Gas exchange of water vapor and CO2 are controlled by stomata. In many plants, when CO2 increases, there is a reduction of stomatal conductance.
But many species are known to grow more when Co2 increases. How do we reconciliate the two facts ?
If you provide other things such as water, elements, temperature etc, you can increase phothosynthesis . also you have to balance sources with sinks.
The time scale is very important. You meant an instantaneous reaction that is occurring during gas exchange in a cuvette and in minutes range. CO2 rise occurs seasonally and both stomatal density and leaf expansion rates vary. In minutes range most leaves open stomata at low CO2 but the relative closure at high CO2 does not compensate Ci inside the the leaf. Ci tends to increase anyway and photosynthesis increases according to leaf photosynthestic type that can be without saturation in C4. http://www.plantphysiol.org/content/71/4/789.full.pdf.
Thank you very much for your interesting answer. I though that plants would tend to maintain the ratio Ci/CO2 ( CO2 intercellular over ambient) pretty well constant (0.7 for C3 plants and 0.4 for C4 plants). How plants manage to change that ratio then in a higher CO2 environment ?
The investement in Rubisco is under tight control assuring the best assimilation for a given transpiration andt relative Ci can decrease in high CO2. Apparently competitive evolution of leaf population selects the ones than produce more carbon for every transpiration unit. Plants appeared on earth under very high CO2 You may follow hundreds of infos about diffrent sytems by following a good review like http://www.ncbi.nlm.nih.gov/pubmed/24311129
To add to the above discussion, stomatal control is linked not only to CO2 content of the leaf, but also regulates water loss through transpiration. This is a balancing act, whereby the stomata must admit sufficient CO2, whilst not allowing too much water to escape. In a higher CO2 atmosphere the plant is able to close the stomata somewhat and still achieve good internal CO2 levels, whilst reducing water loss, and it is the balance between these processes that matters, more than the actual ratio of internal to external CO2.
As earlier contributors write - increasing CO2 decreases stomatal conductance but also increases substomatal CO2 concentration (Ci). The ratios of Ci/Ca are relatively constant but not rigidly fixed. As at current atmospheric CO2 Ci with open stomata does not saturate photosynthesis in saturating light, increasing Ci increases the rate of photosynthesis. The two are not proportional and what controls the balance is unclear (but perhaps there is an explanation I have missed?) - I feel it is related to increased carbohydrate supply. As Ci rises so does photosynthesis and sucrose production which may then (indirectly) decreases stomatal conductance so ensuring that photosynthesis is always greater with elevated CO2 even if stomatal conductance decreases. The resultant decrease in transpiration is a `bonus' but may even be a secondary effect. This applies mainly to C3's but does occur in C4's.
Hope this helps
David Lawlor
I would like to add that, under drought, C4 crops indeed benefit from elevated atmospheric CO2 to roughly the same extent as C3 crops. Probably, this is due to a stomatal limitation of photosynthesis that also occurs in C4 crops if drought leads to increased stomata closure and a consequently decreased Ci.
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