SE—Structures and Environment: Fitting the Nicolet Lettuce Growth Model to Plant-spacing Experimental Data

The Nicolet model had been developed to predict the rate of growth and the nitrate content of a greenhouse lettuce crop. This simple, two-state variable model has more than a dozen parameters, whose values have been established by fitting the model to various experimental data sets. The original model describes a continuous crop canopy, which eventually covers the ground completely. In the present study, the original model is modified to improve the treatment of a sparse canopy. Other refinements are also introduced. The modified model is fitted to published data of a lettuce spacing experiment (Scaife & Jones, 1976), in an attempt to identify the parameters which are most responsive to changes in plant spacing. The results show that the limited domain (range of treatments) of the data restricts the number of adjustable parameters to essentially one. This parameter expresses the relative magnitudes of maintenance and growth respiration and is not directly related to the spacing treatments. Adjusting this parameter results in a good fit to growth data for crop densities which span two orders of magnitude (32–3200 plant m−2). None of the density treatments was sparse enough to evaluate the spacing parameter.