A Model for Calculating Light Interception by a Grain Sorghum Canopy

A simplified model for calculating intercepted photo-synthetically active radiation (0.4-0.ly) was developed to eliminate the necessity for determining the de-tailed plant foliage characteristics required in many light interception models. To simplify predicting foliar light interception, plant structure is idealized using geometric shapes and a concept of effective light inter-cepting leaf area is introduced. The effective light inter-cepting leaf area is a function of that portion of foliage actually intercepting light and, hence, accounts for mutual shading of plant leaves and shadows cast by neighboring plants. As plant leaf area increases, the equivalent idealized areas representing plant leaf sur-faces intercepting light interact. Interplant competi-tion is determined with mensuration formulas. Photosynthetically active radiation measurements were made simultaneously above and below three grain sorghum canopies for use in developing and validating the model. Different canopy architecture was achieved by using three row spacing regimes (25.4, 50.8, and 101.6 cm, with average within-row plant spacings of 21.2, 10.6 and 5.3 cm, respectively) and equipopulated plantings (185,000 plants/ha). Analysis of light penetration measurements shows the sensitivity of the extinction coefficient to interplant competition and solar altitude. Light attenuation in the model is computed using a Bouguer-Lambert (Beer's) Law relationship in which the extinction coefficient is sensitive to adjacent plant light competition interactions. Comparison of measured and modeled transmitted light data for both clear and cloudy days indicated that the model is reliable. The applicability of this model is ex-tended by inclusion of a relationship for equating radiant energy units to quantum units.