Irrigation and Seed Quality Development in Rapid-cycling Brassica: Soluble Carbohydrates and Heat-stable Proteins

Changes in soluble carbohydrates and heat-stable proteins have been examined in relation to the acquisition of desiccation tolerance and/or potential seed longevity during seed development in rapid-cycling brassica [Brassica campestris (rapa)L.]. Rates of seed development were moderated by different irrigation regimes. At the early stages, glucose, fructose and sucrose predominated. The raffinose series oligosaccharides accumulated during seed maturation, and occurred earliest in seeds from plants irrigated only until 16 days after pollination. Stachyose content correlated positively, and monosaccharide content correlated negatively, with the ability of seeds to tolerate rapid desiccation and with their potential longevity (the constantKiof the seed viability equation). Similarly, the ratio of oligosaccharide:total sugars provided strong positive correlations with ability to tolerate desiccation and with potential longevity. Most of the heat-stable proteins selected for study accumulated comparatively late, i.e. during maturation drying. The imposition of water stress induced earlier accumulation of heat-stable proteins. The ability to tolerate desiccation was correlated with the content of selected heat-stable proteins, but potential longevity provided stronger correlations. The content of a 58 kDa heat-stable protein provided the strongest positive correlation with potential longevity. A simple multiple regression model of the relations between potential longevity and both the oligosaccharide:total sugar ratio and the 58 kDa heat-stable protein content was developed for all three plant irrigation regimes to show the combined effect of certain sugars and proteins on seed quality. The model suggests that these sugars and proteins are equally likely to be required for seed quality development, and that initially the sugars tend to accumulate at a greater rate than the proteins, but that during maturation drying the heat-stable proteins accumulate at the greater rate.