1970 Volume 39 Issue 2 Pages 185-192
CO2 concentrations in vegetable growing were measured by means of an infra-red gas analyser in order to discuss the CO2 depletion, being considered as one of serious problems for sufficient photosynthesis. Sampling gas was taken occasionally with suction pump controlled by automatic setting box at the rate of 0.3 litre per minute through the plastic tube distributed within and above the plants.
In the open field, minimum CO2 concentrations within the canopy measured on a clear day with and without wind were 275-285ppm and 205-250 ppm. This depletion was caused by mean wind speed below 1.5m/sec. over the plants and by dense plant community.
In the atmosphere under the glass- and plastic-houses, CO2 shortage was more remarkable than that outside. High CO2 accumulated by leaf and soil respiration was avaiable for photosynthesis in the early morning. Very soon, however, the content fell below ambient. Thus, critical light intensity for increasing dC/dt could be assumed to be 3-5 klux on the plant top. Finally, CO2 concentration fell to 80-85ppm provided the vents remained closed.
Measurements of the CO2 profile at the time when plants were well grown in a plastic-house on a sunny day, showed that CO2 depletion was observed twice a day, before and after ventilation. In the first depletion CO2 level dropped to nearly 105ppm and the height of minimum concentration (Zm) was found at the middle part of plant height. When ridge ventilator was opened toa maximum of 25cm, the second depletion was slightly decreased and the concentration remained at 140ppm.
On the other hand, a ventilating fan prevented greatly CO2 depletion and Zm was observed over a wide range of plant height. To prevent the CO2 depletion in the particular part within the canopy, air circulating fan was operated in the morning when the vents were keeping closed. The resulting profile 8m away from the fan, showed that it had no effect on CO2 homogenization in and above the plant.
CO2 depletion was also greatly affected by leaf area density. The concentration in the air of nursery bed covered with vinyl film was depleted to 65-70ppm. High leaf area density lengthened the time course of equilibrium until ventilated.
All of these observations indicate that the ventilation speed and leaf area density are the predominant factors for the CO2 depletion under covered conditions on a sunny day. It may be argued that by keeping the ventilators open CO2 depletion would be avoided. The curves of N. A. R. against the ventilation rate, however, would suggest the air changes of 20-30 times for house volume per hour often occurred in commercial glass-and plastic-houses are not enough CO2 supply within the plant canopy for sufficient photosynthesis.
Beneficial effect of CO2 enrichment during the first CO2 depletion was discussed from practical point of view.