Fig. 1. Rate of respiration (R) plotted against time of day, for 5 plants, in increasing order of weight from the bottom of the chart. The plants had been in a cycle of 12 hours or light at 100W m^-2 of photosynthetically active radiation (400 to 700 nm), and 12 hours of darkness. At zero time, which is the start of the normal photoperiod, the irradiance was changed to 5W m^-2, in the case of the 5 lowest curves on the chart. The top set of 4 curves was obtained with one large plant, the irradiance being reduced 0, 3, 6 and 9 hours after the start of the photoperiod.