Studies on the role of photosynthesis in the photoperiodic induction of flowering in the short-day plants Kalanchoe blossfeldiana Poellniz and Xanthium pensylvanicum Wallr. I. The requirement for CO2 during photoperiodic induction

It has been established that Kalanchoe blossfeldiana and Xanthium pensylvanicum require CO2 during the light period of short days for successful photoperiodic induction of flowering, even if all but the induced leaf are held in normal air. In X. pensylvanicum floral induction in normal air was independent of the starch status of the leaves but when reserves were reduced, lack of CO2 in the light suppressed floral induction to an even greater extent. Injection into the induced leaf (Kalanchoe) or leaf tip feeding (Xanthium) of carbohydrates, organic and amino acids or several other metabolites failed to substitute for the CO2 requirement for induction. A small response was produced by 10 mg ml-1 sucrose in X. pensylvanicum while in normal air 25 parts 10-6 ATP reduced the time to flowering in K. blossfeldiana and 10-4 M proline was inhibitory. An experiment on the light requirement established a need for red light (λ max 660 nm) during photoperiods but red light alone did not facilitate maximal induction. It is concluded that some early, possibly labile, product of photosynthetic CO2 fixation is essential to floral induction in these species.