CHANGES IN GROWTH AND RADIATION USE BY LETTUCE CROPS IN RELATION TO TEMPERATURE AND ONTOGENY

A crop of lettuce was grown within two polyethylene-covered tunnels along each of which a temperature gradient was imposed. Between transplanting and harvest maturity the mean temperature differed by up to 5.5 degrees C along the tunnels. Initial increase in plant dry weight was more rapid in the warmer plots. Similarly, maximum ground cover was attained earlier in warmer plots. However, dry weights at the final harvest were greater in plants from cooler plots. Accumulated dry matter was linearly related to accumulated total intercepted radiation, and this relationship differed among plots (P < 0.001). When these data were analysed over each harvest interval, the slope of this relationship, the radiation conversion coefficient, varied systematically during vegetative growth (P < 0.001). Estimates of the radiation conversion coefficient compared after set thermal periods among all plots increased progressively with thermal time from 0.33 g MJ(-1) soon after transplanting to a peak of 1.39 g MJ(-1) after 400 degrees Cd, and thereafter declined progressively to a minimum of 0.33 g MJ(-1) at 650 degrees Cd. Reanalysis of published data for both lettuce and other field crops revealed further examples in which similar ontogenetic trends in radiation conversion coefficient were apparent.