GERMINATION AND VIGOUR OF PRIMED ASPARAGUS SEEDS

The osmotic conditioning or priming is considered a promising technique to speed up germination and to improve seed performance. Four seed lots of asparagus, Mary Washington cultivar, were primed at 25°C for seven or 14 days using PEG 6000 at -1.0 or -1.2 MPa, or sea water at -3.3 MPa; or for three days in distilled water. The physiological quality of the seeds was evaluated by standard germination, first count germination, speed of seedling emergence, and germination percentage and seedling fresh and dry weights after controlled deterioration test. Primed seeds presented higher germination speed, independently of their initial physiological quality. Beneficial effects of priming on germination and vigour were more expressive in the seed lot of low physiological quality. Priming in PEG 6000 at –1.0 MPa for 14 days was the most beneficial treatment to improve asparagus seeds performance.


INTRODUCTION
Depending on temperature and soil water potential, it takes from four to six weeks sowing to seedling emergence of asparagus.Techniques that speed up germination, such as seed priming, can enhance seed performance.This technique consists of seed imbibition in osmotic solutions with sufficient hydration to permit pregerminative metabolic events, but insufficient to allow radicle protrusion (Heydecker et al., 1975).
Salt solutions of MgSO 4 , NaCl and KNO 3 , and others have been used to obtain a controlled hydration of seeds (Haigh & Barlow, 1987), and synthetic sea water (Frett et al., 1991) also has been used for this purpose.
According to Heydecker et al. (1975) polyethylene glycol (PEG 6000 or PEG 8000) is commonly used as osmotic-priming agent, because of its relatively inert and non-toxic nature.Seed priming response depends on several factors such as osmotic potential, temperature and duration of the treatment.
It is important, therefore, to evaluate the effects of priming on seed lots of different physiological quality.The objective of this study was to evaluate the effects of priming treatments on asparagus seed lots with different vigour levels.

MATERIAL AND METHODS
Seeds of four distinct seed lots of 'Mary Washington' asparagus (Asparagus officinalis L.) with 7.5 to 8.0% moisture contents were spread on two sheets of paper towel (JProlab ® -pH 6.5, density 68.4 g m -2 , and resistance to rupture 2.67 kg cm -2 ) moistened with 20 mL of polyethylene glycol (PEG 6000) solution at -1.0 (296 g L -1 ) or -1.2 MPa (326 g L -1 ) according to Villela et al. (1991), or with natural sea water (-3.3MPa osmotic potential), and kept in germination plastic boxes (11 × 11 × 4 cm) for either seven or 14 days.The sea water osmotic potential (25°C), as determined by osmometer was 1.3672 Osmol, or 3,332 MPa.The period for each treatment was determined previously by Bittencourt et al. (2004a).
A sample of seeds was also imbibed in distilled water for three days.Fungal growth was controlled by adding 0.15% (a.i.) Captan to the solutions.All germination boxes were placed in an incubator at 25°C.Non-primed seeds were used as control.After priming, seeds were rinsed in running tap water and then kept for three hours on towel paper (25 ± 2°C; 60% RH) for superficial drying.The seeds were submitted to the following tests: Seed moisture content: Determined by oven-drying duplicated samples at 130 ± 3°C for one hour (Brasil, 1992).Percentage moisture content was expressed on a fresh weight basis.
Standard germination: Four replications of 50 seeds were distributed over three layers of paper towel (JProlab ® ) moistened with distilled water equivalent to three times the dry weight of the paper, in germination boxes, and incubated at 25°C.The results were calculated as the average of four replicates of 50 seeds and expressed as a percentage by number of normal seedlings obtained at 13 days (first count) and 28 days after sowing (Brasil, 1992).
Speed of seedling emergence: Twenty-five seeds from each treatment were sown 3-mm deep in plastic trays (230 × 160 × 60 mm) filled with moistened sand (70% water retention capacity) to a depth of 40 mm.Trays were kept at 25°C in a greenhouse.Emerged seedlings were counted daily for 23 days.The speed of seedling emergence was expressed in days and calculated according to Maguirre (1962).
Controlled deterioration: After each priming treatment, seeds were oven-dried (30°C) back to 20% moisture.Non-primed (control) seeds were imbibed on moistened paper towel in an incubator at 25°C until they reached 20% moisture.The target weight of seed samples at this moisture content was calculated according to Powell (1995).Once seed samples achieved the target weight, they were sealed in aluminium foil packet and held overnight at 10°C.Samples then were placed in a waterbath at 45°C for 24 hours.Immediately after deterioration, germination tests were set up with four replicates of 50 seeds, as described above.The results were expressed as a percentage of normal seedlings obtained at 28 days after sowing Seedling dry weight: Normal seedlings obtained at 13-days germination in the controlled deterioration test were weighed (g) after seed coat removal.Seedlings dry weight (mg seedling -1 ) was determined by drying in an oven at 70°C for 48 hours.
Statistical design and analysis: Trials were arranged in completely randomized block design (n=4); data were subjected to analysis of variance, and differences between means separated by Duncan's test (P = 0.05).

