| Pith explants of Nicotiana tabacum L. (3.5 cm long, 0.43 cm in diameter) were cultured 14 days in media with IAA, kinetin, or with both hormones (10 (mu)moles/1 each), under 12 hr light/dark cycles (2,310 lux) or in darkness. In addition, some explants received pressure. A significant frequency of failure to produce callus and xylem occurred under IAA-light/dark cycles, as compared to IAA-darkness, or to IAA-kinetin irrespective of light. Failure to respond was significantly more frequent at the base or top than at the middle of the stem. Unresponsiveness is apparently caused by lack of cytokinin and/or the presence of an inhibitor of growth. Auxin-kinetin-mechanical stresses produced 100 percent frequencies of xylem differentiation.;Treatment effects on average total yields of xylem were analyzed by t-test. Kinetin was equally effective in inducing xylem under light/dark cycles and in darkness. The mean yield of xylem for any of these two treatments was much higher than for IAA-light/dark cycles. Auxin-darkness significantly increased the yield of xylem, as compared to auxin-light/dark cycles. The mean yields for kinetin-auxin were significantly higher than those for auxin-darkness. Irrespective of light, mechanical stresses significantly increased the yields of xylem in IAA-kinetin medium, as compared to other treatments.;Differentiation of fiber-like cells occurred in both media, irrespective of light. However, there was a light-kinetin significant positive interactive effect. The combination IAA-kinetin with light/dark cycles was as fibrogenic as IAA-kinetin with light/dark cycles plus pressure. Under darkness, IAA-kinetin-pressure produced no fibers.;Yield patterns of xylem were characteristic for each treatment. Kinetin increased the formation of xylem at the middle of the stem. Light significantly decreased it at the top, whereas pressure significantly increased it both at the top and base. |
