Effects of nutrient solution EC, plant microclimate and cultivars on fruit quality and yield of hydroponic tomatoes (Lycopersicon esculentum)

Four cultivars (Blitz, Mariachi, Quest and Rapsodie) of tomato were grown hydroponically on rockwool in two microclimates (east and west) inside the greenhouse (Tucson, AZ) under two nutrient solution electrical conductivity (EC) levels (2.6 or 4.5 dS m -1 ), adjusted by adding NaCl and CaCl2 after the setting of first fruit truss. In all cultivars, total soluble solid (TSS, %Brix at 20 o C) and lycopene concentration of fruits increased by 12-23 % and 34-85 %, respectively, with increasing EC level. Fruits harvested from the east side of the greenhouse had higher TSS than those from the west side, due to the different plant microclimate varying by daily PPF (photosynthetic photon flux) and VPD (vapor pressure deficit). However, lycopene concentration in fruits was not significantly affected by plant microclimate regardless of cultivars or EC. The cultivar of Mariachi showed the strongest effect in response to nutrient solution EC levels regarding both TSS and lycopene concentration among the cultivars examined. The cumulative yield at 7 weeks showed no significant differences between nutrient solution EC and locations, regardless of cultivars. The results indicated that value added tomato fruits could be produced by manipulating EC and plant microclimate in the greenhouse without causing yield reduction. INTRODUCTION Tomato is an important crop in fresh vegetable market around the world. Recently, there is great interest for growers to improve the fruit quality by introducing better cultivation methods. Total soluble solid concentration (TSS, measured as %Brix) of tomato fruits is one of the important variables that determines the fruit flavor and quality because TSS is the most common index associated directly with sugars and organic acids concentrations in the juice (Stevens et al., 1977; Young et al., 1993). Manipulation of nutrient solution electrical conductivity (EC, dS m -1 ) is a well known technique to grow flavor-enhanced tomato because the elevated salinity in nutrient solution restricts the water transport to fruits and thus increase the TSS (Adams, 1991; Mitchell et al., 1991; Cornish, 1992; Lin and Glass, 1999). However, too severe water stress from high EC nutrient solution may cause a significant yield reduction (Adams, 1991). The aerial environmental factors influence fruit growth and quality, and they also interact with the nutrient solution EC effects on the plant. It was reported that increasing photosynthetic photon flux (PPF) significantly increased the leaf net photosynthetic rate and total carbon fixed by tomato plants (Schwarz et al., 2002). When the PPF was lower than 200 mol m -2 s -1 , there was no effect on plant growth up to a nutrient solution EC of 8 dS m -1 ; however, when the plants were grown at a higher light intensity (1000 mol m -2 s -1 ), the tomato plant growth was reduced by the same EC treatments (Xu et al., 1995).