The Effect of Air Supply on Apparent Photosynthesis

It has been recognized for a long time that the rate of absorption of C02 by a green plant shoot or leaf enclosed in a vessel is affected by the rate at which air is supplied. Much attention has been given to the problem of maintaining a "normal" air supply ; that is, a C02 supply comparable to that found under natural conditions. The early literature was reviewed and discussed by Heinicke and Hoffman (5), who presented results showing that the rate of photosynthesis was below normal when the air supply was less than approximately two liters per hour per square centimeter of leaf area. Verduin and Loomis (9) presented results from which they concluded that photosynthesis of corn leaves "was affected surprisingly little" by 70% depletion of the C02 within the leaf chambers. Brown and Escombe (1) presented results which they interpreted as indicating an approximate proportionality between the rate of apparent photosynthesis and the mean C02 concentration. They pointed out that "in all cases where the illumination of the leaf was good, although the amount of intake of C02 into the leaf was approximately proportional to the increased partial pressure, the photosynthetic work was always somewhat in excess of what might be expected from the increased amount of C02. ' ' They were obviously expecting strict proportionality and had overlooked some of the following facts. Proportionality implies a linear relationship. General linear relationship is described by the equation y = a + bx ; but strict proportionality exists only when a = 0 and y = bx. Apparent photosynthesis becomes zero at a measurable C02 concentration (the compensation point) ; therefore a ^ 0, and strict proportionality cannot exist. The data of Brown and Escombe actually contain very little information concerning the real relationship between photosynthesis and C02 concentration. They made only two observations on each plant, one at high C02 concentration and one at low concentration. Thus only two points were established for each plant, and a linear function can be derived from any two points whether established experimentally or selected at random. Further, a variety of functions other than linear can be fitted to any two points, for example, y = axn, y = a + 1/x. Data presented by Deneke (4) indicated that the rate of photosynthesis increased with the velocity of air over the leaves and apparently approached a maximum limit at an air velocity of approximately 100 meters per minute. He circulated air over a plant in a closed system and measured the time required for a certain reduction of C02 content. One set of data, his figure 8, is reproduced here as figure 1. This and similar curves suggested to