Among and within-cultivar variability for histological traits of lucerne (Medicago sativa L.) stem

The histological structure of lucerne stems is a limiting factor for digestibility of the forage. Two experiments were carried out to compare cultivars and genotypes within cultivars for histological traits of the stems. In the first one, the among-cultivar genetic differences were assessed on 6 cultivars differing indigestibility. In the second one, the within-cultivar differences were evaluated on twenty genotypes belonging to two cultivars. Two sets of quantitative histological traits were measured using image analysis on the basal portion of the stems corresponding to the mature zone: the proportions of the different tissues of the lucerne stems (cortex, xylem, pithparenchyma), and the traits describing the xylem in vascular bundles (xylem cell wall thickness, xylem cell wall surface density, mean area of xylem vessel and mean area of fibre lumen). Few significant differences were observed among-cultivars, where as numerous within-cultivar differences were observed for histological traits. When the stem length was introduced as covariate, the differences among- and within-cultivars remained significant. Xylem proportion was negatively correlated to pith parenchyma proportion only. Xylem cell wall surface density and thickness were correlated to the cross-section radius but not to xylem proportion. As digestibility of lucerne is limited by histological structures, the large within-cultivar differences for histological traits of lucerne stems could be used in a breeding program for improved digestibility. This revised version was published online in July 2006 with corrections to the Cover Date.