Effect of leaf position and age on anatomical structure, photosynthesis, stomatal conductance and transpiration of Asian pear

The effects of leaf position and leaf age on gas exchange, transpiration, stomatal resistance, and anatomical structure of seven-year-old trees of Asian pear cv. Huanghua and Jingshiu II (Pyrus serotina Rehd. cv. culta rehd.) were examined under field conditions. Individual leaves were monitored from unfolding to maturation, and detailed measurements were made on leaves at various node positions and leaf ages. The anatomical structure varied according to the node position of the leaf, with palisade thickness increasing and the thickness of spongy mesophyll decreasing from the base to the apex of the branch. Intercellular space fraction of leaves decreased from the apex to the base of a shoot. ”Huanghua” leaves took 20 days for full expansion while ”Jingshiu II” required 25 days. Net photosynthesis (Pn) and stomatal conductance (Gs) of both genotypes increased with leaf age, particularly in the early stage. Pn reached a maximum value when the leaf was completely expanded. Leaves at different developmental stages from the apical position to the base performed differently. The transpiration rate and the vapor pressure deficit of older leaves on the base of the branch were higher than those of the younger leaves at the apex of the branch. Leaves at node 3 to 16 had higher saturation vapor pressure and transpiration rates. Both the apical and basal leaves had higher stomatal resistance and lower net photosynthesis than leaves in an intermediate position.