Growth of Lettuce, Onion and Red Beet. 2. Growth Modelling

Data from a field experiment carried out on growth of lettuce, onion, and red beet were used: ( a) to fit logistic, Gompertz, expolinear and ‘Scaife and Jones’ ( Journal of Agricultural Science, Cambridge 86: 83–91, 1976) functions using time, day-degrees and effective day-degrees; and ( b) to test a mechanistically-based model that combines the effects of potentially limiting variables, such as temperature and light, and allows for plant zone area in light interception (Aikman and Benjamin, Annals of Botany 73: 185–194, 1994). The use of day-degrees and effective day-degrees instead of time, in general, improved the fit and gave a better estimate of growth parameters. The best fit was obtained by the Gompertz function for lettuce, and by the expolinear function for red beet and for onion. The expolinear function seemed the most reliable function in estimating the early relative growth rate which is the crucial value in all the mechanistic models. The zone area model showed very good simulations for lettuce and red beet, but it requires a modification for canopy senescence in onion.