Turning over a new ‘leaf’: multiple functional significances of leaves versus phyllodes in Hawaiian Acacia koa

Abstract

Hawaiian endemic tree Acacia koa is a model for heteroblasty with bipinnately compound leaves and phyllodes. Previous studies suggested three hypotheses for their functional differentiation: an advantage of leaves for early growth or shade tolerance, and an advantage of phyllodes for drought tolerance. We tested the ability of these hypotheses to explain differences between leaf types for potted plants in 104 physiological and morphological traits, including gas exchange, structure and composition, hydraulic conductance, and responses to varying light, intercellular CO2, vapour pressure deficit (VPD) and drought. Leaf types were similar in numerous traits including stomatal pore area per leaf area, leaf area-based gas exchange rates and cuticular conductance. Each hypothesis was directly supported by key differences in function. Leaves had higher mass-based gas exchange rates, while the water storage tissue in phyllodes contributed to greater capacitance per area; phyllodes also showed stronger stomatal closure at high VPD, and higher maximum hydraulic conductance per area, with stronger decline during desiccation and recovery with rehydration. While no single hypothesis completely explained the differences between leaf types, together the three hypotheses explained 91% of differences. These findings indicate that the heteroblasty confers multiple benefits, realized across different developmental stages and environmental contexts.

How Does Leaf Anatomy Influence Water Transport outside the Xylem?

“…Previous studies found these criteria to be sufficient for the stabilization of transpiration rate, water potential, and K leaf . Tests for any change in transpiration rate, leaf water potential, and K leaf with measurement time (after stable flow was established) across leaves of a given species for seven species with a wide range of leaf capacitance showed no relationship of K leaf to measurement time (Scoffoni et al, 2008;Pasquet-Kok et al, 2010).…”

“…In particular, the evaporative flux method requires steady-state transpiration and stable leaf water potential to enable the determination of K leaf . We followed the procedure tested and established for a wide range of species in previous work (Scoffoni et al, 2008Pasquet-Kok et al, 2010;Guyot et al, 2012). In measuring K leaf , 30 min was chosen as a minimum to ensure that leaves had acclimated to high irradiance and stomatal conductance had stabilized.…”

Making the best of the worst of times: traits underlying combined shade and drought tolerance of Ruscus aculeatus and Ruscus microglossum (Asparagaceae)

“…Although the detailed functional trait survey conducted here is relatively novel in its breadth (see also Pasquet-Kok et al 2010), this is a logical extension of the traditional approach for understanding the basis for plant adaptation to environment, i.e. testing expectations for individual traits established by previous studies of the functional significance of these traits in other species.…”

“…Such traits include a high water-use efficiency (Wright and Westoby 2003), as well as water storage tissue with high water storage capacitance associated with low bulk leaf modulus of elasticity, high relative water content at turgor loss point, and low cuticular conductance (Sack et al 2003a;Pasquet-Kok et al 2010;Ogburn and Edwards 2012). Traits potentially contributing to shade tolerance include low rates of maximum photosynthetic CO 2 assimilation per leaf area and per leaf mass, low light compensation point, low maximum rate of carboxylation, low maximum rate of electron transport, and more negative carbon isotope ratios (Walters and Reich 1999).…”

Botany, Ecology and Diversity of <i>Acacia koa</i> in the Hawaiian Islands

“…Multiple hypotheses have been made to explain the change from true leaves to phyllodes; true leaves may be more important in earlier developmental fast-growing stages, while phyllodes provide the plant higher tolerance to drought (Walters and Bartholomew, 1984;1990;Hansen, 1986;Hansen and Steig, 1993). Pasquet-kok et al (2010) tested these hypotheses on A. koa and observed that true leaves showed higher massbased gas exchange necessary for growth; on the other hand, phyllodes had characteristics for drought tolerance, which is due to greater capacitance of the water storage tissue and its ability to keep stomata closed under hot and dry conditions. Furthermore, the orientation of the leaf types may support these hypotheses; the true leaves, which spread horizontally, absorb more irradiance within small canopies under shadows, whereas the vertically arranged phyllodes protect the plant from intense sunlight (Walters and Bartholomew, 1984;Hansen, 1986;.…”