Effect of harvest stage and nitrogen fertilization on the postharvest shelf life of black nightshade (Solanum nigrum L.) and collard (Brassica oleracea var. acephala L.)

Leafy vegetables play a crucial role in the human diet providing numerous nutrients and health benefiting compounds. Leafy vegetables like collard (Brassica oleracea var. acephala) and black nightshade (Solanum nigrum l.) are commonly consumed leafy vegetables in Kenya. However, their high perishability and short shelf life (usually 1-2 days at ambient temperature) limits their utilization resulting in significant high postharvest losses. This study assessed the effect of harvest stage and nitrogen fertilization on the postharvest shelf life of collard and black nightshade. Experiments were conducted at Kabete field station, University of Nairobi, using collard and black nightshade. Field experimental layout was a 4 x 3 factorial arrangement in randomized complete block design with three replicates. Factors were nitrogen levels and harvest stage. Four levels of nitrogen (0, 30, 60 and 90 kg N/ha) were applied on black nightshade and (0, 55.5, 111.1 and 166.6 kg N/ha) in collard where 0 kg N/ha was the control. Collard and black night shade were harvested at three harvest stages: 4 weeks, 6 weeks and 8 weeks after transplanting. The harvested vegetables were kept at ambient room condition (20 ºC, 55% relative humidity). Data collection was performed daily for quality related parameters which included color change, wilting index and cumulative weight loss. Results show that there was a progressive deterioration in quality of the collard and black nightshade with storage time regardless of harvest stage and nitrogen level. Harvesting at 8 weeks after transplanting resulted in longer shelf life in collard (three days) and black nightshade (two days) when compared to harvesting at 4 weeks or 6 weeks after transplanting. Collard and black nightshade showed reduced hue angles over storage time at different harvest stages. Black nightshade subjected to 90 kg N/ha and harvested at 4 weeks after transplanting had the highest wilting index of 33%. The highest cumulative weight loss of 29% was recorded in collard that were harvested at 8 weeks after transplanting. Black nightshade subjected to 90 kg N/ha and harvested at 6 weeks after transplanting showed the best color at a hue angle of 145°. Overall, harvesting at 8 weeks after transplanting resulted in the longest shelf life of both black nightshade and collard. These results show that low application of nitrogen fertilizer in black nightshade (30 kg N/ha) and in collard (55.5 kg N/ha) had minimal effects on weight loss and wilting and resulted in good keeping quality.