Penetration of Photosynthetically Active Radiation in Corn Canopies 1

The penetration and distribution of light in leaf canopies are essential data to understanding the photosynthetic productivity of a crop. Light sensors which measure photosynthetically active radiation were constructed and traversed through the leaf canopies of six types of corn ( Zea mays L.) to observe radiation penetration. Under clear sky conditions two major irradiance levels were observed in the corn canopies. One irradiance level was slightly less than the total radiation above the crop and represented the occurrence of sunflecks penetrating into the canopy. The second irradiance level was much less than the first and was comprised of sky radiation and radiation scattered by leaves. However, a decrease in the amount of direct solar radiation by either an overcast sky or a low sun angle resulted in a single, broad‐band irradiance level. The data on sunfleck penetration also provided a basis for testing two physical models of light penetration in leaf canopies. Data from corn planted in a uniform, hexagonal pattern was in good agreement with the Poisson model which assumes a random leaf distribution. However, the Markov chain model which introduces a frequency gap parameter, λ, to account for nonuniform leaf distribution was required to obtain good agreement with data recorded in the canopy of corn planted in rows 76‐cm apart. This was especially true for the top part of the row‐planted corn where small, short leaves resulted in large gaps for radiation penetration. The Markov model would be necessary, for instance, to determine the penetration of photosynthetically active radiation to the leaves in the middle of the canopy. Nevertheless, the Poisson model gave good agreement with the measured values for radiation intercepted by the entire canopy of the row‐planted crop.