The Interrelation between CO 2 Metabolism and Photoperiodism in Kalanchoë.

Abstract

Many studies have been made of the flowering response of plants exposed to alternating conditions of light and darkness. The present state of knowledge on this subject is reviewed by Lang (12). These studies have given rise to a number of schemes which attempt to explain photoperiodism by means of specific reactions which are supposed to occur in the dark and light phases of photoperiodic cycles, and by the formation in the leaf of flower-promoting and/or inhibiting substances under the influence of light and dark reactions. Diurnal endogenous rhythms of alter-niating "photophil" and "skotophil" phases have also been invoked to account for the varying behavior of long and short day plants (4), but no direct evidence of diurnal changes in metabolism has so far been presented to substantiate this theory. In contrast with this wealth of speculation little has been done to investigate changes in the metabolism of plants, resulting from alterations in the photo-periodic regimes to which the plants are submitted. In spite of the well-established fact that a supply of CO2 during the light phase is essential for the flowering response of short day plants (soybean: Parker and Borthwick, 13; Kalanchoe: Harder and von Witsch, 11) the change in assimilation rate during induction has not been directly investigated, nor has the need for CO2 during the dark phase of the cycle. Some interesting experiments have been carried out on isolated leaves and shoots taken from plants exposed to different day-lengths (Bode, 2; Schmitz, 15). These data, which will be taken up in more detail later, apparently show that, in Kalanchoe plants, the length of day affects both the photosynthetic rate in the light and the pattern of CO2 production in the dark. The suggestive nature of these findings, and particularly the general absence of basic information in this field, indicated the need for a more thorough investigation of assimilatory and respiratory changes in relation to photoperiodism. Such an investigation, it was felt, must of necessity be carried out with whole plants, at least at first, so that correlations between metabolic behavior and flowering could be made where possible. It is well known that the group of plants known as succulents have diurnal fluctuations in their organic acid level and that they have the ability Texas. amounts of carbon dioxide in the dark (1, 3, 5, 18, 19, 20). With these facts in mind, it was decided to study the CO2 metabolism of a succulent plant as a function of photoperiod. The plant selected for study was Kalanchoe Bloss-feldiana, a short-day plant, whose compactness, ease of handling, and slow growth rate make it well suited to the present purpose. It had already been used for investigations both of flowering (Harder, 9) and of leaf metabolism (2, 15). The only disadvantage of Kalanchoe is the large number of short days needed for floral induction. Under the conditions of temperature and light used, a minimum of 13 or 14 short days are required for any flower primordia to be subsequently formed, while a maximum response is only obtained with 20 to 25 short days. This disadvantage has so far not proved serious. MATERIALS AND MIETHODS Kalanchoe Blossfeldiana var. Tom Thumb was used for most of the experiments described, but in the preliminary experiments at Imperial College, London, another dwarf variety, Ernst Theide, was employed. The plants at Harvard were raised from cuttings taken from plants which had been maintained on long days. They were then grown in air-conditioned light-rooms where the temperature was maintained at 19°C and the relative humidity at 75 %. The length of day was maintained at 16 hours. A light intensity of 1500 fc at the level of the plants was obtained by using incandescent and fluorescent lamps, balanced in such a way as to produce a spectral distribution of light intensity similar to that of sunlight. For most of these experiments, the plants were enclosed in gas-tight chambers as shown in figure 1. The seal is effected by affixing a slit piece of 7/16" (O.D.) gum rubber tubing around the lower portion of the stem by filling in the space with latex. The tubing is slightly larger than the hole in the brass base plate of the chamber. The base plate is assembled and when bolted together forms a gas-tight seal around the rubber tubing enclosing the stem. The Lucite cylinder slips over the plant, and is pressed against a rubber gasket in the base plate by another brass plate at the upper end. Air is circulated by means of a small inlet nozzle in the base plate and an outlet in the top of the Lucite cylinder. The orifice of the inlet is only 0.5 mm in diameter, and this causes the air to enter in a jet, thereby facilitating mixing. When tested, the chamber did not leak even when subjected to 14 to 20 lbs pressure per square inch, which is many times that used. This type of chamber has proven to be very effective, especially since long-term experiments with the same plant are possible. The use of a gas-tight chamber, though essential 220