Effect of sulphate on photosynthesis in greenhouse-grown tomato plants

Sulphate accumulates in the rhizosphere of plants gown in hydroponic systems. To avoid such sulphate accumulation and promote the use of environmentally sound hydroponic systems, we examined the effects of four sulphate concentrations (0.1, 5.2, 10.4 and 20.8 mM) on photosynthesis, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activities and related physiological processes in green house-grown tomato plants (Lycopersicon esculentum Mill. cv. Trust). The lowest sulphate concentration (0.1 mM) significantly decreased photosynthetic capacity (P-c) and Rubisco activities on a leaf area basis. This result was supported by our data for dry matter per plant, which was low for plants in the 0.1 mM treatment. The photosynthesis-related variables such as leaf conductance, chlorophyll and soluble protein were lowest for the 0.1 mM treatment. Both total Rubisco activity and the activated ratio were reduced with this treatment. However, Rubisco activities expressed per g of protein or per g of chlorophyll were not significantly affected. These results suggest that sulphur deficiency depressed P-c by reducing the amount of both Rubisco and chlorophyll and by causing an inactivation of Rubisco. The ratio of organic sulphur vs organic nitrogen (S/N) in plants of the 0.1 mM treatment was far below the normal values. This low S/N ratio might be accountable for the negative effect of low sulphate on P-c and plant growth. P-c and dry matter were not affected until sulphate concentration in the nutrient solution reached a high level of 20.8 mM.