Incontinence in aging leaves: deteriorating water relations with leaf age in Agastachys odorata (Proteaceae), a shrub with very long-lived leaves
Gregory J. Jordan A B and Timothy J. Brodribb AA School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia.
B Corresponding author. Email: greg.jordan@utas.edu.au
Functional Plant Biology 34(10) 918-924 https://doi.org/10.1071/FP07166
Submitted: 28 June 2007 Accepted: 2 August 2007 Published: 13 September 2007
Abstract
This paper examines physiological characteristics of the leaves of Agastachys odorata R.Br., 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 10 years of age. Maximum carbon assimilation peaked 1 year after full leaf expansion, 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 five 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 appears to be a consequence of accumulated damage and may contribute to the need for leaf senescence in evergreen species with little self shading.
Additional keywords: carbon assimilation, cuticular conductance, epidermal conductance, sclerophylly, water use efficiency.
Acknowledgements
The authors thank Clare Brooker for technical assistance.
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