Effects of solar radiation on growth of cassava (Manihot esculenta crantz.). I. Canopy development and dry matter growth

Cassava cv. M Aus 7 plants were grown under shade cloths (78% and 32% of full solar radiation) and without shade cloth (control) to examine effects of solar radiation input on crop growth. Plants were at three different stages of growth when shading treatments were imposed. Leaf area development and light interception by the canopies were examined during the experiments, and dry matter samplings were made at the beginning and the end of the treatment periods. Specific leaf area under full sun decreased with ageing of the plants. Low radiation led to leaves with high specific leaf area, particularly in young plants. However, because of reduced leaf dry matter increase, leaf area index was slightly reduced under low solar input. Light measurement in the canopies showed an exponential attenuation of horizontal light flux density with accumulated leaf area index down the canopy. In the canopies with full ground cover, more than 90% of the incident radiation was intercepted by the top half. In such canopies, light extinction coefficients were 0.59-0.76, while the canopy with partial ground cover had a lower value. Coefficients were higher in the canopies under heavy shade, indicating an adaptation in canopy structure to reduced light environment. Reduction in solar input to 78% had significant effects on both tuber and leaf growth (reduction of 86 and 47% respectively) of the younger plants, while it had very little effect on plants in the late bulking stage. Reduction in solar input to 32% reduced crop growth rate to about half that of the control regardless of the plant age. It affected dry matter growth of tubers more than tops, and hence reduced harvest index of plants of all ages. Tuber number was significantly reduced by shading during the tuber initiation stage, but not during the other stages