The effect of photoperiod length on respiration in leaves of Saxifraga cernua L., an arctic herb

Rates of oxygen uptake were measured in leaves of Saxifraga cernua which had been exposed to an 18‐h photoperiod. These rates were compared to those in plants which had been exposed to continuous light. Rates of total dark respiration and alternative pathway respiration measured at the end of the photoperiod gradually decreased over the initial 3 d of exposure to an 18‐h photoperiod. Thereafter, respiratory rates were constant. Rates of total dark respiration and alternative pathway respiration decreased during the 6h dark period. Rates of normal and alternative pathway respiration are equally affected during the dark period. The respiratory rates had reached a new minimum level 3 d after the initiation of a dark period. These results suggest that respiration rates in arctic plants are high because of the long photoperiod in the arctic. The kinetics of photoperiod induced changes in respiration are slow enough to suggest the involvement of the biological clock in setting respiration rates. Indeed, total dark respiration and alternative pathway respiration show a definite circadian rhythm. Free‐running experiments show that normal respiration changes much less (has a smaller amplitude of variation) than alternative pathway respiration and that alternative pathway respiration accounts for most of the rhythmicity of respiration.