Reduction of Lettuce Tipburn by Shortening Day/night Cycle

Abstract

Lettuce tipburn is a calcium-related physiological disorder. Tipburn occurs mainly in inner developing leaves which have lower calcium concentrations and release of latex accompanied by rupture of laticifier in the leaves is responsible for tipburn. Shortening day/night cycles was tested to reduce injury of lettuce tipburn. Growth experiment was carried out to compare development of tipburn under 3- and 24-hour day/night cycles with the same total integrated light intensities in the cultivating period. The 24-hour cycle had a 14-hour light period and a 10-hour dark period and the 3-hour cycle had a 105-minute light period and a 75-minute dark period. Lettuce plants were exposed to the different day/night cycles from the 21st day to harvesting time. Air temperatures in the light period and dark period were 24°C and 20°C, respectively, and RHs were 85% and 90%. PPFD was 264μmol m-2s-1, and CO2 concentration was 1, 500μmol mol-1. Tipburn development was slower at the 3-hour cycle than at the 24-hour cycle though growth rate was the same for both cycles. Time course in turgor pressure in laticifiers in young leaves was measured using the thermocouple psychrometer technique under the two day/night cycles. Maximum turgor pressure in the light period was significantly lower at the 3-hour cycle than at the 24-hour cycle. The results show that frequent repetitions of shorter light and dark periods without changing the total light period decrease the accumulation of metabolites in the latex, lower excessive turgor pressure in laticifiers, and as a result, reduce tipburn without sacrificing a rapid growth rate.