EFFECTS OF WATER AVAILABILITY AND VINE HARVESTING FREQUENCY ON THE PRODUCTIVITY OF SWEET POTATO IN SOUTHERN MOZAMBIQUE. IV. RADIATION INTERCEPTION, DRY MATTER PRODUCTION AND PARTITIONING

Abstract

In Mozambique, the sweet potato (Ipomoea batatas) is grown both as a leafy vegetable, the terminal shoots or vines being progressively harvested during the season, and as a root crop. It is produced in the dry season, in areas with a high water table or with irrigation, and in the rainy season. This paper reports the results of measurements made during the 1995 dry season to evaluate the effects of water availability and the frequency of vine harvesting on the processes of radiation interception, dry matter production and partitioning. An irrigated crop (cv. TIS 2534) with a single, end of season vine-harvest (H₁) intercepted 71% of the total solar radiation, reducing to 52% with weekly (H₄) harvests. The corresponding values for a rain-fed crop were 33 and 20% respectively. When the leaf area index (L) exceeded 3–4, virtually full interception of photosynthetically active radiation (PAR) occurred, except when frequent vine-harvests modified the architecture of the leaf canopy. The extinction coefficient (k, PAR) was highly variable: at a given leaf area, interception was greatest earlier in the season, with infrequent harvests and with irrigation. Total dry matter production (vines, foliage and storage roots), over a period of 161 days, reached 23 t ha⁻¹ in the single harvest (H₁), irrigated crop, less when the vines were harvested weekly (H₄). The corresponding value for the rain-fed crop, which was able to resume active growth immediately after a rainfall event, following a prolonged dry period, was 7 t ha⁻¹. The efficiency of conversion of PAR into dry matter (ε_s) was 2.74 g MJ⁻¹ in the irrigated treatments combined, decreasing to around 73% of this without irrigation. The value of ε_s appeared not to be influenced by dry air or high temperatures. The reduction in ε_s due to drought was less than the corresponding reduction in intercepted PAR. Increasing the frequency of vine harvests increased the partitioning of assimilates to vines. Irrigation had similar effects early in the season, but afterwards it encouraged preferential growth of the storage roots. The end of season harvest index (h) for vines increased with harvesting frequency from, for example, 0.08 (H₁) to 0.38 (H₄) in the irrigated crop. By comparison, irrigation increased ‘h’ for storage roots from 0.24 (rain-fed) to 0.57 in the H₁ treatment. The total harvest index (vines plus roots) increased with the number of vine harvests from 0.32 (H₁) to 0.55 (H₄) in the rain-fed crop, and from 0.62 (H₁) to 0.72 (H₄) in the irrigated crop. Clearly, ‘h’ is not a conservative parameter in the sweet potato, but is sensitive to crop management practices and to soil water availability.