Prediction of growth rate at different light levels from measured photosynthesis and respiration rates

Light integrators with a linear response are not suitable for measuring the light climates of plants because plants are not linear integrators. It should be possible to make a quantitative allowance for this nonlinearity by using the CO₂ uptake curve of the plant. To test this, we have subjected white clover plants to different levels of constant light, comparing the rate of increase of total dry matter with the net rate of uptake of CO₂ per day. Temperature, humidity, daylength and nutrient supply were kept constant. The growth rate calculated from CO₂ uptake agreed well with the observed rate over the light levels tested (3.7-88 w·m⁻², 0.4-0.7 micron: 1 w·m⁻² = 10³ erg · sec⁻¹ cm⁻²). All plants put on weight over the few days of the experiment, even those placed at light levels below their compensation point. The plants adapted their respiration rates to be a constant proportion of their growth rates. Most of the adaptation occurred within 24 hours of the light change. The adaptation of respiration has implications for models of light/growth relations in plant communities, almost all of which assume that respiration is proportional to leaf area and independent of growth rate or light level. The only model which does not is that of de Wit, and this gave good agreement with our results.