Abstract

We investigated the long-term responses of tobacco tissues to salt stress, with aparticular interest for growth parameters, proline (Pro) accumulation, and carbohydrate metabolism. Exposure of 17-day-old tobacco plants to 0.2 M NaCl was followed by a higher decrease in dry matter in roots than shoots with a decrease of both Chlorophyll a and b. Analysis of potassium, sodium and chloral accumulation showed dramatic effect to K⁺ accumulation especially in shoots, whereas Na⁺ and Cl^-accumulation were strongly increased in shoots with NaCl treatment, but with minor effects in the roots. Salinity stress conditions induced a strong increase in sugar free content in the shoot and root and Pro and 2-oxoglutarate biosynthesis in tobacco tissues. However, salt-stress resulted in significant increase in the activity of NADPH and NADH malate dehydrogenase dependency, particularly in the roots, and this effect was reversed with NADP⁺ and NAD⁺-dependent malate dehydrogenase, but the activity of these enzymes was also maintained significantly higher in the absence of NaCl. Significant decreases in NADPH-depend isocitrate dehydrogenase was also observed after long-term salt treatment in tobacco plants. In addition, a gradual increase was observed in the in vitro aminating GDH activity, but the in vitrodeaminating activity remained unchanged under salinity stress. These results confirmed the hypothesis that the salinity-generated MDH and GDH induction act as anti-stress enzymes in ammonia detoxification and production of Glu for Pro synthesis.

Key words: Tobacco, carbohydrate metabolism, salinity stress.