Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic delta C-13 discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in delta C-13 between carbon pools along the plant axis and in respired CO2. Little is known about the within-plant variation in delta C-13 under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in delta C-13 of water soluble organic matter (delta C-13(WSOM)) of leaves, phloem and roots, as well as dark-respired delta(CO2)-C-13 (delta C-13(res)) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs-Halimium halimifolium and Rosmarinus officinalis-and a woody invader-Acacia longifolia). Spatial patterns in delta C-13(WSOM) along the plant vertical axis and between respired delta(CO2)-C-13 and its putative substrate were clearly species specific and the most delta C-13-enriched and depleted values were found in delta C-13 of leaf dark-respired CO2 and phloem sugars, similar to-15 and similar to-33 aEuro degrees, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem delta C-13(WSOM) and delta C-13(res) varied by up to 4 aEuro degrees between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using delta C-13 signatures to trace changes in photosynthetic discrimination.