Incontinence in aging leaves: Deteriorating water relations with leaf age in Agastachys odorata R.Br. (Proteaceae), a shrub with very long-lived leaves

This paper examines physiological characteristics of the leaves of Agastachys odorata, a wet-climate, sclerophyllous shrub with very long-lived leaves. It addresses the hypothesis that cuticles become leakier to water vapour as leaves age. Astomatous cuticular conductance, whole-leaf minimum epidermal conductance, leaf damage and accumulation of epiphylls all increased several-fold with leaf age from first year growth to ten years of age. Maximum carbon assimilation peaked after 1 year, then declined. Intrinsic water use efficiency was highest in mid-aged leaves and declined markedly in the oldest leaves. Stomatal density, stomatal size and cuticle thickness did not vary significantly among ages. The older leaves were less effective at controlling water loss, resulting in decreases in water use efficiency. A differential increase in the conductance of the stomatal surface of the leaves relative to astomatous surface suggested that stomatal leakiness was significant in leaves over 5 years old. Although data for other species is ambiguous, the deterioration in A. odorata appears to be consistent with changes in the oldest leaves of other species. Thus, decreasing ability to use water efficiently may contribute to the need for leaf senescence in evergreen species with little self shading.