The rapid spectrographic method of measuring carbon dioxide developed in this laboratory meets this need. In one of these, fluorescence intensity and carbon dioxide uptake rate are inversely related; in the other there is a direct relationship. In each pair, the upper curve is the recording of fluorescence intensity; the lower the carbon dioxide uptake rate is recorded.
Thus, in the recording of the rate of carbon dioxide uptake, there occurs both a "smearing" of the plant response (as discussed above) and a lateral shift due to the time of transit of gas from the plant chamber to the absorption cell. At the same time, the rate of carbon dioxide uptake goes through a series of windings which seem to be correlated with the fluorescence changes. The carbon dioxide readings here are very erratic due to incomplete premixing of the carbon dioxide added to the tank nitrogen.
I for 0.03 percent carbon dioxide in air, and in curve 2 for 0.03 percent carbon dioxide in nitrogen. Unfortunately, our apparatus does not allow us to follow changes in the rate of carbon dioxide uptake that accompany such changes in carbon dioxide pressure, or to measure the final rate of photosynthesis in 4 percent carbon dioxide. The behavior of cells grown in 4 percent carbon dioxide resembles that of wheat, as shown by curves i and 2 of figure 8.
As shown in curves 3 to 8, the rate of carbon dioxide uptake follows a pattern similar to that found by Aufdemgarten (1939a) for Stichococcus. From these curves it is clear that there is a relationship between the minima in the fluorescence intensity and the rate of carbon dioxide uptake.
DISCUSSION
No.6 PHOTOSYNTHESIS McALISTER AND MYERS 21 From our studies over a wide range of intensities it is clear that in low light the proportional changes in fluorescence intensity are small. The fluorescence curves obtained by Wassink and Katz (1939) for Chlorella vulgaris are also not directly comparable to ours due to the very low light intensities used. This discussion will therefore be primarily of a phenomenological nature, considering the relationships between the various observed effects rather than the mechanism by which they are brought about.
INTERPRETATION OF INDUCTION CURVES
During the initial rapid "swallowing" of carbon dioxide (curves i and 2 of Fig. 3) the maximum change in carbon dioxide passes through the absorption cell in much less than 4 seconds. A number of the curves shown above have been redrawn to eliminate as much as possible the instrumental delay in recording the carbon dioxide assimilation rate. This is done by tracing the original curves and moving the carbon dioxide curve to the left, a distance equivalent to the transit time between the plant chamber and the absorption cell.
The curves are also corrected for the reduced sensitivity at higher carbon dioxide concentration, which is characteristic of the spectrographic method. In addition, a dashed line ( ) has been added to show the likely course of carbon dioxide assimilation in the plant, which would give the recorded curve. Here there is a strictly inverse relationship between the rate of carbon dioxide assimilation and the intensity of fluorescence.
However, the near-perfect mirror-image relationship of the two curves is merely coincidental in terms of ordinate height, since fluorescence intensity and carbon dioxide assimilation rate are recorded in arbitrary and independently chosen units. Let us assume that the broken line (behavior in low oxygen) represents an approach to an idealized case in which there is a strictly inverse relationship between the rate of assimilation of carbon dioxide and the intensity of fluorescence. Therefore, the hatched area between the lines would represent the extent of a reaction that reduces both the fluorescence intensity and the carbon dioxide assimilation rate.
In one of these, the rate of carbon dioxide uptake and fluorescence intensity are inversely related; in the other, directly. A dashed line was then plotted on the carbon dioxide curve so that the rate of uptake shown is always inversely proportional to the fluorescence intensity. In the case of wheat with a high content of carbon dioxide (Fig. 13, A), it is clear that the reaction takes place during the time of the fluorescence minimum, which involves a direct relationship between carbon dioxide and fluorescence.
For convenience, the ordinate scale for the carbon dioxide uptake rate at A has been reduced to -l. 14.-Induction behavior at 0.03 percent carbon dioxide and high light after 10 minutes dark rest for Chlorella grown in air (A) and for wheat (B)., curves derived from fig. 6.
STEADY-STATE RELATIONS
14.-Induction behavior in 0.03 percent carbon dioxide and high light after 10 minutes of dark rest for Chlorella grown in air (A) and for wheat (B)., The curves are derived from 6 of fig. this difference is simply due to the predominance of one or the other of at least two different processes in these extreme cases. collect additional data that may be of help in interpreting the relationship between fluorescence and photosynthesis shown by induction studies. Similar studies were made by Wassink et al. 1938), who measured the intensity of fluorescence and rate of oxygen production under steady-state conditions. They report no change in the state of fluorescence (i.e., intensity of fluorescence proportional to incident intensity) in the transition from a light-limiting state of photosynthesis to light saturation.
