Diurnal Variation in Physical Attributes Associated With Photosynthesis of a Tobacco Canopy

Abstract

Diurnal changes in vertical profiles of sunlit foliage and distributions of the angle between the leaves and the sun were derived from computer representations of the actual foliage display of a tobacco crop. The crop was analysed at five stages of development.

Diurnal variations in the proportion of sunlit leaf area were significant only at lower solar elevations. For a large part of the day, most of the foliage of small, isolated plants was directly lit, whereas the proportion of beam-lit foliage fell to approximately 0.4 in larger crops.

The distributions of sunlit foliage with respect to the leaf-beam angle were used to assess the efficiency with which photosynthetically active radiation was captured by the component layers of the crop. At each stage of development, larger proportions of sunlit foliage at intermediate and higher leaf-beam angles coincided with higher solar elevations. These results are discussed in the context of the photosynthetic response of tobacco leaves.

Sunlit leaf area and its distribution with respect to the leaf-beam angle were also estimated with a model of the foliage display of tobacco plants. The model accounted for the presence and influence of neighbouring plants in the crop. Comparisons between model estimates and those from geometrical representations of the crop are discussed in assessing model performance.