Nutritional water productivity of selected sweet potato cultivars (Ipomoea batatas L.)

Versatile crops such as sweet potatoes have potential to assist in addressing challenges related to food and nutrition security under conditions of water scarcity hence addressing the water-food-nutrition-health nexus. The objective of this study was to determine nutritional water productivity (NWP) of three locally bred sweet potato cultivars (A40, A45 and 199062.1) in response to varying water regimes. The experiment was conducted under controlled environment conditions (similar to 33/18 degrees C day/night and 65% relative humidity). The experimental design was a split-plot with water regimes [30 and 100% crop water requirement (ETc)] as main plots and cultivars (A40, A45 and 199062.1) as sub-plots arranged in randomized complete blocks, replicated three times. Cultivars A45 and 199062.1 are orange-fleshed sweet potato varieties (OFSPs) and A40 is cream-fleshed. Yield and water productivity (WP) were determined at harvest. Thereafter, samples were analyzed for nutrient (energy, protein and fat) and micro-nutrient (beta-carotene, calcium, zinc and iron) content. Results of nutrient content (NC) and WP were then used to calculate NWP. Yield did not vary in response to water regimes nor were there differences (P>0.05) among cultivars. Cultivars 199062.1 (26.4 t ha(-1)) and A45 (16.7 t ha(-1)) out-yielded A40 (14.9 t ha(-1)). Similar to yield, WP did not differ significantly between water regimes and among cultivars. Based on mean values, WP was higher under 30% ETc relative to 100% ETc. This was attributed to yield maintenance under low water availability. Consistent with results of yield, 199062.1 (13.4 kg m(-3)) and A45 (8.8 kg m(-3)) had higher WP than A40 (7.5 kg m(-3)). Results of NWP for energy (E), protein (P) and fat (F) showed no significant differences between water regimes and among cultivars. The trend for NWP ((E, P, F)) was such that 30% ETc>100% ETc and 199062.1>A45>A40 for water regimes and cultivars, respectively. NWP for calcium, zinc and iron mirrored this trend. However, NWP beta-carotene varied significantly (P<0.05) between water regimes and among cultivars. Consistent with other variables, NWP beta-carotene was higher at 30% ETc than 100% ETc. Cultivars A45 and 199062.1 had significantly higher NWP beta-carotene than A40. This confirmed that OFSPs are nutritious and offer greater diversity, especially in areas where vitamin A deficiency is a problem. The fact that NWP did not vary significantly across water regimes implies that OFSPs have potential to contribute to human nutrition in water scarce areas. The use of NWP as a metric allows for an analysis of how agriculture can contribute to food and nutrition security under water scarce conditions.