Rice root morphological traits are related to isozyme group and adaptation

Rice accessions from the International Rice Research Institute (IRRI) germp lasm bank were evaluated for root traits of 40-day-old plants grown in soil in the greenhouse. The 136 accessions represented six groups defined on th e basis of isozyme classification, with isozyme group six further subdivide d on the basis of origin and morphology. An additional 28 rice cultivars we re evaluated for seminal root xylem vessel diameter when grown in pots in a growth chamber. Rice groups differed in root thickness, root xylem vessel diameter, root:shoot ratio, and patterns of root distribution. Isozyme grou p 1, which corresponds generally to the indica subspecies, had thin, superf icial roots with narrow vessels and a low root:shoot ratio. The other major isozyme group, group 6, comprising japonica types, was characterized by th ick roots with wider vessels, a greater proportion of the root weight below 15 cm, and a larger root:shoot ratio. On an average, the bulu and temperat e group 6 accessions were similar to the non-bulu types except that their r oot:shoot ratios and proportion of root weight above 15 cm were more simila r to group 1, Group 2, with aus types from South Asia, was characterized by intermediate root thickness, but vertical root distribution and root:shoot ratio were more similar to group 6. The minor isozyme groups 3-5 were repr esented by few accessions, and in general, they had root thickness and root distribution profiles more similar to group 1 than to group 6. While signi ficant differences were observed among isozyme groups for all the traits un der study, there was significant variation within groups and groups overlap ped for all traits measured. This study highlights the wide range of variab ility for constitutive root traits in rice. For example, root thickness ran ged from 0.68 to 1.04 mm, seminal root xylem vessel diameters from 30 to 58 mum, root:shoot ratios from 0.05 to 0.21, and accessions had from 44 to 73 % of the total root weight concentrated in the surface 15 cm of soil. For t he 28 cultivars evaluated, root xylem vessel diameter was highly correlated with reported values of leaf epicuticular wax content (r = 0.89). These va lues indicate the range of genetic variation within the rice genome for roo t morphological traits. (C) 2001 Published by Elsevier Science B.V.