10. Loss of turgor pressure in guard cells causes an uptake of?

tha answer is H+.u may refer the following explanation.

Theory of K+ ion transport by Levitt and elaboration by Raschke and Bowling:-In light starch is converted into phosphorylated hexoses and then to phosphoenol pyruvic acid which combines with carbondioxide to produce malic acid.Malic acid dissociate into malate anion and H+ ion in the guard cell.H+ ions are  transported to epidermal cells   and K+ions are taken into the guard cells in exchange of H+ ions and is called ion exchange.Increased concentraton oh H+ ions and malate ions in the vacuole of guard cells causes sufficient osmotic pressure to absorb water from surrounding cells.It results in the opening of stomata.

                   In the dark carbondioxide concentration is increased in the substomatal cavity which prevents proton gradient across the protoplasmic membrane in guard cells.As a result active transport of K+ ions into guard cells ceases.As soon as the PH of guard cells decreases the abscissic acid inhibits K+ ion uptake by changing the diffusion and permeability of guard cells .Malate ion in the guard cell cytoplasm combine with H+ ion to  produce malic acid.These changes cause reversal of concentration movement  .So the K+ ion is transported out of guard cells into the surrounding epidermal cells.The osmotic pressure of guard cell is decreased which results in the movement of water from guard cells to surrounding cells and guard cells becomes flaccid and stomata closes