FRUCTAN METABOLISM IN LEAVES OF LOLIUM-RIGIDUM GAUDIN .1. SYNTHESIS OF FRUCTAN

Accumulation of water-soluble carbohydrates (WSC) in leaves of intact seedlings of annual ryegrass (Lolium rigidum Gaudin cv. Wimmera) was studied using a novel model system. The roots and leaf bases of intact seedlings, grown in nutrient solution, were cooled to 5-degrees-C to reduce sink activity and the production of photosynthates was enhanced by continuous illumination of the plants. This resulted in accumulation of WSC in the shoots. Thin-layer chromatography showed the presence of a complex series of fructans, distinct from those found in tubers of Helianthus tuberosus. Two trisaccharides, 1-kestose and neokestose, were present in various proportions, but no 6-kestose was detected in the shoots. After feeding (CO2)-2-14 to the seedlings in a pulse-chase experiment, high specific radioactivity was measured in monosaccharides, sucrose and fructan oligosaccharide fractions to DP 5 within 1 hr. These fructan pools were rapidly turned over without significant accumulation of fructans during the first 16 h of the accumulation phase. Neokestose and 1-kestose contained equally high specific radioactivity, 1 h after feeding. These results indicated that neokestose was possibly as important as 1-kestose as a possible precursor for synthesis of fructan. Concentrations of WSC increased linearly from 2 to 14 mg g-1 f. wt of the leaves after 16 h and reached a concentration of 30 mg g-1 f. wt after 64 h. Sucrose, glucose and fructose were the first sugars to be accumulated and reached concentrations of 10, 3.5 and 2.5 mg g-1 f. wt, respectively. Tri- and tetrasaccharide accumulated to measurable concentrations after approximately 16 h of treatment and reached concentrations of 2.5 and 2.0 mg g-1 f. wt, respectively. The appearance of these oligosaccharides occurred when sucrose and monosaccharides reached stable concentrations in the leaves. The WSC concentration declined at a rate of about 0.8 mg g-1 f. wt h-1, when the root temperature was raised to 24-degrees-C after 64 h and plants were darkened. The decline in WSC concentration was accompanied by a 70% decline in sucrose concentration and a decline in all other fructan concentrations, while the concentration of fructose increased 2-fold.