Plant water uptake and water use efficiency of greenhouse tomato cultivars irrigated with saline water

Effects of salinity on tomato (Lycopersicon esculentum Mill.) fruit yield, plant water uptake and water use efficiency (WUE) have been quantified in experiments carried out under greenhouse and soil-less cultivation with four cultivars (Floradade, L1, L5 and L9) and four salinity levels (0, 25, 50, and 75 mM NaCl). Fruit represented 70% of plant fresh weight while leaves and stems represented 22 and 8%, respectively. Fruit were the most sensitive part of the plant, with the four cultivars showing similar significant fruit yield reduction, namely about 28 g/mM NaCl or 290 g/dS m−1. Yield threshold varied from 0 to 3.4 dS m−1, values lower than or close to the electrical conductivity (EC) of nutrient solutions used in commercial greenhouses. Yield reduction from threshold to upper salinities was about 8% of maximum yield per dS m−1 increase. Blossom end rot increased with salinity although the pattern of increase depended on the cultivar. Tomato fruit grown under saline conditions had higher soluble solids and acid content than those from the control (0 mM) plants. Plants grown under the most saline conditions consumed, on average, 40% less water than control plants. The relationship between total plant water uptake and salinity was linear (negligible threshold) and salinity of the nutrient solution almost entirely explained the variations in plant water uptake (R2 from 0.94 to 1); therefore, salinity of the irrigation water has to be taken into account when calculating tomato water requirements. However, significant differences in the negative slopes of the correlation lines indicate that decreases in plant water uptake, from 3.5 to 5% per dS m−1, are cultivar-specific and cannot be generalised. Vegetative (stem and leaves) dry weight was a better indicator of tomato plant water uptake in saline conditions irrespective of cultivars than fruit yield or plant and fruit dry weight. Tomato plants in the control averaged a higher WUE than the most salinized plants when WUE was expressed as a function of fruit yield (25 and 13 g fruit L−1); however WUE was independent of salinity if expressed as a function of plant and fruit dry matter (approximately 3.0 g dry matter L−1). Tomato plants absorbed only a small proportion of the Na+ present in the nutrient solution (from 2.3 to 3.2%) but there were significant differences among the four cultivars which suggest that plant ability to select ions is a trait to be taken into account when selecting tomato genotypes for salt tolerance.