Salinity represents an increasing threat to agriculture around the world, while at the same time agricultural food production will need to increase to meet the needs of the increasing world population. This increases the likelihood that agricultural production will expand into marginal areas, which may be salt-affected (Flowers, 2006). Salt-affected soils occur in more than 100 countries around the world (Rengasamy, 2006), and according to a report published in 2000 by the Food and Agriculture Organization of the United Nations (FAO), the total global area of salt-affected soils, including saline and sodic soils, was 831 million ha (Martinez-Beltran and Manzur, 2005). Besides the occurrence of naturally saline soils, agriculture has contributed to salinization through use of poor-quality irrigation water, high fertilization rates, and forest clearance. Although NaCl is the dominant salt in many saline soils, calcium, magnesium, potassium, iron, boron, sulfate, carbonate, and bicarbonate can also accumulate (Szabolcs, 1989).
Greenhouse cultivation is one agrosystem that may be highly sensitive to salt stress because of the areas in which it usually occurs, the climatic conditions under which it is carried out, and the cultural techniques adopted (Leonardi and Martorana, 2005). Tomato is one of the most important crops grown under protected conditions. Tomato production is concentrated in arid or semi-arid areas, where it is often cultivated in saline soils or with poor-quality irrigation water. According to production data from 2005, ≈30% of world production comes from countries around the Mediterranean basin and ≈10% from the United States (FAO, 2006), primarily California and Florida. These regions are characterized by intensive agriculture along coastal areas, where use of saline water and consequent soil salinization are common.
Many detrimental effects of salt stress have been reported on tomato, mainly concerning reductions in yield (Cuartero and Fernández-Muñoz, 1999). Although saline conditions are linked to reductions in growth and yield, in some cases moderate salt stress can improve fruit quality (Cornish, 1992; Plaut, 1997; Sakamoto et al., 1999; Serio et al., 2004). In soilless cultivation, growers may increase the electrical conductivity (EC) of the nutrient solution by adding NaCl or by increasing the overall nutrient concentration to apply a controlled salt stress to the plant and increase fruit quality.
Saline conditions usually refer to the presence of high NaCl concentrations in the irrigation water or in the soil, but few studies looked at Na-specific vs. nonspecific effects on tomato growth and physiology. The presence of salt in the nutrient solution decreases the osmotic potential of the root environment, but not all plant parameters are influenced the same way by different salinity sources (Adams, 1991).
Altering fertilization practices has been suggested as one possible way to increase the salt tolerance of tomato. In particular, providing additional Ca has been shown to reduce some of the detrimental effects of Na on tomato and other crops (Cabañero et al., 2004; Carvajal et al., 2000; Lopez and Satti, 1996; Navarro et al., 2000, 2005). On the basis of these findings, the objectives of our study were to quantify detrimental effects of NaCl on growth, leaf photosynthetic parameters, and nutrient uptake of hydroponically grown tomatoes; to determine which of these effects are ion-specific and which ones are not; and to determine if additional Ca can minimize or prevent detrimental ion-specific effects.
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