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Influence of Stand Geometry on Light Interception and Net Photosynthesis in Cotton¹

The following geometric variables interact to determine the nature of shade patterns in row crops: solar altitude, the angle of the row with respect to the solar azimuth, plant size and planting pattern. Great change during the day was observed in the relative percent interception in all stands when the crop was young. In the early morning and late afternoon NS rows intercepted more light than EW rows. Percent interception began to level off at a leaf area index of about 3 in the conventionally planted cotton. Little or no convergence in the percent interception vs. LAI curves for noon and noon ± 5 hours was noted in the NS-skip cotton up to an LAI of 4 on a per lanted acre basis. This indicates that expenditure of net photosynthate in the further elaboration of new leaf tissue would be a good investment as long as the fruit ltimately produced by the plant would have time to mature. Row direction had no significant effect on the total daily net photosynthesis by cotton conventionally planted in 40-inch rows. On a per planted acre basis the NS-skip planted cotton consistently outperformed the NSsolid stand and the EW-skip planted stand. In all of the stands the daily time course of net photosynthesis parallelled very closely the course of interception of solar energy. The light data from a flat surface receiver may be adjusted for percent interception by any stand, regardless of geometry, to give an accurate prediction of net photosynthesis.

² Research Soil Scientist, Boll Weevil Research Laboratory, State College, Miss.

³ Former Plant Physiologist, Boll Weevil Research Laboratory. Present address: Department of Agronomy, Texas Technological College, Lubbock, Texas.

Received for publication May 20, 1965.