Light

1. Light
Sankrita Gaonkar
Assistant Professor in Botany
sankrita002@gmail.com

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

4. • https://www.youtube.com/watch?v=CIWEJEYD-bg

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.