Effect of a change in photoperiod on subsequent CO2 exchange.

The effect of a shift from a long to a short photoperiod on CO2 exchange of Chenopodium polyspermum was studied. Equal quantities of photosynthetic energy were given daily to the plants, long photoperiods being produced by low intensity red light extension. A change in the photoperiod was shown to affect the pattern of CO2 loss at the beginning of the night period and the onset of CO2 intake at the beginning of day time. These events seem to be under phytochrome control.The photoperiod had an effect on the slope of the CO2 curve of photosynthesis, efficiency of photosynthesis being increased after a short day. This effect was not due to a variation in the stomatal resistance.The action of O2 concentration on photosynthesis (Warburg effect) was affected by the photoperiodic treatment, being less important after a long day than after a short day.Involvement of phytochrome in photosynthetic efficiency and photorespiration is discussed.