Fructan metabolism in expanding leaves, mature leaf sheaths and mature leaf blades of Lolium perenne. Fructan synthesis, fructosyltransferase and invertase activities

Abstract

Accumulation of water-soluble carbohydrates was studied in leaf tissues of 8-wk-old plants of perennial ryegrass (Lolium perenne L. var. Bravo). The roots and leaf bases were cooled to low temperatures to reduce sink activity while source activity was enhanced by continuous illumination of the shoots. This resulted in accumulation of fructans, first in sheaths, subsequently in expanding leaves, and finally in blades. Fructan concentration increased from 6 to 23 mg g-1 f. wt in sheaths, from 8 to 30 mg g-1 f. wt in expanding leaves and from 6 to only 17 mg g-1 f. wt in mature leaf blades. Increase in concentration of low-DP fructans preceded accumulation of high-DP fructans. Expanding leaves accumulated significantly more glucose and fructose than did mature leaf tissue. Leaf blades contained a higher concentration of sucrose than either leaf sheaths or expanding leaves. Expanding leaves exhibited the greatest activity of SST (0·77 nkat g-1 f. wt), the mature leaf blades the least (0·09 nkat g-1 f. wt), whilst that of mature leaf sheaths was intermediate (0·27 nkat g-1 f. wt). In each leaf tissue, invertase activity was higher than SST activity. Under conditions of fructan accumulation, SST activity increased threefold in mature leaf sheaths and 1·2-fold in expanding leaves. In mature leaf blades, however, the activity remained constant at a low level. Invertase activity never increased. Inhibitors of protein synthesis (cycloheximide) and of RNA metabolism (actinomycin D) prevented any increase of SST activity in leaf sheaths. The results are discussed in relation to fructan metabolism in leaf tissues of Lolium perenne.