ERRORS IN MEASURING ABSORBED RADIATION AND COMPUTING CROP RADIATION USE EFFICIENCY

Radiation use efficiency (RUE) is often a crucial component of crop gr owth models that relate dry matter production to energy received by th e 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 ene rgy content of phytomass per unit of energy received. Both the numerat or and denominator in computation of RUE can vary with experimental as sumptions and methodologies. The objectives of this study were to exam ine the effect that different methods of measuring the numerator and d enominator have on the RUE of corn (Zea mays L.) and to illustrate thi s variation with experimental: data. Computational methods examined in cluded (i) direct measurements of the fraction of photosynthetically a ctive radiation absorbed(f(A)), (ii) estimates of f(A) derived from le af 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 PA R was lower than the RUE calculated using the indirect measures of abs orbed PAR. For crops that contain senesced vegetation, green LAI and t he spectral vegetation indices provide appropriate estimates of the fr action of PAR absorbed by a crop canopy and, thus, accurate estimates of crop radiation use efficiency.