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 transporte rs and that of PCMBS, a non-permeant water soluble chemical modifier o f membrane sulphydryl groups, on proline and sugar accumulation and su gar efflux have been studied in tomato leaf discs (Lycopersicon escule ntum L. Mill. cv. Volgogradskij) subjected to stress conditions when i ncubated in 300 mM NaCl or iso-osmotic solution of non-permeant PEG 60 00. 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 unde r saline and PEG conditions respectively, while 49.4 and 91.7 mu mol o f sucrose were found under these conditions. The application of inhibi tors provoked a different sugar efflux to the bathing medium: PCMBS en hanced the sugars efflux up to 10 times in saline medium and up to thr ee times under PEG conditions. Surprisingly, the sucrose content in th e tissue was always similar to that of the same treatment in the absen ce of the inhibitor. When exogenous sucrose was supplied to the treate d leaf discs, it was found that it exerts two opposite effects on prol ine accumulation since (1) it prevented the inhibitory effect of both phlorizin and PCMBS in some treatments and (2) it enhanced proline acc umulation provoked by these inhibitors in control discs and those trea ted 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 str ess conditions are discussed.