POOR SEED GERMINATION OF <em>MELIA</em> <em>DUBIA</em>— UNRAVELLING THE BIOLOGICAL CAUSES AND DESIGNING AN APPROPRIATE SEED TREATMENT PROTOCOL
Keywords:Fruit structure, physical dormancy, mechanical dormancy, seed vigour, seed treatment, relative humidity
Melia dubia produces indehiscent drupes which are sown for seed germination. Studies revealed that the germination is highly protracted, sporadic and the final germination is very low (< 10 %). The exocarp and mesocarp of M. dubia fruits are impermeable to water (physical dormancy), while the endocarp causes mechanical resistance to germinating embryo (mechanical dormancy). The kernels extracted from the drupes responded well to GA3 (250 mg L-1) treatment, when applied through seed soaking (24 hours) +
humid priming (2 days), compared to seed soaking (24 hours) in GA3 (250 mg L-1) alone. Excavation of ungerminated seeds from nursery beds revealed that the remaining kernels were either fresh ungerminated (FUG) or decayed. This might be due to presence of a large proportion of low vigour seeds within a seed lot. Exposing the kernels to dry heat (40 oC for 4 hours) followed by seed soaking in GA3 (250 mg L-1) for 24 hours + humid priming with GA3 (250 mg L-1) for two days, and sowing in a polyhouse with high relative
humidity (80 ± 5 % RH) resulted in highest improvement in synchrony of germination, with corresponding decrease in FUG and seed decay. The protocol enabled to increase the seed germination of M. dubia to 42.3% and reduced the period of seed germination to 21 days. The poor germination behavior of M. dubia is attributed to multiple factors such as physical dormancy imposed by exocarp and mesocarp, mechanical dormancy caused by endocarp and higher proportion of low vigour seeds in a seed lot.