Carbon dioxide compensation points of leaves and stems and their relation to net photosynthesis

The interactions between CO2 and H2O vapor exchange of the leaf and respirant organs like stems were studied in tobacco plants. The results were analyzed according to a suggested model. Good agreement between open and closed system measurements supported the validity of the model. The measured over-all resistance to CO2 of a leaf and a stem enclosed in a measuring cuvette was the same as the measured resistance of the leaf when measured alone provided the resistance of the stem to CO2 is relatively high. The combined CO2 compensation concentration of a leaf and stem having high resistance to CO2 was higher than the CO2 compensation point of the leaf alone, by the magnitude of rate of CO2 evolution from the stem multiplied by the overall resistance of the leaf. CO2 evolution into CO2-free air was found to be higher in light than in dark in leaves, while the reverse was true for stems. It was concluded that normally the CO2 compensation point of a leaf is unaffected by stomata and boundary layer resistance while the combined CO2 compensation point of a leaf and a stem differs in its nature since it represents a steady state of photosynthesis in which stem contribution, $I_{a}$, is equal to net photosynthesis, $I_{s}$. Interpretation of the experimental data shows tht respiration in the light is unaffected by external CO2 concentration (at the range of 0-300 μl liter) and by intensity of photosynthesis.