Changes in Seed Reserves and Cell Wall Composition of Component Organs During Germination of Cabbage ( Brassica oleracea) Seeds

Abstract

Seeds of cabbage ( Brassica oleracea L. var. capitata cv. Golden Acre Primo) were germinated and grown in darkness at 25°C. The seedlings were harvested at 24 h intervals for 7 days and analysed either as intact whole seedlings (minus seed coat which became detatched within 48 h) or after dissection into cotyledons (including plumule), hypocotyls and roots. Intact whole seedlings and excised organs were analysed for fresh weight, dry weight, lipid content, protein content, insoluble and soluble carbohydrate. Hypocotyl and root lengths were also recorded. The cotyledons were rich in food reserves consisting of protein, lipid and possibly some water-soluble carbohydrate. They exhibited little change in fresh weight during the growth period, though their dry weight decreased by 50% by day 5. In the cotyledons of fully-imbibed seeds, lipid, protein, soluble carbohydrate and insoluble carbohydrate were present in the ratios 1: 0.33: 0.03 : 0.06. No starch was detected. During the first 2 days post imbibition, protein decreased rapidly, whilst levels of lipid remained constant. However, after day 2, only lipid reserves decreased. These became depleted by day 5 after which protein was again mobilised. Soluble sugar levels decreased by over 60% during the growth period whilst levels of insoluble (cell wall) carbohydrate showed a slight increase. Arabinose, the most abundant sugar residue in the cell walls of imbibed cotyledons, decreased by over 70% by day 7. This was accompanied by a concomitant increase in the levels of glucose and uronic acid residues, indicating cell wall turnover. Whilst radicle emergence preceded hypocotyl elongation, the hypocotyl quickly became the dominant growing organ, exhibiting much greater increases in fresh weight, dry weight, protein, lipid and carbohydrate (soluble and insoluble). During hypocotyl elongation the level of cell wall arabinose (the most abundant cell wall sugar in 1-day-old hypocotyls) decreased relative to the other component sugar residues. It is likely that this was due not to metabolism of arabinose-containing polysaccharides (as in the cotyledons) but to the deposition of primary cell wall polysaccharides relatively poor in arabinose during cell extension. No such changes were observed in the cell walls of growing roots.