Effects of defoliation on growth and carbon dioxide exchange of subterranean clover swards

Abstract

From 4 weeks after germination, three defoliation regimes – undisturbed, cut to 3 cm above ground every 4 weeks, or cut to 1.5 cm every week – were imposed on small swards of subterranean clover cv. Tallarook grown at constant 20´C and a 10 hr day length of 23,700 1x intensity.

The treatments induced marked differences in morphology, growth, and survival. Within 16 weeks from sowing 50% of uncut plants had died compared with 5% under weekly defoliation. The rate of ³⁵S uptake was consistently higher in uncut swards, which were estimated to have produced over 60% more dry matter than defoliated swards, yet yielded some 30% less because of high rates of death and decay within them. An optimum leaf area index (LAI) of 2.5–3 for net photosynthesis resulted from the rapid decomposition of dead tissues. Photosynthetic efficiency increased with age in uncut swards, and respiration rates over all treatments were linearly related to total plant weight.

Plants adapted to weekly cutting by developing a prostrate network of stolons with a dense population of sites of leaf production which led to rapid canopy development following defoliation, and increasing rates of ³⁵S uptake as time progressed. Five times as many leaves were harvested as from the uncut treatment and 70% of the herbage was leaf, compared with 25% in uncut swards. After 8 weeks of cutting, the photosynthetic rate of ground cover left following defoliation was about two-thirds of that achieved by complete leaf cover.

The usual confounding of LAI with age, and different susceptibilities of dead tissues within low and high density populations to decompose rapidly, may explain previous conflicting reports of whether or not optimum LAIS for growth exist in clover swards.