Effect of water stress on photosynthetic light phase in leaves of two ecotypes of Platanus orientalis L. plants

Platanus orientalis is considered endangered species that is almost extinct in the natural ecosystems of Western Europe and, because of its hydrophilic habitat, may also be strongly affected by increasing water limitations in Eastern Europe. The effects of drought stress were studied and compared in young plane trees of two ecotypes from Bulgarian and Italian regions. The dynamics of drying and following re-watering was monitored in vivo by changes in activity of photosynthetic light phase reactions in leaves of 4-5 month old seedlings subject to controlled moderate water limiting regime for 12 days and subsequent recovery by controlled gradual irrigation for 10 days. The physiological state of photosynthetic machinery was estimated by analysis of signals of prompt and delayed chlorophyll a fluorescence measured in attached plane leaves by the Multifunctional Plant Efficiency Analyzer (Hansatech Instruments Ltd., UK). Prompt chlorophyll a fluorescence directly correlates to the redox state of the electron carriers in photosystem II at light conditions, while delayed fluorescence indicates rate constants of direct and back electron transport reactions within the same structure. The fluorometer allows simultaneous measurement of both prompt and delayed chlorophyll fluorescence signals that provide complementary information concerning the reactions in the photosynthetic light stage. We show that both donor and acceptor sides of photosystem II as well as intersystem electron transport are inhibited during drought stress. Moreover, the intersystem electron carriers appear to be the most sensitive part, indicated by strongly reduced size of their pool in the thylakoid membrane. The stress reaction of Bulgarian ecotype is expressed clearly. However, fluorescent parameters undergo partial recovery during re-watering. On the other hand, parameters in Italian ecotype exhibit weak changes during drought stress but the effect proceeds throughout the whole monitoring period.