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Sucrose in barley leaf blades can be considered as being compartmented between two types of pool: transport (including mesophyll cytosol and apoplast, and vascular tissue) and vacuolar. The relative sizes of these pools, and their changes over a light/dark cycle, have been estimated using [SUP14]C labeling of the intact leaf and washout of [SUP14]C from leaf discs. Both types of pool increase during the photoperiod, the proportional distribution between them remaining the same; roughly one-fifth of the sucrose is in the transport pool. The rate-constants describing translocation, and loading and unloading of the vacuole, show diel changes, and fluxes of sucrose across the tonoplast are likely to be critical in controlling fluxes of photosynthetically fixed carbon within and out of barley leaves. The unloading of sucrose from vacuoles is probably the site where control is exercised; this conclusion is supported by a simple simulation model. [ABSTRACT FROM AUTHOR]

Gas exchange, mass of carbohydrates, and turnover of carbohydrates, have been followed diurnally in barley leaf blades. Mature second leaf blades store sucrose, which occurs in at least two types of pool which turn over at different rates. Over a light-dark cycle, photosynthesis, sucrose synthesis and translocation all show appreciable changes in rate but are not obviously related; nor is the rate of translocation a function of the total concentration of sucrose in the leaf. In the dark, starch mobilization starts immediately and that of sucrose increases, maintaining the mean rate of translocation in the dark at 77 % of that in the light. Fructans are turned over but show little diurnal variability in mass. [ABSTRACT FROM AUTHOR]