Diurnal and seasonal variation in the carbon isotope composition of leaf dark‐respired CO₂ in velvet mesquite (<i>Prosopis velutina</i>)

ABSTRACT We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark‐respired CO 2 ( δ 13 C l ) in the C 3 perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south‐western USA. δ 13 C l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night‐time periods to minimum values at pre‐dawn. The magnitude of the diurnal shift in δ 13 C l was strongly influenced by seasonal and habitat‐related differences in soil water availability and leaf surface vapour pressure deficit. δ 13 C l and the cumulative flux‐weighted δ 13 C value of photosynthates were positively correlated, suggesting that progressive 13 C enrichment of the CO 2 evolved by darkened leaves during the daytime mainly resulted from short‐term changes in photosynthetic 13 C discrimination and associated shifts in the δ 13 C signature of primary respiratory substrates. The 13 C enrichment of dark‐respired CO 2 relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre‐dawn (0300 h). The origin of night‐time and daytime variations in δ 13 C l is discussed in terms of the carbon source(s) feeding respiration and the drought‐induced changes in carbon metabolism.