The photosynthetic responses to moderately high temperatures (38 degrees C, imposed at 21% or 2% O(2) in air and 1500 mumol m(-2) s(-1)) were compared in wheat (Triticum aestivum L.) cultivars grown in northern regions of Ukraine and expected to be relatively sensitive to high temperatures ('North' cultivars) and in cultivars grown in southern regions and expected to be relatively heat-tolerant ('South' cultivars). Heating intact leaves in 21% O(2) for 1 h decreased CO(2) assimilation by c. 63% in 'North' cultivars and only c. 32% in 'South' cultivars, with a decrease in PSII activity being observed in only one of the 'North' cultivars. Carboxylation efficiency was decreased by about 2.7-fold in 'North' cultivars with no significant effect in 'South' cultivars. The maximum rates of carboxylation by Rubisco in vivo, V(cmax), estimated from Farquhar's model, increased more than 2-fold in 'South' cultivars and remained unchanged in 'North' cultivars while the maximum rate of RuBP regeneration, J(max), decreased by 53% and 21% in 'North' and 'South' cultivars, respectively. Where the heat treatment was imposed in 2% O(2) this increased (as compared with treatment at 21% O(2)) the inhibitory effect on CO(2) assimilation in tolerant cultivars, but decreased it in sensitive ones. The results suggested that differences in tolerance of moderately high temperatures in wheat relate to the stability of the Rubisco function and to RuBP regeneration activity rather than to the effects on PSII activity or stomatal control.

The problem of drought impact on crop productivity is especially relevant for the leading cereal – wheat, since significant cultivation areas of this crop are located in risky farming zones. The aim of our studies was to compare the peculiarities of drought impact during the period of vegetative growth and reproductive development on the chlorophyll content, activity of chloroplast antioxidant enzymes, the rate of carbon dioxide gas exchange and transpiration as well as productivity of the wheat plants. The studies were carried out on bread winter wheat plants (Triticum aestivum L.) of the Astarta and Natalka varieties grown in a pot experiment. The plants were watered daily to maintain the soil moisture level in the pots in the range 70–60% of field capacity. Drought treatment involving decrease in soil moisture to 30% of the field capacity was applied to some of the plants at late stem elongation stage (BBCH 37-45) and to other plants at heading-anthesis (BBCH 59-65) while control plants were watered as usual. After 7 days of drought, the watering of treated plants was resumed to maintain the soil moisture at the level of control plants until the harvest. The penultimate leaf at the late stem elongation stage and flag leaf at the heading–anthesis period were used for measurements of photosynthetic apparatus response to drought. Plants of both varieties treated at stem elongation stage showed lower decline in the photosynthetic activity, chlorophyll content and grain productivity than those treated at heading-anthesis. The results obtained indicated that photosynthetic apparatus of wheat has a higher drought tolerance at the stem elongation stage than during the heading-anthesis stage. Furthermore, plants stressed at the stem elongation stage revealed better compensatory growth of lateral productive shoots due to superior supply of plants with assimilates after drought termination. Astarta variety had a more drought-tolerant photosynthetic apparatus and higher grain productivity in both well-watered and drought conditions as compared to Natalka variety. The differences between varieties were more contrasting when drought was applied at heading-anthesis.

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