Significance of solar radiation in crop production

I. Effect of light Duration

• Based on the duration of light period required, plants are classified as a. long day plants b. short day plant and c. day neutral plants. It has been observed that the initiation of reproductive phase in plant is initiated when certain duration of light period is obtained. Short day plants are those whose reproductive phase is initiated when period of shorter light duration (<12 hours) are provided. Rice, maize, soybean etc. are the short day plants. If longer day duration (>12 hours) for the initiation of reproductive phase is required, it is long day plant. Wheat, barley, oat etc. belong to long day plants; and those plants which don’t have any specific requirement of duration of light period for the initiation of reproductive phase are day neutral plants.
• Photoperiodism in the plant has also been found to be influenced by temperature conditions. This effect is referred as thermophotoperiodism (response of the plants to both temperature and day length for flowering). The occurrence of phonological events particularly the reproductive phase in the plants, at the location of higher altitudes has been found to be delayed due to lower environmental temperature.
• Not only the duration of the day length, but also the duration of darkness is equally important in the occurrence of the reproductive phase in the plants. It has been observed that even if light is induced momentarily during the period of darkness, the phasic development in the plants is altered.

ii. Effect of solar radiation intensity

• Affects physical processes like photosynthesis, respiration, transpiration
• Very low intensity reduces the rate of photosynthesis and may even result in the closing of stomata; results in plant growth
• Rate of photosynthesis increases with the increase in intensity of solar radiation up to a limit and further increase in intensity has no contribution on the rate of photosynthesis; it is called light saturation point.
• When the plants are gradually exposed to the increasing light, from darkness to sunlight, a minimum light threshold is reached, where the amount of the CO2 taken up in photosynthesis exactly equals the amount given off in the respiration. This is light compensation point. Light compensation point is lower for the shade loving plants. As light intensities are increased beyond the light compensation point, rate of photosynthesis of well-exposed leaves increased.
• High light intensity has dwarfing effect on plants whereas low light intensity causes etiolation of the plants.

iii. Effect of solar radiation (light)quality:

• The quality of solar radiation affects flowering, germination and elongation.
• It is observed that red light with wavelength of 0.66μ is by far the most effective inhibitor of flowering in the case of long day plants. The red light helps mature apples to turn red.
• The germination of seed is inhibited when they are exposed to green blue and other short wavelength color and again in the infrared part of spectrum.
• Germination is however induced when the seeds are exposed to the red portion of the spectrum. Stem elongation is promoted by the exposure to infrared wavelengths whereas the red part of the spectrum suppresses the elongation.