Dietary Fibre, Starch, Amino Acids and Nutritive Value of Potatoes as Affected by Sulfur, Nitrogen, Phosphorus, Potassium, Calcium and Water Stress

Abstract

In pot experiments with greatly differing rates of N, P, K and S, two levels of Ca and three levels of water, dry matter (DM) yields of tubers varied from 35 to 402 g/pot. K-Deficiency and water stress reduced the starch content of boiled potatoes, for K from 77 to 62% in DM. S-Deficiency increased total dietary fibre (TDF), from about 7.7 to 9.5%. Water stress, N-application and P-, K-, Ca- and S-deficiency increased total- and NO₃-N concentrations, which varied from 0.88 to 3.73% and from 14 to 230 ppm in DM. S-Applications increased total-S content from 0.06 to 0.15% in DM, of which between 81 and 45% were found in methionine + cystine. Total-N/total-S ratios decreased from 33.5 to 10.8 with S-applications. An N/S ratio of about 13 or less seems necessary for high yield and quality. Increasing N in DM, due to water stress, high N-rates or deficiencies of other nutrients, decreased concentrations in crude protein (CP) of all essential amino acids, whereas aspartic acid (asparagine) in most cases increased. S-Deficiency caused particularly strong decreases in concentrations of essential amino acids, from 1.36 to 0.56, 1.60 to 1.10, 4.93 to 3.88 and 5.41 to 2.86 g/16 g N of cystine, methionine, lysine and leucine, respectively. Glutamic acid (glutamine) content was increased from 15.3 to 20.6 g/16 g N by S-deficiency. Expressed as g amino acid/kg DM, all amino acid concentrations increased with increasing% N in DM. In N-balance trials with rats increasing crude protein concentrations in DM of boiled potatoes increased the true digestibility (TD) of the protein from 78 to 89 but decreased the biological value (BV) from 94 to 66. S-Deficiency caused a further reduction of the BV to 55. Excluding S-deficiency treatments, linear regression equations between CP concentrations and BV and TD gave correlation coefficients, r, of −0.90∗∗∗ and 0.89∗∗∗, respectively. There was close agreement between changes of BV and concentrations of first limiting amino acids (chemical score), with r = 0.94∗∗∗.

Key Words:

• total-S
• N/S ratios
• cystine
• methionine
• N-balance in rats
• digestible energy