Effects of Soil Drought on Photosynthesis and Chlorophyll Fluorescence in Bean Plants

The effects of soil drought on photosynthesis and chlorophyll fluorescence in the leaves of three bean (Phaseolus vulgaris L.) genotypes were studied. Drought was imposed 14 days after plants growing up. In the primary leaf of all the cultivars, water stress led to a noticeable decrease in both the initial slope of the An/Ci curve and Amax. The most strongly marked reduction in leaf CO2 exchange was observed in cv. Dobrudjanski ran. Maximal carboxilation efficiency (α) and CO2 assimilation (Amax) was reduced over five folds. At normal ambient CO2 concentration (Ca 350 μmol mol -1), leaf water deficit resulted in a dramatic reduction (92.2%) of An. CO2 compensation point (Ã) increased with 127.5%. Stomatal limitation of photosynthesis (SL) increased significantly (131.5%), which suggests a stronger influence of stomatal factors. Lowest reduction in leaf gas exchange parameters were observed in cv. Prelom. Cv. Plovdiv 10 showed moderate behavior. In the primary and in the first trifoliate leaf of all genotypes studied, drought stress induced an increase in the minimal chlorophyll fluorescence (F0), accompanied by a decrease in the maximal one (Fm). Cv. Prelom was less affected. The Fv/Fm ratio practically was not changed and showed a slight tendency to decrease in all genotypes. Cv. Dobrudjanski ran presented the highest decrease (52% and 43%) in photochemical quenching (qP), in contrast to cv. Prelom (29% and an 18%) in primary and first trifoliate leaves, respectively. The quantum yield of electron transport (Y) strongly decreased in cvs. Dobrudjanski ran and Plovdiv 10, while in cv. Prelom Y it was less affected. At the end of the drought period, in the primary and first trifoliate * Corresponding author, e-mail: [email protected]