Studies of the Hearting of Butterhead Lettuce: Temperature Effects

Abstract

The effects of temperature on the growth and hearting of butterhead lettuce cv Cobham Green were investigated in two experiments which used respectively a range of sowing dates and soil mulching treatments to provide different soil temperatures. In both experiments the rate of seedling emergence was directly related to mean soil temperature, and in the sowing date experiment alone the early relative growth rate of the crop was positively related to mean soil temperature but not related to air temperature. There were relationships between leaf number and soil temperature 2 cm deep (measured on an accumulated day-degree scale above 0 °C) which were described by two separate straight lines with different slopes, one for data up to and one for data beyond ten leaves, the latter having a higher rate of leaf production. Lettuces sown in mid-May, when the soil temperature was 20 °C, had heavier heads at maturity than those sown earlier or later, but there was no relationship between head weight and early relative growth rate or mean soil temperature during early growth. Changing soil temperature in the same macro-environment by using soil mulches had large effects on early growth but small effects at maturity. The warmest treatments during early growth gave the lightest heads at maturity, suggesting that factors other than temperature alone are responsible for the differences in head weight which occur with different sowing dates.