In Vitro Evaluation of Tipburn Resistance In Lettuce (Lactuca sativa L.)

Lettuce (Lactuca sativa L.) tipburn is a physiological disorder caused by calcium deficiency that decreases the crop value. It is a serious problem in controlled environments such as plant factories where vegetables are grown rapidly in an artificial environment. The more rapidly plants grow in appropriate environmental conditions with excessive nitrate fertilizer, the higher the risk of tipburn development. Losses from tipburn can reach up to 50% of lettuce production. Breeding a tipburn-resistant cultivar is the only causal therapy in many cases. However, the methods used to evaluate tipburn vary among different research organization or companies. Universal evaluation of tipburn resistance in cultivation trials is difficult, especially in the field, because many environmental and nutritional factors are correlated with tipburn. 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 Ca²⁺. 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. From average values, the tipburn susceptibility of the lettuce types was in the rank order Leaf Lettuce > Butterhead Lettuce > 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.