PRIMED CARROT SEEDS PERFORMANCE UNDER WATER AND TEMPERATURE STRESS

Uniformity and percentage of seedling emergence of direct-seeded vegetables have a major impact on their yield and quality. Depending on temperature and soil water potential, carrot seeds performance in the field is poor justifying the use of techniques to accelerate the germination and seedling emergence. The objective of this work was to evaluate the effects of priming treatments, with and without aeration, on carrot seeds performance under water and temperature stress. Two seed lots of carrots, Brasília cultivar, were primed in PEG 6000 –1.0 and –1.2 MPa solutions, for four and eight days, at 20oC, using two imbibition methods: priming in paper towel moistened with the respective solutions and immersion into aerated osmotic solutions. Non-primed seeds were used as control. After priming, the seeds were dried at room temperature until reach initial seed moisture content. The seeds were submitted to the following tests: germination, first count, seedling emergence in the field, germination at sub-optimal (15oC) and supra-optimal temperature (30oC) and germination under water stress (PEG 6000 at –0.4 MPa). Priming in –1.0 and –1.2 MPa PEG 6000 during four and eight days can be useful for improving carrot seedling emergence in the field and seed performance under supra and sub-optimal temperatures. Aeration of the peg solution was beneficial when seeds were primed at –1.2 MPa for eight days.


INTRODUCTION
Rapid and uniform field emergence are essential prerequisites to increase yield and products quality, especially for direct-seeded vegetables such as carrot.Frequently, adverse environmental conditions led to a slow and non-uniform emergence.Therefore, treatments carried out before sowing, as seed priming, can be very useful for faster and synchronized germination and better performance under adverse conditions.This technique consists of seed hydration in a solution whose osmotic potential is sufficient to permit initial germination events, but not enough for radicle protrusion (Bradford, 1986).Beneficial effects from Sci. Agric.(Piracicaba, Braz.), v.66, n.2, p.174-179, March/April 2009 priming have been reported for several vegetable seeds included carrot (Pelluzio et al., 1999;Balbinot & Lopes, 2006).However, for several crops, the positive effects of priming are more evident under field stress conditions, such as low and high temperatures (Demir & Oztokat, 2003;Bittencourt et al., 2004), water stress (Bittencourt et al., 2004), and salinity stress (Pill et al., 1991).
The response to seed priming is strongly dependent on several factors, including aeration during treatment.Some reports recommend this procedure (Bradford et al., 1988;Nascimento, 2003), whereas others indicate deleterious effects (Akers & Holley, 1986;Nascimento, 2003).No differences in the lettuce germination were observed after seed priming in aerated or non-aerated solutions (Cantliffe, 1981).According to Guedes & Cantliffe (1980), aeration reduces the time needed for lettuce seed priming.
The objective of this study was to evaluate the effects of priming treatments, with and without aeration, on carrot seeds performance under water and temperature stress.

