Abstract
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 (delta C-13) 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 delta C-13 of the phloem (delta C-13(ph)). We measured gas exchange, online C-13-discrimination, and delta C-13(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 degrees 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 delta C-13(ph) agreed with iWUE from gas exchange, but only after incorporating g(m). delta C-13(ph) was also correlated with whole-tree C-13-discrimination, but offset by -2.5 +/- 0.7 parts per thousand, presumably due to post-photosynthetic fractionations.
We conclude that delta C-13(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.