and non-osmoconditioned plants were observed. Keywords: onion; polyethylene glycol; seed germination; seed priming; tomato. Abbreviations: non-osmo=non-osmoconditioned; osmo=osmoconditioned; PEG-8000 =polyethylene glycol molecular weight ~ 8000; Tso= days to 50% seed germination.
INTRODUCTION C o l d w e t soils, i n t o w h i c h c r o p s e e d s a r e s o w n i n t h e s p r i n g , a r e n o t f a v o r able for rapid germination and emergence. Often the seedlings grow slowly and are susceptible to damage by seed and root-rot organisms, resulting in an uneven plant stand of low vigor and poor yield potential. A wide variety of seed treatments has been proposed (Heydecker and Coolbear, 1977) and attempts have been made to improve germination rate, stand uniformity and y i e l d ( B o d s w o r t h a n d B e w l e y , 1 9 8 1 ; W o l f e a n d S i m s , 1 9 8 2 ). Seed treatment with osmotic solutions, known as "priming" (Heydecker
0304-4238/90/$03.50 © 1990 - - Elsevier Science Publishers B.V.
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A. ALI ET AL.
and Coolbear, 1977 ) or "osmoconditioning" (Khan et al., 1978), is one of several physiological methods designed to improve performance and appears to be useful for inducing early and synchronous seed germination. Because of its large molecular size and chemically inert properties, PEG-8000 is considered to be a more desirable osmoticum than salts, which readily penetrate the cell membranes and could affect the seed physiology. In our study, the germination response at several temperatures in the laboratory as well as field responses were examined in relation to various osmoconditioning treatments of tomato and onion seeds. Tomato is a warm season crop and, because of a short growing season in Ontario, growers rely on transplants imported from the Southern U.S.A., which means higher input costs compared to direct seeding and the possibility of pathogen introduction. To assess the potential of osmoconditioning, tomato seeds were direct seeded in the field and tested for early maturity and yield responses at Cambridge and Harrow, Ontario. Onions are considered a cool season crop and were included for comparison. MATERIALS AND METHODS
Laboratory studies Experiment L - Initial investigations were designed to evaluate the effects of seed osmoconditioning techniques on germination. Tomato (Lycopersicon eseulentum Mill. cultivar E-870) and onion (Allium cepa L. cultivar 'Rocket') seeds were imbibed in aqueous PEG-8000 solutions of varying osmotic potentials of - 5.8, - 8.6, - 11.9 and - 14.9 bars at 15 °C. These osmotic potentials were obtained as described by Michel and Kaufmann ( 1973 ). Fifty seeds were placed in a 9-cm Petri plate lined with Whatman No. 1 filter paper and contained 8 ml PEG-8000 solution. The plates were sealed with parafilm-M and incubated in the dark for a period of 1, 2, 3 or 4 weeks. Subsequently, the seeds were washed with distilled water and placed in a filter paper-lined Petri plate containing 6 ml distilled water. These plates were sealed with parafilm, transferred to germination cabinets and stored in the clark at 10, 18 or 26 °C. Seeds germination was recorded daily. Seeds were considered germinated when the radicle had emerged through the seed coat. Tso, the number of days required for germination of 50% of seeds, was determined by plotting germination response against time. Some germination occurred in the osmotic solution when the seeds were treated either at sub-optimal concentration or for an extended period. Therefore the treatments which resulted in germination of > 20% seeds in the osmoticum were not included for determination of Tso values. Experiment II. - To assess the m i n i m u m osmoconditioning period required to enhance germination, tomato and onion seeds were osmotreated for periods ranging from 1 to 7 days, using 250 g 1-1 H 2 0 ( --8.6 bars) and 300 g
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
215
1- ~ H 2 0 ( - 11.9 bars) P E G - 8 0 0 0 solutions, respectively. The germination response o f the osmo seeds was recorded at 15 ° C. E x p e r i m e n t I I I . - Seedling emergence in flats was evaluated for osmo seeds
