CAM photosynthesis saturation / trying to parse a paper

  • #1
100
6
Hi - I'm working to accumulate data on photosynthetic flux saturation levels across a range of plant species, and am having a bit of trouble understanding this paper:

https://onlinelibrary.wiley.com/doi/epdf/10.3732/ajb.94.8.1344

Table 5 shows PPFD vs. ETR, and shows photosynthesis saturating out at varying levels between different cactus species (CAM), but generally on the order of several hundred umol/m²/s. There are two things I find strange things with this.

Issue #1: The saturation fluxes are at far below a typical maximum sunlight flux (~2200 umol/m²/s), unusually low levels, despite these being all "full sun" plants. My two hypotheses as to how this could be the case seem to be contradicted by the paper.

1a) "It's saturating at low light levels, but strong sunlight is still needed to prevent etoliation". But the paper talks about their "low values for dissipation of thermal energy (NPQ) ... indicate that these plants use a great proportion of the light that they absorb for photosynthesis". They couldn't be calling "saturation at a couple hundred umol/m²/s" as meaning "a great proportion of the light", could they? Plants like tomatoes and bananas don't saturate even in full sunlight. Also, what would even be the point of etoliation if you didn't need (and couldn't use) more light?

1b) "This data only applies to seedlings" - but the paper repeatedly stresses otherwise. For example: "It is worth mentioning that the photosynthetic efficiency of the young seedlings (7 d old) is comparable to that of the adult plants and the saturating light levels for ETR (PPFDsat ranges from 231 to 361 umol/m²/s) for nocturanl CO2 update and maximal nocturnal acid accumulation of agaves and cacti. The response of photosynthetic efficiency and ETR to increasing PPDF could indicate that there was no immaturity in the photosynthetic apparatus of the seedlings, as Loza-cornejo et al. (2003) inferred"

Issue #2: Conflating umol/m²/s and mol/m²/d gets confusing, particularly when talking about CAM plants; the former is instantaneous while the latter is accumulative. The paper seems to both simultaneously suggest that the limiting factor is CO2 fixation, a factor for which only mol/m²/d should matter (e.g.: "... seedlings are close to the saturating light levels (500 umol/m²/s) for nocturnal CO2 update and maximal nocturnal acid accumulation..."), but also that the limiting factor is PSII, for which only instantaneous umol/m²/s should matter ("The light level at which PSII becomes saturated (PPFDsat) is related to acclimatization to the light environment in which the seedlings grew (Nobel, 1988; Einhom et al, 2004), and this is supported by our results. That is, PSII is saturated at lower light levels in the seedlings that grew under LL conditions...").

Related to that: if nighttime CO2 fixation / acid accumulation is the true daily limit, are they *actually* testing an instantaneous ETR response to a given umol/m²/s flux, or are they (unstated) converting a daily mol/m²/s to an "average umol/m²/s" over some unspecified photoperiod?

Can anyone help clear up what's going on here?
 
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Answers and Replies

  • #3
100
6
Partial answer - your expectations may not match what is really out there. If I understand your confusion

CAM and C4 and C3 have different photosynthetic efficiencies - See pp 200-201
of:

https://www.eeb.ucla.edu/Faculty/Nobel/Reprints/223 Nobel 1991 New Phytol.pdf

And part 2: it does appear that nighttime CO2 uptake for CAM is a limiting factor.
The question is not about efficiency, but maximum useful PPFD. I'm (as mentioned) trying to gather saturation levels (for the design of cultivation environments). The paper reports saturation at levels a small fraction of full sunlight (a few hundred umol/m²/s, versus full sun of around 2200 umol/m²/s), on plants that are well known to be full-sun plants, and states that they use a great proportion of the light that they intercept. The numbers seem suspect, and the explanations seem contradictory.

If nighttime CO2 uptake is the limiting factor than why is the paper talking about instantaneous limits, making umol/m²/s measurements, and talking about PSII saturation?
 

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