• More than 70 per cent of the solar radiation absorbed by plants is converted into heat.
• A portion of solar radiation, upto 28 per cent in terms of energy, is used in photosynthesis.
• Radiation upto 0.25 μm (ultraviolet) is harmful to most plants.
• Solar radiation in the region of 0.30 to 0.55 μm has photoperiodic effect and from 0.40 to 0.70 μm, it is most effective in photosynthesis.
• Above 0.74 μm, it has practically no effect on photosynthesis, its main effect is thermal.
• Near infrared radiation has influence on seed germination and dormancy of seeds.
• Chlorophyll formation is promoted by light in the region of 0.300 to 0.338 μm.
• Blue light even at very low light intensities cause inclination of wheat leaf.
• With higher light intensities, leaves become horizontal.
• Plants tend to grow prostrate or develop rosette form under high light intensity.
• Interception and utilization of solar radiation can be increased by proper management practices, such as adjustment of row spacing, plant population and selection of the most advantageous time for planting.
• Light interception increases with increase in leaf area index.
• As solar radiation enters the crop canopy, its quantity is gradually decreased due to interception or attenuation by the crop.
• The amount of solar radiation present in any layer of the crop can be calculated by the formula
I = I0 e-kLAI
I = light intensity at a point,
I0 = light intensity on top of the canopy,
LAI = Leaf Area Index,
K= extinction coefficient,
e = exponential constant
• The extinction coefficient or rate at which light is being attenuated with increase in depth into canopy is dependent on the crop.
• Extinction coefficient in rice is 0.65, 0.84 in maize and 0.70 in sorghum.
• In groundnut, low light intensity during peak flowering reduced number of flowers per plant.
• Flowers opened during cloudy period do not produce pegs.
• Low light intensity at pegging and pod filling reduces peg and pod number.
• In cereals, number of tillers increase with increase in light intensity.
• Low light intensity during flowering increases spikelet sterility in cereals.
• The low light intensity from panicle initiation to grain formation is critical but flowering to grain formation is more critical.
• Low light during ripening reduces yield due to lesser number of filled grains per panicle and lower grain number.
• Reduction in grain yield of rice in wet season compared to dry season is attributed to solar radiation.
• Dry matter production increases in proportion to the amount of intercepted radiation; but yield may or may not.
• Most plants are influenced by relative length of day and night, especially for floral initiation.
• The duration of the night or complete darkness is more important than the daylight for floral initiation. This effect of light on plants is known as photoperiodism.
• Long-day plants require comparatively long days (usually more than 14 hours) for floral initiation.
• Long-day plants put forth more vegetative growth when days are short.
• Temperate crops like wheat, barley, oats belong to long day plants.
• In short-day plants flower initiation takes place when the days are short (less than 10 hours) or when the dark period is long.
• Most of the tropical crops like rice, sorghum, maize etc. are short-day plants.
• Crops that have an absolute requirement for a given day length will remain vegetative for an indefinite period as long as they are not exposed to floral inductive day length.
• Cocklebur is a qualitative short-day plant.
• Quantitative short day plants initiate flowering even in long-days but initiation of flowering is delayed by a few days to months.
• Most of the tropical crops are quantitative short-day plants.
• Day-neutral plants do not require either long or short dark periods. Photoperiod does not have much influence for phasic changes for these plants.
• Cotton, sunflower and buckwheat are day-neutral.
• Shorter the days, more rapid is initiation of flowering in short-day plants.
• Longer the days, more rapid is the initiation of flowering in longday plants.
• Alternate high and low light intensity influences photoperiodism.
• Low light intensity following high light intensity, is more effective on photoperiodism than high light intensity following low light intensity.
• Day length is a function of latitude and position of the sun.
• At equator both day and night are equal in length.
• At the time of equinoxes, the length of the day is practically the same at all latitudes.
• Day length changes due to position of the sun during different months.
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