Protein, Methionine and Cysteine Upregulation in Phaseolus vulgaris Black Turtle Bean Seeds through Sulfur Fertilization at V2 and R2 Stages of Growth
Barry, H. P.  2017.  M.S. Thesis.

The purpose of this research was to determine whether the nutritional quality of the common bean (Phaseolus vulgaris L.) could be enhanced by fertilization. Specifically, this research investigated whether sulfur fertilization could increase the content of protein, methionine, and cysteine (sulfur-based amino acids), which have been shown to improve iron and protein absorption in the body. Previous research has shown that sulfur fertilization could increase total protein, methionine and cysteine content in other crop species as well as inhibitory compounds like tannin. The objective of this study was to upregulate total protein, methionine, and cysteine in relation to the inhibitory compound tannin, effectively producing a bean seed with a better nutrient profile for rural populations in developing countries like Guatemala. Two experiments were conducted where sulfur was applied at 0 kg S ha-1, 10 kg S ha-1, 20 kg S ha-1, 40 kg S ha-1, 60 kg S ha-1, and 80 kg S ha-1. In the first experiment, S was applied by hand in the form of granular gypsum (16% S) at the V2 stage of growth (second trifoliate leaf has unfolded at node 4). In the second experiment, S was applied at the R2 growth stage (50% or more of the flowers were open). Soluble protein, crude protein, methionine, cysteine and tannin content were compared to untreated controls. Ratios of soluble protein: tannin, crude protein: tannin, methionine: tannin, and cysteine: tannin content were also compared. Yield depression occurred at 20 kg S ha-1 for both sulfur application timings. Protein, amino acid, and tannin contents varied between treatment levels and fertilizer application timing. Fertilization at the V2 stage of growth decreased soluble protein production at 10 kg S ha-1 compared to controls. Crude protein increased at 10 and 40 kg S ha-1 compared to 80 kg S ha-1, though drought may have somewhat influenced this outcome. For V2 application, methionine peaked at 20 kg S ha-1 while cysteine increased as more sulfur was applied, peaking at 80 kg S ha-1. At R2 application, crude protein decreased at 10 kg S ha-1, 40 kg S ha-1, and 80 kg S ha-1, and cysteine was lower at 60 kg S ha-1 compared to controls. Tannin contents were significantly higher at 80 kg S ha-1. These results all point toward nutrient limitation at the stated sulfur rates, perhaps due to nutrient imbalance in the soil. Application of S at the V2 stage of growth produced the highest protein to tannin ratios compared to R2 fertilization, though amino acid to tannin ratios were similar for both fertilization timings. We conclude that sulfur fertilization at the V2 stage of growth gives the most improvement in nutritional content compared to sulfur application at the R2 stage of growth with 20 kg S ha-1 having the most beneficial effect at V2 application in Phaseolus vulgaris bean seeds.