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Diurnal changes in microclimate in a greenhouse are often greater than changes in daily averages over weeks or months. Thus, one may hypothesize that changing the nutrient solution concentration supplied to plants at intervals less than one day would improve tomato yield and quality. To test this hypothesis research was conducted to compare four nutrient control strategies for their effects on plant growth, fruit yield, fruit quality, and root characteristics of `Counter' tomato [ Lycopersicon esculentum  (L.) Mill.]. The four strategies were 1) EC variable , adjustment of nutrient solution electrical conductivity (EC) at 15-min intervals according to greenhouse microclimate over the previous 15-min and empirical models of photosynthesis and transpiration; 2) EC daily , daily adjustment of nutrient solution EC based on each morning's 24-hour weather forecast; 3) EC 3.7 , supply of a single high nutrient solution of 3.7 dS·m -1 ; or 4) EC 1.5 , low nutrient solution EC of 1.5 dS·m -1  for the entire growth period. Mean effluent EC levels were 1.8 dS·m -1  for treatment EC 1.5 , 5.1 dS·m -1  for treatment EC 3.7 , 3.6 dS·m -1  for treatment EC daily,  and 3.4 dS·m -1  for treatment EC variable . Except for fresh weight (FW) of roots, growth characteristics did not differ significantly among treatments. Total production averaged 12.2 kg·m -2  FW and 1.0 kg·m -2  dry weight (DW); and fruit yield averaged 6.7 kg·m -2 . Dry matter content, yield loss to blossom-end rot, and firmness responded linearly to treatment EC. In general, EC daily  yielded higher fruit quality and EC variable  lower fruit quality than that predicted by linear regression. Although our strategy of short-term dynamic changes of nutrient solution EC according to changes in climate variables did not increase yield, daily adjustment of nutrient solution EC improved external fruit quality characteristics and may be practical for grower adoption.

Model-based Control of Nutrient Solution Concentration Influences Tomato Growth and Fruit Quality