ANTIOXIDANTS AND PHOTOSYNTHESIS IN THE LEAVES OF TRITICUM-DURUM DESF SEEDLINGS ACCLIMATED TO NON-STRESSING HIGH-TEMPERATURE

The foliar antioxidant status and the photosynthetic capacity were com pared in the leaves of non-acclimated Triticum durum Desf. cv. Duilio seedlings grown at an optimal temperature of 25 +/- 0.4 degrees C or a t a supraoptimal temperature of 30 +/- 0.3 degrees C, under moderate l ight levels and ad libitum available water. The plants grown at 30 +/- 0.3 degrees C did not show the symptoms commonly observed in response to high temperature stress, such as acceleration of development, redu ction in size, loss of photosynthetic pigments and reduction in the ph otochemical efficiency of PSII. However, mesophyll conductance to CO2 uptake, net photosynthesis, and photon yield were drastically reduced in leaves grown at 30 +/- 0.3 degrees C. Neither were the extractable capacities of superoxide dismutase, catalase and guaiacol peroxidase i ncreased nor were lipid peroxidation and electrolyte leakage stimulate d in leaves grown at 30 +/- 0.3 degrees C. Limited hyperthermia caused 60 - 80% increases in the contents of dehydroascorbic acid and glutat hione disulfide, thus lowering significantly the redox ratios of the a scorbic acid and glutathione pools, respectively. The above results in dicate that the photosynthetic performance of T. durum seedlings can b e negatively affected even by moderately elevated, non-injurious tempe ratures, which could favour the photo-oxidative carbon cycle over the photo-reductive one. This might lead to an overproduction of reactive oxygen species, with the ascorbic acid and glutathione foliar pools ac ting as a ''first line'' of antioxidant defense. Analogies were found with previous data concerning the effects of suboptimal, non-chilling growth temperatures on the same plant material.