Model-based control of concentration of nutrient solution in glasshouse tomato cultivation

A strategy to control the concentration of the nutrient solution, derived f rom models of photosynthesis and transpiration, was tested in a greenhouse experiment with tomato grown in rockwool in a closed growing system. The se t point for the concentration of the nutrient solution (electrical conducti vity) was calculated and adjusted once a day, as a function of the water va pour pressure deficit inside the greenhouse. This strategy was compared wit h a constant low, and a constant moderate concentration of the nutrient sol ution in the root environment of 2 dS m(-1) and 4 dS m(-1). Changes in the concentration of the nutrient solution supplied were necessary to achieve t hese concentrations. Electrical conductivity of the nutrient solution suppl ied during the growing period ranged, in the low concentration treatment fr om 1.2 to 1.8 dS m(-1) (mean 1.5 dS m(-1)), in the moderate concentration t reatment from 1.9 to 2.8 dS m(-1) (mean 2.2 dS m(-1)) and in model based co ncentration treatment from 1.1 to 3.4 dS m(-1) (mean 2.2 dS m(-1)). No diff erences in yield on either a fresh or a dry weight basis were observed betw een the moderate and model-based treatments. However, the percentage of fru it with blossom-end rot was significantly lower in the model-based treatmen t. The low concentration treatment resulted in the least incidence of bloss om-end rot, and in increased yield related to the decreased dry matter cont ent of the fruit.