Remobilisation of vacuolar stored nitrate in barley root cells

Double-barrelled nitrate-selective microelectrodes have been used to measure the time course of the remobilisation of vacuolar stored nitrate in barley (Hordeum vulgare L. cv. Klaxon) root cells during 24 h of nitrate deprivation. These measurements showed that there are different time courses for this process in epidermal and cortical cells of the same root. The remobilisation was much slower from cortical cell vacuoles and had a time course which was similar to that obtained for tissue digests of the roots. The microelectrodes were also used to measure the nitrate concentration in sap exuding from detopped seedlings. These measurements showed that there was a gradual decrease in the delivery of nitrate to the shoot during this time. Root nitrate reductase activity of neither shoots nor roots changed significantly during the first 24 h. Direct measurement of the cytosolic nitrate in a root epidermal cell showed that during short-term changes, such as a 20-min exposure to zero external nitrate supply, cytosolic nitrate was maintained relatively unchanged. Net nitrate efflux from the roots was measurable during the initial 5 h of the zero-nitrate incubation period; after this time no further nitrate efflux was detectable. These measurements are discussed in relation to the nitrate budget of a root cell and we conclude that during the first 24 h of nitrate withdrawal vacuolar nitrate can be readily mobilised to supply the nitrogen demands of the seedling and to maintain the cytosolic nitrate concentration.