Xylem and nonxylem cell formation in cytokinin-stimulated root tissue: Quantitative analysis of temperature effects.

Xylogenesis in mature root parenchyma is elicited in vitro by the concerted action of the hormones cytokinin and auxin. Decreasing the temperature delayed the initiation of xylogenesis relative to the initiation of nonxylem cell formation. Once xylem or nonxylem cell formation was initiated, its rate was not temperature-sensitive within the 10-25 degrees C range. Lowering the temperature after xylem formation had been initiated had little effect on xylem cell formation. Xylogenesis sensitivity to low temperature was demonstrable from the time cultures were established, even though cell division did not occur until 48-72 hr later and overt xylogenesis did not occur until 6 or 7 days later. The cold sensitivity of xylogenesis is not limited to a unique or early phase of culture.