Transgenic tobacco plants (M51-1) constitutively over-expressing a modified gene for the proline biosynthetic enzyme △<sup>2</sup>-pyrroline-5-carboxylate synthetase (P5CSF129A) and the corresponding wild-type plants (WT) were compared during drought or heat stress and under combination of both stresses. The proline content in M51-1 was several times higher than in WT plants. Under optimal conditions, the transpiration rate and stomatal conductance of M51-1 plants were lower than those in WT plants. The differences in net photosynthetic rate were not significant and water use efficiency and contents of chlorophyll and xanthophyll cycle pigments were higher in M51-1 than in WT plants. Drought induced by cessation of watering for 7 d resulted in decrease of all gas exchange parameters and chlorophyll content, but in an increase of the content of xanthophyll cycle pigments and degree of their de-epoxidation. After application of heat stress (40°C/60 min) to control or water-stressed plants the gas exchange parameters decreased considerably. Short-term heat stress alone, however, did not affect pigment contents. The responses of M51-1 and WT plants to the tested stresses did not differ significantly. Therefore, a decisive contribution of elevated proline content to drought or heat stress tolerance of tobacco was not proved.
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