Can stomatal closure caused by xylem ABA explain the inhibition of leaf photosynthesis under soil drying?

Effects of leaf water deficit and increase in endogenous ABA on photosynthesis of two tropical trees, t Acacia confusa and t Leucaena leucocephala, were investigated with two soil-drying methods, i.e. half or whole root system was subjected to soil drying. Half-root drying was achieved by allowing upper layer of soil column to dry and lower layer of soil column to remain watered. Half-root drying had little effect on leaf water potential, but when compared to the well-watered control, both methods of soil drying substantially increased the ABA concentration in xylem and reduced leaf conductance in both species. There was a significant relationship between leaf conductance and xylem ABA concentrations in both species, which was comparable to the same relationship that was generated by feeding ABA to excised twigs. The rate of photosynthesis was inhibited substantially in both soil-drying treatments and in both species, but photochchemical efficiency, measured as a ratio of variable fluorescence to a peak fluorescence emission of a dark-adapted leaf (F_v/F_m), was not reduced except in the whole root-dried t L. leucocephala plants where leaf water potential was reduced to –2.5 MPa. In all the cases where photosynthesis was inhibited, there was a concomitant reduction in both leaf conductance and calculated internal CO₂ concentration. After two days of rewatering, leaf water potential and xylem ABA concentration rapidly returned to pre-treatment levels, but leaf conductance and photosynthesis of both whole-root and half root dried t L. leucocephala remained inhibited substantially. Rewatering led to a full recovery of both stomatal conductance and photosynthesis in soil-dried t A. confusa, although its photosynthesis of whole-root dried plants did not recover fully but such difference was not significant statistically. These results suggest that drought-induced decline of photosynthesis was mainly a result of the stomatal factor caused by the increase of ABA concentration in the xylem sap. Non-stomatal factors, e.g. reduced photochemical activity and/or carbon metabolic activity, were species-specific and were brought about only at very low water potential.