Lignification in relation to the biennial growth habit in brassicas

The forage brassicas are a useful model system for the study of wood formation because the thickened cell walls of their vascular tissue can vary widely in lignin content. Solid-state 13C NMR spectroscopy was used to quantify lignin, and determine features of its structure, in the vascular cell walls of forage rape (Brassica napus L.), and Thousandhead and marrowstem cultivars of kale (Brassica oleracea L. var. acephala). During the first season of vegetative growth, lignin levels in these cell walls remained low in the upper part of the stems despite the physical resemblance of this tissue to wood. The extended flowering stems produced in the following year were thinner and their vascular tissue contained much more strongly lignified cell walls. The structure of the lignin was typical of angiosperm wood. It showed only small variations in syringyl/guaiacyl ratio, but this ratio increased with lignin content and thus with the proportion of the lignin that was associated with secondary cell-wall layers.