Question

Write an essay on the theories explaining mechanism of stomatal movement.

Answer 1

Theories of Stomatal movements

The opening and closing of stomata take place due to changes in the turgor of guard cells. Generally, stomata are open during the day and close at night. The actual mechanism responsible for the entry and exit of water to and from the guard cells has been explained by several theories. These theories are described below:

1. The Starch - Sugar interconversion Theory: This theory was put forward by Steward in $1964$. According to him, during the day time phosphorylase enzymes convert the starch into sugar due to which the osmotic potential of the guard cell increases and allows the entry of water into the cell. During the night the same reaction occurs in the reverse direction which closes the guard cell i.e. stomata is closed during the night.

2. Proton - Potassium Pump Hypothesis: Levit in $1974$ combined the points in Scarth's and Steward's hypothesis and gave a modified version of the mechanism of stomatal movement which was called the proton - potassium pump hypothesis. According to this hypothesis, $K^{+}$ ions are transported into the guard cells in the presence of light. The sequence of events taking place are as follows:

➢ Under the influence of light, protons formed by dissociation of malic acid move from cytoplasm into the chloroplasts of guard cells.

➢ To counter the exit of protons, $K^{+}$ ions enter the guard cells from the surrounding mesophyll cells.

➢ $K^{+}$ ions react with the malate ions present in the guard cells to form potassium malate.

➢ Potassium malate causes an increase in the osmotic potential of guard cells causing entry of water into the guard cells as a result of which the stoma opens.

➢ At night the dissociation of potassium malate takes place and $K^{+}$ ions exit out of guard cells causing loss of water from guard cells and so the stoma closes

3. Synthesis of organic solutes: blue light also stimulates the starch degradation and malate biosynthesis. Malate is an organic acid. In plants, malate is synthesized in the guard cell cytosol from the compound generated from the hydrolysis of starch. The enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) binds carbon dioxide to PEP to produce oxaloacetate, which is then reduced to malate and stored in the vacuole. In the presence of light, guard cells also perform photosynthesis. It increases the osmotically active solutes such as sucrose. Thus, in the presence of light, concentration of $K^{+}$, $C{l}^{-}$, malate, and sucrose increases. The increase in these osmotically active substances in the guard cells causes water to move passively into these cells and as their turgidity increases, the stomatal pore opens.