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The Effect of Light on the Transpiration of Leaves

In photosynthesis, which takes place in the leaves of most plants, light is used as energy to convert carbon dioxide and water to sugar and oxygen. This is how plants get their energy to grow and live. They take up water from the soil through their roots, and bring in carbon dioxide from the atmosphere through their leaves. They then give off oxygen as a byproduct of photosynthesis, which humans and other animals use to breathe. This exchange of carbon dioxide and oxygen is called transpiration. Conversely, animals take in oxygen and release carbon dioxide through respiration. Thus, each needs the other to survive.

Stomata
- Carbon dioxide must have a way to enter the leaf, and oxygen a way to exit. Plants achieve this through tiny openings on the underside of leaves called stomata. Stomata open and close, much like a mouth opens and closes, to regulate the intake and output of gasses in the leaf. Flanking each stoma is a guard cell, which affects the shape of the stoma. When the guard cells fill with water, stomata open, and when guard cells lose water, stomata close.
Opening Stomata with Light
- When sunlight strikes a leaf, as happens at sunrise, a protein called phototropin absorbs the blue wavelengths of light. This activates a proton pump embedded in the guard cell membrane, the "skin" of the guard cell. When activated, the pump opens and pumps hydrogen ions (small charged atoms) out of the cell. This creates a charged gradient that acts to draw in potassium ions. By osmosis, water follows these potassium ions and the guard cell swells, opening the stomata. The light also warms the leaf, increasing the rate of transpiration, as gasses move faster when heated.
Closing Stomata
- Stomata closure is regulated not by light, but by availability of water. When soil water declines, plants produce abscisic acid, which binds to receptors on the plasma membrane of guard cells. This catalyzes a reaction that results in an increase in pH in the cell (a decrease in acidity), which causes potassium to leave the guard cells. Water follows by osmosis and the cells lose pressure, closing the stomata.
Summary
- Light acts to open the stomata of leaves, which increases transpiration. It also heats up the leaves, further increasing the rate of transpiration. Other factors that affect transpiration include humidity (an increase causes a decrease in transpiration), wind and air movement (increased wind causes an increase in transpiration), soil moisture (decrease in moisture over time leads to less leaves and thus less transpiration) and type of plant (some plants of dry and arid regions transpire less).

Nature