Seasonal changes in growth and nodulation of perennial tropical pasture legumes in the field. II. Effects of controlled defoliation levels on nodulation of Desmodium uncinatum and Phaseolus atropurpureus

Abstract

In a field experiment comparing the effects of varying levels of defoliation on the nodulation of two tropical legumes, D. intortum cv. Greenleaf and P. atropurpureus cv. Siratro, five treatments were imposed: (1) control, (2) cutting at 3 in., (3) removal of all leaves and petioles, (4) removal of half the leaves, taking the young leaves, (5) removal of half the leaves, taking the old leaves. The defoliation treatments were imposed twice, at plant age 73 and 103 days, and sampled twice, at 18 and 26 days, after each defoliation. The effects of defoliation were not evident for at least 18 days, but subsequently the nodule weight per plant was reduced by defoliation, the reduction being related to the severity of the initial defoliation. Cutting reduced both the nodule number and weight per nodule in both species. In P. atropurpureus removing all leaves had a similar effect to cutting. In the other defoliation treatments in both species, the weight per nodule declined even though the nodule number increased, and thus the nodule weight per plant increased or remained constant. This provided evidence that changes in nodule weight induced by defoliation were related to a loss of part of the original nodule population and initiation of new nodules.