Model-based control of nutrient solution concentration influences tomato growth and fruit quality

Diurnal changes in microclimate in a greenhouse are often greater than chan ges in daily averages over weeks or months. Thus, one may hypothesize that changing the nutrient solution concentration supplied to plants at interval s less than one day would improve tomato yield and quality. To test this hy pothesis research was conducted to compare four nutrient control strategies for their effects on plant growth, fruit yield, fruit quality, and root ch aracteristics of 'Counter' tomato [Lycopersicon esculentum (L.) Mill.]. The four strategies were 1) ECvariable, adjustment of nutrient solution electr ical conductivity (EC) at 15-min intervals according to greenhouse microcli mate over the previous 15-min and empirical models of photosynthesis and tr anspiration; 2) ECdaily, daily adjustment of nutrient solution EC based on each morning's 24-hour weather forecast; 3) EC3.7, supply of a single high nutrient solution of 3.7 dS.m(-1); or 4) EC1.5, 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 EC1.5, 5.1 dS.m(-1) for treatment EC3.7, 3.6 dS.m( -1) for treatment ECdaily, and 3.4 dS.m(-1) for treatment ECvariable. Excep t for fresh weight (FW) of roots, growth characteristics did not differ sig nificantly 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 m atter content, yield loss to blossom-end rot, and firmness responded linear ly to treatment EC. In general, ECdaily yielded higher fruit quality and EC variable lower fruit quality than that predicted by linear regression. Alth ough our strategy of short-term dynamic changes of nutrient solution EC acc ording to changes in climate variables did not increase yield, daily adjust ment of nutrient solution EC improved external fruit quality characteristic s and may be practical for grower adoption.