Greenhouse baby leaf production of lettuce and komatsuna in semi-arid climate: Seasonal effects on yield and quality

Greenhouse production of baby salad leaves is a means to provide a clean, safe product for consumers. However, productivity and nutritional quality is not well known especially under semi-arid greenhouse conditions. Three leafy greens, Lactuca sativa 'Red Salad Bowl', L. sativa 'Cimmaron Romaine' and Brassica rapa var. komatsuna were grown during two years (2008-2009) in a pad-and-fan cooled acrylic greenhouse in Southern Arizona using ebb-and-flood irrigation with perlitebased substrate contained in 72-cell plug trays. Nutrient solution was half-strength general purpose hydroponic formulation. Seeds were germinated in a temperaturecontrolled room for four days then moved to greenhouse for three weeks after which it was harvested. Leaves were harvested and the fresh weight recorded on a weekly basis. Carotenoids, ascorbic acid, phenolics and anthocyanins were the phytonutrients measured. Yield and quality were significantly affected by seasonal environment. Fresh weight ranged from 0.08 to 2.18 kg·m^-2 for lettuce and 0.36 to 1.82 kg·m^-2 for komatsuna per 3-week production cycle and was greatest during spring/summer season. Anthocyanin, strongly affected by season, ranged from 4.5 to 121.9 mg·kg^-1 FW for lettuces. Total phenolics showed seasonal variation, concentration ranging from 526.9 to 1914 and 568.4 to 1602 mg·kg^-1 FW for lettuces and komatsuna respectively. Anthocyanin and phenolics concentrations were greatest during winter season on fresh weight basis. Other phytonutrient concentrations were not clearly affected by season. Total carotenoids and ascorbic acid of lettuce grown in our greenhouse were comparable to, but anthocyanins were lower than, levels reported in the USDA database. Yield and quality of leafy greens is satisfactory but could be improved by further optimizing the greenhouse environmental conditions.