Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions
P 3.2 in Ökosysteme: Funktion und Leistungen
Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ13C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes,
leading to differences in δ13C between carbon pools along the plant axis and in respired CO2. Little is known about the within-plant variation in δ13C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ13C of water soluble organic matter (δ13CWSOM) of leaves, phloem and roots, as well as dark-respired δ13CO2 (δ13Cres) 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 d13CWSOM along the plant vertical axis and between respired δ13CO2 and its putative substrate were clearly species specific and the most δ13C enriched and depleted values were found in δ13C of leaf dark respired CO2 and phloem sugars, -15 and -33 %,respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ13CWSOM and δ13Cres varied by up to 4 % between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ13C signatures to trace changes in photosynthetic discrimination.