Competition for Solar Radiation: Solar radiation falling above 700nm is called the near infra red radiation (NIR). PAR is commonly known as light and s directly related to photosynthetic rates of component crop while NIR is the primarily energy source deriving evapo transpiration, sensible heat exchange and other photomorphogenic process. These two types of radiation considered together under this section.
Beer’s law describes the light penetration into a crop canopy if the foliage distribution is uniform in horizontal line.
I = Ice – KL
Where I = light flux density, to a horizontal surface below L units of LAI,
Io is the light flux density to a similar surface above canopy.
e- Base of natural logarithm.
k- Is the extinction coefficient.
In sparse plant stands or with low LAI beers law under estimate light interception.PAR is directly related to photosynthetic rate of component crops. Intercropping can increase light interception by as much as 30-40%. The taller crop in the intercropping systems intercepts most of the solar radiation while shorter component suffers. In some intercropping systems, both crops utilize solar radiation efficiently. In Ground nut Red gram intercropping system, light interception is prolonged as red gram starts growing after harvesting of groundnut. If the component crops have different growth durations the peak demand for light occurs at different times. In maize + green grams intercropping system, green gram flowers in 35 days after sowing and is harvested 65 days after sowing, peak light demand for maize occurs at 60 days after sowing when green gram is ready for harvest. In such intercrops, there is less competition among component crops and higher solar radiation is intercepted in intercropping system than in pure crops.
Proper choice of crops and varieties, adjustment of planting density and pattern are the techniques to reduce competition and increase the light use efficiency.
1). When one component is taller than the other in an intercropping system, the taller component intercepts greater share of light. As a result, the growth rates of the two components will be proportional to the quantity of the two component will be proportional to the quantity of PAR
they intercept provided other growth factors are no limiting and the crops are in their vegetative.
2). The inclination of leaves greatly influences the amount of light intercepted by the taller component and the amount that is available to shorter components. For example, one unit of LAI of prostrate-leaved perennial rye grass (Trfolium ripens) absorbed 50% of the incoming light where as the some LAI of erect leaved perennial rye grass (Lolium pereme) absorbed only 26% (Breugham, 1958).
3). Light interception in a sorghum (Sorghum bicolor) based intercropping system was studied by Selvaraj (1978) Light was measured influx units from 45th-90th day after sowing sorghum at 15 day interval. Light interception was expressed as percentages of light on the top of the canopy of each crop.
The red ram/sorghum intercropping system makes better use of growth resources, particularly light, was brought out by Willey et al. (1981).
Ideally the taller component should have more erect leaves and the shorter component more horizontal leaves. If these to be planted in alternate rows, there would be less competition for light.
Under intercropping situations, the component crops are grown in such a way that competition for light is minimized (Okigbo, 1981); this can be achieved by proper choice of crops and genotypes, the shorter components being harvested sufficiently early so that the later harvested component is not greatly affected.
Effect of solar radiation on Evapotranspiration, sensible heat flux & photomorphogenic effect has not been investigated in detail in mixed crop communities. Evapotranspiration would be much less for the lower canopy in intercropping, leading to less water stress. Finally, the success of the intercropping system would be very much dependent on the photoperiod adaptability of the component crops.