I'm interested in modelling leaf photosynthesis with Farquhar model.
Briefly, this approach assumes that the rate of carboxylation (Vc) is the minimum of three processes such that Vc = min (Ac, Aj, Ap), where Ap is the triose phosphates limited photosynthetic rate.
- In some cases, when net assimilation rate (An) is written as
An = Vc - Rd, the TPU limited photosynthetic rate is given by
Ap = 3TPU (Kim and Lieth, 2003; Prieto et al., 2012, Chen et al., 2008; Sharkey et al., 2007).
- However, in the first papers dealing with TPU limitation (Harley et al, 1991 and 1992),
An = (1- gamma*/Ci)* Vc - Rd and TPU is then given by
Ap = 3TPU + betaVo.
As a result, when TPU is limiting,
An = (1- gamma*/Ci)*(3TPU + betaVo) - Rd (also found in Braune et al., 2009), whereas in the former point, it would be An = 3TPU - Rd.
I therefore would like to ask you 2 questions:
- I found many writings for Ap in the literature, sometimes An = Ap, sometimes An = (1- gamma*/Ci)*Ap...it's a little bit confuse for me, can you explain to me which equation should be considered?
- Why the term "betaVo" is sometimes omitted when estimating Ap?
There are two types of limitations by TPU, which can be separated by the behavior of An at high concentrations of Ci. In the first case it is a straight line (Ap = 3TPU) and the in the second case, decreases at high values of Ci. The "betaVo" is used only in the second case
Sharkey (Bernacchi Farquhar Singsaas) in 2007 PCE wrote the reference (correct) model of triose phosphate use but made clear also the quite rare combination of factors that can generate a TPU limited phase. One question about these models is producing good evidences that electron transport measured with chl fluorescence decreases with increases in CO2 or that CO2 concentration in leaf is the right one. TPU limitation is a stress probably due to unbalance between source and sinks or temperature gradients and its recovery trend is worth of attention. J or Vcm limitations are not a stress.
Many thanks for your explanations.
There is still a point that is unclear to me:
If I write An = min (Ac,Aj,Ap) in a model, which equation(s) is (are) correct for Ap?
(a) Ap = 3TPU*(1-gamma*/Ci) or,
(b) Ap = 3TPU
Thank you
We could spend time about what is rightness. I guess that the reference of Braune is like the book chapter with Muller and Diepenbrock in 2009 and this sort of paper review is concerned with barley and as far I know the original earlier papers of (Sharkey) and Harley in 1991-2 . The safer assumption is TD Sharkey 2007 one, that is 3TPU-Rd. To take a) you should have a better fitting considering a Cc dependence (P. heck the diffrence between Cc and Ci). Leave the choices to an editor but be shure about J lowering when Cc is increased. Probably a leaf chamber with a feeder ring can produce quite stable P limitations.
Sorry for verbosity, literally from Sharkey et al 2007: (P limitation is apparent under variation in CO2 with)...........no increase in A with increasing [CO2]or a decrease (of A) with increasing [CO2], but the reverse sensitivity is not reliable enough to model'.
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