Directional change in leaf dry matter γ13C during leaf development is widespread in C3 plants.

BACKGROUND AND AIMS The stable carbon isotope ratio of leaf dry matter (γ13Cp) is generally a reliable recorder of intrinsic water-use efficiency in C3 plants. Here, we investigated a previously reported pattern of developmental change in leaf δ13Cp during leaf expansion, whereby emerging leaves are initially 13C-enriched compared to mature leaves on the same plant, with their δ13Cp decreasing during leaf expansion until they eventually take on the δ13Cp of other mature leaves. METHODS We compiled data to test whether the difference between mature and young leaf δ13Cp differs between temperate and tropical species, or between deciduous and evergreen species. We also tested whether the developmental change in δ13Cp is indicative of a concomitant change in intrinsic water-use efficiency. To gain further insight, we made online measurements of 13C discrimination (Δ13C) in young and mature leaves. KEY RESULTS We found that the γ13Cp difference between mature and young leaves was significantly larger for deciduous than for evergreen species (-2.1‰ vs. -1.4‰, respectively). Counter to expectation based on the change in δ13Cp, intrinsic water-use efficiency did not decrease between young and mature leaves; rather, it did the opposite. The ratio of intercellular to ambient CO2 concentrations (ci/ca) was significantly higher in young than in mature leaves (0.86 vs. 0.72, respectively), corresponding to lower intrinsic water-use efficiency. Accordingly, instantaneous Δ13C was also higher in young than in mature leaves. Elevated ci/ca and Δ13C in young leaves resulted from a combination of low photosynthetic capacity and high day respiration rates. CONCLUSION The decline in leaf δ13Cp during leaf expansion appears to reflect the addition of the expanding leaf's own 13C-depleted photosynthetic carbon to that imported from outside the leaf as the leaf develops. This mixing of carbon sources results in an unusual case of isotopic deception: less negative δ13Cp in young leaves belies their low intrinsic water-use efficiency.