Deviation Between δ13C and Leaf Intercellular Co2 in Salix Interior Cuttings Developing Under Low Light

Abstract

Leaf δ13C values of perennial species are sometimes less negative than model‐predicted δ13C values derived from instantaneous measures of pi/pa. It has been hypothesized that the less negative δ13C values could be caused by 13C‐enriched stored carbon imported during the early stages of leaf growth. The δ13C values of newly emerging leaves could thus represent δ13C values of stem‐stored carbohydrates and may also provide integral measures of pi/pa at the end of the past growing season. We tested these hypotheses by sprouting cuttings of Salix interior under wet and dry soil‐moisture conditions in a controlled environmental chamber. Plants were defoliated after 56 d, and watering treatments were then reversed for half of the plants in each treatment. The δ13C values of newly emerging leaves did not correlate with pi/pa ratios of newly emerging leaves or of mature leaves prior to defoliation, thereby indicative that δ13C values of newly emerging leaves are not a simple reflection of prior pi/pa. Also, the δ13C values of newly emerging leaves were more enriched in 13C relative to the δ13C values of stem carbohydrates in the treatments where water regimes were reversed. Newly emerging leaves after defoliation had higher δ13C values despite the lower instantaneous water‐use efficiency and similar values of pi/pa to older photosynthetic leaves. Large differences between observed and model‐predicted pi/pa values also occurred in older, more mature leaves, and this may be because large proportions of their total mass were derived from carbon import.