Response to mild water stress in transgenic Pssu-ipt tobacco

The response of antioxidant enzymes to cyclic drought was studied in contro l non-transformed tobacco (Nicotiana tabacum L. ev. Petit Havana SR1) and t wo types of transgenic Pssu-ipt tobacco (grafted on wild rootstock and poor ly rooted progeny of F1 generation) grown under different conditions of irr adiation (greenhouse, referred as high light, versus growth chamber, referr ed as low light). Water stress cycles started with plants at two contrastin g developmental stages, i.e., at the stage of vegetative growth (young) and at the onset of flowering (old). Drought reduced the growth of SR1 plants compared with transgenic ones, particularly, when treatment started in earl ier stage of plant development. Relative leaf water content was significant ly lower (below 70%) in all transgenic grafts and plants compared with the wild type, irrespective of age, drought, and growth conditions. The respons e of antioxidant enzymes was significantly dependent on plant type and plan t age; nevertheless, growth conditions and water stress also affected enzym e activities. Contrary to non-transgenic tobacco, where about half of gluta thione reductase activity was found in older plants, both transgenic types exhibited unchanged activities throughout plant development and stress trea tment. No differences were found in catalase activity, although the growth in the greenhouse caused a moderate increase in all older plants. In contra st to non-transgenic and Pssu-ipt rooted plants, peroxidase activities (asc orbate, guaiacol, and syringaldazine peroxidase) in older Pssu-ipt grafts w ere up to four times higher, irrespective of growth and stress, nevertheles s, the effect seemed to be age-dependent. Superoxide dismutase (SOD) activi ty was affected particularly by plant age but also by growth conditions. Un like in older plants, water stress caused an increase of SOD activities in all younger plants. The differences observed in activities of enzymes of in termediary metabolism (i.e., malic enzyme and glucose-6-phosphate dehydroge nase) revealed that transgenic grafts probably compensated differently for a decrease of ATP and NADPH than control and transgenic rooted plants under stress.