Improving Romaine Lettuce Production in Greenhouse Hydroponic Systems

Abstract

Due to the high temperature and humidity in the Southeastern U.S. in summer, lettuce grows too fast in the greenhouse hydroponic system. Tipburn and bolting occur severely, damaging lettuce market value. Previous studies have been concentrating on alleviating lettuce tipburn and bolting but have not made great improvements. Root-zone solution cooling to 18C was investigated to slow down lettuce growth. Six Romaine lettuce cultivars and tipburn sensitive ‘Bambi’ lettuce were grown in the greenhouse Nutrient Film Technique system. The results showed that Root-zone cooling significantly slowed down growth rate. Root-zone cooling had no influence on tipburn but alleviated bolting. Physiologically, Ca2+ deficiency readily occurs due to limitation of apoplastic pathway, which is highly dominated by the low transpiration rate of young leaves. Increasing horizontal air flow speed was investigated to speed up transpiration rate. Tipburn sensitive lettuce cultivar ‘Bambi’ was grown to trail the effects of air flow speed of 70m min-1, 140m min-1 and 210m min-1 in the Deep Water Culture system. Tipburn lettuce was counted according to light, moderate and severe levels. Linear trend of increasing light to decreasing severe with increasing air flow speed using model regression at P ChiSq < 0.001 (***) was observed. The optimal air speed was at 140m min-1. Cultivar ‘Newham’, ‘Breen’ and ‘Thinker’ performed better in greenhouse hydroponic production. Total phenolic contents depend on cultivars and environmental conditions.