Light
- 2. Contents • Light • Light intensity • Effect on plants • References
- 3. Light • Most essential abiotic factor • Non-lethel limiting factor both at maximum and minimum levels • Natural sources – sunlight, moonlight, star light and luminescent organisms • Radiant energy from sun – visible spectrum – light or luminous energy • Electromagnetic waves – radiation that penetrates earth’s atmosphere – wide range of wavelength • Beam of light – shower of particles – Photons • Photon carries certain amount of energy – Quantum • Energy ∝ 1 Wavelength • Solar radiations penetrate earth’s atmosphere – visible (to man) light and small portion of UV and infrared radiations
- 5. Light intensity • Brightness of light • Differs from place to place • Measured by photometer or luxmeter • Variation caused due to following reasons: A) Effect of atmosphere • Small proportion of radiations (shorter wavelength) absorbed by atmosphere • High altitude place – brighter light than low altitude places • Atmospheric vapour – screening effect – light intensity higher in dry areas • Atmosphere saturated with fog and clouds – longer wavelengths absorbed by atmospheric moisture, shorter wavelength absorbed by gas molecules and water vapours – light reaching earth – diffused light or skylight
- 6. B) Effect of water • Light intensity is reduced in water – decrease with water depth • 10% sunlight reflected by water • 90% penetrates – modified intensity, spectral composition, refraction and time distribution • Phytoplanktons, zooplanktons and suspended particles reflect or absorb light rays • Submerged plants get weaker light than floating ones • Rough water surface – reflection is increased • a) Euphotic zone – upto 50 m depth • b) Disphotic zone – 80-200 m depth • c) Aphotic zone – below 200 m depth
- 7. • Oceanic algae are distributed according to wavelengths of light to which their colours are best suited to absorb and utilize • Green algae – intertidal zone • Brown algae – descend deeper • Red algae – deep ocean water • Light got sufficient power of penetration • Photosynthesis can occur even in plants covered by 40 cm thick snow
- 8. C) Effect of suspended particles • Dust, smoke, and other solid particles dispersed in air or water – screening effect • Smoky industrial sites – 90% reduction in light D) Effects of vegetation layers • Complex plant community (e.g., forest) – tallest plants receive full sunlight, undershrub receive diffused light, herbs receive still weaker light • <1% of total sunlight reaches surface
- 9. Effect of light on plants Photosynthesis – most important role • Abundant light is received on earth’s surface – 2-3% used in primary productivity • Based on light preferences plants are classified as: • i) Heliophytes – grow in open sun • ii) Sciophytes – grow in shade • Facultative sciophytes – heliophytes growing in shade no so well • Obligate heliophytes – heliophytes fail to grow in shade • Facultative heliophytes – sciophytes which may grow in light • Obligate sciophytes – sciophyte that fail to grow in sun • High intensity – photo-oxidation of enzymes – reduce carbohydrate and protein synthesis
- 10. Respiration • No direct effect • In presence of light – respiratory substrates are synthesized • In shade and under water – photosynthesis rate just sufficient to meet need of respiration – compensation point (dry weight does not increase) • Compensation point – differs with species and age • In many plants – respiration rate increases with increase in light intensity Opening and closing of stomata • Stomata open in light and close in dark • Light bring about phosphorylation and conversion of starch into soluble sugars – causes widening of gap between guard cells • Stomatal opening – gaseous exchange and transpiration • Increase in light intensity above optimum – increased transpiration – injurious to plants
- 11. Growth and flowering of plants • Growth of plants – depends on intensity, quality, duration, and direction of light • High light intensity – inhibits auxin production – influence shape and size of plants • Red light favours growth • Shorter wavelength light (except violet) – detrimental to plant • Duration of light or length of the day (photoperiod) – important in growth and flowering • Long day plants – plants which bloom when light duration is more than 12 hrs/day – e.g., potato, spinach, radish • Short day plants – which bloom when light duration is less than 12 hrs/day – e.g., cereals, dahelia • Day neutral plants – plants which show little response to length of daylight – e.g., tomato, cotton
- 12. Movement • Light affects movement of some plants • Stem, root and leaves show different responses to light • Effect of sunlight on plant movement – heliotropic effect • Stem elongate towards light – positively photrophic • Roots are negatively phototropic • Leaves grow towards light –to receive maximum light • Leaf orientation – do not overlap one another
- 13. Seed germination • Retarded in light in some • Light quantity needed for embryo stimulation varies in different seeds • Most cases, red light promotes germination and far-red light inhibits germination • Typha – seed germinate in yellow light
- 14. References • Shukla JP, Pandey A and Pandey K. 2009. Environmental Biology and Ecology. Narendra Publishing House. 7-22. • Ambasht RS and Ambasht NK. 2000. A Textbook of Plant Ecology. 13th edition. SK Jain for CBS Publishers and Distributors, New Delhi.