Evaluation of some important aspects of seed vigour and viability in soybean (Glycine max L. Merrill)
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This study is in three sections. The first section investigated the environmental and seed quality factors affecting seed vigour and viability in soybean. In addition an experiment was performed which attempted to correlate laboratory viability tests to field emergence. The second section studied the effect of ageing and of the rate of water uptake on germinability using polyethylene glycol (PEG). Finally in section three numerous soybean cultivars were evaluated for storage potential and resistance to imbibition damage.Laboratory germination at 25°C, 30°C or 35°C, seedling fresh weight, leachate conductivity and tetrazolium chloride topographic staining tests were highly correlated with field emergence at North West Frontier Province (NWFP), Agricultural University Research Station, Mingora, Pakistan. However, seeds aged for more than 4 days progressively lost the ability to germinate under field conditions compared to laboratory germination. Sealed storage at 8% seed moisture content was superior to storage at 12 or 16% seed moisture content. Compared to seeds with 12 or 16% initial moisture content those at 6% initial moisture showed reduced germinability. Reduced germinability of seeds at 6% moisture content was associated with high solute leakage. Solute leakage increased as soaking temperature increased, but at a higher rate when initial seed moisture content was low (6%). Smaller seeded cultivars were susceptible to etching (cut on both the cotyledons), but showed better storage potential compared to large seeded cultivars.Accelerated ageing (41°C; 95-100% RH for 3 days), and soaking for 5 or 10 h in distilled water produced fewer normal seedlings and delayed 50% emergence by 1.5 days probably due to high solute leakage. The seeds germinated earlier and produced more normal seedlings when soaking occurred in 25% PEG compared to distilled water. Seeds with lower vigour due to ageing, produced a higher number of normal seedlings, greater shoot length, shoot fresh weight and shoot dry weight when pre-imbibed in 25% PEG compared to distilled water. Osmotic stress applied by means of 15, 20 or 25% PEG drastically reduced germination and seeding fresh weight. Stress applied with 10% PEG did not reduce germination, but seedling fresh weight was lowered by about 20% compared to the control. Reduced germination and shoot fresh weight were associated with osmotic stress.Cultivars that were resistant to accelerated ageing were also resistant to soaking injury and vice versa. Generally, smaller or yellow seeded cultivars performed better with between 16 and 28% more normal seedlings being produced and 13 to 19% lower leachate conductivity after accelerated ageing. However, larger or light coloured seeds had poor germinability after accelerated ageing or soaking in distilled water. Solute leakage was higher from the larger seeded cultivars compared to the smaller and medium seeded cultivars. Medium seeded cultivars generally leaked more solutes than smaller seeded cultivars, but this relationship was not entirely consistent. The superiority of smaller seeded cultivars over larger seeded cultivars after accelerated ageing and against soaking injury was mostly associated with hard and waxy type seed coats.