The Branch Roots Of Zea. I. First Order Branches, Their Number, Sizes And Division Into Classes

Recent studies of the axile roots of maize have shown that they have immature late metaxylem for the first 30 cm, and so have a high axial resistance to water flow. This has focused attention on the older regions of the roots as sites of water uptake, and especially on the branch roots they bear. The first-order branches of field-grown roots have been measured, and values found for: (a) the numbers along the axile roots (average 12 cm-1 with small variances); (b) diameters of the branches; (c) diameters of their largest tracheary elements; and (d) branch lengths (mode 6-10 mm, mean 30 mm). On the basis of root diameters and vascular anatomy, four classes of branch root have been distinguished. Class 1 branches have several small tracheary elements and Classes 2-4 have one or more large central tracheary elements besides smaller peripheral ones. Branch diameters are grouped around the means: Class 1 (214 micromole), Class 2 (350 micromole), Class 3 (525 micromole) and Class 4 (675 micromole). Seventy-six percent of the branches are in Classes 1 and 2. Branch length and diameter were weakly correlated. The shortness of the branches is particularly striking, and seems characteristic of maize rather than other cereals or grasses. This study defines the range of material in which water and nutrient uptakes need to be investigated.