Sulfur limitation increases nitrate and amino acid pools in tropical forages

Abstract

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.