Sulfur limitation increases nitrate and amino acid pools in tropical forages

Increasing the supply of sulfur (S) to forage plants can change their nitrogen (N) metabolism, causing changes in the N : S ratio that can potentially affect forage production and quality. The present study was focussed on revealing how supply (low, intermediate, high) of S affects amino acid composition and concentrations of total S, total N, sulfate-S, nitrate-N, and soluble protein in the leaves of tropical pasture species. Greenhouse experiments were conducted in ground quartz (inert solid substrate) culture to examine the effect of S supply in two tropical species: Panicum maximum cv. Tanzania (Guinea grass) and Stylosanthes guianensis cv. Mineirão (stylo). Because legumes have greater S requirement than do grass species, application levels of S varied according to the species. Guinea grass was grown with 0.10, 0.55, 1.00, 1.45, and 1.90 mmol L−1 of S, and stylo with 0.10, 0.70, 1.30, 1.90 and 2.50 mmol L−1 of S. Plants of both species were harvested on two occasions. Low S availability (0.10 mmol L−1) caused a nutritional imbalance with N in Guinea grass and stylo plants, as shown by a high N : S ratio (>60 : 1), and high concentrations of nitrate-N and free amino acids in plant tissues. Increased S supply regulated the N : S ratio at values close to 20 : 1, which provided N and S concentrations more suitable for protein synthesis and optimum forage production for both forage species. Asparagine was the predominant amino acid present in S-limited Guinea grass, whereas arginine was more abundant in S-limited stylo. This result indicates that a limitation of S increases nitrate-N and free amino acids while decreasing plant growth rates and soluble protein concentrations in these forage species.