Partial Reactions in the Formation of the Floral Stimulus in Xanthium

Abstract

When the photoinductive dark period of Xanthium is immediately followed by a second exposure to darkness a reduction in the flowering response is observed. The effectiveness of the second dark period is enhanced by indoleacetic acid. When the plants are exposed to more than 5 hr of high intensity light following the photoinductive dark period, the second dark treatment is no longer effective, but it is shown that the floral stimulus remains in the leaf for 10 hr following a standard photoinductive dark period. It is concluded that some change must taken place in the nature of the stimulus during the exposure to light rendering the stimulus no longer sensitive to destruction by the second dark treatment. A second dark treatment at low (5[degree]C) temperature is ineffective, but when this treatment is followed by a dark treatment at normal temperature destruction of the stimulus occurs. Since high intensity light at low temperature is fully effective in rendering the stimulus stable to subsequent dark treatment, it is concluded that a photochemical process resulting in a change in the nature of the stimulus normally occurs during photoinduction of Xanthium.