Carbohydrate Levels and Fructan Metabolism in Barley (Hordeum vulgare L.) Leaves : effects of manipulating mineral nutrition and photosynthesis

Abstract: Carbohydrate levels and fructan metabolism were studied in the leaves of barley (Hordeum vulgare L. cv. Agneta) plants subjected to phosphorous (P)-deficiency, nitrogen (N)-deficiency and N-resupply following the N-deficiency. Carbohydrate levels, indole-3-acetic acid (IAA) levels, photosynthesis and dark respiration were also investigated in the apical and basal tissues of a single barley leaf following the manipulation of photosynthesis in those tissues.After short term P-deficiency, fructan was the first carbohydrate to accumulate among those investigated. The results indicate that the mechanism for partitioning of carbon into fructan is more responsive to low P conditions than the mechanisms for partitioning of carbon into starch and other carbohydrates.N-deficiency caused increases in fructan and starch levels of both sink and source leaves. Fructan, the major carbohydrate reserve accumulated in the leaves under N-deficiency, together with starch fell to the same levels as those in the controls, after N-resupply. Analyses of fructan metabolizing enzyme activities, foliar soluble proteins, sucrose:sucrose 1-fructosyltransferase (1-SST) and sucrose:fructan 6-fructosyltransferase (6-SFT) mRNA accumulations in the sink leaves revealed, that the changes in fructan content at various levels of N nutrition are intimately connected with the regulation of fructan synthesis rate. The latter is mainly controlled by 6-SFT at the transcriptional level.Only small changes in the sucrose content were observed under P- or N-deficient conditions. The buffering role of fructan in carbohydrate metabolism is discussed.An intrinsic distribution of non-structural carbohydrates along the youngest fully expanded barley leaf was observed. Greater accumulation of carbohydrates occurred in the upper (apical) region of the leaf. The variation in carbohydrate levels, between upper and lower (basal) part of the leaf, was mainly caused by the variation in fructan level. Levels of starch and fructan in the apical and basal parts, were dramatically altered by shading of the leaf parts. This treatment led to a decrease in the dark respiration rate rather than a change in the photosynthesis. Leaf parts which had low dark respiration rates, contained higher levels of IAA. The regulation of carbon partitioning, photosynthesis and dark respiration within a single leaf, during partial shading, is discussed.