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UPTAKE AND TRANSLOCATION OF SULPHATE IN TOMATO SEEDLINGS IN RELATION TO SULPHATE SUPPLY

ABSTRACT Uptake and translocation of 35SO4 2− sulfate in intact tomato seedlings (Lycopersicon esculentum cv. Solairo) incubated in nutrient solutions containing four concentrations of sulfate were investigated. The 35S fractions were measured in roots, stem, old leaves, and young leaves over a 24-hour period. At deficient and excessive sulfate levels, the instantaneous sulfate uptake (Φoc) in tomato seedlings was strongly influenced by the sulfate concentration in the rhizosphere. Increasing the sulfate concentration in the rhizosphere from 0.1 to 10.4 mM resulted in a proportional increase in net uptake into the whole seedling. The transport of sulfate from root to shoot (Φcx) was almost the same in seedlings incubated at both 20.8 mM sulfate and 10.4 mM sulfate. Young tomato plants can regulate sulfate influx from the cytoplasm to the xylem and the net uptake. The highest influx of SO4 2− from the external solution to the cytoplasm (Φoc) and the highest efflux (Φco) was observed at the highest sulfate concentrations (20.8 mM), indicating extensive recycling at the plasma membrane. Our results suggest for the greenhouse tomato a sulfate concentration of 10.4 mM in the nutrient solution, which allowed an optimum uptake. The data provided further information on the events associated with sulfur redistribution.