RESULTS AND DISCUSSION
Seed moisture content after priming ranged from 34.4 to 36.0%, with no difference among the seeds from different treatments (data not shown).In general, priming increased total germination percentage (Table 1), especially for seed lot 1 (SL-1), in which germination increased from 40% (non-primed) to 61% with priming for 14 days in -1.2 MPa PEG.A minimum of 60% germination is required for marketing asparagus seeds in Brazil.Therefore, priming may improve germination to levels closer to commercial standards.For seed lot 2 (SL-2), with high initial germination percentage, there was no effect of any treatment.However, in SL-3, also with high physiological quality, positive effects were obtained with 14-day priming in -1.2MPa PEG.All priming treatments improved the germination of SL-4, except priming for 14 days in seawater.The initial seed quality has a marked effect on the success of the priming (Nascimento, 1998).However, Evans & Pill (1989) did not obtain a priming benefit for asparagus seeds with high physiological quality.
All priming treatments improved germination at first count of SL-1, although the best results were obtained after priming for 14 days in -1.0 MPa PEG (Table Sci. Agric. (Piracicaba, Braz.), v.62, n.4, p.319-324, July/Aug. 2005 2).For SL-2, PEG treatments at -1.0 MPa for seven days or at -1.2 MPa for seven or 14 days gave greater germination percentage than other treatments, while for SL-3 and SL-4, priming in -1.2 MPa PEG for 14 days resulted in the highest germination percentage.In general, a comparison among the lots showed that SL-1 and SL-4 had low and medium vigour, respectively, while the SL-2 and SL-3 had high vigour.The priming effects were more evident on the first count germination (Table 2) than on the total germination percentage, obtained 28 days after sowing (Table 1).Krarup (1991), Gray et al. (1991) and Heydecker & Coolbear (1977) reported that the main effect of priming is increased germination percentage.In general, non-primed seeds had lower germination percentage independent of their vigour level, which justifies the use of priming for seed lots of both low and high physiological quality.The improvement in germination performance resulting from priming has been suggested as a result of repair mechanisms that occur during seed imbibition (Bray, 1995).
The speed of seedling emergence from nonprimed seeds was low, independent of the initial physiological quality (Table 3), but after priming in -1.0 MPa PEG for 14 days, the emergence of SL-1, SL-3 and SL-4 was, on average, 4.6 days earlier than that of the control.For SL-2, all treatments, except sea water, enhanced speed of emergence as compared to the control.Priming in distilled water increased the speed emergence for all seed lots.These results reveal the benefit of priming asparagus seed lots of either low or high physiological quality.Krarup (1991) reported that while germination percentage of asparagus seeds was not affected by priming, the germination and emergence rates were increased.The fact that seed priming can increase germination rate is well-documented for several species (Heydecker & Coolbear, 1977;Bradford, 1986;Evans & Pill, 1989;Pill et al., 1991;Demir & Van de Venter, 1999;Demir & Oztokat, 2003;Nascimento, 2003).The beneficial effect of priming has been attributed to membrane repair, increased protein synthesis, and more ef-   Table 2 -Germination at first count (13 days after sowing) of four asparagus seed lots subjected to different priming treatments.
Means followed by the same letter, capital in each column and small in each row, did not differ by the Duncan's test (P < 0.05).ficient mobilization of sugars and proteins (Srinivasan et al., 1999).

