Oliveira Vogado, Nara O., Winter, Klaus, Ubierna, Nerea, Farquhar, Graham D., and Cernusak, Lucas A. (2020) Directional change in leaf dry matter delta C-13 during leaf development is widespread in C-3 plants. Annals of Botany, 126 (6). pp. 981-990.

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Abstract

Background and aims: The stable carbon isotope ratio of leaf dry matter (delta C-13(p)) is generally a reliable recorder of intrinsic water-use efficiency in C-3 plants. Here, we investigated a previously reported pattern of developmental change in leaf delta C-13(p) during leaf expansion. whereby emerging leaves are initially C-13-enriched compared to mature leaves on the same plant, with their delta C-13(p) decreasing during leaf expansion until they eventually take on the delta C-13(p) of other mature leaves.

Methods: We compiled data to test whether the difference between mature and young leaf delta C-13(p) differs between temperate and tropical species, or between deciduous and evergreen species. We also tested whether the developmental change in delta C-13(p) is indicative of a concomitant change in intrinsic water-use efficiency. To gain further insight, we made online measurements of (13)C( )discrimination (Delta C-13) in young and mature leaves.

Key Results: We found that the delta C-13(p) difference between mature and young leaves was significantly larger for deciduous than for evergreen species (-2.1 parts per thousand vs. -1.4 parts per thousand, respectively). Counter to expectation based on the change in delta C-13(p), 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 (c(i)/c(a))was significantly higher in young than in mature leaves (0.86 vs. 0.72, respectively), corresponding to lower intrinsic water-use efficiency. Accordingly, instantaneous Delta C-13 was also higher in young than in mature leaves. Elevated c(i)/c(a) and Delta C-13 in young leaves resulted from a combination of low photosynthetic capacity and high day respiration rates.

Conclusion: The decline in leaf delta C-13(p) during leaf expansion appears to reflect the addition of the expanding leaf's own C-13-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 delta C-13(p) in young leaves belies their low intrinsic water-use efficiency.

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