Effects of solar radiation on growth of cassava (Manihot esculenta crantz.). II. Fibrous root length

Abstract

Effects of solar radiation on the growth of fibrous roots of cassava cv. M Aus 7 were examined by growing plants without shading (control) and under shade cloths which reduced input to 78% and 32% respectively of full solar radiation. Plants were at two different stages of growth when shading treatments were imposed. The experimental results indicate that cassava can develop fibrous root systems rapidly and extensively. Within 50 days of planting, root length density in the 0-20 cm soil layer was 1.6 cm cm-3 and by 84 days it had increased to 3.6 cm cm-3. Maximum root depth also increased from 60-80 cm at 29 days after planting to 100-120 cm by day 50 and beyond 120 cm by day 72. Estimated total fibrous root length per m2 ground area to a depth of 120 cm was 32 km m-2 and 23 km m-2 at the end of the experiment for the two crops, 133 days old and 84 days old, respectively. Shading the plants for 35 days reduced the rate of increase in root length density and hence decreased total root length in the two crops. Reduction in solar input to 32% resulted in a 50% reduction in elongation rate of fibrous roots. Reduction to 78% resulted in a similar reduction in elongation rate when plants were at tuber bulking stage, but much less reduction when plants were at the tuber initiation stage. The results indicate that under limited carbohydrate supply shoot was a stronger sink than both fibrous roots and tubers. At tuber initiation stage tuber was a weaker sink than fibrous roots, but at bulking stage it was about equal to fibrous roots.