TOLERANCE TO LOW LEAF WATER POTENTIAL IN SOYBEAN GENOTYPES

Photosynthetic activity of water stressed plants could be used to assess drought tolerance in soybean (Glycine max L. Merr.). Changes in Photosystem II (PS II) activity during stress integrates the direct effect on PS II activity, analyzed by chlorophyll fluorescence, and responses to decreased CO2 fixation activity due to stomatal closure and photoprotective energy dissipation within the photochemical apparatus. This study analyzed genotypic variation for drought tolerance at low leaf water potentials. Plants were grown in pots under natural or controlled environmental conditions. A PAM modulation fluorometer was used to measure the effects of dehydration on the activity of the photosystems of detached or attached leaves at 35-mu-LL-1 ambient CO2. The fluorescence parameters which were examined, were i. the photosystem II efficiency defined by (Fm-Fo)/Fm where Fm is maximum fluorescence with saturating light and Fo the initial fluorescence, ii. the photochemical fluorescence quenching (Q(P)) and iii. the ratio of the fluorescence decrease, Rfd = (Fp-Ft)/Ft, from the maximum fluorescence Fp to the steady-state fluorescence Ft. As leaf water potential declined, the decrease in these fluorescence parameters differed between 'Hodgson' and 'Kingsoy' and paralleled the decrease in stomatal conductance. The parameter Rdf, which is associated with photosystem activity and CO2 exchanges, was the most discriminating factor of the ability of genotypes to withstand low leaf water potential. Seven genotypes showed a wide variability in the response of Rfd to leaf dehydration. Effects observed on detached leaves under irradiance during dehydration were representative of the response of the attached leaves of plants undergoing a soil water deficiency. Readily and rapidly measurable fluorescence parameters, such as Rfd obtained with the PAM modulation fluorometer at the steady-state fluorescence, thus appear to be valuable selection criteria for drought tolerance in soybean.