EFFECT OF THERMAL-STRESS COMBINED WITH DIFFERENT IRRADIANCE ON SOME PHOTOSYNTHETIC CHARACTERISTICS OF BARLEY (HORDEUM-VULGARE L) PLANTS

The treatment of plants for 4 h with high temperature, HT (42 degrees C) at 100 mu mol m(-2) s(-1) (LI) photon flux density (PFD) led to abo ut 27 % decrease in CO2 uptake in comparison to the controls (LI and 2 5 degrees C = LT). Similar lowering in the photosynthetic rate (about 20 % in comparison to the combination LT plus 1000 mu mol m(-2) s(-1) = HI) was observed after subjecting the plants for 4 h at HT, HI. The rate of electron transport, including photosystem (PS) 2 + PS1, was gr eatly reduced at HT. HI did not enhance the inhibition due to HT. The electron transport of PS1 measured under the same conditions was almos t resistant to the effect of thermal stress. Under HI the formation of zeaxanthin was observed both under LT and HT.