Temperatures and net energy gain in a growing, mature head of lettuce

A method was developed to determine the net heat gain per unit of time (G) in a mature, growing head of lettuce (Lactuca sativa L., var. capitata from measured temperatures, by adapting a formula used to calculate internal energy changes in columns of air, water, or soil ( Sellers , 1965). G also was calculated for hypothetical cubes (sides 1 cm), one of which represented the cool side of the head, and the other the warm side, and each having a thermocouple at its center. The mean for the two cubes differs from that for the entire head by less than 10%, and avoids the cumbersome determination of the mean temperature of the head. Tipburn of lettuce was discussed in relation to internal differences in G.