INSTANTANEOUS MEASUREMENT OF RADIATION AND WATER-USE EFFICIENCIES OF A MAIZE CROP

Field-scale measurement of CO2 Aux between vegetation and the atmosphe re is a direct way to quantify short-term performance of an agricultur al crop through the growing season. This study measured CO2 and H2O ex change in a maize (Zea mays L.) held during a growing season, in order to (i) estimate crop net photosynthesis (P-n) and canopy nighttime re spiration acid (ii) calculate crop radiation and water use efficiencie s. Maize net photosynthesis was calculated for 850 h (day) and canopy respiration for 175 h (night) from measurements of CO2 fluxes above th e crop and estimates of CO2 fluxes at the soil surface. Under high rad iation, 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 d ecreased until first frost. Two negative exponential equations were pr oposed to describe the relationship between P-n and intercepted photos ynthetically active radiation (IPAR) during the growing season: one fr om 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 e fficiency of diffuse than of direct beam radiation was documented. Ins tantaneous radiation use efficiency (IRUE) was shown to decrease by 66 % from cloudy to clear sky conditions, so it is desirable to incorpora te 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 decreasi ng 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 su rface (at low LAI) or from wet plant parts resulted in considerable no ise 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 slo pe 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.