δ¹³C_p as proxy for water use efficiency is deceptive in young leaves

The stable carbon isotope ratio of leaf dry matter (δ¹³C_p) has been considered a reliable measure for water use efficiency in C₃ plants. However, it has been reported that there is a shift during a new leaf’s maturation phase. As it is expanding, the young leaf is ¹³C-enriched compared to fully mature leaves. The δ¹³C_p drops through the period of leaf expansion until it has taken on the value of surrounding mature leaves.

In an article published earlier this year in Annals of Botany, lead author Nara O. Vogado and colleagues set out to determine whether the difference between juvenile and mature leaf δ¹³C_p is the same for both temperate and tropical species, and for deciduous versus evergreen species. They also tested whether the changing δ¹³C_p was a sign of decreasing water use efficiency during a young leaf’s maturation.

The researchers found that deciduous trees have a significantly larger δ¹³C_p difference between juvenile and mature leaves than do evergreens. No significant difference was found between tropical and temperate species.

Contrary to expectations based on δ¹³C_p, leaf water use efficiency increased rather than decreased during maturation. The counterintuitive δ¹³C_p value is the result of a combination of low photosynthetic capacity and high daytime respiration rates in young leaves, leading to the addition of ¹³C-depleted photosynthetic carbon from the young leaf to imported carbon from older leaves.

“[O]ne of the most useful applications of leaf δ¹³C_p for C₃ plants is as an indicator of intrinsic water-use efficiency,” write the authors. “The relationship between intrinsic water-use efficiency and mature leaf δ¹³C_p has generally been observed to be robust. However, our analysis suggests that for young vs. mature leaves of C₃ plants, higher δ¹³C_p should not be interpreted as an indication of higher intrinsic water-use efficiency. Indeed, the pattern of change for intrinsic water-use efficiency as C₃ leaves expand is just the opposite.”

They note that further research will be necessary to uncover the mechanism behind the δ¹³C_p differences between the young and mature leaves of deciduous and evergreen trees.