Abstract
Lettuce constitutes an important part of greenhouse production and much research has been devoted to investigating appropriate control strategies for that crop. In particular, recent studies have been concerned with avoiding high nitrate content of the harvested crop. This paper supplements previous studies by developing optimal control rules for the application of artificial light (when justified). It utilizes an existing lettuce model, Nicolet, in conjunction with a simple greenhouse model, to explore the control possibilities for hydroponic greenhouses where plant spacing is constant and marketing is limited by quota. Optimal control theory is used to solve the problem. It is shown that the optimal policy for the control of temperature and nitrate supply is unaffected by adding light control. Supplemental lighting starts when the crop coverage of the ground reaches a certain level and then increases continuously up to the installed capacity, if justified. Numerical simulations are used to illustrate this behaviour.
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