THE INFLUENCE OF SOLAR-RADIATION AND TEMPERATURE ON THE HEAD WEIGHT OF CRISP LETTUCE

Five experiments in 1986, 1987 and 1988 using cv. Saladin examined the influence of shading, which halved the amount of solar radiation the crop received, on the growth of lettuce heads and head weight at maturity. There were significant positive correlations between head weight and mean solar radiation in the seven-day periods beginning five days before hearting and continuing until 11 days after hearting. There were also significant negative correlations between head weight and mean temperature in the seven-day periods beginning at least 13 days before hearting and continuing until ten days after hearting. This suggests that head weights at maturity are increased by high solar radiation in a specific period beginning just before hearting and by low temperature in a longer period up to and around hearting, though it is accepted that other, as yet unexplained, factors also affect head weight. The Relative Growth Rate (RGR) of plants was positively correlated with solar radiation at hearting and post-hearting and with temperature at hearting. However, there were no significant correlations between RGR at hearting and head weight. It is suggested that as head weight is not associated with the ability of the plant to accumulate dry matter at hearting, it may be influenced by environmental effects on plant morphology, particularly leaf shape, around the time of hearting.