Effects of irrigation interval and planting density on biomass yield and chemical composition of nightshade (solanum retroflexum) in Limpopo Province, South Africa

Abstract

Nightshade (Solanum retroflexum Dun.) is among the most important indigenous leafy vegetables in Vhembe District, Limpopo Province, South Africa, due to its high values of beta-carotene, vitamin E, folic acid, ascorbic acid, calcium, iron and protein. Vhembe District occurs in the tropical regions of Limpopo Province and the production of vegetables is dependent upon the availability of irrigation water. An Integrated Drip Irrigation System (IDIS) and a 3S planter were developed to save water by planting several plants/hole of drip irrigation system. The subsystems in IDIS allow for the production of different crops with different water requirements, whereas the 3S planter can be used for planting from one to nine plants/hole of drip irrigation system. Also, the subsystems could be used in assessing irrigation interval for crops under various planting densities. The interaction of irrigation interval and planting density of S. retroflexum had not been documented. The objective of this study, therefore, was to determine the interactive effects of irrigation interval and planting density on biomass yield and chemical nutrient elements (summer harvest only) of S. retroflexum under field conditions. The irrigation interval and planting density/hole were arranged in a split-plot experimental design, with eight replications. The main plot was irrigation interval and the subplot was the planting densities. Harvesting was done twice for both summer and winter experiments. The first harvest (H1) was done at 6 weeks after transplanting, with the second harvest (H2) being done at six weeks after the first harvest. Fresh shoots were oven-dried at 60°C for 72 h for the determination of dry matter. Mature leaves were powdered and analysed for mineral content (Ca, P, K, Mg, Na, Fe, Zn, Mn and Cu) using the ICPE-9000. Data were subjected to analysis of variance using SAS software. In the summer experiment, the interaction was significant (P ≤ 0.05) for dry shoot mass at H1 and H2. However, the contribution of xv

the interaction in the total treatment variation (TTV) of the variable was negligent and therefore, only single factors were reported. Irrigation interval and planting density had highly significant (P ≤ 0.01) effects on plant variables during H1 and H2 in summer and winter. However, irrigation interval effects for dry shoot mass were not significant for summer H2. Interaction effects were significant for Ca, P, K, Mg, Mn and Cu in leaf tissues during summer H1, but were not significant for Na, Fe and Zn. Also, irrigation interval was significant for Ca, Mg, P, K, Na, Fe, Zn, Mn and Cu during summer H1, whereas planting density had no significant effects for all chemical nutrients except for Ca, P and K during summer H1. Dry shoot mass of S. retroflexum increased linearly with increasing irrigation interval and planting density. Results suggested that most nutrient elements increased with deficit irrigation water and higher planting density, whilst P decreased under high planting density. The study showed that there is a high potential for saving water through longer irrigation intervals and produce good high yields at a higher planting density. In conclusion, the use of IDIS and 3S planter to promote growth and accumulation of essential nutrient elements on S. retroflexum demonstrated that longer irrigation interval and higher plant density per drip irrigation hole could be suitable for cultivation of this indigenous vegetable. The recommendation of this study is that higher planting density and longer irrigation intervals are key determinants of higher biomass yield and water saving strategies for large-scale production of the crop. Further, the mineral composition of the crop was under the influence of higher planting density and irrigation intervals.