Dynamics of Reactivation of Rubisco Carboxylase Activity after Short-Term UV-Irradiation of Wheat Leaves

The attached leaves of a three-week-old plant of durum wheat (Triticum durum L.) was irradiated with different doses of UV radiation (248 or 302 nm), then the plant was exposed to continuous white light (150 W/m2) for 48 h. During the first 3 h after UV treatment, a significant inhibition of the enzyme activity was observed in both preparations (by 35% for UV-B and 60% for UV-C in purified Rubisco—PRP; 20% for UV-B and 35% for UV-C in protein extract—TPE). At 10–12 h, a peak-like increase in Rubisco activity (by 50% for UV-C and 70% for UV-B) was observed in the TPE only. UV wavelength and dose had a significant and regular effect on the amplitude of the peaks. By the 24th hour of photoreactivation, a decrease in the oscillations of the enzymatic activity was observed, reaching the control value 48 h after UV irradiation. Measurement of 14CO2 assimilation by wheat leaves under UV stress and during a subsequent 48-h period in white light showed a slight decrease in the first 3 h, which changed to an increase in photosynthetic rate by 6–12 h after irradiation. As a result of the non-specific action of UV on leaf mesophyll cells and directly on chloroplasts, two processes develop in parallel. One of these processes is probably destructive and is directly linked to Rubisco. The other, which activates the work of the enzyme, is most likely localized at the level of the chloroplast and the mesophyll cells.