Lettuce tipburn is an irreversible physiological disorder caused by calcium deficiency that decreases the crop value. Breeding a tipburn-resistant cultivar is the only causal therapy in many cases. In this study, we investigated an efficient method to evaluate lettuce resistance to tipburn in vitro. Seedlings of 19 lettuce cultivars representing three head types were cultured on agar medium containing EGTA, which chelates Ca 2? . The percentage of tipburned leaves decreased proportionally with EGTA concentration. Susceptible cultivars were distinguished at 0.01 mM EGTA, whereas resistant cultivars were classified at 1.0 mM EGTA. Based on mean values of tipburn measurements, tipburn susceptibility was highest for 'Leaf Lettuce', followed by 'Butterhead Lettuce', and then 'Crisphead Lettuce'. Two cultivars were selected for further tests using hydroponic and pot culture. The rank order of susceptibility to tipburn in these experiments was consistent with that of the in vitro assay. The in vitro evaluation of lettuce susceptibility to calcium deficiency is useful for initial screening of lettuce cultivars against tipburn incidence. Resistant cultivars identified in this study are practical candidates for cultivation in controlled environments, such as a plant factory, while sensitive cultivars are also useful as indicator plants to monitor environmental conditions.

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