Agronomic and physiological approaches to improving productivity of selected sweet potato (Ipomoea Batatas L.) cultivars in KwaZulu-Natal : a focus on drought tolerance.
Sweet potato (Ipomoea batatas L) is a resilient food security crop with wide adaptation characteristics and hence can fit well under smallholder production. Its importance as a food security crop in relation to drought is still underestimated and fails to attract sufficient attention from agricultural researchers. The adaptive responses of different sweet potato cultivars to different agro-ecological areas may vary, and sweet potato is an important crop for small-holder farmers in KwaZulu-Natal (KZN) of South Africa, which has diverse agro-ecological areas. Adaptive responses of sweet potato cultivars in KZN’s ecological regions are not known. The possible varying adaptive responses may impact on the food and nutrition security role of sweet potato. This study evaluated the ecophysiology, growth, yield and nutritional composition of three locally bred sweet potato cultivars in response to a range of climates and soils from KZN, South Africa. With the help of smallholder farmers, field experiments were conducted at three sites located in three different agro-ecological areas (Deepdale, Umbumbulu and Richards Bay) of KZN. Agronomic, physiology and yield data were collected. Harvested roots were further analysed for selected nutrients (starch and β-carotene) and other metabolic responses to drought stress. A separate study on physiological and yield response of sweet potato to water stress was conducted under controlled environment. Metabolic analyses were conducted continuously during plant growth. Plant growth, physiological responses and yield were significantly (P ≤ 0.05) influenced by growth environment. Drought stress in Richards Bay resulted in poor plant growth, low yields and low nutritional content (starch and β-carotene content). High temperatures and evapotranspiration (ETo) were associated with drought stress. The other locations (Deepdale and Umbumbulu) where ETo was low showed increased plant growth, yields and nutritional content. The cultivars’ ecophysiology, growth and yield were not suitable for the Richards Bay agro-ecology/bioresource group. They were more suitable for Umbumbulu and Deepdale agro-ecology/bioresource groups. Under controlled conditions, the cultivars adapted to water stress through reduced canopy size. When fully-irrigated, they increased vegetative growth than storage root growth, thus resulted in low storage root yield. This suggested that the cultivars were drought tolerant and suitable for production in marginal areas. Leaf phytochemical content was high in sweet potato leaves compared to other common leafy vegetables. It was even higher in leaves of water stressed plants. This indicated that both leaves and storage roots can be utilised for improved food and nutrition security. Under marginal areas where storage root yield is compromised, the leaves can contribute to food and nutrition security. In areas where rainfall is not limited, communities can benefit from both storage roots and leaves.