Nor was fluorescence intensity affected by any of the many conditions that markedly affected the rate of photosynthesis (temperature, partial inhibition by cyanide, oxygen pressure). In a later paper Wassink and Katz (1939) showed an increase in fluorescence intensity due to complete inhibition of photosynthesis by cyanide. Full saturation was apparently reached at this intensity in the carbonate-bicarbonate buffer number 9 of Warburg (1920).
For the two cases in air (duplicate experiments), the intensity of fluorescence is seen to increase above the original straight line simultaneously with a marked deviation from light-limiting conditions. Here, the wheat was probably completely under light-limiting conditions at 4 percent carbon dioxide, but at 0.03 percent carbon dioxide, carbon dioxide limitation begins at a relatively low incident intensity (cf. Fig. 15), and the fluorescence intensity rises above the initial line. Warburg (1920) has shown that Chlorella in high light and high carbon dioxide produces oxygen at a considerably greater rate in 2 per cent than in 20 per cent oxygen.
As Warburg pointed out, this suggests that going from high to low oxygen reduces the rate of a reaction involving oxygen and opposing photosynthesis, and consequently the rate of carbon dioxide assimilation. The intensity of fluorescence is lower in this experiment in 0.5 percent oxygen than in normal air. The reduction of the oxygen pressure then allows a greater rate of photosynthesis and the fluorescence is consequently reduced.
Further experiments, where the transition from the induction phase to the steady state is monitored more closely, are.
DATA OF 5-7-40
ON WHEAT
LIGHT INTENSITY (ERGS/CM*/SEC)
The only marked departure we find seems to be due to a limitation of carbon dioxide, whereas with number 9. At this higher intensity we find that the assimilation of carbon dioxide at 0.4 percent carbon dioxide. The sharp end of these curves (in darkness) represents the clearance from the system of a 15-second accumulation of respiratory carbon dioxide after resumption of airflow.
It is conceivable that three types of reactions evolve towards a steady state during this period: I. photosynthesis, which involves the reduction of carbon dioxide; Either of these two hypotheses can be made to account for the induction of both carbon dioxide uptake and wheat fluorescence in 0.03 percent carbon dioxide (Fig. 12). The induction of carbon dioxide uptake is due to the carbon dioxide (or carbon dioxide sparing substance) produced by the initial photooxidation.
This also means that there is a direct relationship between the rate of carbon dioxide production and the intensity of fluorescence, i.e. inverse relationship between carbon dioxide uptake rate and fluorescence intensity. There is an initial "gulp" of carbon dioxide and a momentary quenching of fluorescence, corresponding to the amount of "intermediate". The first is certainly inconsistent with a burst of fluorescence caused by a sudden increase in carbon dioxide concentration (curves 3 and 4 in Fig. 6), and complicates any interpretation of the several observed cases in which carbon dioxide and fluorescence are directly related (Fig.
The second does not explain the inverse relationship between the second carbon dioxide minimum and the second fluorescence maximum found in wheat at high carbon dioxide concentration (fig. 13, A). From this second point of view the induction shown by wheat in 0.03 per cent carbon dioxide in nitrogen is mainly the build-up of the rate of photosynthesis accompanied by a decaying fluorescence." Cells grown in 4 per cent carbon dioxide show , when first studied, an induction generally similar to that of wheat (as fig. 13, B).
After several hours of light in 0.03 percent carbon dioxide, the fluorescence curve develops a distinct minimum during the induction period. 34; This experience can be related to the observation of Aufdemgarten (1939b) that the minimum in the induction curve of carbon dioxide uptake depends on the composition of the nutrient media used.
SUMMARY
At the same time, a minimum rate of carbon dioxide uptake is observed, which clearly shows an inverse relationship with this second. Curves relating the intensity of the fluorescence and the rate of carbon dioxide uptake to the intensity of the incident were obtained from measurements performed under steady-state conditions after the induction period. These show a clear change in fluorescence upon transition from light-limiting to carbon dioxide-limiting conditions.
The fluorescence intensity rises above the initial flat line simultaneously with a marked departure from light-limiting conditions. The rate of carbon dioxide assimilation in wheat under bright light and 0.03 percent carbon dioxide is 30 to 50 percent greater at 0.5 percent than at 20 percent oxygen. This suggests that, in young wheat, a large-scale reaction opposing photosynthesis always reduces the rate of carbon dioxide assimilation under natural growing conditions.
LITERATURE CITED
