According to the Active Potassium Transport Ion concept given by Levitt et.al, stomata open due to turgidity in guard cells. It seems that guard cells turn the starch produced by photosynthesis into malate (which is a stable solute and should increase the osmotic pressure of the cell) but then go on to invest energy to exchange the hydrogen ions of malate with potassium ions.

Thus guard cells basically invest energy to turn malate into lots of potassium malate (so that water will enter due to increase in osmotic pressure), but why should such a process evolve if they can become turgid by just producing lots of malate?


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