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The present study investigated the interactive effects of three environmental stress factors elevated CO2, temperature, and drought stress on soybean growth and yield. Experiments were conducted in the sunlit, controlled environment Soil–Plant–Atmosphere–Research chambers under two-level of irrigation (WW-well water and WS-water stress-35%WW) and CO2 (aCO2-ambient 400 µmol mol−1 and eCO2-elevated 800 µmol mol−1) and each at the three day/night temperature regimes of 24/18 °C (MLT-moderately low), 28/22 °C (OT-optimum), and 32/26 °C (MHT-moderately high). Results showed the greatest negative impact of WS on plant traits such as canopy photosynthesis (PCnet), total dry weight (TDwt), and seed yield. The decreases in these traits under WS ranged between 40 and 70% averaged across temperature regimes with a greater detrimental impact in plants grown under aCO2 than eCO2. The MHT had an increased PCnet, TDwt, and seed yield primarily under eCO2, with a greater increase under WW than WS conditions. The eCO2 stimulated PCnet, TDwt, and seed yield more under WS than WW. For instance, on average across T regimes, eCO2 stimulated around 25% and 90% dry mass under WW and WS, respectively, relative to aCO2. Overall, eCO2 appears to benefit soybean productivity, at least partially, under WS and the moderately warmer temperature of this study. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSC) are the dominant functional vegetation unit in some of the harshest habitats in the world. We assessed BSC response to stress through changes in biotic composition, CO2 gas exchange and carbon allocation in three lichen-dominated BSC from habitats with different stress levels, two more extreme sites in Antarctica and one moderate site in Germany. Maximal net photosynthesis (NP) was identical, whereas the water content to achieve maximal NP was substantially lower in the Antarctic sites, this apparently being achieved by changes in biomass allocation. Optimal NP temperatures reflected local climate. The Antarctic BSC allocated fixed carbon (tracked using 14CO2) mostly to the alcohol soluble pool (low-molecular weight sugars, sugar alcohols), which has an important role in desiccation and freezing resistance and antioxidant protection. In contrast, BSC at the moderate site showed greater carbon allocation into the polysaccharide pool, indicating a tendency towards growth. The results indicate that the BSC of the more stressed Antarctic sites emphasise survival rather than growth. Changes in BSC are adaptive and at multiple levels and we identify benefits and risks attached to changing life traits, as well as describing the ecophysiological mechanisms that underlie them. [ABSTRACT FROM AUTHOR]

Plant stomata display a wide range of short-term behavioural and long-term morphological responses to atmospheric carbon dioxide concentration ([CO]). The diversity of responses suggests that plants may have different strategies for controlling gas exchange, yet it is not known whether these strategies are co-ordinated in some way. Here, we test the hypothesis that there is co-ordination of physiological (via aperture change) and morphological (via stomatal density change) control of gas exchange by plants. We examined the response of stomatal conductance ( G) to instantaneous changes in external [CO] ( C) in an evolutionary cross-section of vascular plants grown in atmospheres of elevated [CO] (1,500 ppm) and sub-ambient [O] (13.0 %) compared to control conditions (380 ppm CO, 20.9 % O). We found that active control of stomatal aperture to [CO] above current ambient levels was not restricted to angiosperms, occurring in the gymnosperms Lepidozamia peroffskyana and Nageia nagi. The angiosperm species analysed appeared to possess a greater respiratory demand for stomatal movement than gymnosperm species displaying active stomatal control. Those species with little or no control of stomatal aperture (termed passive) to C were more likely to exhibit a reduction in stomatal density than species with active stomatal control when grown in atmospheres of elevated [CO]. The relationship between the degree of stomatal aperture control to C above ambient and the extent of any reduction in stomatal density may suggest the co-ordination of physiological and morphological responses of stomata to [CO] in the optimisation of water use efficiency. This trade-off between stomatal control strategies may have developed due to selective pressures exerted by the costs associated with passive and active stomatal control. [ABSTRACT FROM AUTHOR]

mmonia (NH) fluxes between beech leaves ( Fagus sylvatica) and the atmosphere were investigated in a 90-year-old forest canopy and related to leaf nitrogen (N) pools and glutamine synthetase (GS) activities. The stomatal ammonia compensation point, χ, was measured by both a twig cuvette and bioassay techniques involving measurements of pH and ammonium (NH) concentration in the leaf apoplastic solution. The χ determined on the basis of the gas exchange measurements followed a seasonal variation with early-season peaks during leaf expansion (9.6 nmol NH mol air) and late-season peaks during leaf senescence (7.3 nmol NH mol air). In the mid-season, the χ of mature green leaves was much lower (around 3 nmol NH mol air) and dropped below the NH concentration in the ambient atmosphere. For comparison, χ obtained by the apoplastic bioassay were 7.0, 3.7 and 6.4 nmol NH mol air in early-, mid-, and late -season, thus agreeing reasonably well with χ values derived from the gas exchange measurements. Potential NH emission fluxes during early and late season were 1.31 and 0.51 nmol m leaf surface area s, respectively, while leaves were a sink for NH during mid-season. During leaf establishment and senescence, both apoplastic and bulk tissue NH concentrations were relatively high coinciding with low activities of glutamine synthetase, which is a key enzyme in leaf N metabolism. In conclusion, the exchange of NH between beech leaves and the atmosphere followed a seasonal variation with NH emission peaks being related to N mobilization during early leaf establishment and remobilization during late leaf senescence. [ABSTRACT FROM AUTHOR]

Sky conditions play an important role in the Earth's climate system and CO uptake by plants. We used eddy covariance and meteorological data, including global and diffuse photosynthetic photon flux density (PPFD), recorded over the 2008 and 2009 growing season at two Sitka spruce [ Picea sitchensis ( Bong.) Carr.] forest sites in northern Britain, in order to establish relationships between physiological properties under diverse sky conditions, i.e. (1) sunny, (2) cloudy, and (3) overcast, and several canopy activity-related properties. These properties are: (1) response to PPFD, (2) photosynthetic light use efficiency, and (3) canopy stomatal conductance. We found that Sitka spruce forests utilise PPFD in a more efficient way when solar radiation is dominated by diffuse radiation. Furthermore, our results show that diffuse radiation enhances canopy stomatal conductance, an effect which may be the result of both blue light enrichment within the canopy and the reduction in vapour pressure deficit during cloudy and overcast weather. Diffuse radiation does not only influence short-term (hourly, daily, monthly) canopy activity but also long-term forest growth. [ABSTRACT FROM AUTHOR]

Deserts are characterized by low productivity and substantial unvegetated space, which is often covered by soil microbial crust communities. Microbial crusts are important for nitrogen fixation, soil stabilization and water infiltration, but their role in ecosystem production is not well understood. This study addresses the following questions: what are the CO2 exchange responses of crusts to pulses of water, does the contribution of crusts to ecosystem flux differ from the soil respiratory flux, and is this contribution pulse size dependent? Following water application to crusts and soils, CO2 exchange was measured and respiration was partitioned through mixing model analysis of Keeling plots across treatments. Following small precipitation pulse sizes, crusts contributed 80% of soil-level CO2 fluxes to the atmosphere. However, following a large pulse event, roots and soil microbes contributed nearly 100% of the soil-level flux. Rainfall events in southern Arizona are dominated by small pulse sizes, suggesting that crusts may frequently contribute to ecosystem production. Carbon cycle studies of arid land systems should consider crusts as important contributors because of their dynamic responses to different pulse sizes as compared to the remaining ecosystem components. [ABSTRACT FROM AUTHOR]