INTERACTIONS OF CARBON DIOXIDE CONCENTRATION LIGHT INTENSITY AND TEMPERATURE ON PLANT RESISTANCES TO WATER VAPOUR AND CARBON DIOXIDE DIFFUSION

3Exchange of water vapor and carbon dioxide, between whole sunflower plants and an atomospherphere controlled to required CO2 and water vapor concentrations, was measured in a polythene chamber. Plant resistances to diffusion of CO2 and H2O were calculated. From the combined data of net flux of CO2 ("P"), resistances to CO2 uptake, and atmospheric CO2 concentration [CO2]atm the intercellular space CO2 concentration [CO2]int could be calculated. [CO2W was linearly related to [CO2Jatm and this relationship was modified by light intensity. Plant age, between 3 and 6 weeks, did not alter the relationship. The ratio of [CO2]int, to [CO2]atm was shown to be equivalent to the ratio of mesophyll resistance to CO2 diffusion (rm) to total resistance to CO2 diffusion (RcO2)- At a given [CO2]atm [CO2]int decreased with increasing light intensity between 500 and 5500 ft. -candles but was little affected by leaf temperatures between 18 and 34[degree]C. On the other hand, the minimum internal CO2 concentration r, measured as the equilibrium CO2 compensation point, increased linearly over the same temperature range but was little affected by light intensity above 500 ft. -candles. The effects of temperature and light on [CO2]int are in contrast to their effects on the r value. Therefore, it is less likely that temperature effects on [CO2]int, measured in the normal photosynthetic [CO2]atm range, are responsible for "midday stomatal closure" as inferred from observations of temperature effects on r alone. By comparing the effects of temperature and light intensities on stomatal resistance over the same range of [CO2]int values, direct effects of light and temperature, not mediated through changes in [C02]int were demonstrated. The interactions of the factors involved are discussed.