Root and microbial involvement in the kinetics of C-14-partitioning to rhizosphere respiration after a pulse labelling of maize assimilates

In a previous study, we examined the kinetics of radioactivity evolution fr om rhizosphere respiration after the pulse labelling of maize shoots with ( CO2)-C-14 (Nguyen et al., 1999). The specific activity of rhizosphere respi ration demonstrated two peaks of (CO2)-C-14 production. The first one occur red a few hours after the pulse of (CO2)-C-14 and was followed by a second peak, which took place during the night following the labelling. In the pre sent work, we demonstrate that the second phase of activity occurred in bot h sterile and non sterile plant-soil systems. This was inconsistent with th e results obtained for wheat by Warembourg and Billes (1979) who observed t he second peak solely in the case of non-sterile cultures. These authors su ggested that this second phase of (CO2)-C-14 production was related to micr obial mineralisation of labelled complex compounds. Their synthesis and bre akdown into smaller molecules delayed their utilisation by micro-organisms. However, in the present work, we also demonstrate that the second phase of activity was closely related to photoperiod. When plants were transferred from a 16 h to 20 h photoperiod, the second mineralisation of labelled rhiz osphere compounds occurred sooner after the initiation of the dark period a nd it was strongly attenuated. Therefore, we suggest that the second phase of activity resulted from the utilisation by roots and by micro-organisms o f stored C-14-compounds, which accumulated during the previous light period .