Tag: photosynthesis in higher plants

According to the most widely accepted theory of stomatal opening and closing, potassium ions are involved in turgor movement of guard cells. Inflow of potassium ions decreases water potential in guard cells, which leads to inflow of water. Consequently guard cells swell and stomatal pore opens. When potassium ions move out, these events are reversed and stomatal pore closes.

Transpiration is the process of loss of water, in vapour form, through stomata. The process depends on the turgidity of guard cell. An increase in turgidity of guard cells opens stomatal pore, whereas a decrease in turgidity of guard cells closes stomatal pore. The change in turgidity of guard cells is brought about by reversible influx and efflux of potassium ions.

Starch is osmotically inactive and does not contribute to decrease in water potential. Conversion of starch to organic acids like malic acid promotes uptake of ions like potassium. This decreases water potential and helps in uptake of water. Thus making guard cells turgid and opening of stomata.

Changes in turgidity of guard cells, regulate the opening and closing of stomata. When guard cells are turgid, stomatal pore opens and when guard cells are flaccid stomatal pore closes.

Potassium ions movement into and out of guard cells is related to the turgidity and consequent opening and closing of stomatal pore.

Guard cells have chloroplast, on exposure to light, there is synthesis of sugar in guard cells. The increase in sugar concentration causes water to move in from surrounding cells, thus making guard cells turgid. Increase in turgidity of guard cells leads to stomatal opening.

Amophila are xerophytic grasses, which show rolling of leaf as a xerophytic adaptation to conserve water. Nerium leaves show multilayered epidermis, thick layer of cuticle and sunken stomata, but do not show rolling of leaves. Melia is not a xerophyte.

The starch-sugar interconversion theory, which suggests that starch is converted to sugar (glucose) in light to increase osmotic pressure, was proposed by Steward.

Stomata open and closes due to water and ion movement. When the light falls on the parts of the plant, chloroplast carries out photosynthesis. ATP from photosynthesis powers sodium and potassium pump. Then the K$^+$ ions are pumped (influx of K$^+$) into the guard cells, by creating a concentration gradient and also water enter into the guard cell through osmosis. As a result of this, guard cells swells and stomata open. The reverse process occurs in the closing of stomata, i.e, loss of K$^+$ ions (efflux of K$^+$) by guard cells and water also leave the guard cells by the process osmosis. As a result, guard cells become flaccid and stomata close. 

Botanist Sayre observed that stomatal opening is connected to change in ph of guard cells due to carbon dioxide.