MATERIAL AND METHODS
Two commercial seed lots of carrots, cv.Brasília, with 10% and 12% moisture content were used.The seeds of each lot were primed in polyethylene glycol (PEG 6000) solution at -1.0 (296 g L -1 ) e -1.2 MPa (326 g L -1 ), obtained according to Villela et al. (1991), at 20ºC, for four and eight days.Fungal growth was controlled by adding 0.15% (a.i.) captan 750 TS 24 to the solutions.The time period for each treatment was determined previously by Pereira (2007).Two priming methods were used: 1. Paper towel moistened: in germination boxes, two subsamples of 2.0 g of seeds were placed on two sheets of paper towel moistened with 4.5 mL of PEG solutions.The germination boxes containing the seeds were placed in an incubator at 20ºC. 2. Soaking in aerated solution: a sample of 4.0 g of seeds was soaked in PEG aerated solutions (10.0 mL solution g -1 of seed) in 250 mL erlenmeyers.The erlenmeyers were closed and connected to an aquarium pump to provide aeration.The solutions were not changed.
After priming the seeds were rinsed in running tap water for two minutes and the seed moisture content was determined in an oven at 105 ± 3ºC for 24 hours (Brasil, 1992).Then, they were dried back in ambient condition (25 ± 2ºC and 65% RH) to their initial moisture content (10 and 12%).Unprimed seeds were used as control.
The seeds of each treatment were submitted to the following tests: Standard germination: four replications of 50 seeds each were distributed over two layers of paper towel (11 × 11 cm) previously moistened with water equivalent to three times the dry weight of the paper, in germination boxes, and incubated at 20ºC.The germinated seeds were counted seven and fourteen days after seeding and the results were expressed as mean percentage of normal seedlings (Brasil, 1992).
Seedling emergence in the field: sowing was done on February 10, 2006 in conventionally prepared seedbeds.Four replicates of 50 seeds for each treatment were sown in furrows 5.0 m long spaced 0.20 m apart and 0.01 m deep.Soil moisture was kept sufficiently wet for germination.The percentage of emerged seedlings was counted on the 25 th day after sowing.
Germination at sub and supra-optimal temperature: the germination was tested at 30 and 15ºC using the method described for standard germination test (Brasil, 1992).
Germination under water stress: the method used was similar to that described for the standard germination test except that the paper towel was moistened with 15 mL of the -0.4 MPa (178 g L -1 ) PEG-6000 solution (Villela et al., 1991).

Statistical design and analysis:
all the experiments were done in a completely randomized block design with four replications, and data were analyzed in a factorial design: two lots × two imbibition methods × five priming treatments.Treatment means were compared using Tukey's test (p = 0.05).

