MUSKMELON SEED GERMINATION AND SEEDLING DEVELOPMENT IN RESPONSE TO SEED PRIMING

Important factors affecting seed priming have not been extensively reported in muskmelon (Cucumis melo L.) studies. The optimization of the seed priming technique becomes very important at the commercial scale. Little information has been reported on seedling development of muskmelon subsequent to seed priming. Seeds of muskmelon were primed in darkness at 25°C in different solutions and three osmotic potentials. Seeds were also primed with and without aeration during different periods. In relation to osmotic solutions, an osmotic potential around -1.30 MPa is most adequate for muskmelon priming. Salt solutions gave better germination rate but were deleterious for seed germination, especially at higher osmotic potentials. Aeration of the soaking salt solution gave faster germination at 17°C, and because of the early germination, these treatments probably presented a better seedling development. Deleterious effect on total seed germination was observed for long soaking periods with aeration. Fungal growth increased on seeds primed in aerated solutions. Seeds from priming treatments had a better germination rate and seedling development under 17 and 25°C.


INTRODUCTION
Seed priming (osmoconditioning, osmopriming, osmotic priming) is a pre-sowing treatment that involves exposure of seeds to a low external water potential that limits hydration.This hydration is sufficient to permit pregerminative metabolic events but insufficient to allow radicle protrusion through the seed coat (Heydecker et al., 1975).This technique has become a common seed treatment that can increase rate, percentage and uniformity of germination or seedling emergence, mainly under unfavorable environmental conditions.
To better understand the priming technique applied to muskmelon, this study screened several priming solutions of different osmotic potentials, to investigate the need for aeration of the soaking solution and to clarify the correlation between aeration and soaking period duration.

MATERIAL AND METHODS
Experiment 1: One lot of muskmelon cv.Mission seeds (Asgrow Seed Co, San Juan Bautista, CA) was primed in darkness at 25°C for six days, in five aerated solutions (10 mL of solution g -1 of seed), using two osmotic potentials (Table 1).A third osmotic potential for each solution ranging from -0.8 to -0.9 MPa was also used, but these treatments were discarded due to the high number of germinated seeds during the soaking period.The osmotic potential at 25°C of each solution was determined with a Wescor 5130 B Vapor Pressure Osmometer (Wescor Inc., Logan, Utah).Solution aeration was provided by an aquarium pump.The air was moistened by bubbling through water to minimize evaporation of the priming solution.The solution was changed every other day.After the six-day period, seeds were rinsed in running tap water (2 min) and then dried at 25 ± 2°C and 50% RH for four days.Seed moisture after priming and drying was determined by the oven method (104°C / 24 h).Four replications of 50 primed and nonprimed seeds were placed in petri dishes containing two germination papers and 10 mL of deionized water and incubated in a germination chamber, in darkness, at 17 or 25°C.Radicle protrusion was scored daily.After 7 and 10 days of incubation, seedlings were evaluated for abnormality (AOSA, 1993).Shoot and root fresh weights and lengths were measured at 5 and 10 days.All data were subjected to analysis of variance.
Experiment 2: Seeds from the same lot were primed for 3, 6, 9 and 12 days in darkness at 25°C in KNO 3 (Ψ = -1.4MPa) solution (10 mL of solution g -1 of seed).Seeds were either aerated as described in Experiment 1 or not aerated, which formed a 4 x 3 factorial of priming duration and aeration.The solution was changed every other day.Rinsing and drying procedures were the same as in Experiment 1. Seed germination and seedling growth data at 17 or 25°C were collected and analyzed as described in Experiment 1. Four replications of 50 seeds were used in each treatment using a complete randomized design.Analysis of variance and factorial analysis were performed using the Statistical Analysis System.

