**Acknowledgements**

intensities and under drought stress. So, increasing light intensity activates photosynthetic performance of plants. At low and moderate intensities, when the plant photosynthetic apparatus copes with coming light energy, the efficiency of photosynthetic conversion of light energy is very high, when photochemical quenching factor is near to its maximum and nonphotochemical quenching is negligibly low. Long-term drought stress due to stomatal and non-stomatal limitations to photosynthesis induces enhancement of photorespiration as an alternative sink for transported electrons in reaction centers of photosynthesis. However, further increase of light intensity increases non-photochemical quenching, and in droughtstressed plants, it is higher than in well-watered ones. This causes faster decrease of ФPSII and

Experiments with the measurement of chlorophyll fluorescence and the gas-exchange in different cotton genotypes showed that under drought stress, CO2 uptake slightly decreases, while ETR increases considerably. Simultaneously measuring these two parameters of photosynthesis allowed us to estimate the magnitude of photorespiration in the plant leaves, assuming that changes in the ETR/4-*A*<sup>G</sup> reflect the changes in photorespiration. Photorespira‐ tion increases with increasing light intensity and decreasing CO2 concentration. Moderate drought stress noticeably increases the rate of photorespiration, which can be considered as a

Leaves of drought-stressed cotton plants displayed higher ФPSII and photorespiration at low and moderate light intensities, and non-photochemical quenching, NPQ, was stronger in drought-stressed plant than that in well-watered one. Obviously, higher levels of photorespi‐ ration in plant leaves during the drought stress exerts the "pressure" to the rate of electron

The photosynthetic apparatus of plants supports higher performance of electron transport chain through enhancement of quantum efficiency of photochemistry in Photosystem II under drought stress. The accumulated energy in this state of over-excitation may be utilized in enhanced photorespiration. This protective reaction of the plant photosynthetic apparatus to drought stress has different magnitude depending on its drought tolerance. Field measure‐ ments of the chlorophyll fluorescence parameters simultaneously with morpho-physiological indicators of the cotton genotypes studied have displayed direct correlations between these parameters under drought stress. These correlations together with possible calibration of chlorophyll fluorescence parameters by photoacoustic characteristics determined at applica‐ tion of low-frequency-modulated light to plant leaves give new opportunities in monitoring

*q*p in drought-stressed plants.

104 Applied Photosynthesis - New Progress

**6. Conclusion**

characteristic response of C3 plants to a drought [44].

flow and makes Photosystem II to operate with higher efficiency.

of drought tolerance of various cotton genotypes in the field.

The authors thank Dr. A. Massacci from the Institute of Agro-environmental and Forest Biology, CNR, Roma, Italy, and Dr. Y. Fracheboud and Dr. J. Leipner from the Institute of Plant Sciences, ETH, Zurich, Switzerland, for fruitful discussions on the photorespiration mecha‐ nisms in plants.
