Metabolism of carbohydrate and lipid reserves in germinated cotton seeds

Abstract

Utilization of reserve lipid and carbohydrates during germination (0-12 h) and postgerminative growth (12-48 h) was studied in cotton (Gossypium hirsutum L.) seedlings. Raffinose and stachyose were utilized during the germination period and early growth; mobilization was associated with α-galactosidase (EC 3.2.1.22) activity. Results from pulse-chase experiments with [(3)H]raffinose supplied exogenously to 4-h soaked seeds indicated that raffinose-derived catabolites contributed to the coincident increase in cotyledon sucrose and starch, and to the small increase in axis dry weight. Starch appears to be an alternative sink for end products of hydrolysis of reserve carbohydrates prior to the onset of rapid axis growth and cotyledon expansion. Mobilization of neutral lipid commenced at about 16 h after soaking, concomitant with development of key glyoxylate-cycle and other gluconeogenesis-related enzyme activities. Axis dry weight increased three-fold between 24 and 48 h. Results from pulse-chase (3 h, 16 h) experiments in which [2-(14)C]acetate was supplied to cotyledons of intact 22-h-old seedlings showed that acetate-derived metabolites were not transported exclusively to the axes, but were partitioned between axes and cotyledons. Only 27% of total incorporated radioactivity was recovered in axes following the chase, 18% was evolved as CO2, and the rest was recovered in water-soluble substances (20%) and polymers (31%) within the cotyledons. Of the polymers, 55% of the activity was in polysaccharides (Starch, pectic substances, hemicellulose, cellulose), 25% in protein, and 20% in unidentified neutral and acidic compounds. Considering these data, the amount of lipid mobilized, and various routes by which supplied [2-(14)C]acetate could be metabolized, it appears that lipidderived compounds contribute only 25-40% of axis dry-weight gain. Lipid-derived substances retained in the cotyledons likely are utilized for expansion and differentiation of the cotyledons into photosynthetic organs.