Instantaneous measurement of radiation and water use efficiencies of a maize crop

Field-scale measurement of CO2 Aux between vegetation and the atmosphere is a direct way to quantify short-term performance of an agricultural crop through the growing season. This study measured CO2 and H2O exchange in a maize (Zea mays L.) held during a growing season, in order to (i) estimate crop net photosynthesis (P-n) and canopy nighttime respiration acid (ii) calculate crop radiation and water use efficiencies. Maize net photosynthesis was calculated for 850 h (day) and canopy respiration for 175 h (night) from measurements of CO2 fluxes above the crop and estimates of CO2 fluxes at the soil surface. Under high radiation, P-n reached 2 mg m(-2) s(-1) at 30 d after emergence, remained between 2 and 2.5 mg m(-2) s(-1) for the next 30 d, and then slowly decreased until first frost. Two negative exponential equations were proposed to describe the relationship between P-n and intercepted photosynthetically active radiation (IPAR) during the growing season: one from planting to maximum leaf area index (LAI(max)) and one from LAI(max) to the first fall frost. These results confirmed that, in absence of water stress and with adequate fertilization, simple models based on IPAR could account for about 90% of the variation in P-n, The higher efficiency of diffuse than of direct beam radiation was documented. Instantaneous radiation use efficiency (IRUE) was shown to decrease by 66% from cloudy to clear sky conditions, so it is desirable to incorporate IRUE for estimation of short-term (hourly) P-n. Canopy respiration rates at night ranged from 0.1 to 0.2 mg m(-2) s(-1) for LAI between 1 and 3, or about 10% of daily photosynthesis. The relationship between P-n and water vapor flux (F-q,F-a) was nonlinear, with slope decreasing with increasing F-q,F-a. Water use efficiency at LAI(max) was 17 mg g(-1) for F-q,F-a = 0.05 g m(-2) s(-1) and about 10 mg g(-1) for F-q,F-a = 0.20 g m(-2) s(-1). Direct evaporation of water from the soil surface (at low LAI) or from wet plant parts resulted in considerable noise in the P-n - F-q,F-a relationship. The normalization of F-q,F-a by the vapor pressure deficit (VPD) linearized the relationship. The slope of the P-n - F-q,F-a/VPD curve was larger for cloudy than for clear sky conditions, probably as a result of the larger RUE under diffuse radiation.