Circadian rhythms in Kalanchoë: effects of irradiance and temperature on gas exchange and carbon metabolism

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Abstract
Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO2 uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m-2 and 24.0 h at 10 W m-2. In bright light (≧80 W m-2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO2-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO2 uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m-2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.