Tomato has been an important horticultural crop in the U.S. market (Jones, 1999; Rick, 1995). For fresh tomato production, 159,664 and 1,594,241 tons of tomatoes were produced in the greenhouse and field, respectively, in the United States in 2003 (Cook and Calvin, 2005). Since 1985, the consumption of fresh tomatoes in the United States increased about 30%, with an annual per capita consumption level estimated at 8.8 kg in 2003 (Cook and Calvin, 2005). The percentage of greenhouse tomatoes available in the U.S. retail markets has increased dramatically during the past decade and accounts for 37% of the weekly quantity of tomatoes sold in the average U.S. supermarket in 2003 (Cook and Calvin, 2005). In Mexico, the total area of greenhouse used for production of vegetables is increasing rapidly, reportedly as high as 30% annually (Steta, 2004), and growers are now shifting toward production of higher quality tomato fruit to obtain premium price. One such shift is pursuing better tomato fruit flavor similar to what consumers perceive as ‘home garden’ flavor, and another shift is a reduced use of pesticides to produce a safer fruit.
Sugar and organic acids are the major components of tomato flavor (Stevens et al., 1977). Total soluble solid concentration [TSS; percent (w/v) at 20 °C] is the most common index for overall flavor of tomato fruit associated directly with sugar and organic acid concentrations in tomato juice (Stevens et al., 1977; Young et al., 1993). In hydroponic tomato production, increasing electrical conductivity (EC) of nutrient solution is a well-known technique to increase TSS of tomato fruit because the decreased osmotic potential (ψS) of nutrient solution restricts the water transport to fruit, resulting in higher concentrations of soluble solids (Adams, 1991; Cornish, 1992; Dorais et al., 2001; Lin and Glass, 1999; Mitchell et al., 1991). EC can be increased by increasing overall strength (total concentration) of the nutrient solution or by adding sodium chloride (NaCl). The former method can be achieved by altering dilution rate of injectors for the stock solutions, but the latter is more widely accepted by commercial growers as being economically feasible.
One of the disadvantages of increasing TSS by high EC treatment is a reduction in fruit size by reducing water content in fresh fruit (Adams and Ho, 1989). The EC of nutrient solution used for commercial hydroponic tomato production generally ranges between 1.6 and 5.0 dS·m−1. Dorais et al. (2001) examined the effects of EC on tomato fruit yield and found that tomato yield was not reduced when EC ranged from 2.1 to 5.1 dS·m−1. Adams (1991) reported that, compared with the control treatment of 3.0 dS·m−1 EC, application of 8 dS·m−1 EC decreased tomato yield by 4% to 5% per dS·m−1, whereas 12 dS·m−1 EC decreased tomato yield by 6% to 8% per dS·m−1, where both high EC treatments were achieved by adding NaCl to the nutrient solution. Another report showed that there was no significant difference in yield between plants grown under 2.7 and 4.5 dS·m−1; however, the yield was reduced linearly when the EC was increased from 4.5 to 6.0, 7.4, or 8.6 dS·m−1 (Leonardi et al., 2004). These results suggest that when EC was increased moderately to around 5 dS·m−1, TSS of fruit could be enhanced without yield reduction.
Under high EC, the tomato plant may be affected by water stress from the low water potential of the nutrient solution, which is caused by the decreased (or more negative) ψS of the solution, or affected by excessive ion uptake because of greater ion concentrations in solution (Greenway and Munns, 1980). Photosynthesis, transpiration, and stomatal conductance (gS) under high EC were affected by limited irrigation or increased salt concentrations in nutrient solution (Romero-Aranda et al., 2001). These physiological parameters are closely related to plant growth, as well as to fruit yield and quality. Xu et al. (1995) studied the effects of EC of hydroponic nutrient solution, growth medium (substrate), and irrigation frequency on tomato plant photosynthetic response and found that the maximum leaf photosynthetic rate was increased by 15.4% and 14.1% when EC was increased from 2.5 to 4.0 dS·m−1 for plants grown in nutrient film technique and rockwool systems, respectively. But a further increase of EC to 5.5 dS·m−1 resulted in a 10% lower maximum photosynthetic rate compared with that under 4.0 dS·m−1 EC when plants were grown in a rockwool system. Schwarz et al. (2002) found that an increase of EC from 1.25 dS·m−1 up to 8.75 dS·m−1 did not reduce the leaf photosynthetic rate of tomato. In experiments reported by Xu et al. (1995) and Schwarz et al. (2002), the EC was enhanced by increasing the overall strength of nutrient solutions.
Romero-Aranda et al. (2001) showed that the leaf net photosynthetic rate of tomato plants was reduced proportionally as NaCl concentration increased in the nutrient solution (0, 35, and 70 mm NaCl), and stated that the decrease might have resulted from the reduction in gS and stomatal density. The nutrient solution examined in their experiment had 4.0 to 5.4 and 8.1 to 9.2 dS·m−1 EC for 35 and 70 mm NaCl treatments, and 1.8 to 2.0 dS·m−1 for the control (0 mm NaCl; R. Romero-Aranda, personal communication). The decrease in net photosynthetic rate observed at EC of 4.0 dS·m−1 or greater may be because of accumulated sodium in the plant tissue.
The plant photosynthetic and transpirational responses of commercially important cultivars to nutrient solutions of varied EC would provide critical information to facilitate optimization of tomato fruit quality, provide a basis for a reference study, and/or promote other long-term investigations in greenhouse production. The objective of this study was to evaluate the effects of EC of nutrient solution on tomato plant leaf photosynthetic response, transpiration rate, and stomatal leaf conductance and its interaction with cultivars and plant developmental stages.
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