Fructan synthesis is inhibited by phosphate in warm-grown, but not in cold-treated,excised barley leaves

Abstract

The inhibition by phosphate of fructan accumulation has been investigated in warm3 grown and cold-treated barley (Hordeum vulgare L.) plants. • Detached leaves were incubated in water or phosphate for 24 h under lighting or in darkness. Fructosyltransferase, sucrose phosphate synthase (SPS) and cytosolic fructose- 1, 6 - bisphosphatase (FBPase) activities were subsequently analysed, as well as the content of carbohydrates, hexose-phosphates, phosphate, amino acids and protein. • In warm-grown leaves, phosphate decreased fructan accumulation and total carbon in carbohydrates and did not affect protein contents. Phosphate increased hexose phosphates, phosphate and amino acids. Fructosyltransferase and FBPase activities were not affected by phosphate feeding, while SPS activity was inhibited by phosphate in incubations both in light and in darkness. In cold-treated leaves, which prior to incubation had higher SPS activities than warm-grown leaves, phosphate had no inhibitory effect on fructan accumulation, carbohydrate content or total carbon in carbohydrates. SPS and FBPase activities were unaffected by phosphate. • The results indicate that phosphate decreases fructan accumulation through an inhibition of SPS whenever this activity is not high prior to a rise in phosphate content.