INTERCEPTION AND ABSORPTION OF PHOTOSYNTHETICALLY ACTIVE RADIATION BY CORN CANOPIES

KEVIN PAUL GALLO, Purdue University

Abstract

Photosynthetically active radiation (PAR; 400 to 700 nm) is an important energy source for photosynthesis. PAR can only be utilized when it is absorbed by a plant. A study was conducted to develop techniques for measurement of absorbed PAR (APAR) in corn (Zea mays. L.) canopies, to evaluate several estimators of APAR, and to examine the relationship between canopy APAR and final grain yield. Corn was planted on a Typic Argiaquoll on 14 May and 24 June 1982 and thinned to densities of 50 and 100 thousand plants/ha. APAR was computed from clear sky measurements of incident PAR, PAR transmitted through and to the floor of the canopy (TPAR), and PAR reflected from the soil and canopy. APAR was proportioned by the amount of incident PAR. Intercepted PAR was calculated as 1.0 minus the proportion of incident PAR that was TPAR. Spectral reflectances of these canopies were measured with a radiometer that has wavelength bands corresponding to the Landsat Thematic Mapper. IPAR estimated APAR better than models based on Beer-Bouger's Law that predicted APAR as a function of leaf area index and solar zenith and azimuth angles. TPAR was significantly affected by stage of canopy development, plant density, leaf area index, and solar zenith and azimuth angles. APAR in corn canopies increased as a function of green leaf area index up to silking and then decreased at a slower rate due to absorption by stalks and nongreen leaves. A spectral variable 'greenness index' (GI) derived from the reflectance data was associated with 94% of the variance in APAR. An energy-crop growth (ECG) variable that combines temperature, moisture stress, and APAR was used to simulate the daily interactive effects of weather on corn growth and yield. The ECG variable, summed from planting to physiological maturity, was associated with 83% of the variance in grain yields when APAR was estimated with LAI. The ECG variable was associated with 85% of the variance in grain yields when APAR was estimated with the spectral variable GI.

Degree

Ph.D.

Subject Area

Agronomy

COinS