EFFECTS OF SHORT-TERM PHOSPHORUS DEFICIENCY ON CARBOHYDRATE STORAGE IN SINK AND SOURCE LEAVES OF BARLEY (HORDEUM-VULGARE)

Barley (Hordeum vulgare L. cv. Agneta) was cultivated hydroponically u nder continuous light, constant temperature and relative humidity. The relative growth rate (RGR) was kept at 25% by an exponential supply o f complete medium for 2 wk. The cultures were then transferred to phos phorus-free media and the amounts of total P, fructans, starch, sucros e, glucose and fructose in sink and source leaves were measured at 0, 12, 21, 48, 72, 120 and 156 h. The capacity of net fructan synthesis ( i.e. total activity of sucrose:sucrose 1-fructosyltransferase (1-SST) and sucrose:fructan 6-fructosyItransferase (6-SFT)), the activity of f ructan exohydrolase (FEH), as well as the activity of acid invertase w ere also measured in the two types of leaves. Omission of P from the g rowth medium caused a decrease in the P content in both sink and sourc e leaves, whereas the RGR was not affected during the 156 h of the exp eriment. The fructan concentration increased in low-P plants whilst th ere was little or no effect on the concentrations of starch, sucrose, glucose and fructose. There was a slight increase in the capacity of n et fructan synthesis in both sink and source leaves at 156 h after the onset of P-starvation. There was no change in the activity of FEH or acid invertase. The results indicate that in barley leaves the mechani sm for carbon partitioning into fructans is more sensitive towards low P conditions than the mechanism for carbon partitioning into starch.