ESTIMATION OF THE NET CO2 ASSIMILATION RATE OF A MAIZE (ZEA-MAYS L) CANOPY FROM LEAF CHAMBER MEASUREMENTS

The net CO2 assimilation rate of a maize canopy (A(c)) was estimated by vertically integrating leaf chamber CO2 exchange rates, taken between 6 and 11 weeks after emergence, using four methods: (i) the CUPRAD model based on five sunlit inclination angle classes; (ii) the CUPRAD model based on a mean leaf angle following the spherical distribution; (iii) the three-point integration method of Gauss with five sunlit inclination angle classes; (iv) the Gaussian method with leaves in a horizontal position. Canopy assimilation rates were measured using the eddy correlation method combined with soil respiration measurements obtained with a portable gas exchange system. They were used as a reference to test the vertical integration approaches. The Gaussian method and the CUPRAD model with several inclination angle classes provided similar estimates. CUPRAD based on a mean angle overestimated the crop assimilation rate by 5% compared with estimates based on several inclination angle classes. Leaves in a horizontal position integrated with the Gaussian method gave estimates 5-15% higher than those estimated by CUPRAD. CUPRAD based on several leaf angle classes overestimated the reference by 15-25% in the morning and by 30-40% in the afternoon. The Gaussian method, with five sunlit inclination angle classes, gave the best estimates of A(c) if the leaf chamber measurements were corrected to account for the leaf boundary layer conductances of the canopy. Morning estimates remained within the range of precision of the reference, i.e. 7%, but afternoon estimates were 10-30% higher than the reference. The afternoon discrepancy could not be explained by diurnal water stress or other physiological causes. The discrepancy is more likely to be a result of the fact that the boundary layer conductance equation used in this approach neglected the shelter effects. The upper layer of the canopy, which represented 28% of leaf area index (LAI) according to Gauss partition, contributed 38-44% of canopy assimilation rate around noon. The middle part, which accounted for 44% of LAI had a similar contribution, whereas the lower part, which accounted for 28% of LAI, contributed only 16% of canopy assimilation rate.