Techniques for Measuring Intercepted and Absorbed Photosynthetically Active Radiation in Corn Canopies1

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Abstract
A portable system that would enable rapid measurement of photosynthetically active radiation (PAR) would be useful in studies that include measurements of interception and absorption of PAR in crop canopies. The objectives of this study were to develop and evaluate a portable system for measuring intercepted and absorbed PAR in corn (Zea mays L.) canopies. This study consisted of two field experiments with corn planted in north‐south rows at two densities with two planting dates in each of 2 yr. The soil was a dark (10YR 4/1) Chalmers silt loam (Typic Argiaquoll). A handle, two levels, and a data logger were added to a line quantum sensor to form a portable system for rapidly measuring the ascending and descending fluxes associated with absorbed PAR (APAR). The effects of sensor orientation and surface length on measurement of transmitted PAR and the errors induced by use of intercepted PAR as an estimate of absorbed PAR were examined. Transmitted PAR (TPAR) was optimally measured with the sensor leveled and positioned perpendicular to the row direction with the length of the sensor equal to the row spacing. Intercepted PAR (IPAR), the difference in descending PAR fluxes above and below the canopy, required fewer measurements than APAR and differed from APAR by the difference between canopy and soil surface reflectances. Prior to the dent stage (R5) of grain maturity, differences between APAR and IPAR were less than 3.5% for this study. Thus IPAR was a reasonable approximation of APAR and required fewer measurements.