PHOTO-PHYSIOLOGICAL PROCESSES

 The number of photons (or quanta) required to release one molecule of oxygen in photosynthesis is called Quantum Requirement (QR) . A minimum quantum requirement of 8-10 is required for photosynthesis. On the other hand, the number of O2 molecules released per photon of light in photosynthesis is known as Quantum Yield (QY) . The QY is always less than one . It is well known that the rate of photosynthesis is maximum in red light followed by the blue band of the visible spectrum. Each photon of red light is known to contain about 40 k.cal of energy.

 The phenomenon of fluorescence greatly helps in the understanding of the transfer of light energy in between the photosynthetic pigments. Some photosynthetic pigments absorb light rays of shorter wavelengths (with more energy) and while the others absorb longer wavelengths (with lesser energy). The pigments, which absorb light rays of shorter wavelengths, will emit fluorescent light (of higher wavelengths) which in turn will be absorbed by other pigments, which otherwise normally absorb light rays of higher wavelengths.

  Light rays consist of tiny particles called photons . The energy carried by a photon is called quantum . Light rays of shorter wavelengths contain more energy per photon of light than the light rays of longer wavelengths. for example, one photon of blue light contains about 70K.cal. of energy while one photon of red light contains about 40k.cal. of energy only. Diagram showing excitation of molecules of fluorescence or phosphorescence The normal state of the molecules or atoms is called as ground state or singlet state . When an electron of a molecule or atom absorbs a quantum of light, it is raised to a higher energy level, which is called as exciting second singlet state. This is unstable and has a half-life of 10-12 seconds. * The electron comes to the next higher energy level by the loss of some of its extra energy in the form of heat. This is called an exciting first singlet state and is also unstable with a half-life of 10-9 second. * From the first singlet state, the ex...

 * Chief source of light energy for photosynthesis is the sun * The earth receives only about 40% (or about 5x1020 K.cal.) of the total solar energy. The rest is either absorbed by the atmosphere or is scattered into space. * All the incident light energy falling on green parts of the plants is not absorbed and utilized by the pigments. Some of the incident light is reflected , some are transmitted through them while only a small portion is absorbed by the pigments. * Photosynthetic pigments absorb light energy only in the visible part of the spectrum. However, certain photosynthetic bacteria use infrared light of comparatively shorter wavelengths. * Only about 1% of the total solar energy received by the earth is absorbed by the pigments and is utilized in photosynthesis. * There is very weak absorption and strong reflection of light by the pigments in the green part of the spectrum and hence, the chloroplasts appear green in green plants. * All the pigments except chlorophyll A...

 The term photo-physiology refers to the study of physiological reactions as influenced by light. Light influences various physiological processes directly or indirectly like photosynthesis, flowering phenomena(photoperiodism), phototropism, chlorophyll synthesis, seed germination, protoplasmic streaming, etc. Visible Spectrum of Light * Sun is the ultimate and huge source of light energy. The earth intercepts very little amount of this energy. * The so-called light is the visible part of the spectrum of electromagnetic radiation. ` It ranges from 380 to 760. * A white light consists of violet (390-430nm), blue (430-470nm), blue-green (470-500nm), green(500-560nm), yellow(560-600nm), orange (600-650nm), and red(650-76nm). * But, there are several physiological processes, which are affected by 280 to 800 nm of which, the wavelength of light ranging from 400to 700 nm is very efficient in increasing the rate of photosynthesis. This range of wavelength (400-700nm) of the visible spectr...