The development and evaluation of techniques to predict when to harvest iceberg lettuce heads

Abstract

Twenty-seven crops of iceberg lettuce, cv. Saladin, were grown in 1988 and 1989 at four sites and sampled twice weekly to study heart growth under a range of environmental conditions. The aim of the work was to develop a technique to predict when heads reached a specified weight or size. Changes in heart fresh weight were best described by a Gompertz curve using effective day-degrees from transplanting as the explanatory variable. Changes in heart diameter were best described by a logistic curve using effective day-degrees from transplanting as the explanatory variable. The accuracy of prediction of the day of maturity was tested on nine independent crops using four mechanistic models describing heart growth and four other empirical models based on the duration of crop growth from transplanting. The deviations of actual from predicted dates of maturity were smaller using the simple empirical models based on days, day-degrees and effective day-degrees from transplanting than using the more general mechanistic models based on heart fresh weight or diameter. This suggests that heart formation and growth is affected in a complex way by environmental conditions during growth.