Free and Total Amino Acid Composition of Edible Parts of Beans, Kale, Spinach, Cauliflower and Potatoes as Influenced by Nitrogen Fertilisation and Phosphorus and Potassium Deficiency

Abstract

Vegetables were grown in pots at widely differing nutrient levels, which greatly affected dry matter (DM) yields and total-N concentrations in all crops. Nitrate-N contents were low and little affected in cauliflower and potatoes, and highest, and strongly affected, in spinach and kale. The sum of free amino acid-N as percentage of total-N of bean pods, kale, spinach, cauliflower curds and potato tubers varied between 12 and 27%, 10 and 21%, 5 and 12%, 7 and 36% and 34 and 56%, respectively. In beans and potatoes asparagine was the dominant free amino compound (29–55% and 33–59% free amino-N as percentage of total free amino-N, respectively), whereas in kale, spinach and cauliflower free glutamine was dominant (17–52%, 31–48% and 14–54%, respectively). Free essential amino acids were generally found in very low concentrations, especially cysteine (which can partly replace essential methionine in nutrition), tryptophan and phenylalanine. With some exceptions in beans, the concentrations of all total amino acids in DM increased linearly with increasing total-N content, and with correlation coefficients very close to 1·00 in most cases. P- and K-deficiency affected free and total amino acid composition mainly through their effects on total-N content but had some specific effect on arginine concentrations. Generally, chemical scores of the crude protein decreased with increasing N content of DM, which was mainly due to low contents of S-amino acids.