The Initiation, Growth, and Characteristics of a Tissue 2

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Abstract
The rate and extent of initiation of callus from potato tuber discs depends on the concentrations of auxin and kinetin in the medium on which they are grown. NAA is the most effective auxin, initiating callus at a concentration (0. 01 mg/1) an order of magnitude lower than for IAA or 2,4-D. There is a week's lag before initiation begins with IAA or 2,4-D. In combination with each auxin, kinetin is inhibitory to initiation of callus and its growth on the explant. High-intensity light and low temperature are also inhibitory. In isolated callus subcultured so as to prevent dilution of its accumulated auxin, the only effect of varying exogenous auxin levels is as a progressive inhibition by NAA. If this dilution is permitted, however, 2,4-D and IAA have an optimum growth promoting activity at 1 mg/1, whereas the effect of NAA increases up to 10 mg/1. The growth of the callus is affected by agar concentration (1 per cent optimum), and is halted by pH values below 5. The callus grows on various carbon sources but 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 growth curve. The phenolic content follows a pattern different from that of growth, protein, and RNA content, and phenolics are rapidly synthesized as active growth ceases. In contrast to the callus tissue, the suspension culture grows at a wide range of pH values and buffers the medium. At low temperatures in the light, potato discs produce green callus with a chlorophyll content corresponding to that of the discs from which they grew. The isolated callus tissue does not require kinetin and produces and excretes its own cytokinin(s). The amount synthesized varies over the growth cycle.

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