EFFECT OF SULFATE ON PHOTOSYNTHESIS IN GREENHOUSE-GROWN TOMATO PLANTS

Sulphate accumulates in the rhizosphere of plants gown in hydroponic s ystems. To avoid such sulphate accumulation and promote the use of env ironmentally sound hydroponic systems, we examined the effects of four sulphate concentrations (0.1, 5.2, 10.4 and 20.8 mM) on photosynthesi s, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1. 39) activities and related physiological processes in green house-grow n 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 resu lt was supported by our data for dry matter per plant, which was low f or plants in the 0.1 mM treatment. The photosynthesis-related variable s such as leaf conductance, chlorophyll and soluble protein were lowes t for the 0.1 mM treatment. Both total Rubisco activity and the activa ted ratio were reduced with this treatment. However, Rubisco activitie s expressed per g of protein or per g of chlorophyll were not signific antly affected. These results suggest that sulphur deficiency depresse d P-c by reducing the amount of both Rubisco and chlorophyll and by ca using an inactivation of Rubisco. The ratio of organic sulphur vs orga nic 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 negati ve effect of low sulphate on P-c and plant growth. P-c and dry matter were not affected until sulphate concentration in the nutrient solutio n reached a high level of 20.8 mM.