Photosynthetically Active Radiation
This is usually in direct proportion with the incident solar radiation, you know that tropic$ are probably the best illuminated part of the globe. Also, variation in day length is minimal in this zone. No wonder, light-dependent life has attained maximal density in the tropics. Within a dense population of plants, different organisms may have different degrees of exposure.
However, if a plant is exposed to increasing light intensity it shows a corresponding increase in the rate of photosynthesis up to a certain point, beyond which further increase in light causes either no change in the rate or causes an inhibition. Thus, each plant species shows a characteristic photo saturation and/or photo inhibition of photosynthesis. In other words, each species has its own optimum with respect to light intensity. It is under stress if exposed to intensity above or below this optimum.