Stomatal conductance and transpiration of Acacia under field conditions: similarities and differences between leaves and phyllodes

Leaf diffusive conductance and transpiration rates in response to situations of high evaporative demand were measured in 40 Acacia species varying widely with regard to the morphological and anatomical characters of their assimilatory organs. The measurements took place in south-eastern and central Australia, central Africa and south-western Europe and included species of all three subgenera of Acacia Mill. Soil moisture conditions and consequently the water status of the experimental plants varied between the different measuring sites, some of which were regularly watered. All the species investigated showed a similar daily pattern of diffusive conductance with a morning peak and a subsequent decrease, which was more pronounced in plants growing under water stress, indicating a decisive stomatal regulation of transpiration. A relationship between the structure of assimilatory organs and leaf diffusive conductance or transpiration rates per unit surface area could not be detected in the Australian acacias. However, there are indications that the leaves of the non-Australian species operate on higher conductances than the foliage of the Australian ones. It is suggested that the observed differences in the performance of African and Australian acacias reflect the deciduous or evergreen nature of foliage rather than structural differences. In regard to taxon-specific differentiation this might implicate an ecophysiological character which separates the evergreen species of the geographically isolated subgenus Heterophyllum from the deciduous species of the subgenera Aculeiferum and Acacia with an overlapping area of distribution.