Root distribution in space and time in Trifolium subterraneum

Abstract

Distribution of length and weight of roots of subterranean clover cultivars Northarn and Nungarin were measured in the field throughout a growing season. Plants were grown at seven populations from 2 to 1510 plants per m2 in coarse sand which had a bulk density of 1.4 g cm-3. Root growth was described by its vertical and horizontal distribution as functions of population and time after emergence. Differences in growth between cultivars were small. Root depth, which reached a maximum (plateau) at about 90 days, did not vary with plant population. Root density (length per volume of soil) increased faster at high population due to a higher density of' roots beneath the mainstream and an attenuation with distance away from the mainstem which did not appear to be affected by population. Root densities were 10 cm cm-3 in the uppermost core (0-10 cm depth) and 1.6 -3.5 cm cm-3 throughout the rooting zone (0-50 cm) at 119 days after emergence. Root death, estimated from dichlorotrianzinyl staining, was negligible. Leaf area per plant declined with population above 16 plants per m2; leaf area per unit root length (in cores beneath the mainstream) declined 90-fold with increase in population. Top weight and burr yield per plant at final harvest also declined with increasing population; burr yield per m2 also fell significantly at the highest population. It is concluded that the amount of root per unit area increases with population and that plants appear to adjust conservatively in some respects (to much lower leaf area per root length) while being unable to sustain dry matter partitioning (reducing burr/top yields) at the highest population studied.