Sci
In the controlled deterioration test (Table 4), the germination percentage of SL-1 increased after all priming treatments, and the highest performance occurred after priming in sea water (seven and 14 days) and in PEG -1.0 MPa for 14 days.Using synthetic seawater may result in overpriming, because of its very low osmotic potential, a response not observed in PEG-primed seeds (Owen & Pill, 1994).However, the observed lack of harmful effect on seed vigour could be attributed to the tolerance of asparagus seeds to salts (François, 1987).Priming did not enhance germination performance of SL-2, SL-3 and SL-4.
The seed lot of lower physiological quality (SL-1) had greater benefit from priming, especially after a stress condition, as obtained in the controlled deterioration test.Several authors reported benefits of seed priming on germination under stress conditions such as low and high temperatures (Demir & Van de Venter, 1999;Demir & Oztokat, 2003;Bittencourt et al., 2004b).The effects of priming were less pronounced when priming was followed by controlled deterioration.When primed in PEG -1.0 MPa for 14 days seeds of SL-1, had their germination percentage doubled in comparison to the control (Table 4).Such improved germination of low vigour seed lots after priming and drying back was already observed by Tilden & West (1985).Improved germinability of aged sunflower seeds by priming was associated with a lower rate of lipid peroxidation and higher antioxidant enzyme activities (Bailly et al., 2000).
Most priming treatments increased seedling dry weights (Table 5).In general, priming in PEG -1.0 MPa for 14 days was efficient for all seed lots, especially SL-2.Increased seedling fresh and dry weights following priming was also registered for carrot seeds (Finch-Savage & Pill, 1990).The fact that priming can increase germination rate is well-documented (Heydecker & Coolbear, 1977).However, a question arises as to whether post-germination growth is also increased.The major ef- Table 3 -Speed of seedling emergence of four asparagus seed lots subjected to different priming treatments.
Means followed by the same letter, capital in each column and small in each row, did not differ by the Duncan's test (P < 0.05).
Table 4 -Germination (28 days after sowing) in the controlled deterioration test of four asparagus seed lots subjected to different priming treatments.
Means followed by the same letter, capital in each column and small in each row, did not differ by the Duncan's test (P < 0.05).
t fect of seed priming on seedling growth resulted from earlier germination, which gave the seedlings more time to grow.Similar results were found by Nascimento &West (1999) andNascimento (2003).
SL-1 was inferior to SL-2, SL-3 and SL-4 in all treatments (Tables 1 to 5).In general, unprimed seeds of SL-2 and SL-3 showed similar physiological quality and were superior to SL-4.In general, primed asparagus seeds, regardless of priming conditions, germinated more rapidly than non-primed seeds (Table 3).Similar results have been obtained for asparagus seeds by Krarup (1991) and Gray et al. (1991).However, the beneficial effect of priming was more pronounced for seeds of low physiological quality, as also reported by Szafirowska et al. (1981), Bray (1995) and Nascimento & Aragão (2004).
Effects of priming on seeds of different initial quality seems to depend on the stress that they encounter during germination.In general, PEG-priming at -1.0 for 14 days was the best treatment to improve asparagus seed performance, while priming in distilled water or seawater appeared to be less adequate.Probably, these results can be attributed to imbibitional damage in dry seeds caused by rapid imbibition in distilled water, leading to solute leakage and consequent cellular constituent losses (Heydecker at al., 1975).There is no consistent information regarding the use of natural seawater as the priming agent.Despite its osmotic potential being lower than that of PEG, no harmful effect on asparagus seed germination was found.However, seawater contains several ions which can penetrate the seed tissue, destroying enzymes or other cellular constituents, while PEG 6000 molecules have colloidal size and do not penetrate seed tissue.Hydropriming also improved performance of watermelon seeds, at low temperature, but was generally less effective than osmopriming in salt solutions (Demir & Van de Venter, 1999).According to Evans & Pill (1989), asparagus seeds primed for seven days in -0.8 MPa PEG showed higher germination, and synthetic sea water was as effective as PEG in accelerating seed germination (Frett et al., 1991).
Effect of priming improved seed performance in the controlled deterioration test (Table 4), but were generally more evident on germination at first count (Table 2), speed of seedling emergence (Table 3) and seedling growth (Table 5).Asparagus seeds primed in -1.0 MPa PEG for 14 days showed higher germination rate and speed of emergence independent of their physiological quality, but for all characteristics evaluated, beneficial effects of priming were more expressive on lower quality seed lots.Table 5 -Seedling dry weight at 13 days after sowing of four asparagus seed lots subjected to different priming treatments.
Means followed by the same letter, capital in each column and small in each row, did not differ by the Duncan's test (P < 0.05).

Table 1 -
Germination (28 days after sowing) of four asparagus seed lots subjected to different priming treatments.
Means followed by the same letter, capital in each column and small in each row, did not differ by the Duncan's test (P < 0.05).