Combined experimental drought and nitrogen loading: the role of species-dependent leaf level control of carbon and water exchange in a temperate grassland.

Nitrogen (N) loading and extreme drought strongly alter biomass production, species composition and carbon and water fluxes of temperate grasslands. Such changes at the community level are often attributed to species- and functional group-specific responses in phenology and/or physiology. In a multifactorial field experiment, we studied the responses of three abundant grassland species (forb Centaurea jacea, grasses Arrhenatherum elatius and Dactylis glomerata) to N loading and extreme drought, focusing on responses of carbon and water relations at the leaf level. We analysed (1) changes in bulk leaf N (uptake efficiency of additional N), (2) adaptation of plant water status (leaf water potential) and (3) impact on leaf carbon and water fluxes. We observed more efficient N utilization in the two grasses compared to C. jacea. Naturally occurring summer drought significantly impacted the plant water status of all species, while extreme drought treatment only further affected water status during and after summer drought. C. jacea was able to maintain much lower leaf water potentials compared to the grasses during drought. Despite these clear species-specific responses to N loading and drought, the species were able to maintain homeostasis of leaf carbon and water fluxes. Thus, strong declines in the (community) carbon sequestration observed at this site during the (natural) summer drought were not related to leaf physiological responses in assimilation, but were driven by phenological adaptions of the species community: the drought-sensitive grasses, even though exhibiting higher N uptake efficiency, responded with a shortened life cycle to severe summer drought.

Response pattern of amino compounds in phloem and xylem of trees to soil drought depends on drought intensity and root symbiosis.

This study aimed to identify drought-mediated differences in amino nitrogen (N) composition and content of xylem and phloem in trees having different symbiotic N(2)-fixing bacteria. Under controlled water availability, 1-year-old seedlings of Robinia pseudoacacia (nodules with Rhizobium), Hippophae rhamnoides (symbiosis with Frankia) and Buddleja alternifolia (no such root symbiosis) were exposed to control, medium drought and severe drought, corresponding soil water content of 70-75%, 45-50% and 30-35% of field capacity, respectively. Composition and content of amino compounds in xylem sap and phloem exudates were analysed as a measure of N nutrition. Drought strongly reduced biomass accumulation in all species, but amino N content in xylem and phloem remained unaffected only in R. pseudoacacia. In H. rhamnoides and B. alternifolia, amino N in phloem remained constant, but increased in xylem of both species in response to drought. There were differences in composition of amino compounds in xylem and phloem of the three species in response to drought. Proline concentrations in long-distance transport pathways of all three species were very low, below the limit of detection in phloem of H. rhamnoides and in phloem and xylem of B. alternifolia. Apparently, drought-mediated changes in N composition were much more connected with species-specific changes in C:N ratios. Irrespective of soil water content, the two species with root symbioses did not show similar features for the different types of symbiosis, neither in N composition nor in N content. There was no immediate correlation between symbiotic N fixation and drought-mediated changes in amino N in the transport pathways.

Carbon balance in leaves of young poplar trees.

In the present study, important components of carbon metabolism of mature leaves of young poplar trees (Populus x canescens) were determined. Carbohydrate concentrations in leaves and xylem sap were quantified at five different times during the day and compared with photosynthetic gas exchange measurements (net assimilation, transpiration and rates of isoprene emission). Continuously measured xylem sap flow rates, with a time resolution of 15 min, were used to calculate diurnal balances of carbon metabolism of whole mature poplar leaves on different days. Loss of photosynthetically fixed carbon by isoprene emission and dark respiration amounted to 1% and 20%. The most abundant soluble carbohydrates in leaves and xylem sap were glucose, fructose and sucrose, with amounts of approx. 2 to 12 mmol m(-2) leaf area in leaves and about 0.2 to 15 mM in xylem sap. Clear diurnal patterns of carbohydrate concentration in xylem sap and leaves, however, were not observed. Calculations of the carbon transport rates in the xylem to the leaves were based on carbohydrate concentrations in xylem sap and xylem sap flow rates. This carbon delivery amounted to about 3 micromol C m(-2) s(-1) during the day and approx. 1 micromol C m(-2) s(-1) at night. The data demonstrated that between 9 and 28 % of total carbon delivered to poplar leaves during 24 h resulted from xylem transport and, hence, provide a strong indication for a significant rate of carbon cycling within young trees.

Enhanced ozone-tolerance in wheat grown at an elevated CO2 concentration: ozone exclusion and detoxification.

Elevated [CO2 ] has been shown to protect photosynthesis and growth of wheat against moderately elevated [O3 ]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat (Triticum aestivum L. ev. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO2 ] at 350 or 700 µmol mol-1 and [O3 ] peaking at < 5 or 60 nmol mol-1 , respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [CO2 ] and/or [O3 ]. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced by elevated [CO2 ]. In wheat, at moderately elevated [O3 ], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO2 ] against O3 damage.