Effects of foliage spray of methionine and tryptophan with surfactant active agent on growth and bolting stem glucosinolate composition of Chinese kale

Chinese kale (Brassica alboglabra L.) is a native endemic variety of Brassica vegetables in China. It is highly rich in anti-cancer glucosinolates (GSs). Recent investigations have shown that methionine and tryptophan were the synthetic precursors of aliphatic and indolyl GSs, respectively. Therefore foliage spray with methionine and tryptophan has been thought to be a potential mode for increasing GSs concentrations in Chinese kale. However, the heavy surface wax layers of Chinese kale affect absorption efficiency of the exogenous amino acids. In this study, pot experiments were carried out to investigate foliage spraying effects of methionine and tryptophan with or without surfactant active agent (SAA) on the growth and GSs composition and concentration in bolting stems of Chinese kale. There was no significant difference in growth indexes and GSs compounds of Chinese kale among the treatments. Eleven individual GSs were separated and identified by liquid chromatography-mass spectrometry (LC-MS) in edible bolting stems of Chinese kale in all the treatments, including seven kinds of aliphatic GSs and four kinds of indolyl GSs. Foliage spraying methionine, tryptophan and tryptophan with SAA had no effect on concentrations of total GSs, total aliphatic GSs and total indolyl GSs. Compared with foliage spray of water, total GSs and total aliphatic GSs concentrations increased by 28.5% and 31.9%, respectively, under foliage spray of methionine with SAA. However, there was no significant difference in total indolyl GSs concentrations between the treatments. Compared with foliage spring water, foliage spraying methionine combined with SAA significantly increased especially the concentration of Gluconapin, increasing on the average by 44.3%. Also foliage spraying methionine combined with SAA increased sulfur (S) content in bolting stems of Chinese kale. This in turn decreased nitrogen/sulfur (N/S) ratio, while the changes in S and N/S ratio influenced GSs concentrations in Chinese kale bolting stems. Moreover, the concentrations of individual GSs within a specific GS group (i.e., aliphatic or indolyl) did not change consistently. The above results suggested that methionine absorbed by Chinese kale promoted the synthesis and accumulation of GSs while SAA enhanced the absorption of methionine through Chinese kale leaves covered with heavy layers of wax. However, the action mechanism of these substances needed further studies.