The Initial Burst of CO2 Uptake

The exact causes of the negative effects of drought on CO2 fixation during the day remain to be studied in more detail. However, it seems reasonable to postulate that the stomata are at least partially responsible for the effect. Stomata tend to close as the plant water potential becomes more negative (see Chap. 5.3.4). It seems most unlikely that water stress sufficient to inhibit opening of stomata would have a dramatic effect on photosynthesis itself, thus inhibiting daytime CO2 uptake. 

Under both laboratory and field conditions, a burst of CO2 uptake occurs immediately after the onset of illumination (see Chap.5.1.3.1 and Fig. 5.1). Normally, this initial burst reaches its maximum within 30-60 min after illumination. Subsequently, the CO2 exchange curve declines steeply towards the compensation point. 

An unequivocal explanation of the initial burst cannot yet be provided. It has been observed by Kluge (1968 a) that CO2 produced from malic acid stored during the previous night is utilized after the onset of illumination only after a lag phase of about 30-60 min. The lag phase corresponds with the time needed to reach the maximum CO2 burst. Hence, it has been concluded that during the lag phase of malic acid utilization, photosynthesis depends on ambient CO2. Only later does malic acid-derived CO2 become available for photosynthesis. However, this is only a partial explanation, because the basic question about the cause of the malic acid lag phase is unresolved. This problem is discussed elsewhere (see Chap. 3.1.7). 

---