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An arable crop rotation (winter barley-sugar beet-winter wheat) was exposed to elevated atmospheric CO(2) concentrations ([CO(2) ]) using a FACE facility (Free-Air CO(2) Enrichment) during two rotation periods. The atmospheric [CO(2) ] of the treatment plots was elevated to 550 ppm during daylight hours (T>5°C). Canopy transpiration (E(C) ) and conductance (G(C) ) were measured at selected intervals (>10% of total growing season) using a dynamic CO(2) /H(2) O chamber measuring system. Plant available soil water content (gravimetry and TDR probes) and canopy microclimate conditions were recorded in parallel. Averaged across both growing seasons, elevated [CO(2) ] reduced E(C) by 9%, 18% and 12%, and G(C) by 9%, 17% and 12% in barley, sugar beet and wheat, respectively. Both global radiation (Rg) and vapour pressure deficit (VPD) were the main driving forces of E(C) , whereas G(C) was mostly related to Rg. The responses of E(C) and especially G(C) to [CO(2) ] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO(2) ] impact on E(C) and thus, at least in part, explained the variability of seasonal [CO(2) ] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO(2) ] enrichment increased plant available soil water content in the course of the season by ca. 15 mm. This was true for all crops and years. Lower transpirational cooling due to a [CO(2) ]-induced reduction of E(C) increased canopy surface and air temperature by up to 2 °C and 0.5 °C, respectively. This is the first study to address effects of FACE on both water fluxes at canopy scale and water status of a European crop rotation., (© 2010 German Botanical Society and The Royal Botanical Society of the Netherlands.)

Sugar beet (Beta vulgaris ssp. altissima Döll) was grown in the field under free-air CO(2) enrichment (FACE, 550 ppm) and different nitrogen (N) supply (2001: 126 (N100) and 63 kg.ha(-1) (N50); 2004: 156 (N100) and 75 kg.ha(-1)) during two crop rotations. Canopy CO(2) exchange rates (CCER) were measured during the main growth phase (leaf area index > or =2) using a dynamic chamber system. Canopy CO(2) exchange data were analysed with respect to treatment effects on seasonal means and light use efficiency and light response characteristics. CO(2) enrichment enhanced CCER throughout the season. However, in both years, CCER declined after the second half of August independent of radiation and [CO(2)]. Elevated [CO(2)] strongly stimulated CCER on a seasonal basis, whereas the reduction of CCER caused by low N was below 10% and not significant. There were no effects of N on daily radiation use efficiency of carbon gain calculated from CCER data, but a strong enhancement by CO(2) enrichment. CCER closely tracked diurnal variations in incident photosynthetic photon flux density (PPFD, mumol.m(-2).s(-1)). The relationship between CCER and incident PPFD was curvilinear. In both seasons, initial slopes and maximum rates (CCER(max)) were determined from two 6-day periods using these relationships. The first period was measured after canopy closure (first half of July) and the second in the second half of August. In the first period, elevated [CO(2)] increased the initial slopes. Low N supply affected neither the initial slopes nor their response to elevated [CO(2)] in either period. In contrast to initial slopes, N stress limited the [CO(2)] response of CCER(max) in the first period. In the second period, however, this interaction of [CO(2)] and N on CCER(max) was completely dominated by a general decline of CCER(max) whereas no general decline of the initial slopes occurred in the second period. This response of light response parameters to [CO(2)] and N suggests that, in sugar beet, the decline of CCER in the late season may rely on limiting mechanisms such as photosynthetic acclimation responses to elevated [CO(2)] caused by sink limitations.

Sugar beet ( Beta vulgaris ssp . altissima Döll) was grown in the field under free-air CO2 enrichment (FACE, 550 ppm) and different nitrogen (N) supply (2001: 126 (N100) and 63 kg·ha−1 (N50); 2004: 156 (N100) and 75 kg·ha−1) during two crop rotations. Canopy CO2 exchange rates (CCER) were measured during the main growth phase (leaf area index ≥2) using a dynamic chamber system. Canopy CO2 exchange data were analysed with respect to treatment effects on seasonal means and light use efficiency and light response characteristics. CO2 enrichment enhanced CCER throughout the season. However, in both years, CCER declined after the second half of August independent of radiation and [CO2]. Elevated [CO2] strongly stimulated CCER on a seasonal basis, whereas the reduction of CCER caused by low N was below 10% and not significant. There were no effects of N on daily radiation use efficiency of carbon gain calculated from CCER data, but a strong enhancement by CO2 enrichment. CCER closely tracked diurnal variations in incident photosynthetic photon flux density (PPFD, μmol·m−2·s−1). The relationship between CCER and incident PPFD was curvilinear. In both seasons, initial slopes and maximum rates (CCERmax) were determined from two 6-day periods using these relationships. The first period was measured after canopy closure (first half of July) and the second in the second half of August. In the first period, elevated [CO2] increased the initial slopes. Low N supply affected neither the initial slopes nor their response to elevated [CO2] in either period. In contrast to initial slopes, N stress limited the [CO2] response of CCERmax in the first period. In the second period, however, this interaction of [CO2] and N on CCERmax was completely dominated by a general decline of CCERmax whereas no general decline of the initial slopes occurred in the second period. This response of light response parameters to [CO2] and N suggests that, in sugar beet, the decline of CCER in the late season may rely on limiting mechanisms such as photosynthetic acclimation responses to elevated [CO2] caused by sink limitations. [ABSTRACT FROM AUTHOR]

Summary • The predictive capacity of two simulation models with different degrees of complexity for the calculation of assimilate production, was tested at different time scales, using a data set of wheat grown in an open-top-chamber experiment at two CO2 concentrations. • Observed values of net canopy assimilation (Pn) were obtained from wheat plants grown at ambient (410 µmol mol-1 ) and elevated (680 µmol mol-1 ) CO2 mole fractions. Pn was simulated by using either simple multiple regression equations (AFRCWHEAT2) or by highly detailed calculations of leaf energy balances and the coupling of photosynthesis with stomatal conductance (LINTULCC2). • Irrespective of the developmental stage of the crop or variation in weather, the models accurately simulated canopy assimilation and growth. We conclude that the response of aboveground-biomass production to elevated CO2 concentrations was explained primarily by the effects of CO2 on radiation-use efficiency and assimilate production. • The models explained satisfactorily the daily course of Pn, its integrated daily totals, and the seasonally produced aboveground biomass, both at ambient and elevated CO2 concentrations. Specific problems in the simulations were identified and discussed. [ABSTRACT FROM AUTHOR]