Carbon levels drop by 37 % in Arabidopsis leaves during senescence. This drop could result from C mobilization. How- ever, respiration, to provide energy for senescence, is likely to result in significant CO2 loss. As little CO2 assimilation occurs during senescence, respiration can result in a net loss of C (Noodén and Guiamet 1989). It is unclear whether mobiliza- tion or respiration or both are responsible for the observed decrease in C levels.