AN APPRAISAL OF RESEARCH ON WET AND DRY PHYSIOLOGICAL SEED TREATMENTSAND THEIR APPLICABILITY WITH SPECIAL REFERENCE TO TROPICAL AND SUBTROPICAL COUNTRIES

Physiological seed treatments for improved performance include presowi ng, prestorage and midstorage wet and dry treatments. As a presowing t reatment, osmoconditioning of seeds, especially for vegetables, using polyethylene glycol has shown more consistent positive effects on seed performance than the conventional 2 to 3 cycles of 24 h wetting-dryin g treatments. Similar benefits are obtained with low-temperature holdi ng of partially imbibed seeds. Single short-duration soaking for 4 to 8 h, followed by light air-drying also improves the overall germinabil ity, rapidity and synchrony of germination of nonleguminous seeds. For leguminous seeds, instead of soaking-drying, moisture equilibration a t 100% relative humidity for 24 to 48 h, or preincubation with moist s and (5% moisture content; 3 kg sand per kg of seed) for 16 to 24 h, or matriconditioning with highly porous, chemically inert, water insolub le, inorganic carriers like Micro-Cel E, are recommended. Prestorage d ry-dressing of freshly harvested seed using the common bleaching powde r calcium oxychlorite at the rate of 3 g per kg of seed, or mixing al the same rate with an inert carrier like calcium carbonate containing very low concentrations of iodine or alcohol, would improve storabilit y of many dry-stored agricultural and horticultural seeds. Midstorage hydration-dehydration treatments which include short-term soaking-dryi ng for low and medium vigour seeds and dipping-drying, moisture equili bration-drying and moist sand conditioning-drying for medium and high- medium vigour seeds, are very effective in extending storability and s ubsequent field performance. The seed following hydration must be thor oughly dried back before restorage. Use of very low concentrations of chemicals such as potassium or sodium phosphate (mono- and dibasic, 10 (-4) M) gives some additional advantage over water. The beneficial eff ects of the treatments on germinability are also reflected upon field emergence, crop growth and final agricultural productivity. More recen tly, beneficial effects of chemicals like ferrous sulfate (2 x 10(-3) M) and 3-methyl-2-benzo-thiazolinone hydrazone (MBTH, 10(-4) M) on pre - and post-storage germinability have been recorded. The mode of actio n of the different dry and wet seed treatments is yet to be clearly un derstood, but evidence so far indicates that retention of better membr ane functions and a reduced production of toxic metabolites including volatile aldehydes, presumed to be products of free radical and lipid peroxidation reactions, are consistently associated with improved seed performance. In this review, the practical implications of the physio logical seed treatments have been discussed with special reference to tropical and sub-tropical countries.