SUCROSE AND PROLINE ACCUMULATION AND SUGAR EFFLUX IN TOMATO LEAF DISCS AFFECTED BY NACL AND POLYETHYLENE-GLYCOL-6000 ISOSMOTIC STRESSES

The effect of phlorizin, a reversible inhibitor for glucose transporters and that of PCMBS, a non-permeant water soluble chemical modifier of membrane sulphydryl groups, on proline and sugar accumulation and sugar efflux have been studied in tomato leaf discs (Lycopersicon esculentum L. Mill. cv. Volgogradskij) subjected to stress conditions when incubated in 300 mM NaCl or iso-osmotic solution of non-permeant PEG 6000. The amounts of proline and sucrose found in the tissue were mainly related to the structure of the osmoticum used for the treatment: 12.5 and 56 mu mol of proline per gram of dry weight were registered under saline and PEG conditions respectively, while 49.4 and 91.7 mu mol of sucrose were found under these conditions. The application of inhibitors provoked a different sugar efflux to the bathing medium: PCMBS enhanced the sugars efflux up to 10 times in saline medium and up to three times under PEG conditions. Surprisingly, the sucrose content in the tissue was always similar to that of the same treatment in the absence of the inhibitor. When exogenous sucrose was supplied to the treated leaf discs, it was found that it exerts two opposite effects on proline accumulation since (1) it prevented the inhibitory effect of both phlorizin and PCMBS in some treatments and (2) it enhanced proline accumulation provoked by these inhibitors in control discs and those treated with PEG. The proline response in relation to the accumulation of sugars for osmotic adjustment and the possibility of an osmoregulated process concerning sucrose synthesis and carbohydrate export under stress conditions are discussed.