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I understand how plants open stomata, with the H+ ion removal and the resulting K+ ion influx in the guard cells to induce turgor (wikipedia article here), though not how this process relates to presence/absence of light.

However, I don't understand how CAM plants manage to keep their stomata closed during the day and how it relates to CAM photosynthesis (if at all). I know that malate ions are involved in the opening of the stomata in C3 plants, could the malic acid production in CAM photosynthesis contribute to this?

So my question is: How do CAM plants induce closing of the stomata and what signal triggers this?

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    $\begingroup$ This isn't homework, I'm just genuinely curious. CAM and C4 photosynthesis isn't part of the Scottish curriculum, so I haven't covered this and nor can I find anything about this online. $\endgroup$ – Phototroph Jan 25 '16 at 15:40
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    $\begingroup$ Yeah I've been searching for this casually for about 3 months now and am yet to find anything, thank you for the link but it hasn't turned over anything new. I apologise if the wording makes it out to seem like I haven't done reading of my own. I asked my interviewers at a university interview this question and they said they knew someone who could answer my question, so I know this has been answered and I'm just hoping someone on here will know it. $\endgroup$ – Phototroph Jan 25 '16 at 16:14
  • $\begingroup$ I think the biggest problem is we don't know the signal transduction pathway in CAM plants in response to CO2 levels. $\endgroup$ – CKM Jan 25 '16 at 21:26
  • $\begingroup$ Is it known what conditions in the cell trigger the H+ ion pumping in normal plants? Again something I've been looking for $\endgroup$ – Phototroph Jan 26 '16 at 11:43
  • $\begingroup$ @Phototroph Blue light phototropins activate the membrane H+-ATPase through phosphorylation events (1). Therein voltage-gated potassium channels open in response to the membrane potential generated by the H+ pumping. $\endgroup$ – CKM Jan 26 '16 at 17:26
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As others already say in comments, the complete answer to this question is still unknown. However, the basic mechanism has been understood and has been summarized in the following diagram (from Wikipedia):

mechanism

As you see, the mechanism is regulated by temperature instead of presence of light, keeping in mind that high temperatures are generally observed during the day and low tempertures at night. Temperature regulates, at the expression level, the activity of an enzyme PEP-C kinase. This enzyme phosphorylates PEP carboxylase (or PEPCase in short), which increases its activity. So lets see what happens when temperture is high:

During day:

  • high temperature inhibits expression of the enzyme PEP-C kinase, which decreases the activity of PEPCase.

  • activity of PEPCase is further reduced by an enzyme PEP-C phosphatase (this enzyme is active all the time, but its effect is reversed by PEP-C kinase).

  • due to respiration and photosynthesis, the amount of cytoplasmic malate is reduced (malate is converted to pyruvate to yield NADPH for photosynthesis, and pyruvate is used up in TCA cycle, whose by-product, $\ce{CO_2}$, is used up in calvin cycle).

  • due to this, malate is released from vacuole into the cytoplasm. This malate is not only used up in photosynthesis and respiration, but also further inhibits activity of PEPCase and expression of PEP-C kinase.

  • because PEPCase is inhibited, it results in depletion of malate in cytoplasm as well as vacuole.

During night:

  • at night, stomata open leading to increase in amount of $\ce{CO_2}$. Also, low temperature is unable to inhibit expression of PEP-C kinase.

  • when activity of PEPCase increases, PEP is converted to malate, which is then transported into vacuole. Also, increased concentration of $\ce{CO_2}$ increases photosynthesis.

Where is light required? As already said, the exact mechanism is still unknown. The mostly accepted mechanism is potassium ion pump hypothesis, but problems arise when we think about the initiation of this process in CAM plants. There are many hypotheses regarding this, and I will follow the one presented by Lee, 2010. Lets begin with the diagram (don't pay attention to the spellings):

hypothesis

  • initial stomatal opening is mediated by phytochrome in guard cells. The $\ce{H^+}$ pump in guard cells is also activated by darkness.

  • the maleic acid synthesized in other cells is also transported to the vacuoles in guard cells. There, the intracellular $\ce{H^+}$ (from maleic acid) is exchanged for intercellular $\ce{K^+}$.

  • increased concentration of $\ce{CO_2}$ also regenerates glyceraldehyde-3-phosphate and ribulose-1,5-bisphosphate, which lead to formation of sucrose via calvin cycle.

  • this sucrose is again transported to vacuole of guard cells, adding up to osmotic pressure. However, the major effect comes from maleic acid.

  • increased $\ce{CO_2}$ concentration and increased activity of PEPCase (as already explained) form malate and lead to further increase in the osmotic concentration. All this causes stomata to open.

  • during the day, malate is decarboxylated to pyruvate (as already explained), leading to decrease in osmotic pressure and thus, closing of stomata.

I hope this makes the concept more clear, but bear in mind that the above mechanism could later prove to be wrong as further studies on CAM plants help to find out the actual mechanism.

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    $\begingroup$ It's unbelievable that this answer was here for almost 1 day, with 500 views, without a singe vote. People upvote like if it was paid...at StackOverflow it's even worse! (I'll delete this). $\endgroup$ – user24284 Jun 5 '17 at 8:20
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    $\begingroup$ Problem solved! Joking apart, new users normally forget to vote and experienced users apparently see each other like competitors. They don't understand that the gamification aspect of SE, despite a lot of people denying it, is what maintain SE sites active and it's what incentives people to write (good) answers. $\endgroup$ – user24284 Jun 5 '17 at 8:59
  • $\begingroup$ @gerardofurtado +1 for explaining what I call healthy competition :) $\endgroup$ – another 'Homo sapien' Jun 5 '17 at 9:19

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