Fluorescence parameters as early indicators of light stress in barley.

Photosynthetic efficiency of two Syrian barley landraces Arabi Aswad and Arabi Abiad grown under different light intensities were studied by the application of qualitative and quantitative analysis of chlorophyll a fluorescence. Different values of fluorescence parameters, quantum efficiencies, specific and phenomenological energy fluxes were obtained for each cultivar. Both low and high light stresses decreased photosystem II (PSII) activity in barley seedlings depending on the stress type and its duration. Cultivar Arabi Aswad was more tolerant to high light while Arabi Abiad was more tolerant to low light stress. The results allowed us to select chlorophyll a fluorescence parameters related to energy flux within PSII which were specifically affected under low or high light stress. We found that the performance index parameter is a sensitive indicator to explore the effect of light changes on PSII activity immediately after stress application, while maximal quantum yield of PSII and phenomenological parameters were only modified after a long period of stress application indicating PSII damage. Thus, we recommend the former parameter for early detection of light stress.