Daily variation in glucosinolate concentrations in the leaves and roots of cabbage seedlings in two constant temperature regimes

Limited information is available on the glucosinolate variation within the Brassica plant and the relationship between the pattern and concentration of glucosinolates in the aerial parts and the roots has received little attention. Early studies carried out under field conditions have shown that glucosinolate levels may vary considerably throughout a 24 h period. The purpose of the present study was to show whether, under controlled conditions, temperature was a factor in glucosinolate variation and to determine whether such variation might be due to translocation of glucosinolates between the aerial parts of the plant and its roots. Cabbage seedlings were maintained at 20 and 30 degrees C over 2 days and leaves and roots sampled at 02:00 h, 06:00 h, 10:00 h, 14:00 h, 18:00 h and 22:00 h. The glucosinolates 2-propenyl- and 3-methylsulphinylpropyl- with an average of 261 and 167 micromole 100 g-1 DW, respectively, were the two main glucosinolates in the aerial part of the plant whilst in the roots 1-methoxyindol-3-ylmethyl-, 2-phenylethyl- and 3-methylsulphinylpropyl, with 495, 495 and 385 micromole 100 g-1 DW respectively, showed the highest average concentrations. Total and individual glucosinolates in the roots and in the aerial part of the plant showed the highest concentrations in the dark cycle, at 02:00 h and 22:00 h, respectively, whilst the lowest levels were during the light cycle, mainly at 18:00 h. The results suggest that temperature was not a major factor in the short-term variation in glucosinolate levels. Although there was a very high significant difference between the total glucosinolate levels in the aerial part of the plant (581 micromole 100 g-1 DW) and roots (2124 micromole 100 g-1 DW), the results of the present study do not support the concept of translocation between aerial part and roots, suggesting that other factors may be involved.