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Leonid Asipov
Leonid Asipov
L-Data Research
Introduction
In this article we’ll show the relation between CO2 concentration in the air and the physiological parameters, photosynthesis and transpiration. We’ve measured the stomata aperture at different CO2 concentrations and came to interesting conclusions regarding the possible trigger for induction of changes in the stomata aperture.
Responses of photosynthesis and transpiration to changes in CO2 concentration
Elevation in CO2 concentration in the plant surrounding air causes decrease in transpiration rate and increase in the rate of carbon fixation. Decrease in CO2 concentration, causes an exactly opposite effect.
Figure 1: Effect of changes in CO2 concentration on photosynthesis and transpiration of Arabidopsis
CO2 concentration has an effect on the transpiration rate in light and in darkness (Figure 2), which suggests that stomata reaction (which causes the changes in leaf conductance to water) to CO2 is light independent. Although in the light, the reaction is stronger.
Figure 2: Effect of changes in CO2 concentration on the transpiration rate of Solanum lycopersicum in light and darkness.
We’ve photographed stomata of Arabidopsis at different CO2 concentrations of the air (figure 3). At 650 uL L-1 CO2 the stomata appear to be more closed than at 80 uL L-1. This data suggest that CO2 may be the trigger for changes in the stomata aperture.
Figure 3: Stomata of Arabidopsis at 650 (left) and 80 (right) uL L-1 CO2.
What is the effect of light on transpiration and why in the light, the response of the stomata is stronger than in darkness (figure 2)?
The answer may be that during the light period, the concentration of CO2 in the leaf surroundings and the intercellular space is lower than darkness due to photosynthesis. If CO2 concentration is the trigger, we would expect stronger responses of stomata in light than in darkness.
Can the concentration in the intercellular space be the trigger for stomata induction?
In the following experiment (figure 4), transpiration, photosynthesis and intercellular CO2 concentration was measured during plants awakening from a period of darkness.
We can see that up-regulation in the transpiration rate happens not immediately with the light turning on (like the photosynthesis), but after a period of time close to 20 minutes. During this period, the intercellular concentration gets below a certain threshold. This experiment was performed in low light levels of 20 µE.
Figure 4: Photosynthesis, transpiration and intercellular CO2 concentration during plants awakening in low light (28uE).
Similar experiment was conducted in higher light levels of 300 µE. As we would have expected in the case that a decline in the intercellular CO2 causes the stomata response, at higher photosynthesis levels the delay between the light turning on and transpiration response should be shorter. This is exactly what we see in the results (figure 5), the delay was shortened from 19 to 7 minutes.
Figure 5: Photosynthesis, transpiration and intercellular CO2 concentration during plants awakening in high light (300 µE).
According to these experiments, intercellular CO2 concentration can be the trigger for the stomata aperture induction. Further research, however, showed that it is most likely not the intercellular but the concentration of CO2 in the close surroundings of the guard cells, and it is needed to relate to the gradient of CO2 from the stomatal pore to the mesophyll cells which perform the carbon fixation.
The problem with Ci (intercellular CO2) concentration parameter is that it does not consider the gradient of CO2 between the stomatal pore and the photosynthesizing mesophyll cells. The traditional Ci parameter conveys to all the intercellular space the same concentration, which is not accurate, since the concentration of CO2 obviously has a gradient between the stomatal pore and the photosynthesizing mesophyl cells (figure 6).
Figure 6: The CO2 gradient in the intercellular space
In the following experiment we’ve measured photosynthesis, transpiration and intercellular [CO2] during stepwise elevation and decrease in the [CO2] in the leaf surroundings (figure 7).
Figure 7: The effect of stepwise CO2 concentration changes on photosynthesis, transpiration and intercellular CO2 concentration.
We see that after the elevation in CO2 concentration, there is a decrease in the transpiration rate and intercellular [CO2]. High CO2 levels cause stomata closure, and due to high photosynthesis rate, the intercellular space becomes less loaded with CO2. Despite the decrease in Ci to very low levels, the stomata continue to be closed, against our previous expectations (figures 4 and 5). Please notice that the previous two experiments were conducted at 400 uL L-1 CO2 and in the one shown in figure 7, the high CO2 point of this experiment was 600 uL L-1. At lower CO2 levels of 400 uL L-1, the gradient between the [CO2] of the surroundings is less sharp than at 600 uL L-1, and thus at 400 uL L-1, Ci may represent more closely the [CO2] near the guard cells. When the gradient is more sharp, the Ci parameter represents less the [CO2] concentration near the guard cells, which is closer to the [CO2] outside. The stomata do not respond due to relatively high levels of [CO2] near the guard cells.
These results may suggest that the trigger for induction of stomata is CO2 concentration near the guard cells and not in the intercellular space.
Materials and Methods
The photosynthesis and transpiration was measured using Li-Cor LI-6400 portable photosynthesis system.
The data was analyzed using Data-Lightning0.1, visual graphical platform, L-Data, Israel.
http://www.ldata.co.il
The plants were wt Arabidopsis Thaliana Columbia strain (figures 1 and 3),
and wt Solanum lycopersicum, of 30 days old (figures 2,4,5, 7).
© L-Data Research LTD
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