Carbon distribution within the plant and rhizosphere in laboratory and field-grown Lolium perenne at different stages of development

Carbon distribution within perennial ryegrass was determined at different stages of plant development, by pulse-labelling laboratory and field-grown plants with 14C-CO2. During the early stages of growth (23–51 days), C distribution of laboratory grown plants was not markedly affected by plant age, with 12.4–24% of net assimilated label lost into the soil as root-soil respiration. The percentage of net assimilate translocated below ground was 20–28% during this stage of growth. At 65 days, the percentage of the label translocated below ground decreased to 8.1% of the net assimilate, with a subsequent decrease in root-soil respiration to 3.9%. The ability of the plant to fix the label (expressed in MBq g−1 oven dry total plant weight) decreased steadily as the plants aged.

When the 30 day old plants were subjected to water stress (soil water potential −1.5 MPa) for 2 days before pulse-labelling, root-soil respiration of the pulse-label decreased compared with plants grown at field capacity.

The distribution of a 14C pulse-label within perennial ryegrass grown under field conditions was found to be dependent on the age of the plants. For 4 week old plants, 67% of net assimilated label was translocated below ground, with 64.8% of this respired by the roots and soil. Less label was translocated below ground at subsequent pulse-labels from weeks 8 to 24. The proportion of label translocated below ground respired by the roots and soil also decreased. The investment of label in the plant shoots was found to be greater in field grown plants as compared to plants of the same age grown in a controlled, laboratory environment.