Growth and Phosphorus Concentrations of Centrosema pubescens Benth in Flowing Solution Culture under Differing Defoliation Regimes

Abstract

An experiment was carried out to investigate the effect of several defoliation regimes on growth and tissue P concentration of centro grown in flowing nutrient solution at eight constant solution P concentrations, viz. 0.06, 0.24, 0.7, 3, 9, 26, 87 and 264 m. Eight flowing culture units were employed, each being randomly assigned a solution P concentration. Within each unit four methods of defoliation x three cutting intervals x three replications were established using which using a randomized block design. Phophorus treatments were imposed 13 days after pregerminated seeds were transferred into the flowing culture units which contained basal nutrients only. Defoliation treatments were initiated 29 days later. At this time, plants were cut down to stubble heights of 2.5, 7.5 and 12.5 cm, while in a fourth treatment all leaves were removed from the plants. Cutting intervals of 7, 14 and 21 days were imposed on plants in each of the four defoliation treatments over the 42 days experimental period which followed the initial defoliation. These cutting intervals resulted in 6, 3, and 2 harvests respectively. Dry matter were obtained at the final harvest only. Maximum dry matter yields in all defoliation treatments and in non-defoliated control plants were obtained at 3 MP. Total dry matter production decreased with lowered cutting height, the lowest top and root yields being consistenly obtained at all solution P concentrations and all cutting intervals in the 2.5 cm. cutting height and complete defoliation treatments. The effects of cutting height were particularly marked at the 7-day cutting interval. Best growth of defoliated plants was achieved with a cutting interval of 21 days and a cutting height of 7.5 or 12.5 cm. Mean rate of P absorption were depressed by the most severe defoliation treatments, viz. 2.5 cm. cutting height and the complete defoliation treatments. Death of some plants occurred in these treatments. The O concentration in the regrowth and whole tops of defoliated plants were always higher than those in the tops of non-defoliated control plants. Although P concentrations in the regrowth and whole tops increased with lowered cutting height following a single defoliation, subsequent defoliations decreased P concentration in the regrowth, particularly at the lower cutting heights.