stored in moist osmoticum. T o m a t o and onion seeds were o s m o c o n d i t i o n e d for 7 days at - 8.6 and - 11.9 bars, respectively. The osmo seeds submerged in P E G - 8 0 0 0 solution were stored at 5 + 1 °C in the dark, for pe.riods ranging from 0 to 28 days. Following storage, the o s m o seeds were equilibrated at 15 ° C for 1 h and quickly washed with distilled water. The washed seeds were air dried, " coated with thiram fungicide dust, planted in plastic flats ( 175 X 135 X 50 m m ) containing a Fox sandy loam and placed in cabinets at 18 + 2 ° C day, 12 + 2 ° C night temperature and with a 14-h photoperiod. The seeds were planted in rows o f 50 seeds per row and each treatment was replicated four times. Seedling emergence was m o n i t o r e d daily. Field studies Harrow trial 1983. - T o m a t o seeds cultivars H-2653 and TH-318 were os-
m o c o n d i t i o n e d for 7 days by submerging seeds in PEG-8000 solution (250 g I- 1 H 2 0 , - - 8.6 bars). Osmoconditioning was conducted in large stainless steel trays ( 5 7 X 5 7 X 2 c m ) lined with filter paper, sealed and placed in a controlled temperature cabinet at 15 + 1 ° C in the dark. After a 7-day treatment period, the seeds were quickly rinsed with deionized water, air dried and coated with thiram fungicide dust. A Stanhay planter was used for direct seeding o f the n o n - o s m o and osmo t o m a t o seeds on 12 M a y 1983. A clump o f 4 7 seeds was d r o p p e d every 20-22.5 cm in Fox sandy loam, soil fertilized with 8 - 3 2 - 1 6 N P K at a rate o f 560 kg ha -I. The herbicide treatments involved trifluralin pre-plant incorporated at 1.0 kg h a - 1 and metribuzin applied preemergence at 0.125 kg ha-1. A post-emergence application o f metribuzin at 0.5 kg h a - 1 was applied in the 4-leaf stage o f tomatoes. Plots consisted o f two 8-m rows and the rows were 1.5 m apart. Each treatment was replicated four times in a r a n d o m i z e d block design. Cambridge trials 1984 a n d 1985. - T o m a t o seeds cultivar FM-6203 were osm o c o n d i t i o n e d in 1984 for 7 days b y submerging in PEG-8000 solution (250 g 1- i H 2 0 , - 8.6 bars) in a sealed plastic tray (250 X 174 X 45 m m ) lined with filter paper and placed in a controlled temperature cabinet at 15 + 1 oC in the dark. After a 7-day treatment period, the seeds were quickly rinsed with deionized water, air-dried and coated with thiram dust. Using a Planet Junior, two 60-m rows o f o s m o and two rows of non-osmo t o m a t o seeds plus the guard rows were planted on 28 M a y 1984. The plants within the rows were thinned to 15-cm spacing after emergence. In the 1985 trials, seeds o f t o m a t o cultivars FM-6203, H-2653, O H - 7 8 1 4 and O H - 8 2 4 3 were osmoconditioned, as in the previous year 1984, for 4- and
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7-day periods. Using a Planet Junior, 5-m rows of osmo and non-osmo seeds were planted as a randomized block on 22 May 1985. The rows were 1.5 m apart and thinned to 17.5-cm spacing within the row after emergence. Trifluralin at 1.5 kg h a - 1 was shallow pre-plant incorporated for weed control. Weed escapes in 1984 and 1985 were controlled during the growing season by a combination of inter-row cultivation and hand-weeding within the rows. Laboratory as well as field data were analyzed using SAS ( 1985 )procedure GLM and the treatment means separation involved LSD with P = 0.05. RESULTS
Laboratory studies Germination during seed osmoconditioning. - Seed germination in the osmoticum occurred during osmoconditioning and varied with the species, PEG concentration and the duration of osmotreatment (Table 1 ). Osmotreatment with 300 g 1- 1 H20 or a higher PEG for 1 week resulted in minimal, i.e. < 1% germination of onion seeds in the osmoticum. Tomato seeds exhibited minimal or no germination in the osmoticum at all concentrations tested during 1 week of osmoconditioning. A sub-optimal concentration of PEG-8000 solution as well as osmotreatment for an extended period resulted in considerable germination of both onion and tomato seeds in the osmoticum (Table 1 ). Germination following