Directional change in leaf dry matter delta13C during leaf development is widespread in C3 plants

Background and aims: The stable carbon isotope ratio of leaf dry matter (delta13Cp) 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 delta 13Cp during leaf expansion, whereby emerging leaves are initially 13C-enriched compared to mature leaves on the same plant, with their delta13Cp decreasing during leaf expansion until they eventually take on the δ 13Cp of other mature leaves. Methods: We compiled data to test whether the difference between mature and young leaf delta13Cp differs between temperate and tropical species, or between deciduous and evergreen species. We also tested whether the developmental change in delta13Cp is indicative of a concomitant change in intrinsic water-use efficiency. To gain further insight, we made online measurements of 13C discrimination (Delta13C) in young and mature leaves. Key Results: We found that the delta13Cp 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 delta13Cp, 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 Delta13C was also higher in young than in mature leaves. Elevated ci/ca and Delta 13C in young leaves resulted from a combination of low photosynthetic capacity and high day respiration rates. Conclusion: The decline in leaf delta13Cp 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 delta13Cp in young leaves belies their low intrinsic water-use efficiency