Re: Transpiration

Hi Christopher, here is your answer.

Yes, transpiration occuring through general plant body surface, including leaves, is affected by the quality of light, as well as the quantity of light. By quality I mean, the wavelength of the light rays and by quality, I mean the intensity or brightness of the light.

Although transpiration is basically an evaporation process, it is complicated by other physical and physiological conditions prevailing in the plant. Physical factors which affect rate of transpiration include temperature, humidity of the atmosphere, light, wind, atmospheric pressure. A large bulk of the transpiration process occurs through small openings, called stomata, generally present on the undersurface of leaves. A small amount of water is also evaporated through general body surface.

Light is one of the major factors influencing the rate of transpiration beacause of its controlling effect on the opening and closing of stomates. Light of low intensity is, generally speaking, less effective than stronger illumination in inducing stomatal opening. Hence stomates often do not open as wide on cloudy as on clear days, and often do not remain open for as much of the daylight period.

Since leaves in direct sunlight usually have temperatures from one to several degrees higher than that of the surrounding atmosphere light also has a secondary accelerating effect on transpiration through its influence on leaf temperatures.

Infrared rays, a part of the sun's solar spectrum having wavelength greater than 700 nano meters {abbreviated as nm, and 1 nm is equal to 10 raise to the power of -9}, have more heating effect as compared to that of rays of visible spectrum, which lies in the wavelength range between 400 nm to 700 nm. Among the colors of the visible spectrum, the red color have more heating effect than colors occuring earliar in the color series VIBGYOR. Violet has thus the lowest heating effect.

However, since the temperature of the leaf is largely determined by number of rays falling per cm/inch i.e. the brightness or intensity, the effect of quality of light in normal terrestrial conditions is negligible, and hence also on the rate of transpiration.

I hope you gained some understanding of the process from my answer. For more information or something which you didn't understand, please feel free to ask me. My e-mail address is manmohan@csec.ernet.in