osmoconditioning. - Germination responses of the tomato and onion seeds were temperature-dependent. The cultivar E-870 of tomato seeds used was very sensitive to the low temperature and resulted in a very low germination response at 10°C (Fig. 1 ), whereas the onion seeds, osmo as well as non-osmo, germinated well at all temperatures. Compared to non-osmo, the onion seeds which were osmotreated with PEG solution for 1 week showed accelerated germination and reduced Tso values (Fig. 1; Table TABLE 1 G e r m i n a t i o n (%) o f t o m a t o a n d o n i o n in the o s m o t i c u m PEG-8000 solutions. Values are m e a n s o f 12 replications o f 50 seeds each. M e a n in the s a m e c o l u m n followed by the same letter are not significantly different (LSD, P = 0.05 ) PEG-8000
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217
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
2). At 10 ° C, the 7"50 value for osmo onion seeds was reduced, indicating a gain in germination time. At 18 ° C, the Tso values for the osmo onion and tomato seeds were much lower than for non-osmo seeds (Table 2). At 10°C, the non-osmo and osmo tomato seeds resulted in < 5 and 20% germination, respectively, over a 2-week period. However, transferring these seeds to 18 ° C led to germination of > 75% osmo seeds within 2 days compared to < 10% of the non-osmo seeds, indicating that enhanced germination potential was sustained. Osmotreating tomato and onion seeds at 1-day intervals from 1 to 7 days resulted in a progressively faster germination response at 15 °C (Fig. 2 ). The Tomato 100
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Tso value of 5 days for non-osmo seeds was reduced to 1 day for seeds which were osmoconditioned for 7 days. These results indicate that a m i n i m u m of 7 days osmotreatment was necessary for tomato and onion seeds to gain a significant advantage in germination time over the non-osmo seeds. G e r m i n a t i o n a n d e m e r g e n c e o f s e e d s stored in o s m o t i c u m . - To allow for convenience for the grower, the osmo tomato and onion seeds were moist-stored in the osmoticum at 5 _+ 1 °C for a period of 3-28 days. The enhanced germination response in Petri plates of the osmo onion seeds remained unaffected even after storage for 28 days, whereas germination of the stored osmo tomato seeds was much reduced (Fig. 3a). Osmoticum-stored osmo onion seeds emerged earlier than the non-osmo seeds when seeded in sandy loam in flats, however when moist storage was extended for ~>14 days the magnitude of the enhancement effect was reduced (Fig. 3b). The emergence of the osmo tomato seeds was adversely affected by storage in the osmoticum at 5 _+ 1 °C for as little as 3 days (Fig. 3b). F i e l d studies H a r r o w trial 1983. - During early crop establishment in the field, seedlings
from non-osmo seeds ofTH-318 emerged earlier and were sturdier than those of H-2653, indicating seedling vigor differences between the two cultivars. Regardless of the method of weed control, i.e. hand weeded or chemical, the osmo seeds of H-2653 and TH-318 showed early emergence. Non-osmo seeds showed no sign of emergence 12 days after seeding, whereas a 20 or 40%
219
OSMOCONDITION1NG OF TOMATO A N D O N I O N SEEDS a
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emergence o f the osmo seeds was already observed in both cultivars at this time (Fig. 4 ). Fifteen to 16 days after seeding, emergence o f the H-2653 osmo seeds remained considerably higher than that o f the non-osmo seeds, but the non-osmo seeds ofTH-318 soon caught up and at 15 days there was no difference in percent emergence between the non-osmo and the osmoconditioned seeds. Evaluation at 6 weeks after seeding revealed that H-2653 osmo seeds had considerably higher ( > 80%) emergence and more plants at the 4-leaf stage than the non-osmo seeds (Table 3 ). Enhanced emergence and developmental responses were eventually reflected in increased yield and fruit maturity. However with TH-318, no differences with respect to total emergence, seedling development, fruit maturity and yield were observed between the non-osmo and osmo seeds (Table 3).