RESULTS AND DISCUSSION
For both lots, the seed moisture contents determined after four days priming in -1.0 MPa and -1.2 MPa PEG solutions were 45 and 42%, respectively.For both lots, seed moisture contents after the 8-day priming were slightly high reaching values between 45 and 46% at -1.2 and -1.0 MPa, respectively.Therefore, the seed moisture content determined after priming was similar regardless of the treatments and lots.The determination of seed moisture content before and after priming must be considered as important parameter in selecting the adequate procedure for seed priming (Caseiro et al., 2004).
Initial germination of seed lots 1 and 2 increased after soaking for four days in PEG-6000 solutions regardless of the osmotic potential (Table 1).Carrot seeds primed for eight days did not differ from   non primed seeds and had lower germination percentage than four day primed seeds, independent of osmotic potential and imbibition method used.Therefore, the duration of the treatment was the more effective factor to improve primed seeds germination.Similar results were observed by Carneiro et al. (1999) and Peluzio et al. (1999).They also verified an increase in the germination of carrot seeds primed for seven days in PEG 6000 -1,5MPa; however, when the period of priming was increased to 14 and 21 days, there was reduction on the seed germination.Effectiveness of priming may be altered by the duration of the soak, as reported for sugar beet (Costa & Vilela, 2006) and onion (Caseiro & Marcos Filho, 2005) seeds.
The aeration process during priming did not affect total germination of seed lot 1; both priming methods (paper towel moistened and aerated solution) were similar and improve germination percentage in relation to control (Table 1).On the other hand, the germination percentage for seed lot 2 primed in PEG -1.2 MPa aerated solution, for eight days, was superior to primed seeds in paper towel moistened.This difference did not occur in the other treatments.Therefore, aeration was an effective factor when more concentrated solution was associated to longer priming duration.Cantliffe & Abebe (1993) reported that allowing more PEG solution aeration during priming led to an increased germination of tomato seeds.
Seeds with high germination percentage at first count have higher vigor, indicating higher germination rate.At first count (Table 2), four days priming in -1.0 e -1.2 MPa PEG improved the germination rate more than other treatments for seed lot 1, as also observed in the germination test (Table 1).Therefore, the duration of treatment had clear effect on seed performance.Carrot seeds primed during 14 days had lower germination rate, indicating that a longer priming can be harmful to germination (Gray et al., 1991).Peluzio et al. (1999) observed that the osmotic potential reduction in association with higher priming duration de-creased germination rate of carrot seeds.Germination rate of seed lot 2 was enhanced by priming at -1.0 MPa during four days and at -1.2 MPa for 4 and 8 days, regardless of the imbibition method (Table 2).Comparison between imbibition methods showed that, for both lots, there was reduction on germination rate of the seeds primed in PEG 6000 -1.2 MPa aerated solution for four days in relation to seeds imbibed in paper towel moistened with PEG solutions.Heydecker et al. (1975) demonstrated that proper aeration of the osmotic solution was essential to living seeds and obtain synchronized germination.However, Nascimento (2003) verified that soaking muskmelon seeds in aerated solution for 12 days was deleterious for germination.
For both seed lots, all the priming treatments improved field seedling emergence, regardless of the osmotic potential and duration of priming (Table 3).Although all the priming treatments enhanced seedling emergence of lot 1 in relation to control, seeds primed in aerated solutions of PEG 6000 at -1.0 MPa for eight days and at -1.2 MPa for four and eight days had better performance than those primed in moistened paper towel.The effect of priming method was not yet observed by germination test (Tables 1 and 2).
Positive response to aeration process during priming was verified only for seed lot 1 when more concentrated PEG solution was used (-1.2MPa), regardless of the priming duration (Table 4).In general, all the priming treatments contributed to improve germination of both lots at sub-optimal temperature in comparison to the control, with exception of 4-day priming with -1.2 MPa PEG, in moistened paper (lot 1) and in aerated solution (lot 2).The best results were obtained with the 8-day priming treatments regardless of the osmotic potential.Therefore, the time increase of the priming was beneficial to germination at suboptimal temperature, in contrast to the results observed on the germination test (Table 1), where a shorter period (four day priming) was more effective.Positive effects of seed priming on germination have been mainly observed at sub-optimal temperatures as reported in carrot (Pill & Finch-Savage, 1988), muskmelon (Nascimento & Aragão, 2004), watermelon (Demir & Oztokat, 2003) and asparagus (Bittencourt et al., 2004).In contrast, priming did not improve Pyterogines vitens seed germination in sub-optimal conditions (Tonin et al., 2005).
The positive response to aeration process during priming was verified only for seed lot 1 when more concentrated PEG solution was used (-1.2MPa), regardless of the priming duration (Table 4).The germination at supra-optimal temperature of both seed lots was enhanced by all priming treatments (Table 5).In general, there was an interaction between aeration of the solution and duration of priming, as observed by 8-day priming at -1.2 MPa in aerated solution results, which were superior to the same treatment in moistened paper.Viscosity and oxygen diffusivity problems can occur in more concentrated PEG solutions (Nascimento, 2004).In this situation, additional aeration can be important to optimizing the priming procedure.

Table 1 -
Germination of two carrot seed lots submitted to priming treatments by imbibition in moistened paper and immersion in aerated solution.Means followed by the same letter, capital in each column and small in each line, do not differ by Tukey's test at 5%.CV(%) = 5.29.

Table 2 -
Germination at first count of two carrot seed lots submitted to priming treatments by imbibition in moistened paper and immersion in aerated solution.Means followed by the same letter, capital in each column and small in each line, do not differ by Tukey's test at 5%.CV(%) = 5.78.

Table 4 -
Germination at sub-optimal temperature (15ºC) of two carrot seed lots submitted to priming treatments by imbibition in moistened paper and immersion in aerated solution.Means followed by the same letter, capital in each column and small in each line, do not differ by Tukey's test at 5%.CV(%) = 7.11.

Table 3 -
Seedling emergence of two carrot seed lots submitted to priming treatments by imbibition in moistened paper and immersion in aerated solution.