Apparent controls on leaf conductance by soil water availability and via light-acclimation of foliage structural and physiological properties in a mixed deciduous, temperate forest

Controls on leaf stomatal conductances imposed by soil water availability a nd foliage acclimation to longterm integrated irradiance were studied in a natural mixed deciduous stand composed of shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. Positive relationships between ma ximum stomatal conductance and seasonal integrated average daily quantum fl ux density (Q(int), mol m(-2) d(-1)) were observed in both species, whereas the slope of this relationship declined with increasing soil water limitat ions. There were negative correlations between Q(int) and leaf water and os motic potentials, and stomatal conductances reached in conditions of severe water stress were relatively lower in the upper than in the lower canopy i n both species. Thus, even at a constant soil water availability, foliar wa ter stress increased with increasing seasonal average integrated light in t he canopy. A number of plastic structural and chemical adjustments improvin g the apparent tolerance of water deficits were observed in the foliage alo ng the light gradient. Leaf dry mass per unit area and the content of leaf osmotica per unit area increased, and an estimate of symplasmic leaf fracti on-leaf water contents per unit leaf dry mass-decreased with increasing Q(i nt). In addition to stomatal closure, the leaves had a substantial and rapi d capacity for osmotic adjustment of leaf water potential in response to wa ter stress, and because of light-related foliar modifications, this capacit y was greatest in the upper canopy leaves. Across the whole set of data, th ere was a negative correlation between minimum daily leaf water potential a nd stomatal conductance, because both variables covaried with irradiance. W hen the covariation with light was accounted for by a multiple linear regre ssion analysis, minimum leaf water potential had no significant effect on s tomatal aperture. Instead, stomatal conductance correlated positively with soil water potential in both species. Concentration of xylem sap abscisic a cid (C-ABA) was analyzed to assess its role as a potential messenger of soi l water status. Instantaneous C-ABA was negatively correlated with soil wat er potential, and with stomatal conductance only in the drought-stressed le aves. Thus, there was evidence that stomatal sensitivity to abscisic acid ( ABA) increased with advancing water stress. Tilia cordata, stomata of which were more sensitive to water limitations, also appeared to be more respons ive to abscisic acid. We found a negative correlation between C-ABA and Q(i nt), which possibly resulted from greater transpiration rates at greater Q( int). Since stomatal conductances were more responsive to drought in the up per canopy, where ABA concentrations were the lowest, we suggest that stoma tal sensitivity to ABA is not constant along the canopy light gradient.