Differences in the structure and gas exchange physiology of juvenile and adult leaves in Metrosideros excelsa

In many plant species, ontogeny is characterized by the production of different leaf forms, but the functional significance of this phenomenon is unclear. In Metrosideros excelsa (Myrtaceae), vegetative phase change is characterized by a transition from glabrous juvenile foliage to adult leaves that possess a dense pubescence on their abaxial surface. We examined the changes to anatomical and physiological leaf characteristics that accompany phase change in this species. There was no consistent change to delta C-13 during ontogeny in M. excelsa, indicating that the ratio of intercellular to ambient CO2 remained relatively constant. Rates of photosynthesis were lower in adult versus juvenile foliage, apparently because of a reduction in resources invested toward carbon gain. The stomatal conductance to water vapor tended to decline as the development of pubescence progressed but increased in fully pubescent adult leaves. The stomatal limitation to photosynthetic carbon gain (Ls) exceeded 40% in juvenile and transition leaves but was less than 30% in adult leaves. During vegetative phase change in M. excelsa, the control of water loss appears to shift from a physiological to a physical basis, possibly associated with a reallocation of leaf resources away from photosynthesis.