Effects of environmental factors on the growth of two varieties of subterranean clover (Trifolium subterraneum L.)

A water culture experiment is described in which the effects of day and night temperature, photoperiod, light intensity, and phosphate level on the total growth (T + R) and top/root ratio (T / R) of the Dwalganup and Mt. Barker varieties of subterranean clover were studied. The individual or combined effect of increases in photoperiod and in light intensity was to increase (T + R) in both varieties. With Dwalganup the relative increase in top growth was greater than that in root growth, whereas with Mt. Barker the reverse occurred. With a 16-hr photoperiod varietal differences in the response of (T + R) to day temperature occurred depending on the level of phosphate in the solution. In general, T / R increased with day temperature, although the magnitude of the increase was affected by photoperiod, phosphate level, and variety. The response of total growth (T + R) in each variety to phosphate was independent of day temperature at an 8-hr photoperiod, but at 16 hr it was greatest at 17°C for both varieties. The growth (T + R) response to phosphate was greater for Dwalganup than Mt. Barker, the condition favouring the greatest difference being high light intensity, or a 20°C day temperature combined with long photoperiod. These varietal differences in total growth due to different level of phosphate in the presence of other variables were associated with differential effects on the relative top/root ( T / R ) growth within and between the varieties. Different night temperatures had no differential effect on total growth of plants at an 8-hr photoperiod, but at 16 hr there were highly significant interactions between day and night temperature in both varieties. The sense of the interaction was reversed by an increase in phosphate level in the substrate. Runner stem development was a more sensitive indicator of the interaction than total growth (T + R).