Night temperature drop affects the diurnal photosynthetic rhythm of Kalanchoe blossfeldiana

Abstract

Kalanchoe blossfeldiana is a CAM plant. However, it functions like a C3 plant under normal greenhouse conditions with net CO2 uptake during the light period and net CO2 production during the dark period. Stress conditions such as drought induce a change from C3 to CAM metabolism. Dynamic temperature control in greenhouses as a means of saving energy entails a drop in night temperature. The question was whether a night temperature drop would be perceived as a stress signal, causing the plants to switch from C3 to CAM photosynthesis. In the first experiment, the diurnal CO2 exchange rates (CER) of small plant stands of K. blossfeldiana were measured with and without a temperature drop during the dark period from about 20°C to 6–12°C when grown at a photon flux density (PFD) of 220 µmol m-2 s-1 during 16 h day-1 (8 h dark period). At a constant temperature, CER was relatively constant throughout the photoperiod. A drop to 12°C during the night caused a strong diurnal rhythm with high CER during the first and last hours of the light period and a 75% decrease in CER in the middle of the day as compared to the first hours of the light period. A drop to 6°C resulted in net photosynthesis reaching zero at midday the following day. Low night temperature caused a change from negative to positive CER values during the night, demonstrating a CAM metabolism. In a second experiment, CER was measured at light conditions simulating clear days in a greenhouse in late spring (0–1100 µmol m-2 s-1). A drop to 7°C during the night decreased CER by about 50% from morning to midday, before it then started increasing again. By gradually increasing the diurnal minimum night temperature, the midday CER decrease eventually disappeared. Regression analysis showed a close relationship between the temperature drop and the midday decrease as well as increased CAM metabolism. The response illustrated the genetics of K. blossfeldiana, originating in dry, arid conditions in Madagascar where low temperature during the night is a signal for the plant to protect itself against a warm, dry day by closing the stomata. Dynamic temperature control involving a night temperature drop is not recommended for this species due to the negative effect on photosynthesis when the plants change from C3 to the less efficient CAM metabolism under greenhouse conditions.