Role of sugars and proteins in development of desiccation tolerance in fresh and shade-dried onion seeds

Abstract

In a 2-year field study, the umbels of open-pollinated onion cv. ‘Pusa Red’ were tagged on the day of anthesis and were sampled at different intervals until harvest maturity. Each umbel was cut in half and bulked with others in the respective replication. Seeds were immediately taken from half of the umbels for analysis of seed water content, leachate conductivity, seed capsule chlorophyll, seed dry weight and seed germination. They were also subjected to rapid desiccation to identify the stage at which desiccation tolerance occurred. The remaining umbel halves were shade dried and subjected to the same quality analyses except chlorophyll content. With seed maturation, there were rapid declines in chlorophyll content, leachate conductivity and seed water content, whereas seed dry weight and germination increased. In fresh seeds, maximum germination occurred around 50 days after anthesis, when physiological maturity and maximum seed weight were attained. In shade-dried seed, maximum germination occurred at 42 days after anthesis. This coincided with drastic reductions in seed capsule chlorophyll, leachate conductivity and seed water content in fresh seeds. At this stage, reducing sugar concentration declined and non-reducing sugar concentration increased many fold in shade-dried seeds. SDS-PAGE analysis of the heat-stable proteins extracted from the seeds also showed prominent bands in the lower molecular weight region at 42 DAA. Therefore, the shade-dried seeds attained membrane stability at 42 DAA. In cases of adverse weather conditions or disease attack, seed umbels can be harvested as early as 42 DAA and shade dried without compromising seed germination. Desiccation tolerance in fresh onion seeds occurred around 46 DAA and was a gradual event.