Unnerving patterns of hydration in Australian phyllodinous Acacia

Background/Question/Methods Leaves are frequently cited as the main source of resistance to water flow within plants. We examined hydraulic conductance (the inverse of resistance) in Acacia phyllodes (leaves). We measured phyllode traits related to the internal hydration and carbon economy of Australian Acacia growing in three common gardens. We aimed to better understand functional links between the anatomy of the phyllode and flow of water within it. As precipitation has been identified as an important driver for differences in the distribution of the almost 1000 Australian Acacia species, we aimed to determine if phyllode structure and function varied on a precipitation gradient.

Results/Conclusions Phyllode hydraulic conductance (K_phyllode) ranged from 1.1 to 19.7 mmol m^-2 s^–1 MPa^–1, with the highest values in phyllodes with higher mass per area (PMA) (p = <0.001, r² = 0.38). Interestingly, high PMA phyllodes had lower carbon assimilation rates per unit mass (p= <0.001, r² = 0.47) and lower transpiration rates although stomatal pore area per phyllode area did not vary significantly within Acacia. There was no correlation between vessel diameters and K_phyllode. Water flow rates through phyllodes were most strongly related to nerve (vein) patterns. Thicker, denser phyllodes had lower nerve density (p = <.001, r² = 0.40), primarily due to lower density of minor nerves, more major longitudinal nerves per phyllode width (p = <0.001, r² = 0.49), and a more acute angle between branching nerves. The shorter and less tortuous pathway through nerves of high PMA phyllodes may account for their higher K_phyllode. High PMA phyllodes were more common in acacias from areas receiving lower annual precipitation (p = <0.001, r² = 0.55). Maximising efficient water movement through phyllodes may be more important where rainfall is meagre and infrequent, explaining relationships between nerve patterns and the climates of origin in Australian phyllodinous Acacia species.