If carbon dioxide fertilization is not practical on a field scale, crop producers may have to rely on CO2 replenishment by air currents. Based on diffusion theory, Waggoner, Moss, and Hesketh (1963) predicted an increase in photosynthesis by increasing the turbulence of air and hence of CO2 movement, even though they observed that according to previous reports still air was not a common phenomenon outdoors. On a macro or large-scale basis at a 50-cm (20 in) height above a forage crop, calm air occurred less than l%of all daylight hours, and half of the hours had wind speeds in excess of 133 cm per sec (3 mph). On a micro basis, air turbulence around the leaf also affects the rate at which CO2 diffuses into a leaf by modifying leaf resistance to Co movement. A sugarcane leaf in very still air at CO2 concentrations of 200 and 300 ppm showed differences in the rates of photosynthesis (Figure 9-4) Stirring the air containing 200 ppm increased photosynthesis to a level equal to that of the upper curve, i.e., as fast as increasing CO2 levels to 300 ppm in the quiet air