Allelopathic effects of white clover (Trifolium repens L.) on pasture species in high country environments

Abstract

The allelopathic activity of white clover Trifolium repens L. 'Grasslands Huia' on the germination and establishment of pasture species frequently used in oversowing was investigated in the laboratory and in a range of high country environments.

In the laboratory, extracts of white clover shoot material were capable of reducing the germination of the seed of species commonly used in oversowing. Seeds germinating in the presence of white clover shoot material often developed abnormally, with root, root hair and shoot growth being reduced in legumes and root growth reduced in grasses. The allelo-chemicals responsible for these effects were not identified but it is suggested that they may include plant phenolics. The use of a phenolic adsorbent was able to remove up to three-quarters of the observed allelopathic effect, with a consequent increase in seedling vigour.

In the field, the addition of white clover shoot material to the soil surface both promoted and inhibited the establishment of pasture species from seed. These responses depended upon species and moisture status of the site. At the highest rate of applied shoot material (4 t ha⁻¹), inhibition was more common than promotion. At the driest site, white clover shoot material severely inhibited seed establishment, and seedling and plant survival. Legumes were generally more affected than grasses. Allelopathic effects became less as the moisture status of sites improved. At these sites, the addition of shoot material tended to initially inhibit establishment at high rates, but seedling and plant vigour were eventually stimulated by these additions. In all situations, the auto-allelopathic effect of white clover shoot material inhibiting the establishment and survival of white clover seedlings was the most inhibitory system studied. The level of allelochemical production by white clover plants was monitored over a wide range of high country environments. Levels of allelochemicals tended to increase in plants of poor vigour in environments experiencing severe moisture and temperature stresses. It is unlikely that the level of allelochemicals in white clover shoot material from most high country environments would differ from that of lowland white clover.

These effects are discussed in relation to the possible interactions between the rate of allelochemical release and degradation, mineralisation of nitrogen and mulching. The implications of white clover's allelopathic mechanism to high country oversowing situation are also discussed.