Effects of photon flux density on carbon partitioning and rhizosphere carbon flow ofLolium perenne

Abstract

The distribution and partitioning of dry matter and photoassimilate of Lolium perenne was investigated under two light regimes providing photosynthetically active radiation of 350 μmol m^–2 s^–1 (low light treatment) or 1000 μmol m^–2 s^–1 (high light treatment). Plants were grown at specific growth conditions in either soil or sand microcosm units to follow the subsequent release of carbon into the rhizosphere and its consequent incorporation into the microbial biomass (soil system) or recovery as exudates (sand system). The distribution of recent assimilate between the plant and root released carbon pools was determined using ¹⁴CO₂ pulse-chase methodology at both light treatments and for both sand- and soil-grown seedlings. A significant (P≤0.05) increase in partitioning of ¹⁴C-label below-ground occurred for both soil- and sand-grown seedlings at the increased light treatment. Incorporation of recent assimilate into the microbial biomass, however, was unaltered by light treatment. Total plant biomass of L. perenne seedlings grown in the sand microcosm unit was unaffected by light treatment, but differences in partitioning of biomass did occur resulting in an increased root-to-shoot ratio under high light. Soil-grown L. perenne seedlings showed a large (81%) and significant (P≤0.001) increase in shoot biomass under high light with a consequent decrease in root-to-shoot ratio. Specific leaf 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 ¹⁴C-chase period. Comparison of plant C budgets using these two measurements is discussed.