Photosynthesis and its related physiological variables in the leaves of Brassica genotypes as influenced by sulphur fertilization

In the present investigation, we examined the effect of sulphur fertilization on photosynthesis (Pn) and its related physiological variables in the leaves of field grown Brassica genotypes (Brassica juncea [L.] Czern. and Coss. cv. Pusa Jai Kisan and Brassica campestris L. cv. Pusa Cold) over a whole growing season. Sulphur fertilization significantly (P < 0.05) increased the Pn rate on leaf area basis at all the growth stages over -S treatment. The photosynthesis related variables such as soluble protein and Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) protein were significantly higher in the leaves of plants grown with +S treatment, when compared to -S treatment. Sulphur fertilization also improved the chlorophyll, N and S content in the leaves of +S treated plants over -S treatment. Leaf-S content was linearly correlated with Pn rate, N-content and Rubisco protein in the leaves of both genotypes. An interesting relationship between N-content and Pn rate in the leaves of -S and +S treated plants was observed. In -S plants, the relationship between Pn rate and N-content per unit area of fully matured leaves became non-linear when leaf-N exceeded 1.5 g m(-2), while in +S plants the same remained linear, Rubisco protein was linearly related to Pn rate and leaf-N content. The ratio of Rubisco/soluble protein was lesser in the leaves of -S treated plants than +S treated plants. The effect of sulphur fertilization on Pn is discussed in relation to improved nitrogen utilization efficiency of the plants that leads to incorporation of reduced-N into the protein, especially in Rubisco protein rather than the non-protein compounds.