The Interrelation between CO(2) Metabolism and Photoperiodism in Kalanchoë. II. Effect of Prolonged Darkness and High Temperatures.

When Kalanchoe Blossfeldiana is exposed to repeated short-days there is a marked increase in both the amount and the rate of the fixation of carbon dioxide in the dark (3). Plants which have received a series of short day and long night cycles show an uptake of CO2 in each dark period which begins slowly, increases to a maximum and falls again; finally it reaches zero and is followed by a brief period of CO2 production just before the end of a 16-hour dark period. If similar plants were kept in darkness for a period longer than 16 hours, there was no further CO2 uptake but rather a steady loss of CO2 by respiration. During the light period, on the other hand, the opposite phenomenon occurred; CO2 was given off soon after illumination had begun, and this also passed through a maximum and came to an end before the end of the 8-hour light period. It was also found that the amount of CO2 evolved paralleled the amount of CO2 absorbed during the preceding dark period. This uptake of CO2 in dark and release in light only occurred to a very small extent in plants exposed to 8-hour nights or to long nights interrupted with a brief light period. Tentatively the phenomenon was ascribed to the formation of a light-labile or heat-labile CO2 compound, probably an organic acid. The parallelism between the effect of photoperiodic treatment on CO2 metabolism and its influence on flowering in this short-day plant is obviously very suggestive. However, before this parallelism can be pursued, the exact role of the light period in causing release of CO2 needs further investigation. In the preceding experiments (3) it was not determined whether the CO2 release is due to light itself or to high temperature. This uncertainty stems from the fact that when the lights come on, although the temperature of the room remains constant, the temperature within the Lucite chamber containing the plant is raised to about 290 C. The temperature within the leaves doubtless rises even higher. It has been shown by Wolf (8) that organic acids are used up in Bryophyllum leaves kept in the dark at high temperatures, and other workers have demonstrated that acid formation by succulents is depressed at higher temperatures (1, 7). It was possible, therefore, that the production of CO2 in light, particularly if it arises from an organic acid, might have been due merely to the temperature. It must also be borne in mind that the photoperiodic response of many plants, in regard to flowering, can be extensively modified by temperature.