Climate change is causing increase in concentration of ambient CO2. Increase in CO2 concentration increases the specificity of RUBISCO to CO2. Will it reduce unwanted photorespiration, which is a big drain on the energy of plants?
Mostly CO2 is causing climate change and yes it will reduce photorespiration and allow plants to survive in more arid environments. There is a lot of research on this...
Mostly CO2 is causing climate change and yes it will reduce photorespiration and allow plants to survive in more arid environments. There is a lot of research on this...
I n addition to what mentioned by David Stern, noteworthy that increased CO2 concentrations had caused increase in yield as well. In fact this is part of homeostasis of biosphere regarding CO2 level, which causes more CO2 fixation in response to elevated worldwide CO2 emissions. Interestingly this is part of answer to the old question that why RUBISCO had not evolved in such a way to stop the oxidative reaction. Photorespiration has been an essential part of world CO2 homeostasis!
In section 8.4 of the following review paper, I summerized some related studies on this issue.
Wang, K., and R. E. Dickinson (2012), A review of global terrestrial evapotranspiration: Observation, modeling, climatology, and climatic variability, Rev. Geophys., 50, RG2005, doi:10.1029/2011RG000373.
There are several ways to look at the reduction in “photorespiration,” which is better termed as “photosynthetic carbon oxidation,” since it is not an energy-yielding respiration. First, you can look at the empirical response curves of photosynthesis (photosynthetic carbon fixation), A, as a function of internal CO2 level in the leaf (Ci, often used in units of partial pressure of CO2). These A-Ci curves are monotonically increasing with Ci, and Ci rises as ambient CO2 rises. Second, you can look at more fundamental enzyme-kinetic expressions for both light-saturated (Asat) and light-limited (ALL) rates of photosynthesis (by C3 plants; C4s are different):
Asat=Vc,max (Ci – gamma)/(Ci + KCO)
Here, Vc,max is the maximal carboxylation capacity per unit leaf area, closely related to leaf N content per area, and modulated by temperature (activity of Rubisco enzyme increases with T, up to a point). Gamma is a photorespiratory offset, increasing with temperature and with the ratio of O2 to CO2 and adversely affecting photosynthesis. KCO is an effective Michaelis constant for the binding of CO2 to Rubisco, and it, too, increases with temperature and with the ratio of O2 to CO2, also adversely affecting photosynthesis. You can find their formulas, as in the classic paper of Farquhar, von Caemmerer, and Berry (Plant Physiology, 1980). This formula assumes that other limitations are not primary (electron transport or triose-phosphate transport).
For light-limited carbon fixation:
ALL = Q00 IL (Ci – gamma)/(Ci + 2 gamma)
Here Q00 is a basically universal constant and IL is the absorbed PAR flux density. Obviously, this rate decreases with increasing O2:CO2 ratio.
Thus, both rates, and the transitional rate between them, increase as CO2 increases at constant O2. To be more complete, we must relate Ci to the CO2 partial pressure in the air outside the leaf, Ca. This gets a bit complicated. We can equate the, say, light-saturated rate to the rate set by CO2 transport through the stomata and leaf boundary layer,
A = gbs’ (Ca-Ci)/P = Vc,max (Ci – gamma)/(Ci + KCO)
where gbs’ is the molar conductance for CO2 offered by the stomata and boundary layer and P is the total air pressure. Then gather all the terms in a quadratic equation to solve for Ci as a function of Ca. It’s not simple, because stomatal conductance responds to factors that included the photosynthetic rate itself. Suffice it to say that a fair approximation is that Ci is about 0.7 times Ca for C3 plants. So, as Ca (atmospheric CO2) rises, gamma and KCO fall, Ci rises, and so does A, the photosynthetic rate.
There are corollary effects of rising CO2 in the atmosphere that affect plants and their photosynthetic rates. Consider especially rising temperatures and altered precipitation (up in wet areas, down in dry areas, in general). Rising T increases gamma and KCO, countering some gains in photosynthesis, A, from higher Ci itself. Rising T also activates the carboxylation capacity, Vc,max, but for many plants they often do their peak seasonal photosynthesis at T close to their optimum. The magnitude of Vc,max has an intrinsic response to T, a rising exponential, but there is also control of Vc,max by Rubisco activase. Activase has an optimum T, after which increasing T makes its activity fall. Some plants, such as maize, have an alternative Rubisco activase that takes over at higher T and extends the photosynthetic performance to these higher temperatures. The effects of altered precipitation are complex to predict. Broadly speaking, at high CO2 (high Ca), plant water-use efficiency increases, so that lower precipitation, P, may not be deleterious…except that lower annual P usually means greater short-term variability in P, that is, droughts that can damage many plant functions. There is a great deal of research on the combined effects, aimed at developing adaptive methods of farming, policies for agricultural support and for climate-change action, etc. You’ve opened up a very broad, fascinating, and, ultimately, scary set of questions.
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The ultimate end of lowering photorespiration can be reassimilation by C4 transition. If one integrates CO2 balance from sources to night respiration and to consumers, it is evident that even this approach as any other involving rubisco or pepCO2 on annual plants, has the effect of increasing fast carbon pool. It is therefore advisable to store carbon on long term pools like wood, soil protein or even carbonates in the sea. Lastly an increased CO2 in the ambient may increase photorespiration under thermal or drought stress. It is therefore necessary to analyse a system in context (species, carbon turnover, stress, energy and water) in order to assess a phorespiration footprint on the balance
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