EFFECTS OF WATER AVAILABILITY AND VINE HARVESTING FREQUENCY ON THE PRODUCTIVITY OF SWEET POTATO IN SOUTHERN MOZAMBIQUE. I. STORAGE ROOT AND VINE YIELDS

Abstract

In Mozambique the sweet potato (Ipomoea batatas) is often grown as both a leafy vegetable, the terminal shoots or vines being progressively harvested during the season, and as a root crop. This paper reports the results of experiments designed to evaluate the effects of drought and vine harvesting frequency on the productivity of both yield components (cv. TIS 2534). Experiments were conducted during the rainy and dry seasons, with supplementary irrigation treatments superimposed. As the frequency of vine harvesting (equivalent to the number of harvests) increased, the total fresh weight of vines increased. There was a corresponding reduction in the yield of storage roots, however, particularly under well-watered conditions. As a result, the total harvested yield (vines plus roots) was remarkably stable in both wet and dry seasons (43–45 t ha−1 fresh weight). The cumulative dry weight of harvested vines increased with the number of harvests at constant rates, depending on water availability (from +150 to +250 when rain-fed, up to +340 to +440 kg ha−1 harvest−1 when irrigated). The corresponding rates of reduction in storage root yields varied from −3 to −130 (rain-fed) down to 310 to 400 kg ha−1 harvest−1 (irrigated). Total dry weight yields under well-watered conditions were constant at about 11 to 13 t ha−1, or double this on an annual basis. Irrigation water-use efficiencies (by dry weight of harvested crop) were generally higher in the dry season than in the rains. For vine production they increased with the number of harvests from 1.6 to 3.5 (rains) up to 0.9 to 6.7 kg ha−1 mm−1 (dry season). The corresponding values for root production were 7.5 to 13.1 (rains) and 12.2 to 19.1 kg ha−1 mm−1 (dry season). For the combined dry weight yields the water-use efficiencies were, with one exception, independent of harvesting frequency at 11.2 (rains) and 19.0 kg ha−1 mm−1 (dry season). Irrigated plants harvested at weekly intervals yielded vine dry weights of about 0.5–0.6 t ha−1 week−1. Compensatory vine growth (reported elsewhere) was observed in previously droughted plants following a rainfall event. The practical implications of these results are discussed. Future papers describe in more detail the physiological aspects of the observed yield responses.