A PHYSIOLOGICAL COMPARISON OF LEAVES AND PHYLLODES IN ACACIA-MELANOXYLON

Leaves and phyllodes of A. melanoxylon were compared in several aspect s of their physiology. Changes in gas exchange and water use efficienc y (WUE) under controlled conditions of vapour pressure deficit (vpd) a nd foliar water potential were examined. Water use efficiency in phyll odes remained constant under a wide range of evaporative demand due to high stomatal sensitivity to vpd. Leaf stomata were less sensitive to changes in vpd causing decreased WUE with increased vpd. Under water stress phyllodes survived longer and produced higher WUE than leaves. Maximum photosynthetic rates per unit foliar area were higher in phyll odes than leaves. Thus, phyllodes have a number of advantages over lea ves under conditions of long and short term water stress and high irra diance. Leaves had a higher photosynthetic rate per unit of photosynth etic investment than phyllodes, suggesting that their function is to m aximise growth during the seedling phase.