Stomatal and Mesophyll Resistances in Two Clones of Pinus radiata D.don Known to Differ in Transpiration and Survival Rate

Abstract

The gaseous exchange characteristics of two clones of P. radiata were compared. Clone 457, which had a low survival rate in the field, showed a transpiration rate up to twice that of clone 456, which had a high survival rate. Transpiration in both clones was not markedly affected by leaf-air vapour pressure differences varying between 0.4 and 1.4 kPa.

The stomatal resistance of clone 457 was almost half that measured for clone 456 under all experimental conditions; however, increasing the leaf-air vapour pressure difference resulted in increased stomatal resistances in both clones.

The photosynthetic response to CO₂ concentration was also determined. The measured rates for clone 457 were twice those of clone 456 and both clones gave maximum rates of apparent photosynthesis above about 700 ng cm^-3 internal CO₂ concentration (corresponding to about 1400 ng cm^-3 external concentration). The differences in photosynthetic rate could be accounted for by the greater than twofold difference in mesophyll resistance between the clones. Mesophyll resistance was not affected by altering the leaf-air vapour pressure difference.

It is suggested that a factor in determining the relative survival rate of the two clones is the greater ability of clone 456 to limit transpiration water loss and that this more than compensates for the reduced rate of photosynthesis of this clone compared with clone 457.