EFFECTS OF PHOTON FLUX-DENSITY ON CARBON PARTITIONING AND RHIZOSPHERECARBON FLOW OF LOLIUM-PERENNE

The distribution and partitioning of dry matter and photoassimilate of Lolium perenne was investigated under two light regimes providing pho tosynthetically active radiation of 350 mu mol m(-2) s(-1) (low light treats ment) or 1000 mu mol m(-2) s(-1) (high light treatment), Plants were grown at specific growth conditions in either soil or sand micro cosm units to follow the subsequent release of carbon into the rhizosp here and its consequent incorporation into the microbial biomass (soil system) or recovery as exudates (sand system). The distribution of re cent assimilate between the plant and root released carbon pools was d etermined using (CO2)-C-14 pulse-chase methodology at both light treat ments and for both sand-and soil-grown seedlings, A significant (P les s than or equal to 0.05) increase in partitioning of C-14- label below -ground occurred for both soil-and sand-grown seedlings at the increas ed light treatment. Incorporation of recent assimilate into the microb ial biomass, however, was unaltered by light treatment, Total plant bi omass of L. perenne seedlings grown in the sand microcosm unit was una ffected by light treatment, but differences in partitioning of biomass did occur resulting in an increased root-to-shoot ratio under high li ght. Soil-grown L. perenne seedlings showed a large (81%) and signific ant (P less than or equal to 0.001) increase in shoot biomass under hi gh light with a consequent decrease in root-to-shoot ratio, Specific l eaf area was altered in the soil grown plants only, where it decreased under high light, Total organic carbon (TOC) content of the recovered exudate material was measured throughout the 14 d experimental period and during the C-14-chase period. Comparison of plant C budgets using these two measurements is discussed.