Tomato and salinity

Abstract The effects of salinity on tomato plant growth and fruit production, the cultural techniques which can be applied to alleviate the deleterious effects of salt, and the possibilities of breeding salt-tolerant tomatoes are reviewed. Salinity reduces tomato seed germination and lengthens the time needed for germination to such an extent that the establishment of a competitive crop by direct seeding would be difficult in soils where the electrical conductivity (EC) of a saturated extract was equal to or above 8 dS m−1. Priming seeds primed with 1 M NaCl for 36 h seems advisable to establish a crop by direct sowing in saline soils, and seedling conditioning, either by exposure to moderately saline water exposure or by withholding watering until seedlings wilt for 20–24 h, can be recommended for crops that are to be established by transplanting. Yields are reduced when plants are grown with a nutrient solution of 2.5 dS m−1 or higher and above 3.0 dS m−1 an increase of 1 dS m−1 results in a yield reduction of about 9–10%. At low ECs, yield reduction is caused mainly by reduction in the average fruit weight, whilst the declining number of fruits explains the main portion of yield reduction at high ECs. Since the smaller the fruit, the less important the reduction in fruit weight caused by salt, small size tomatoes are recommended to be grown at moderate salinity. Short cycle crops, in which only 4–6 trusses are harvested, are also recommended – especially since upper inflorescences are particularly sensitive to salt. Root growth, which slows when salinity reaches 4–6 dS m−1, appears to be less affected by salt than shoot growth. Salinity raises Na+ concentration in roots and leaves of tomato plants. A higher Na+ concentration in the leaves lowers the osmotic potential and promotes water uptake, but it is the ability to regulate Na+ in older leaves while maintaining a low Na+ concentration in young leaves which seems to be related to salinity tolerance. Ca2+ and K+ concentrations in roots of salinised tomato plants change little under salinity whilst they are greatly reduced in leaves; those plants taking up more Ca2+ and K+ from the salinised medium will have lower Na+/K+ and Na+/Ca2+ ratios and an equilibrium of nutrients more similar to the non-salinised plants. Increasing Ca2+ and K+ concentrations in the nutrient solution is, consequently, advisable. Root NO−3 concentration is maintained for longer periods after salinisation or under higher salinity levels than leaf NO−3 concentration. Salinity enhances tomato fruit taste by increasing both sugars and acids, fruit shelf life and firmness are unchanged or slightly lowered, but the incidence of blossom end rot is much higher. Breeding of tomato cultivars tolerant to moderate salinity will only occur after pyramiding in a single genotype several characteristics such as greater root volume, higher efficiency in water absorption and dry matter formation per unit of water absorbed, higher selectivity in absorption of nutrients, and higher capability to accumulate toxic ions in vacuoles and old leaves.