Penetration of Photosynthetically Active Radiation Into Tobacco Crops

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Abstract
The spatial distribution of photosynthetically active radiation (PAR) was measured on several occasions during the growth of a tobacco crop. The data describe the penetration of PAR into the crop. Penetration was influenced by crop size, row structure, time of day and conditions of incident irradiance. At solar elevations greater than 15° on clear days, there were few regions in the crop with flux densities between 0.1I0 and 0.9I0, where I0 is the flux density of PAR above the canopy. Frequency distributions of flux densities were therefore bimodal. At lower elevations, the direct- beam component was attentuated and frequency distributions were unimodal with few flux densities above 0.6I0. Under diffuse irradiance, the bimodal response was attentuated to give a relatively uniform distribution of flux densities on the transects. These data were used to appraise two estimates of the penetration of PAR into the canopy. The first was based on a physical representation of the display of leaf surfaces in the canopy. Alternative estimates were derived from a mathematical model of the display of foliage of single plants which was extended here to account for the effect of neighbouring plants on the penetration of PAR into the crop.