2018 ASHS Annual Conference

Lettuce (Lactuca sativa L.) is one of the major leafy and salad vegetables. A total of sixty-four lettuce cultivars with green or red leaves in 5 different subtypes (butterhead, crisphead, loose leaf, oakleaf, and romaine) were evaluated for hydroponic culture using the NFT (nutrient film technique) system. Two-week old rockwool-grown seedlings were anchored 6” apart on an NFT system and grown for AN additional 4 weeks with a nutrient solution containing 200 ppm N prepared with a 20-20-20 commercial fertilizer. The lettuce crops reached marketable size within 3-4 weeks of transplanting into the hydroponic system. Performance factors evaluated for each cultivar 4 weeks after transplanting included fresh and dry weights, plant size and shape, stem diameter and length, leaf tissue pH, nitrate (NO₃^-), calcium (Ca²⁺), and sugar contents, and tendency for bolting, molding and leaf tip burns. The cultivars of Adriana, Four Seasons, and Red Cross in butterhead type; Simpson Elite, Green Star, Tropicana, Two Star, Red Sails, and New Red Fire in loose leaf type; 3Sx4203 in oak leaf type; Concept, Magenta, Muir, Nevada, Lettony, and Cherokee in crisphead type; Parris Island, Green Forest, Jericho, Giant Caesar, Parris White, and Outredgeous in romaine type performed well for hydroponic culture on NFT system. Four cultivars (Tango, 3Sx4102, Burpee Bibb, Arctic Crisp) were sensitive to bolting and five cultivars (Tennis Ball, Emerald Oak, 3Sx4403, 3Sx497, 3Sx3410) were identified as developing undesirable leaf tip burns. Evaluation of photosynthetic pigments (chlorophylls, carotenoids, anthocyanin), chlorophyll indices (SPAD) and photosynthetic reflectance index (PRI) associated with lettuce leaf color and type revealed that SPAD values were better indicators of leaf chlorophyll and carotenoid status and PRI values were better indicators of anthocyanin status with a more reliable differentiation between levels. Anthocyanin levels correlated negatively with PRI values. Chlorophyll and carotenoid levels in leaf tissues correlated either positively or negatively, respectively, with SPAD values. The relationship between PRI, SPAD and PSII photochemistry among different lettuce cultivars associated with their leaf color and lettuce type was characterized. Lettuce cultivars greatly varied in their performance under the NFT hydroponic culture system. Information obtained in this study may well be used by greenhouse growers in selecting lettuce cultivars suited for their production practices.