Chemical Control of Vascular Cambium Initiation in Isolated Radish Roots.

Roots of many higher plants increase in diameter through the enlargement of cells formed in a vascular cambium. After the seedling plant is established, thickening begins near the root base by the initiation of cell divisions in the procambial tissue of the central cylinder. How the vascular cambium is initiated, what maintains cambial activity and causes it to extend, with time, along the root, and how the new cells differentiate into secondary xylem and phloem are questions which have received considerable attention from botanists. It is clear from several lines of evidence that the aerial shoot has an influence in some or all of these processes. Dependence upon the shoot involves more than supplying adequate carbohydrate to the heterotrophic root since cambial activity has been observed only rarely in excised roots provided simply with an abundant supply of carbohydrate. Of particular interest are the observations that auxin will induce moderate cambial activity when it is supplied to the stumps of decapitated plants' and of excised roots grown in culture,2' or to the medium in which excised roots are grown.4 5 Interest also centers on several species in which root thickening has been reported to be dependent upon the relative lengths of day and night in the shoot environment. Such dependence can be interpreted as indicating that other substances besides carbohydrate and auxin produced under appropriate photoperiodic stimulus are probably involved in cambial initiation. Root thickening of intact radish plants, Raphanus sativus L., has been reported to be promoted by short days (8-12 hr light)6 7 and optimum temperatures. Excised seedling roots of radish fail to thicken noticeably in culture2' 8' 9 even though a cambium sometimes may be initiated. Jones2 found that cambial initiation and activity in such roots was promoted by auxin supplied to the root base or by small amounts of stem (hypocotyl) tissue left attached to the root. Thus, radish roots which thicken rapidly from a single vascular cambium appear to be particularly well suited for investigations on cambial activity. We have undertaken a study into their behavior. Excised seedling roots were cultured sterilely for a period of time to deplete them of possible endogenous cambial regulators. Transfer tips were then cultured with their cut bases placed in media containing substances to be tested for their influence on cambial initiation.' These experiments have shown that, in addition to auxin, kinins and inositol have striking effects on cambial initiation and activity.