The influence of water stress on the growth and development of Townsville stylo (Stylosanthes humilis) in pure ungrazed swards at Katherine, Northern Territory

Abstract

In order to understand more fully the effect of water stress on the growth of Townsville stylo (Stylosanthes humilis) (TS), five drought treatments, imposed by the use of rain shelters, were compared with an unstressed control in a field experiment. Droughts were imposed during the early vegetative (EV) and late vegetative (LV) stages, and during flowering (F) until leaf relative water content at 2.30 p.m fell to 60 per cent. Further unrelieved drought was re-applied to these three treatments when seeding was well advanced. The other two treatments were subjected to unrelieved drought, one starting at late flowering (LF) and the other seeding (S). The drought treatments had no effect on the final yield of dry matter compared with the control. Stress during the vegetative stage reduced growth, but the relief of stress was followed by a period of rapid growth which largely compensated for the loss. However, because the EV stress killed 40 per cent of the sward (the plants were unable to use water from depths below 75-1 00 cm), the compensatory phase in this treatment was less marked, but more sustained. Because of the decline in growth rate with approaching maturity, stresses during and after flowering had little effect on yield. Stress during vegetative growth hastened flowering by two weeks; however, pod yields were only reduced in the LF treatment. In general these observations demonstrate important aspects of the plants' good adaptation to the dry monsoonal tropics, and explain its stable long-term yields when maintained in near-pure swards.