Morphology of Alfalfa Divergently Selected for Herbage Lignin Concentration

Programs to improve alfalfa (Medicago sativa L.) nutritive value have used selection criteria based on total herbage cell-wall composition. There is only limited information addressing the effect of total-herbage based phenotypic selection on changes in dry-matter partitioning of the plant and on the nutritive value of plant parts. The objective of this study was to determine the effects of divergent herbage selection for alfalfa cell-wall components on plant morphology. Two groups of germplasm were observed. Group 1 consisted of six syn-2 lines derived from two-parent combinations, which were randomly selected from 'Saranac-AR' for high (HL) or low (LL) herbage acid-detergent lignin concentration. Group 2 had 11 HL and 13 LL lines that were full-sib lines produced on pairs of parents from the Group 1 synthetics. Group 1 was grown for two harvests in a greenhouse and one harvest in thefield, and Group 2 was grown for two harvests in the field. Compared with HL lines, LL lines had a greater leaf-to-stem ratio, especially in the fraction above the sixth node from the stem base. For LL lines, this ratio in the upper plant fraction was 53, 26, and 18% greater than HL lines for Group 1 greenhouse, Group 1 field, and Group 2, respectively. When harvested at the same time, LL lines were less mature than HL lines, and, for Group 2, main stem length of LL lines was about 30 mm shorter than HL lines. Dry-matter yield of HL lines was 17, 28, and 28% greater than LL lines for Group 1 greenhouse, Group 1 field, and Group 2, respectively. Most of the differences between divergent types were related to changes above the sixth node. Thus, selection for improved nutritive value in total herbage of alfalfa may result in marked changes in plant morphology.