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TABLE 3 Emergence, plant maturity, fruit maturity and yield in tomato (cultivars H-2653 and TH-318) plants grown in the field from non-osmo and osmo seeds. The seeds were osmoconditioned at 15 °C for 1 week, using PEG-8000 solution of 250 g 1-1 H20 ( - 8 . 6 bars). Values are means of four replications. Means in the same column followed by the same letter are not significantly different within the cultivars (LSD, P = 0.05 ). Emergence and plant maturity were assessed at 6 weeks, fruit maturity and yield were evaluated 16 weeks after seeding (Harrow trial 1983) Cultivar
Seed treatment
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1Based on non-osmo seedlings.
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
221
Cambridge trials 1984 a n d 1985. - Sixteen weeks after seeding, the breakerred fruit count in the two 60-m rows sown with osmo tomato seeds had 83% more breakers than the non-osmo rows, indicating earlier fruit maturity. Subsequent harvesting and evaluation of the trial 17 weeks after seeding revealed that the plant count, the total fruit yield as well as the green fruit yield, showed little difference between osmo and non-osmo treatments. The breakers and the total fruit yield of osmo seed plots were 56 and 18% higher than for the non-osmo seeds. The total yield differences between the non-osmo and osmo seed plots were statistically not significant. In the 1985 trial, the total fruit yield (breakers and green fruits) of the four tomato cultivars FM-6203, H-2653, OH-7814 and OH-8243 showed little difference between the non-osmo and osmo seed plants (data not presented). For FM-6203, OH-7814 and OH-8243, the osmo seed plants produced a greater number and weight of breaker-red fruits than the non-osmo seed plants. These cultivars showed a 1-week delay in fruit ripening as compared to H2653 (data not presented). DISCUSSION
Laboratory studies
Seed osmoconditioning has been claimed to be effective in accelerating germination, seedling emergence and performance of the crop in the field (Heydecker and Coolbear, 1977; Brocklehurst and Dearman, 1983, a,b; Alvarado et al., 1987 ). Seeds of tomato and onion, investigated in our study, have shown that the m i n i m u m period for osmoconditioning treatment is ~ 7 days and enhanced germination of the osmo seeds is dependent on temperature. Following osmoconditioning, the emergence response with tomato and onion was less pronounced than that noted with the germination studies (Figs. 1 and 3 ), which indicates that osmoconditioning is more directly targeted towards the germination process. These observations are partially in agreement with previous reports where osmoconditioning of vegetable and flower seeds resulted in improved germination and seedling emergence (Heydecker and Wainwright, 1976; Simmonds, 1980; Bodsworth and Bewley, 1981; Khan et al., 1983). Previous reports indicated that storage of air-dried osmo seeds, depending on the length of storage, tends to reduce the enhancement of germination potential (Heydecker and Coolbear, 1977; Brocklehurst and Dearman, 1983 a,b). However, Haig et al. (1986) noted that enhanced germination and emergence characteristics of salt-primed tomato and carrot seeds were maintained after air drying and storage for up to 28 days, although air drying of the onion seeds led to a reduced percentage of emergence. In our study, the osmo tomato and onion seeds, stored moist in the PEG-8000 solution at
