Prediction of Soluble Solids of Tomato Fruit Grown in Salinized Nutrient Solution Based on the Electrical Conductivity of the Drainage

The relationship between fruit Brix and the electrical conductivity (EC) of the nutrient solution was investigated under gradually increasing EC conditions to predict and control tomato fruit Brix in commercial greenhouses in Japan. Based on the three experiments, fruit Brix was significantly and highly correlated with the cumulative EC of the drainage during the period from anthesis to harvest (cECd). This relationship followed a linear regression function. We then modelled fruit Brix based on cECd and validated this model to predict and control fruit Brix in four other experiments in different growing seasons using two cultivars, slab substrates, and irrigation systems. Using this model, we calculated the target cECd (cECdt) to achieve a target fruit Brix of 6% or higher and used cECdt as an indicator to manipulate the EC of the nutrient solution. In the validation experiments, cECd was lower than cECdt at the beginning of harvest in all experiments. cECd reached cECdt at 72.3–214.0°C·day after the first harvest. When cECd was higher than cECdt, more than 86.9% of the fruit had a higher than Brix 6%. In addition, the marketable yield was higher than 88.2%. RMSEs between the observed fruit Brix and predicted fruit Brix were 0.60–1.25. These results indicate that our model can predict and control fruit Brix.