ERRORS IN MEASURING ABSORBED RADIATION AND COMPUTING CROP RADIATION USE EFFICIENCY

Radiation use efficiency (RUE) is often a crucial component of crop growth models that relate dry matter production to energy received by the crop. RUB is a ratio that has units g J(-1), if defined as phytomass per unit of energy received, and units J J(-1), if defined as the energy content of phytomass per unit of energy received. Both the numerator and denominator in computation of RUE can vary with experimental assumptions and methodologies. The objectives of this study were to examine the effect that different methods of measuring the numerator and denominator have on the RUE of corn (Zea mays L.) and to illustrate this variation with experimental: data. Computational methods examined included (i) direct measurements of the fraction of photosynthetically active radiation absorbed(f(A)), (ii) estimates of f(A) derived from leaf area index (LAI), and (iii) estimates of f(A) derived from spectral vegetation indices. Direct measurements of absorbed PAR from planting to physiological maturity of corn were consistently greater than the indirect estimates based on green W or the spectral vegetation indices. Consequently, the RUE calculated using directly measured absorbed PAR was lower than the RUE calculated using the indirect measures of absorbed PAR. For crops that contain senesced vegetation, green LAI and the spectral vegetation indices provide appropriate estimates of the fraction of PAR absorbed by a crop canopy and, thus, accurate estimates of crop radiation use efficiency.