222
A. ALl ET AL.
5+ I°C, revealed that the enhanced germination potential and seedling emergence of the osmo onion seeds was sustained after storage for up to 28 days (Fig. 3a and b). In contrast, storage of the osmo tomato seeds under the same conditions for as little as 3 days adversely affected the enhanced germination and emergence potential, possibly due to chilling injury to the embryo (Fig. 3a and b ). Also germination of the osmo tomato seeds, which was practically suspended at 10 ° C (Fig. 1 ), resulted in > 75% seed germination within 2 days when the seeds, after 2 weeks at 10 ° C, were transferred to 18 ° C (data not presented). It is evident that, depending on species, the proper conditions for storage of osmo seeds will vary. To sustain the enhanced germination potential, consideration must be given to the temperature sensitivity of the osmo seeds.
FieM studies Except for a few reports which involved field testing of osmo seeds, most of the published research was conducted under controlled conditions. In several crop species tested under controlled conditions, seed osmoconditioning resulted in earlier seedling emergence (Heydecker and Coolbear, 1977; Wolfe and Sims, 1982; Brocklehurst and Dearman, 1983b; Khan et al., 1983; Haig et al., 1986). Furthermore, enhanced plant development and fruit maturity (Wolfe and Sims, 1982), increased plant weight (Brocklehurst and Dearman, 1983b; Alvarado et al., 1987 ) and harvest yields (Szafirowska et al., 1981 ) were observed with osmoconditioned seeds. Wolfe and Sims (1982) noted enhanced seedling emergence, plant develo p m e n t and fruit maturity which was similar to our resuRs with osmo tomato seeds of the FM-6203 and H-2653 ( 1983 and 1984 trials). Early maturity and increased yield due to rapid emergence of the osmoconditioned seeds has been reported for carrots (Szafirowska et al., 1981 ). However, yield increases were not always associated with osmoconditioned seeds (Wolfe and Sims, 1982; Zbitnew, 1984; Alvarado et al., 1987 ). The increased fruit yield observed in our study with osmo seeds of H-2653 was more a reflection of sustained differences in seedling emergence and development between osmo and non-osmo seeds. It should be noted that the fast emergence of TH-318, except for an initial difference in seedling emergence, showed no developmental or maturity differences and the fruit yield of the osmo and non-osmo plants was similar (Fig. 4, Table 3 ). It therefore appears that the field performance of the osmo seeds could be related to genetic cultivar differences, and the potential benefits of seed osmoconditioning for early crop establishment and maturity are more evident with low-vigor, slow-emerging cultivars. Early crop establishment and fruit maturity in these cultivars is of economic significance, as a reduction in the growing season of only a few days is considered economically important for commercial growers in the tomato processing industry
OSMOCONDITIONING OF TOMATO AND ONION SEEDS
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(Wolfe and Sims, 1982 ). Rapid seedling establishment also reduces the risks associated with early crop development. Contrary to the 1983 and 1984 field trials in which FM-6203 and H-2653 showed developmental and maturity differences between osmo and non-osmo seeds, no such differences were evident during the 1985 growing season. Therefore, in addition to the cultivar differences, the climatic variation during crop establishment could influence field performance of the osmoconditioned seeds. Seed osmoconditioning can therefore be regarded as an insurance measure with potential benefits o f rapid and uniform crop emergence, development and maturity. Zbitnew (1984), working with the onion hybrid cultivar 'Aries' in Ontario, noted that differences between osmo and non-osmo seeds in emergence and subsequent growth, as indicated by Ts0 values at the loop, crook, flag and first leaf stage, decreased as seedlings developed and as growth cabinet temperature regimes increased from day/night levels of 10/6°C to 15/11 °C. The greatest