Renata Nurzyńska-Wierdak
The yield and chemical composition 
of rocket and kohlrabi leaves
in dependence on nitrogen and potassium fertilization

In the years 2001-2005, pot experiments of garden rocket (Eruca sativa Mill.) and kohlrabi (Brassica oleracea var. gongylodes L.) "Gabi" cultivar were carried out in spring, in a periodically warmed green-house of the Experimental Farm of the Agricultural University in Lublin . The purpose of the studies attempted to determine interrelations between the form and dose of nitrogen and potassium, and selected morphological features of plants, yield of fresh leaves and the chemical composition thereof. The plants were grown in pots (2 dm 3), filled with sphagnum peat enriched by a fertilizer chalk to achieve pH 6.2, mixed with loamy-sandy soil in the ratio of 9:1 by volume. Seeds were sown on about 10 March, and after the emergence the plants were thinned out to leave one plant per pot. Each series included eight replications. Harvest was carried out about 50 days after the seeds were sown, and each time a whole leaf rosette was cut. The content of the dry matter, protein, L-ascorbic acid, sugars, nitrates, total nitrogen, potassium, chlorine, sulphur, calcium and magnesium were examined, as well as amino acid composition of protein was subject to analysis in relation to two patterns of human demand for necessary amino acids. The chemical composition of the growing medium after the harvest was also measured. The results were statistically elaborated with variation analysis for double cross classification, and the importance of differences was evaluated by using Tukey’s confidence intervals and calculations of the least significant difference by the level of significance p = 0.05.
Obtaining a significant yield of a fresh weight of leaves from these plants, of significant nutritional value in the springtime greenhouse cultivation, was proved to be possible. Nitrogen fertilization significantly affected growth and yield of the tested plants. Nitrogen applied in larger doses resulted in an increase of yield of fresh weight of rocket, and a decrease of yield of kohlrabi. However, a larger yield of fresh above-ground weight was obtained in treatments with lime saltpeter and urea, the smallest yield was with ammonium sulphate. Increasing the nitrogen dose resulted in an increase in height and number of leaves of the tested plants. A significant influence of form and dose of nitrogen has been recorded for the nutritional value of leaves of the tested rocket and kohlrabi. Those plants nourished with ammonium sulphate contained more dry matter and protein, and less L-ascorbic acid in comparison with lime saltpeter and urea. Increasing the nitrogen dose resulted in a decrease of the average concentration of L-ascorbic acid and an increase of proteins in the leaves of rocket and kohlrabi, whilst the content of sugars changed indirectionally. The leaves of tested plants of rocket and kohlrabi contained significant quantities of mineral compounds, especially of nitrogen, potassium and calcium. The largest quantity of potassium and calcium in the leaves of the tested plants was recorded when the plants were fed with lime saltpeter, whilst the lowest when fed with ammonium sulphate. A clear relation between tested biometric features, the quantity of yield, chemical composition of leaves and applied potassium fertilisers has not been recorded. The most potassium and calcium in the leaves of tested plants was recorded when feeding with potassium chloride in comparison with potassium sulphate. The leaves of kohlrabi contained half the quantity of nitrates in comparison with rocket. The smallest quantity of these compounds was recorded while using ammonium sulphate in comparison with other nitrogen fertilizers. Increasing the nitrogen dose resulted in a significant increase of the content of nitrates in the leaves of rocket and kohlrabi. The influence of chloride or potassium sulphate on the content of nitrates in the leaves of tested plants was not unequivocal.
Kohlrabi protein was shown to be richer in treonina, sulphuric amino acids, aromatic amino acids, histidine and arginine, and it contained less lysine, leucine, iso-leucine and valine, in comparison with rocket. Amino acid, which limits the nutritional value of the protein contained in rocket and kohlrabi, was lysine. The influence of variable nitrogen and potassium fertilization on content of amino acids in proteins of rocket and kohlrabi was confirmed. Increasing the nitrogen dose in the nutritional environment resulted in an increase of the content of glycine, proline, lysine, leucine, iso-leucine, valine and fenyloalanine, and a decrease of the content of alanine, aspartic acid, serine, cisteine, treonine and histidine in rocket protein. Moreover kohlrabi protein showed an increase in the concentration of glycine, proline, histidine, arginine, and a decrease in the quantity of cisteine due to the increase of nitrogen dose. Feeding rocket with potassium sulphate obtains larger concentrations of alanine, glycine, aspartic acid, serine, lysine, iso-leucine, valine, metionine, cisteine and fenyloalanine and smaller concentrations of leucine, treonine, tirosine, histidine and arginine, in comparison with chlorine potassium. Besides larger concentrations of alanine, glycine, aspartic acid, serine, lysine, leucine, iso-leucine, treonine, waline, metionine and tirosine, and smaller concentrations of cisteine, fenyloalanine and arginine in protein of kohlrabi fed with K 2SO 4, in comparison with KCl, has been recorded.
The analysis of the yield of fresh weight of studied plants" leaves, dry matter, protein, sugars, L-ascorbic acid and nitrates content, allow for the assumption that rocket should be cultivated in medium containing 150-220 mg × dm -3 mineral nitrogen (N-NH 4 + N-NO 3) and 300-350 potassium, whilst kohlrabi 120-190 mg × dm -3 N-NH 4 + N-NO 3 and 260-320 mg × dm -3 K.
Studied plants of rocket and kohlrabi were marked with a larger content of leucine, treonine, sulphuric amino acids, aromatic amino acids and histidine in protein, in comparison with the pattern FAO/WHO and Millward’ pattern. Besides this, in the rocket protein a larger concentration of iso-leucine and valine was recorded, in relation to the model proteins . The index value of the limiting amino acid was larger in the case of using potassium, in the form of K 2SO 4, than KCl and by the average dose of nitrogen ( 0.5 g N × dm – 1 in the growing medium), in comparison with the remaining, which was more profitable with respect to the possibility of better use of amino acids from protein of the tested plants.