Tag: homeostasis in plants

Levitt in 1974, proposed a synthetic theory on the mechanism of stomatal movement, taking all earlier observations into account. The observed facts are:

(a) During opening of stoma, starch disappears from the guard cells.

(b) Photoactive opening is dependent on the chloroplast of the guard cells.

(c) Low $CO _2$ concentration causes stomatal opening.

(d) High pH in guard cells cause stomatal opening.

(e) There is a direct relation between $K^+$ accumulation and stomatal opening.

Opening and closing of stomata are controlled by the turgor changes in the guard cells. In the dark, the guard cells lose water, become flaccid and the stomata close. In the light, the guard cells absorb water by osmosis, become turgid and the stomata open.

Glucose increases osmotic concentration of guard cells. On account of it, the guard cells absorb water from neighbouring cells, swell up and create a pole in between them.

Malate or $k^{+}$ ion pump hypothesis theory was put forward by Levitt $(1974)$. It is considered as the modern one. It states that the change in the turgor pressure of the guard cells regulates the opening and closing of stomata. It appears to be an active mechanism which needs ATP.

• Guard cells help for regulating the rate of transpiration by opening and closing the stomata. 
• When the guard cells are swollen, then this turgidity is due to the accumulation of K ions in guard cells. 
• As K ion level increases, the water potential of guard cells also drops and then water enters into the guard cells. 
• Then it requires the energy which is utilized from ATP form of energy. Thus this needs a constant supply of ATP.
• So the correct answer is$ K^{+}$ and ATP.
• Correct option is B.

There are three more theories for mechanism of stomatal movement
(A) Hydrolysis of guard cell photosynthesis
(B) Classical starch hydrolysis theory
(C) Malate or $k^{+}$ ion pump hydrolysis.

Malate or $k^{+}$ ion pump hypothesis theory was put forward by levitt $(1974)$. It is considered as the modern one. It states that the change in the turgor pressure of the guard cells regulates the opening and closing of stomata. 

Glucose increases osmotic concentration of guard cells. On account of it, the guard cell absorb water from neighbouring cells, swell up and create a hole in between item.

• Potassium is a very important macronutrient for plants. Its ions cause the turgor movement of guard cells. 
• The gain of potassium ions by the guard cells causes a decrease in water potential of guard cells. 
• This makes guard cells turgid and hence they swell. Increase in turgidity of guard cells opens the stomatal pore. 
• Movement of potassium ions from the guard cells results in the increase of water potential. It becomes flaccid and stomatal pore closes. This mechanism of stomatal opening and closing was first stated by Levitt.

The correct option is B.

According to sugar concentration theory,during daytime ,the guard cells begin photosynthesis and the sugar produced during the process increases the osmotic pressure which draws in water from the adjoining cells due to endosmosis.