EFFECTS OF NITROGEN DEFICIENCY ON ACCUMULATION OF FRUCTAN AND FRUCTANMETABOLIZING ENZYME-ACTIVITIES IN SINK AND SOURCE LEAVES OF BARLEY (HORDEUM-VULGARE)

Seedlings of barley (Hordeum vulgare L. cv. Agneta) were grown hydropo nically under continuous light, constant temperature and relative humi dity. During the first two weeks, the relative growth rate (RGR) was k ept at 25% by limiting only the supply of nitrogen. The cultures were then transferred to nitrogen-free media and the amounts of fructan, st arch, sucrose, glucose and fructose in sink and source leaves were mea sured at 0, 12, 24, 48, 72, 120 and 156 h. The activities of two key e nzymes in fructan metabolism, sucrose:sucrose fructosyltransferase (SS T), fructan exohydrolase (FEH), as well as acid invertase were also me asured in the two types of leaves. The fructan and starch levels in bo th sink and source leaves increased during nitrogen deficiency. The hi ghest increase in starch was 200% of the control, while for fructans a 700% increase was recorded. The activity of SST increased parallel to fructan accumulation in sink leaves. However, the FEH activity was co nstant and not affected by nitrogen deficiency. The invertase activity both in sink and source leaves was reduced by nitrogen deficiency. Mo re fructans, as well as sucrose and fructose, accumulated in source le aves compared to sink leaves both before and after nitrogen starvation The results show that fructan is the major carbohydrate reserve accum ulating under nitrogen deficiency both in sink and source leaves in ba rley plants. The induction of fructan accumulation in sink leaves caus ed by nitrogen deficiency is intimately connected with the regulation of SST.