Abstract:
The growth and carbon distribution patterns of perennial ryegrass (Lolium perenne L. cv. Yatsyn1) and phalaris (Phalaris aquatica L. cv. Holdfast) were analysed in response to defoliation and limited nitrogen availability stresses, imposed both individually and in combination. Both species showed similar responses to defoliation and nitrogen limitation when applied individually. Defoliation increased proportional export of carbon from residual leaves and carbon distribution to tillers was increased at the expense of roots. Nitrogen deficiency resulted in increased export of carbon to the root system. These results are consistent with source?sink theory. A key finding from this study was that the species responded differently when defoliation and nitrogen limitation were imposed simultaneously. Under these conditions, carbon distribution patterns in phalaris resembled those seen in plants subjected to nitrogen stress, that is, favouring root growth. The response of perennial ryegrass was to continue to support shoot growth at the expense of roots. These results suggest a physiological basis for the poor survival of perennial ryegrass often observed under continuous grazing in dry regions of southern Australia. Results are discussed in relation to the environments to which each species is adapted, and the management requirements for species persistence in marginal environments.