Diurnal and seasonal changes of leaf lamina hydraulic conductance in bur oak (Quercus macrocarpa) and trembling aspen (Populus tremuloides)

The aim of this study was to examine the diurnal and seasonal variations in the sensitivity of leaf lamina (K _lam) hydraulic conductance to irradiance in bur oak (Quercus macrocarpa Michx.) and trembling aspen (Populus tremuloides Michx.), which vary in their responses of K _lam to irradiance. K _lam was determined using the high-pressure method and the measurements were carried out in June, August and September. The irradiance response of K _lam in bur oak was present throughout the day and declined in senescing leaves. In trembling aspen, K _lam declined from morning to late afternoon and drastically decreased before the onset of leaf senescence, but it was not sensitive to irradiance. In both tree species, the capacity of the petioles to supply water to leaf lamina changed during the day in accordance with the ability of the leaf lamina to transport water. Petiole hydraulic conductivity (K _pet) declined during the season in bur oak leaves, while it tended to increase in trembling aspen leaves. There was no correlation between the K _lam values and air temperature or light intensity at the time of leaf collection. For trembling aspen, K _pet was negatively correlated with the air temperature suggesting sensitivity to drought. We conclude that the water transport properties of petioles and leaf lamina in the two studied tree species reflect their ecological adaptations. Trembling aspen leaves have high hydraulic conductivity and high stomatal conductance regardless of the irradiance level, consistent with the rapid growth and high demand for water. In contrast, the increased lamina hydraulic conductivity and stomatal conductance under high irradiance in bur oak trees reflect a water conservation strategy.