14C-assimilate partitioning within white clover plant-soil system: effects of photoperiod/temperature treatments and defoliation

Shoots of white clover plants were pulse-labelled with (14)CO(2) in order to establish the partitioning of recently assimilated carbon among the different parts of the plant-soil system with special interest in below-ground compartments. The (14)C-partitioning did not change significantly after 24 h chase period. Leaves and stolons contained more than half of the total radioactivity recovered (%TRR). Soil residues (microbial biomass and non-metabolised rhizodeposits) accounted for 7%TRR and rhizosphere CO(2) was 25%TRR. In order to investigate seasonal effects on assimilate partitioning below ground, we compared low photoperiod and day/night temperature conditions (10-h 20 degrees C/18 degrees C, PTL treatment) with high photoperiod and day/night temperature conditions (16 h-25 degrees C/20 degrees C, PTH treatment). Plants of PTH conditions favoured (14)C-assimilate partitioning to leaves at the expense of storage organs such as stolons and roots. This was supported by distribution of the relative specific activity (RSA) which indicated a significant higher activity of leaves compared to roots. The reduction of (14)C allocated to roots (from 11%TRR in PTL treatment to 7%TRR in PTH conditions) was accompanied by a reduction of (14)C found in rhizosphere CO(2) (from 25%TRR to 12%TRR) and in soil residues (from 7%TRR to 3%TRR). This indicated that rhizodeposition of recently fixed carbon is correlated to C allocation to roots. A moderate defoliation (27% of leaf biomass removed) did not modify (14)C-partitioning within the plant-soil system. A severe defoliation (51% of leaf biomass removed) increased (14)C allocated to remaining leaves from 28%TRR to 37%TRR at the expense of stolons. Partitioning of labelled assimilates to below ground remained unchanged. It is suggested that the age of a plant strongly influences its response to leaf removal.