The Initiation, Growth, and Characteristics of a Tissue 2

The rate and extent of initiation of callus from potato tuberdiscs depends on the concentrations of auxin and kinetin inthe medium on which they are grown. NAA is the most effectiveauxin, initiating callus at a concentration (0. 01 mg/1) anorder of magnitude lower than for IAA or 2,4-D. There is a week'slag before initiation begins with IAA or 2,4-D. In combinationwith each auxin, kinetin is inhibitory to initiation of callusand its growth on the explant. High-intensity light and lowtemperature are also inhibitory. In isolated callus subcultured so as to prevent dilution ofits accumulated auxin, the only effect of varying exogenousauxin levels is as a progressive inhibition by NAA. If thisdilution is permitted, however, 2,4-D and IAA have an optimumgrowth promoting activity at 1 mg/1, whereas the effect of NAAincreases up to 10 mg/1. The growth of the callus is affectedby agar concentration (1 per cent optimum), and is halted bypH values below 5. The callus grows on various carbon sourcesbut is dependent upon one or more components of N. Z. Amine;it also requires a number of micronutrients. A suspension culture from the callus exhibits the usual growthcurve. The phenolic content follows a pattern different fromthat of growth, protein, and RNA content, and phenolics arerapidly synthesized as active growth ceases. In contrast tothe callus tissue, the suspension culture grows at a wide rangeof pH values and buffers the medium. At low temperatures in the light, potato discs produce greencallus with a chlorophyll content corresponding to that of thediscs from which they grew. The isolated callus tissue doesnot require kinetin and produces and excretes its own cytokinin(s).The amount synthesized varies over the growth cycle.