Tag: blackman's principle of limiting factors

Starch hydrolysis theory is the classical theory for stomatal opening and closing. It was originally proposed by Sayre, 1923. According to this theory, guard cells contain starch, which is hydrolysed to form glucose under high pH caused due to reduced carbon dioxide concentration. The carbon dioxide concentration is reduced due to the process of photosynthesis. Glucose increases the osmotic concentration and hence, DPD of guard cells. The latter absorb water from epidermal cells, swell up and open the stomatal pore.

During day, light photosynthesis occurs in guard cells as they contain chloroplast. The soluble sugars formed in this process may contribute in increasing the osmotic potential of guard cells and hence resulting in stomatal opening. However, very small amounts of soluble sugars (osmotically active) have been extracted from the guard cells which are insufficient to affect water potential. As a result of photosynthesis CO$ _2$ concentration in guard cells decreases which leads to increased pH up of organic acids, chiefly malic acid during this period in guard cells. The formation of malic acid would produce proton that could operate in an ATP-driven proton K+ exchange pump moving protons into the adjacent epidermal cells and K ions into guard cells and thus may contribute in increasing the osmotic pressure of the guard cells and leading to stomatal opening. Reverse process would occur in darkness.

Plants have photoreceptors, which are proteins that are specially designed to perceive light and signal certain biological effects in the plant. There are two types of photoreceptor proteins in plants - Phytochromes and Blue light receptors. Phytochromes absorb red light and far red light and help the plants to sense seasonal changes in night length, or photoperiod, thereby controlling the opening and closing of stomata.

When the leaf is exposed to light, the process of photosynthesis begins. As the photosynthetic reactions proceed in the guard cells, the residual carbon dioxide is converted to carbohydrates. The disappearance of carbon dioxide from the cytosol of the guard cell results in an increase in the cellular pH. 
As the pH rises, the activity of the enzymes, that convert starch and sugars to organic acids increases. The higher concentration of organic acids results in a higher concentration of hydrogen ions. The hydrogen ions of the guard cells are then exchanged for potassium ions in the subsidiary cells.
This increased concentration of potassium, combined with the higher levels of organic acids, lowers the osmotic potential of the guard cells, and, since water moves from regions of high osmotic potential to regions of lower osmotic potential, water will move from the subsidiary cells into the guard cells. This movement of water increases the turgor pressure (inner pressure) of the guard cells and causes them to swell. Thus, the stomata open.

The stomatal opening follows when solutes from near epidermal and mesophyll cell enter the guard cell lowering its osmotic potential and water potential. Also rising in K ions in guard cell allows movement of water into the guard cell from neighbouring cells and makes the guard cell swollen due to water accumulation which results in the opening of stomata. 

Stomata opens and closes due to water and ion movement. When the light falls on the parts of plant, chloroplast carries out photosynthesis. ATP from photosynthesis powers sodium and potassium pump. Then the K+ ions are pumped into the guard cells, by creating a concentration gradient. For this uptake of K+ ions it requires protons (H+) ions then, K+ ions are exchange for the protons H+.  This is an active ionic exchange and requires ATP energy and cytokinin (a plant hormone). In this way, the concentration of K+ ions increases in guard cells and at the same time, the concentration of H+ ions decreases in guard cells.  As a result of this, guard cells swells and stomata opens.So, the major solute taken in by the guard cells is K+, which is responsible for the opening of guard cells.