Variation in root morphology and anatomy among accessions of cultivated rice (Oryza sativa L.) with different genetic backgrounds

Asian cultivated rice (Oryza sativa L.) has genetic diversification of root characteristics, but this variation has not been elucidated fully with reference to the genetic background. To clarify the differences in root anatomical and morphological traits among different varietal groups of cultivated rice, we analyzed four anatomical traits (root thickness, stele transversal area, total transversal area and number of late metaxylem vessels) and two morphological traits (root length index and ratio of deep rooting) in 59 accessions. A previous principal-coordinate analysis study using data on 179 restriction fragment length polymorphisms classified these accessions into three varietal groups: 13 japonica, 21 indica-I, and 25 indica-II. Based on a principal-components analysis of the six traits, the japonica group had wide variation in root anatomy compared to the two indica groups. In particular, japonica upland rice was characterized by a larger stele and xylem structures. By contrast, the two indica groups had wide variation in root morphology compared to the japonica group. Of the two indica groups, on average, the indica-I accessions had deeper, thicker roots than the indica-II accessions. Our results demonstrate that the japonica and indica groups contain different genetic diversity with respect to their root characteristics.