Several of the physiological processes in plants are directly affected by the radiant energy; and photosynthesis is one of them. In addition radiant energy is known to affect protoplasmic streaming, flower induction, chlorophyll synthesis, seed germination and even phototropism. The study of the reactions as affected by light is called photobiology.
The sun is the source of energy where reactions occur at high temperatures and pressures. As a result, it produces a broad spectrum of radiant energy and sends this into space. However, the earth intercepts very small amount of this energy. This energy is passed through space as electromagnetic radiation having waves of variable length (Fig. 12-1). The region of photophysiology ranges from 280 to 800 nm.
It will also be seen that visible portion of spectrum extends between 380 to 760 nm. Electromagnetic radiation is composed of photons or quanta and the energy of a photon is inversly proportional to the wavelength. This energy can be measured in ergs, gram calories, etc. In Fig. 12-1, we have referred wavelengths as nanometers (nm). The radiant energy can be measured by thermal detectors or photoelectric detectors.
There are several physiological processes which are affected by radiation of 280-800 nm.
These processes are briefly mentioned in Table 12.1:
A chlorophyll molecule on absorbing photon becomes excited i.e. it is brought to a high energy state. The initial step comprises absorption of light and the photon must have sufficient energy to accomplish the above task. The energy in the photon is determined from the wavelength of radiation.
Thus, shorter the wavelength, greater is the energy. In plants there are several pigments which are complex organic molecules. Supposing a photon of appropriate energy hits the pigment, and then an electron of the inner shell is raised to an outer shell. We say that the pigment is in an excited state.
Once the electron drops from the excited state to the inner shell, then energy is emitted. The excited pigment may react in several ways depending upon its electronic structure and specific radiation energy. If the total energy of an excited pigment molecule is used in photophysiological reaction, then no energy is emitted as fluorescence or phosphorescence.
Usually pigment involved in a specific reaction is determined on the basis of an action spectrum. The latter is obtained by measuring the rate of photoreaction. The action spectra are variable for different photophysiological processes.
In the following a few photophysiological processes which have been studied in plants, are mentioned:
(i) Synthesis of chlorophyll-a,
(iii) Phototropism, and
(iv) Morphogenesis, etc.