Sulphur as a factor influencing the content of nitrogen compounds in seeds of narrow-leafed lupin

In view of the predicted increase in sulphur deficiency in crop production, a study was undertaken to assess the effect of fertilization with this nutrient on criteria determining the biological value of narrow-leafed lupin seeds (total and true protein, their ratio, nitrates and amino acid composition). The study was based on a three-year, three-factor field experiment, in which the Elf cultivar of narrow-leafed lupin (Lupine Angustifolius L.) was grown on light soil with low sulphur content. The experiment included foliar and soil application of sulphur and its elemental and ionic forms. The application doses were 0, 20, 40 and 60 kg S ha-1. In all years of the study, the sulphur application dose was found to significantly determine the total and true protein content as well as their ratios in the lupin seeds. Application of just 20 kg S ha-1 caused a significant increase in true protein in the lupin seeds and in the proportion of true protein in the total protein, as compared to the control. The CS index for all the essential amino acids showed that the first limiting amino acid in the lupin seed protein was sulphur-containing methionine. As the dose of applied sulphur increased, there was an overall increase in CSmeth. and EAAI. Given the positive effect of sulphur, particularly in the elemental form, on the protein content and amino acid composition of lupin seeds and in reducing the content of harmful nitrates, the use of this nutrient in fertilization of this species is recommended.