Spectral Estimates of Daily Carbon Dioxide Exchange Rates in Wheat Canopies

Spectral indices have been used to predict leaf area index (LAI) and the interception efficiency (e₁) of photosynthetically active radiation (PAR) within a plant canopy. The objectives of this work were to investigate the influence of canopy geometry on the temporal trends in LAI and spectral indices, and to develop equations to predict the daily canopy gross carbon dioxide exchange rates (DGCE) from daily PAR. Winter wheat (Triticum aestivum L. cv. Newton) was planted in north-south and east-west row orientations with 0.18-, 0.35-, 0.53-, and 0.71-m row spacings, during 1982—1983 and 1983—1984. Net CO₂ exchange rates were measured with a transparent dynamic positive-pressure chamber and a differential infrared gas analysis system. Daytime dark respiration rate correction were made to obtain the DGCE. Canopy spectral reflectance was monitored with a multispectral radiometer, while absorption of PAR was measured with quantum sensors. Daily absorbed PAR was derived by taking the product of e₁, estimated from either the LAI or spectral indices, and total incoming PAR. Row orientation did not significantly influence the temporal trends in LAI or the spectral response, whereas significantly greater LAI and spectral response occurred in the narrower- vs. wider-spaced canopies. A linear model characterized the relationship between DGCE and daily absorption of PAR. A common slope model (0.74 g CO₂ mol⁻¹ photon) indicated that the LAI and spectral indices had similar predictive capabilities. Good agreement occurred between an equation developed with 1984 data and measured values from 1983 (i.e., R² 0.49—0.65). Remotely sensed multispectral canopy reflectance data, therefore, can be used to estimate DGCE in wheat canopies. Contribution no. 88-548-J from the Kansas Agric. Exp. Stn.