Effects of Relative Humidity on the Apparent Variability in the Incidence of Tipburn Symptom and Distribution of Mineral Nutrients between Morphologically Different Lettuce (Lactuca sativa L.) Cultivars

The differences in the incidence of tipburned leaves and distribution of mineral nutrients were observed between two morphologically different lettuce cultivars (Lactuca sativa L.) under various relative humidity (RH) conditions at day and night period. The tipburn was observed in the leaf tissue of a butterhead-type lettuce 'Omega', but not in those of a leaf-type lettuce 'Grandrapid'. 'Omega' lettuce grown at 80/80% day/night RH for 15 days showed 2 times more tipburn compared to 80/40% RH treatment. Tipburn occurrence was coincident with the result of the highest relative growth rate (RGR) in 'Omega' lettuce grown at 80/80% RH treatment for 15 days, compared to 40/40, 40/80 or 80/40% RH. However, the high RGR was not associated with the incidence of tipburn in 'Grandrapid' lettuce. In the leaf tissue of 'Omega' lettuce, tipburn symptom occurred 100% under long term exposure at 80/80% RH for 25 days. No tipburn symptom was observed in the leaf tissue of 'Omega' lettuce grown at 40/80% RH treatment for both 15 and 25 days after treatment, in which the lowest RGR at 15 days and the highest RGR at 25 days after treatment were observed. K/Ca ratio of the inner leaves of 'Omega' lettuce was low in the 40/80% RH treatment, but high in the treatment exposed to 80/80% RH for 25 days. The tipburned leaves in 'Omega' lettuce was observed in the ratio of K/Ca higher than 7.0. 'Grandrapid' lettuce had the higher Ca content in the inner leaves compared to those of 'Omega' lettuce, which maintained the range of K/Ca ratio, 5.60-6.68 in the inner leaves. Results indicate that the different pattern in the internal uptake of K and Ca in the morphologically different cultivars may be associated with the cultivar differences in the incidence of tipburn symptom under various humidity conditions. Tipburn symptoms can be alleviated with optimal humidity conditions at day and night period that assists in the distribution of mineral nutrients within the lettuce leaf.