Effect of Nutrient Solution Electrical Conductivity Levels on Lycopene Concentration, Sugar Composition and Concentration of Tomato (Lycopersicon esculentum Mill.)

Abstract

Tomato is an important commodity in fresh vegetable market. Recently, there is great interest for North American hydroponic growers to improve the fruit quality by introducing better cultivation methods. Manipulation of electrical conductivity (EC) of nutrient solution is a well-known technique to increase sugar concentrations of tomato; however, the potential risk of lower yield is the drawback of introducing this technique. Therefore to find a range of EC that can enhance the fruit quality while maintaining overall yield was the goal of this research. For this purpose, plant physiological responses such as transpirational and photosynthetic characteristics and fruit quality attributes including sugars and lycopene were investigated for selected cultivars under different EC. Regardless of cultivar, tomato plants showed a greater net photosynthetic rate at the reproductive growth stage compared to the vegetative growth stage. An increase of EC of influx nutrient solution up to 4.8 dS m-1 did not reduce the leaf photosynthesis, which supported a hypothesis that there is an optimum EC range for enhancing fruit quality without significant yield loss. A following experiment showed that the tomato fruit quality could be significantly enhanced when plants were grown under around 4.5 dS m-1 EC, in terms of total soluble solids (TSS) and lycopene concentration with no significant yield loss. Last experiment was conducted to quantitatively understand the accumulation of lycopene and sugars in fruits as affected by EC and its application timing relative to the fruit ripeness stages. High EC treatment of 4.5 dS m-1, regardless of its application timing, enhanced TSS and sugar concentration in the juice and lycopene concentrations of the fruit. However, the delayed high EC treatment (application of high EC after 4 weeks of anthesis) showed less enhancement for TSS and sugar concentration. Lycopene concentration of the fruit in the delayed EC treatment reached the same level as that in the standard high EC treatment (application since anthesis), which suggests that enhanced lycopene development under high EC is more related to an abiotic stress response during the fruit maturation, rather than fruit mass balance altered by the limited water flux to the fruit.