response to osmoconditioning was noted at the loop stage and at the lower temperature regime. In field trials involving three planting dates, initial differences in seedling development were noted at the early planting date, but not later, with eventually no differences in the final yield or time to maturation. Seed germination is a temperature-sensitive process with its high and low temperature limits, beyond which the rate of germination is adversely affected. The high and low temperature limits of the cultivars used in our study appear to be 26 and < 15°C for tomato, and 18 and < 10°C for onion seeds, respectively. Between these limits, the potential gain in germination time of the osmo seeds was maximal toward the lower temperature limits (Fig. 1, Table 2). Furthermore, the sustained gains due to osmoconditioning expressed at the germination stage can be maintained up to harvest only if the cultivar has a relatively low seedling vigor due to its inherent genetic background. Results from our study indicate that with the tomato and onion cultivars tested, seed osmoconditioning can lead to rapid and synchronous germination, more uniform seedling emergence and, depending on the species and cultivars, enhanced plant development and earlier fruit maturity. Since the enhanced germination and emergence response ofosmo seeds were more pronounced at sub-optimal cooler temperatures, this technique of seed osmoconditioning could be useful with slow-germinating cultivars in areas experiencing cooler spring conditions. REFERENCES Alvarado, A.D., Bradford, K.J. and Hewitt, J.D., 1987. Osmotic priming of tomato seeds: effects on germination, field emergence, seedling growth and fruit yield. J. Am. Soc. Hortic., Sci., 112: 427-432.
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Bodsworth, S. and Bewley, J.D., 1981. Osmotic priming of seeds of crop species with polyethylene glycol as a means of enhancing early and synchronous germination at cool temperatures. Can. J. Bot., 59: 672-676. Brocklehurst, P.A. and Dearman, J., 1983a. Interactions between seed priming treatments and nine seed lots of carrot, celery and onion. I. Laboratory germination. Ann. Appl. Biol., 102: 577-584. Brocklehurst, P.A. and Dearman, J., 1983b. Interaction between seed priming treatments and nine seed lots of carrot, celery and onion. II. Seedling emergence and plant growth. Ann. Appl. Biol., 102: 585-593. Haig, A.M., Barlow, E.W.R. and Milthorpe, F.L., 1986. Field emergence of tomato, carrot, and onion seeds primed in an aerated salt solution. J. Am. Soc. Hortic. Sci., 111: 660-665. Heydecker, W. and Coolbear, P., 1977. Seed treatments for improved performance - survey and attempted prognosis. Seed Sci. Technol., 5: 353-425. Heydecker, W. and Wainwright, H., 1976. More rapid and uniform germination of Cyclamen persicum. L. Scientia Hortic., 5: 183-189. Khan, A.A., Kar-ling, T., Knypl, J.S., Borkowska, B. and Powell, L.E., 1978. Osmotic coconditioning of seeds: physiological and biochemical changes. Acta Hortic., 83: 267-278. Khan, A.A., Peck, N.H,, Taylor, A.G. and Samimy, C., 1983. Osmoconditioning of beet seeds to improve emergence and yield in cold soil. Agron. J., 75: 788-794. Michel, B.E. and Kaufmann, M.R., 1973. The osmotic potential of polyethylene glycol 6000. Plant Physiol., 51: 914-916. SAS Institute, 1985. SAS User's Guide: Statistics, 5th. edn., SAS Institute, Cary, NC, 956 pp. Simmonds, J., 1980. Increased seedling establishment of Impatiens wallerana in response to low temperature or polyethylene glycol seed treatments. Can. J. Plant Sci., 60: 561-569. Szafirowska, A., Khan, A.A. and Peck, N.H., 1981. Osmoconditioning of carrot seeds to improve seedling establishment and yield in cold soil. Agron. J., 73: 845-848. Wolfe, D.W. and Sims, W.L., 1982. Effects of osmoconditioning and fluid drilling of tomato seed on emergence rate and final yield. HortScience, 17: 936-937. Zbitnew, K.D.W., 1984. The effect of osmoconditioning on the germination and development of onion (Allium cepa L. ). M. Sc. Thesis, University of Guelph, Ontario (unpublished).