Whole‐tree mesophyll conductance reconciles isotopic and gas‐exchange estimates of water‐use efficiency
Teresa E. Gimeno, Courtney Campany, John E. Drake, Craig V. M. Barton, Mark G. Tjoelker, Nerea Ubierna, John D. Marshall
Abstract
Summary 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 (δ 13 C) 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 δ 13 C of the phloem (δ 13 C ph ). We measured gas exchange, online 13 C‐discrimination, and δ 13 C 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 δ 13 C ph agreed with iWUE from gas exchange, but only after incorporating g m . δ 13 C ph was also correlated with whole‐tree 13 C‐discrimination, but offset by −2.5 ± 0.7‰, presumably due to post‐photosynthetic fractionations. We conclude that δ 13 C 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.