RESULTS AND DISCUSSION
Experiment 1: The seed moisture content (SMC) after soaking on the different priming treatments ranged from 39.4 to 45.0% (Table 1).Generally, SMC during priming is maintained between 40-45%, which is equivalent to maintain the seeds in a state where it is at approximately 90-95% of the SMC that could allow germination (Bray, 1995).The amount of water taken up by seeds during soaking was dependent on the osmotic solution.In salt solutions, seeds tended to absorb more water than in seeds primed in mannitol or PEG, corroborating wilt the results obtained in other studies (Guedes et al., 1979).After drying, SMC decreased to 5.5 -6.1% (before priming, the initial SMC was 5.8%).Seeds primed in all osmotic solutions with osmotic potential varying from -0.8 to -0.9 MPa germinated during soaking (data not shown), and were discarded.Carpenter & Boucher (1991) reported similar results in pansy seeds primed in a -0.8 MPa PEG solution.
Priming increased the germination rate for both temperatures (Table 2), although it was more evident for 17°C.Thus, seed priming overcame the inhibitory effect of low temperatures and increased germination and the  stand establishment in the field.In general, the effects of seed priming on muskmelon seed germination have been mainly observed at sub-optimal conditions (Bradford, 1985;Bradford, 1986), and this was confirmed by our results.Priming in salt solutions lead to faster germination than priming in mannitol or PEG.Similar results were also observed for tomato and pepper seeds primed in KNO 3 and KNO 3 + K 3 PO 4 solutions (Alvarado et al., 1987;O'Sullivan & Bouw, 1984).However, priming in salt solutions reduced germination percentage, especially at higher osmotic potentials (-1.06 to -1.18 MPa) (  (Brocklehurst & Dearman, 1984).Ions from salt solutions may penetrate the muskmelon seed during priming and cause decreased seed germination.Muskmelon seeds are, however, not damaged by imbibition in salt solutions because some ions (K + or other ions of similar hydrated size) are excluded from the embryo by the perisperm envelope (endosperm + perisperm) Welbaum & Bradford (1989).
Seedling shoot and root fresh weights and lengths were higher at 25°C than at 17°C germination temperature (Table 3).Beneficial effects of priming on seedling growth were more evident at 17°C than 25°C and in those treatments (e.g., salt solutions) which presented better germination rate, suggesting that priming does not accelerate seedling growth per se.The major effects of seed priming on muskmelon seedling growth were observed due to earlier germination, which gave the seedlings a longer time to develop (Nascimento & West, 1999).Osmotic potential around -1.30 MPa in several solutions at 25°C is adequate for melon seed priming.Salt solutions may lead to a better germination rate, but may be deleterious for muskmelon seed germination.
Experiment 2: Although seeds had been previously treated with fungicide (Captan 50 WP at 3 g kg -1 of seed), those from aerated treatments had higher fungal growth during the germination test, especially at 17°C (data not shown).An increased microorganism population was also observed in aerated priming solution of cantaloupe seeds (Akers et al., 1985;Nascimento & West, 1998a).SMC increased with priming duration until 12 days (Table 4).Seeds from aerated solutions tended to absorb more water during the soaking period, suggesting that oxygen enhances water uptake (Yeoung et al., 1995).Priming increased germination rate at both temperatures, although it was more evident at 17°C.Increasing the soaking duration and including aeration improved the germination rate at 17°C, but an interaction between these two factors was not observed (Table 5).Aeration also affected germination percentage for both temperatures.Soaking in aerated solution for 12 days was deleterious for seed germination.An increase of metabolism in aerated solutions, as well as possible salt penetration into the seeds during soaking might have caused the reduction of germination.Akers et al. (1985) reported deleterious effects of aerated salt (NaCl) during cantaloupe seed priming.Shoot and root growth were greater in those treatments that exhibited faster germination (Table 6), suggesting that priming does not accelerate seedling growth, corroborating with the results of experiment 1. Rapid seedling establishment might minimize crop risk due to environmental conditions or insect and disease problems during field emergence, which is another advantage of primed seeds in muskmelon, especially under adverse conditions.Rapid stand establishment may result in a shorter cycle, although no practical effect on earliness was observed in 'Roundpack' melon primed seeds (Passam et al., 1989).In addition, seed priming minimizes seed coat adherence during emergence of muskmelon seeds (Nascimento & West, 1998b).
Aeration of soaking salt solution during muskmelon seed priming lead to faster germination.However, it may be deleterious for muskmelon seed germination due to increased fungal growth.Finally, seeds of priming treatments presented better seed germination, especially at low temperatures, and may have better seedling development.

Table 1 -
Muskmelon seed moisture content after priming in different osmotic solutions and osmotic potentials, and after drying.

Table 2 -
Muskmelon seed germination percentage and germination rate at 17 and 25°C in response to priming at different osmotic solutions and osmotic potentials.
* MGR = average number of days to germinate.** Values within a column followed by the same letter are not different (P = 0.05), according to Duncan's multiple range test.

Table 3 -
Muskmelon seedling development after 10 days at 17 and 25°C in response to seed priming at different osmotic solutions and osmotic potentials.
*FW = Total fresh weight.Length (cm) and fresh weight (mg) of 20 seedlings randomly selected.** Values within a column followed by the same letter are not different (P = 0.05), according to Duncan's multiple range test.

Table 6 -
Muskmelon seedling development at two temperatures in response to seed priming with and without aeration.

Table 4 -
Seed moisture content of muskmelon seeds in response to seed priming with and without aeration.
*Original seed moisture content.

Table 5 -
Muskmelon seed germination at 17 and 25°C in response to seed priming duration and aeration.
NS, **Nonsignificant or significant at P = 0.01, respectively.*MGR = average number of days to germinate.