Whole-tree mesophyll conductance reconciles isotopic and gas-exchange estimates of water-use efficiency

Photosynthetic water-use efficiency (WUE) describes the link between terrestrial carbon (C) and water cycles. Estimates of intrinsic WUE (iWUE) from gas exchange and C isotopic composition (δ¹³C) differ due to an internal conductance in the leaf mesophyll (g_m) that is variable and seldom computed. We present the first direct estimates of whole-tree g_m, together with iWUE from whole-tree gas exchange and δ¹³C of the phloem (δ¹³C_ph). We measured gas exchange, online ¹³C-discrimination, and δ¹³C_ph monthly throughout spring, summer, and autumn in Eucalyptus tereticornis grown in large whole-tree chambers. Six trees were grown at ambient temperatures and six at a 3°C warmer air temperature; a late-summer drought was also imposed. Drought reduced whole-tree g_m. Warming had few direct effects, but amplified drought-induced reductions in whole-tree g_m. Whole-tree g_m was similar to leaf g_m for these same trees. iWUE estimates from δ¹³C_ph agreed with iWUE from gas exchange, but only after incorporating g_m. δ¹³C_ph was also correlated with whole-tree ¹³C-discrimination, but offset by −2.5 ± 0.7‰, presumably due to post-photosynthetic fractionations. We conclude that δ¹³C_ph is a good proxy for whole-tree iWUE, with the caveats that post-photosynthetic fractionations and intrinsic variability of g_m should be incorporated to provide reliable estimates of this trait in response to abiotic stress.