If I'm correct, the guard cells turn turgid during the day and flaccid at nights. What is the reason behind the same and how is it done? (im a noob highschool student, so basic explanations are welcome)

  • $\begingroup$ What is the result of each type of guard cell status (turgid or flaccid) on the stomatal opening? What are the functions of the stomatal opening? $\endgroup$
    – Armand
    Jun 29 '21 at 8:36
  • $\begingroup$ (Umm, I forgot to add the 'how' part to my question...) Coming to that; i guess flaccid = close; - doesn't allow transpiration turgid = open; - does allow transpiration $\endgroup$
    – lilnewt761
    Jun 29 '21 at 10:02

There is a good free recent review by Brodribb et al., 2020 which explains much of this biology.

The most fundamental mechanism, dating back to the evolution of stomata before they were even used to regulate photosynthesis, seems to be potassium regulation. As a rule, water follows dissolved material (osmosis): if you enclose a bag of water and salt underwater in a membrane permeable to water but not salt, the particles of salt will push out on the bag with the same amount of pressure as if they were particles of gas floating in the bag and there were no water present inside or out. Cells usually hoard potassium ions - they will also passively allow some negative ion, such as chloride or malate, to flow in to counteract the charge on the potassium. Without those negative ions, a positive charge would quickly build up inside that would stop the potassium from moving in. Thermal energy inside the cell gives every particle a random energy of motion (which leads to pressure at a boundary) that is equal on average, whether it is potassium, chloride, glucose, or a protein. So when you double the amount of potassium in a cell, also doubling the corresponding negative ions, that is most of the particles in the cell - so the amount of water will also nearly double and the cell will get double the size. In guard cells the amount of potassium can more than double to open stomata.

There is another mechanism to get to the same end: increasing the amount of glucose in the cell. Normally, cells store energy as starch, which has a tremendous number of glucose molecules physically linked together. These don't count as much more than a single particle for purposes of osmotic pressure, so starch isn't going to make the cell turgid. But if you "mobilize energy reserves from starch" - split it up into glucose - those glucose molecules will all be moving around separately, each bashing against the membrane (and supporting cell wall) with its own random thermal energy. Once again the guard cell will expand.

But everything I've described is relative. A pile of dry salt or sugar won't exert any pressure at all, and a guard cell in a desiccated plant will not be able to expand. Passively, a drying plant's guard cells will tend to close from lack of pressure. However, the review explains that it would be dangerous for the plant to leave that to chance - it can't let air bubbles get into its water distribution network - which is one reason for all the active regulation described above.

Active regulation means "thinking" in some sense at the cellular level, so it is much more complicated than the physics of opening and closing. People can tell a beautiful story that is pretty much wrong by trying to present the process without it. You can see some of this complex regulation covered in the related question here about crassulacean acid metabolism plants, which open stomata only at night to conserve water. Our review covers it under "Molecular evidence..." and it is quite complex; I would probably damage it in the retelling. Key steps are abscisic acid production, H+-ATPase, and KAT/AKT-type Shaker channels. But this is pretty much the on/off switch for a plant, so there could be quite a bit untold there, or unknown altogether.

  • $\begingroup$ thank you for the detailed explanation :) $\endgroup$
    – lilnewt761
    Jun 30 '21 at 13:33

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