Effect of Priming on Stored Onion Seeds

Onion seeds were treated with water (hydropriming), sand (80% WHC) (Solid matrix priming), salts of KNO 3 and NaCl at 3% (halopriming) for 12 h and 24 h and PEG (-0.25 MPa) for 8h and 12 h (osmopriming). Seeds grouped into two lots and dried to 7% and 8% MC were packed in Aluminium foil pouch and cloth bag respectively and stored for four months under ambient conditions (33°C and 57% RH). Results of the storage experiment revealed that all the eight priming treatments imposed upon onion seeds increased the speed of germination, germination percentage, seedling length, protein content and enzyme activity but lowered the electrical conductivity of seeds when compared to control. Among the treatments, seeds hydro primed with 80% sand for 24 h bestowed supremacy over the rest of the treatments throughout the period of storage in both containers. Between the containers, seeds stored in Aluminium foil pouch recorded significantly improved percentage of radicle protrusion and germination throughout the period of storage.

Improving the seed quality is an approach which is likely to produce significant benefits in almost all circumstances without any significant increase in risk.The use of seed enhancement techniques is not new to agriculture and earlier practices have been described for such treatments (Kalyani et al., 2009).Theophrastus (372-287 BC) recommended presoaking of cucumber seeds in milk or water to make them germinate quicker and better (Michael Evenari, 1984).Priming is a process in which seeds are imbibed either in water or osmotic solution or a combination of solid matrix carrier and water in specific proportions followed by drying before radicle emergence.In several studies, an increase in the nuclear DNA contents of radicle meristem cells from the G1 to the S or G2 phases of the cell cycle was noticed.An invigoration treatment should bring about qualitative improvement in the seed, which should persist after the treatment is stopped as the treatments are basically physiological in nature.In the last two decades, seed priming -an effective seed invigouration method -has become a common seed treatment method to increase the rate and uniformity of emergence that has been commercialized.
The recorded effects of priming treatments on the storability of seeds are some what contradictory.The advancement of the germination process during priming continuously consumes stored substances and consequently may shorten seed longevity.However, the repair of DNA damage will increase longevity (Osborne, 1983).The results obtained so far are few, limited, contrasting because of the *Corresponding author email: seedmani@yahoo.comvariability of the response to treatments of cultivars and even seed lots (Bradford, 1986) which require a careful choice of the compounds to be used as osmoticum and standardization of the treatment conditions.With this point of view, the present investigation was conducted.

Materials and Methods
Seeds of onion cv.CO5 were obtained from Department of Seed Science and Technology, Tamil Nadu Agricultural University, Coimbatore.Onion seeds were primed with water (hydropriming), sand (80% WHC) (Solid matrix priming), salts of KNO3 and NaCl at 3% concentration (halopriming) for 12 h and 24 h and PEG (-0.25 MPa) for 8h and 12 h (osmopriming).Based on the experimental results of the standardization of seed priming procedures two best durations in each method of priming was selected.The seeds were grouped into two lots and dried to 7% and 8% MC and packed in i) Aluminium foil pouch and (ii) cloth bag respectively.The containers were kept under ambient conditions (33oC and 57% RH) for four months.
Seed samples were drawn initially and subsequently at monthly intervals were subjected to germination test with four replicates of 100 seeds.The seeds were observed daily up to 14 days for radicle protrusion.The seeds showing less than 3mm radicle protrusion were alone counted.The speed of germination was calculated (Maguire, 1962).The number of normal seedlings were counted after 14 days and expressed as germination percentage.The length of the seedlings were measured and expressed in cm and vigour index (12.87)(12.87) (12.87) (7.9) (7.9) (7.9) (7.9) (7.9) (7.9) (5.7) (5.7) (5.7) (7.9) (7.9) (7.9) (8.53) Biochemical evaluations were carried out before storage and four months after storage.The seeds were analyzed for electrical conductivity (Presley, 1958), -amylase activity (Simpson and Naylor, 1962) and protein content (Ali khan and Youngs, 1973).
The observations recorded were statistically analysed using methods described by Panse and Sukatme (1978).Wherever necessary the percentage values were converted into arcsine for easier interpretation.

Results and Discussion
The results of the storage experiment revealed that all the eight priming treatments imposed upon onion seeds increased the speed of germination, germination percentage, seedling length, protein content and enzyme activity and lowered the electrical conductivity of seeds when compared to control.
Significant differences in radicle protrusion were observed for seed treatment, period of storage and containers.Among the treatments the seeds primed with 80% sand matrix (24 h) (16.4%) or hydroprimed for 24 h (12.8%) recorded supremacy over the rest of the treatments throughout the period of storage in both containers.Between the containers, seeds stored in aluminium foil pouch recorded significantly improved radicle protrusion percentage through out the period of storage (Table 1).After 4 months of storage, except halopriming (3% NaCl, 24 h), all the others treatments recorded significantly higher speed of germination than the control.However, sand matrix priming (24 h) and hydropriming (24 h) recorded the highest speed of germination, with increase of 41 and 43% over the control (Table 2).Among the treatments, 80% sand matrix priming (24 h) recorded the mean maximum germination of 91 per cent, followed by hydropriming (24 h) (90%), while control registered the minimum of 85 per cent (Table 3).Significant differences in vigour index noticed among the seed treatments, period of storage, containers and their interactions.The results revealed that vigour index decreased with increase in the period of storage from 766 (initial) to 498 (fourth month) irrespective of treatments and containers.Among the treatments 80% sand matrix primed (24 h) seeds recorded highest vigour index (778) followed by hydropriming (24 h) (712) (Table 4).The highest EC value was recorded by untreated control (0.071 dSm -1 ) and PEG -0.25 MPa (8h) (0.070 dSm -1 ) while the lowest value was registered by 80% sand matrix priming for 24 h (0.062 dSm -1 ).Over the storage period these treatments also recorded the maximum protein content (20.12%) followed by hydropriming (24 h) seeds (20.02%) and untreated control seeds recorded the minimum value of 19 per cent, which was on par with halo priming (3% KNO3, 24 h) (19.05%) irrespective of The initial increment in seed vigour obtained through the different priming methods was reflected in the seed vigour after four months of storage.
Biochemical manifestations of seed priming has not been studied extensively.Protein, sugar and RNA were found to increase in PEG treated seeds of cauliflower (Fujikura and Karsen 1992).Enzyme activities of catalase, peroxidase, amylase and invertase increased in PEG treated seeds (Sing et al., 1985).
These results on enzyme activity and protein synthesis are consistent with the results of the present study where, primed seeds showed higher levels of amylase enzymes and protein synthesis.Priming attributed germination increase might be due to priming -enhanced repair of membranes which were disrupted during maturation drying.This is indirectly supported by the reduced leakage of electrolytes from primed seeds, since electrolyte leakage is in part a result of damaged cell membranes.
The present study consistently revealed that initial advantage obtained through priming gained in the initial stage also persisted even after 4 months of storage.The study also underscored the conjecture that the seed viability was better maintained in primed seeds of onion than in unprimed seeds.Pill (1995) after an extensive review on seed priming concludes that since viability and germination rate were enhanced by priming both before and after seed storage, priming was involved in both delaying the ageing process and in repairing seed deterioration.These well standardized priming techniques can very well serve farmers as well as industry as a no cost technique to increase the seed germination, vigour and storability.