Impact of variable [CO2] and temperature on water transport structure–function relationships in Eucalyptus

Nearly 30 years ago, Whitehead and Jarvis and Whitehead et al. postulated an elegant mechanistic explanation for the observed relationship between tree hydraulic structure and function, hypothesizing that structural adjustments promote physiological homeostasis. To date, this framework has been nearly completely overlooked with regard to varying atmospheric carbon dioxide ([CO2]). Here, we evaluated Whitehead's hypothesis of leaf water potential (Ψl) homeostasis in faster-growing (Eucalyptus saligna) and slower-growing (Eucalyptus sideroxylon) tree saplings grown under three [CO2] (pre-industrial, current and future) and two temperature (ambient and ambient + 4 °C) treatments. We tested for relationships between physiological (stomatal conductance and Ψl) and structural (leaf and sapwood areas (Al, As), height (h), xylem conductivity (ks)) plant variables as a function of the [CO2] and temperature treatments to assess whether structural variables adjusted to maintain physiological homeostasis. Structural components (Al, As, h) generally increased with [CO2] or temperature, while gs was negatively correlated with [CO2]. Contrary to Whitehead's hypothesis, Ψl did not exhibit homeostasis in either species; elevated temperatures were associated with more negative Ψl in faster-growing E. saligna, and less negative Ψl in slower-growing E. sideroxylon. Moreover, individual structural variables were generally uncorrelated with Ψl. However, across both species, the integrated hydraulic property of leaf specific hydraulic conductance (Kl) was positively correlated with an independent calculation of Kl determined exclusively from leaf physiological variables. These results suggest that physiological homeostasis may not apply to saplings exposed to global change drivers including [CO2] and temperature. Nevertheless, Whitehead et al.'s formulation identified Kl as a sensitive measure of plant structural–physiological co-variation across species.