malic acid. Excess of malic acid inhibits its own synthesis by decreasing the activity of PEP carboxylase. These changes
cause reversal of ion movement so that K+ is transported out of guard cells into surrounding epidermal cells.
The osmotic concentration of guard cells is thus decreased, resulting in the movement of water from guard cells to
surrounding cells. The guard cells become flaccid and the stomatal pore gets closed.
Factors affecting stomatal movement
- In majority of plants, the stomata open in light and close in darkness. Maximum opening occurs in red light and blue
light. No opening occurs in green light, UV-light and infrared light. - Increase in temperature leads to opening of stomata upto certain extent. Very high temperature leads to closing
of stomata even in day (mid-day closure). At 38° – 40°C, stomata can open in complete darkness, while at 0°C
they remain closed even in continuous light. Normally high temperature above 30°C, reduces stomatal opening in
many species. - Water stress (or water deficit) brings about stomatal closure due to rise in DPD of epidermal cells.
- Mechanical shock causes closure of stomata.
- High CO 2 concentration in the intercellular spaces of leaves causes closure of stomata even in day time. Low CO 2
concentration usually induces opening of stomata. - Rise in pH results in opening of stomata while fall in pH induces closing.
- Cytokinins are essential for opening of stomata while ABA takes part in stomatal closure.
- A number of minerals are essential for stomatal movements e.g., K, P, N, Mg, Ca etc.
- Oxygen is essential for stomatal opening.
Stomatal pore is measured with the help of instrument called porometer.
Rate of transpiration is measured by potometer, e.g., Ganong’s potometer, Farmer’s potometer, Darwin’s
potometer, etc. Principle of working of all potometers is that absorption is proportional to transpiration.
Cobalt chloride paper method is used to compare rate of transpiration on two surfaces of leaf.Factors affecting transpiration
External factors
- Light increases transpiration through opening of stomata and increased protoplasmic permeability.
- Increase in temperature brings about an increase in the rate of transpiration. Rate of transpiration is generally
doubled with every 10°C rise in temperature. - The rate of transpiration is inversely proportional to the relative humidity, i.e., the rate of transpiration is higher
when the relative humidity is lower and vice versa. - The movement of air increases the rate of transpiration by removing the saturated air around the leaves. Upto
20-30 km/hr, the rate of transpiration increases with the wind velocity. A wind velocity of 40-50 km/hr decreases
transpiration by closing the stomata due to mechanical effect, drying and cooling of the transpiring organs. - Reduced availability of soil water causes wilting or loss of turgidity resulting in drooping and rolling thus
reduces transpirational rate.
Internal factors
- In general, higher the root/shoot ratio, more will be the rate of transpiration. It is due to the fact that an extensive
root system is more efficient in water uptake from the soil. - More is the leaf area, more will be the rate of transpiration.
- Cuticular transpiration decreases with the thickness of cuticle.
- The sunken (deep seated) stomata, present in xerophytes, are a device to reduce the rate of transpiration.