Impact of mesophyll diffusion on estimated global land CO2 fertilization.

In C3 plants, CO2 concentrations drop considerably along mesophyll diffusion pathways from substomatal cavities to chloroplasts where CO2 assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown and therefore overestimate CO2 available for carboxylation and underestimate photosynthetic responsiveness to atmospheric CO2. An explicit consideration of mesophyll diffusion increases the modeled cumulative CO2 fertilization effect (CFE) for global gross primary production (GPP) from 915 to 1,057 PgC for the period of 1901-2010. This increase represents a 16% correction, which is large enough to explain the persistent overestimation of growth rates of historical atmospheric CO2 by Earth system models. Without this correction, the CFE for global GPP is underestimated by 0.05 PgC/y/ppm. This finding implies that the contemporary terrestrial biosphere is more CO2 limited than previously thought.

Ecosystem-scale volatile organic compound fluxes during an extreme drought in a broadleaf temperate forest of the Missouri Ozarks (central USA).

Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately represented for accurately modeling the coupled biosphere-atmosphere-climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m(-2)  h(-1) at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO2 reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7-17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The meganv2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events.

Asymmetrical effects of mesophyll conductance on fundamental photosynthetic parameters and their relationships estimated from leaf gas exchange measurements.

Worldwide measurements of nearly 130 C3 species covering all major plant functional types are analysed in conjunction with model simulations to determine the effects of mesophyll conductance (g(m)) on photosynthetic parameters and their relationships estimated from A/Ci curves. We find that an assumption of infinite g(m) results in up to 75% underestimation for maximum carboxylation rate V(cmax), 60% for maximum electron transport rate J(max), and 40% for triose phosphate utilization rate T(u) . V(cmax) is most sensitive, J(max) is less sensitive, and T(u) has the least sensitivity to the variation of g(m). Because of this asymmetrical effect of g(m), the ratios of J(max) to V(cmax), T(u) to V(cmax) and T(u) to J(max) are all overestimated. An infinite g(m) assumption also limits the freedom of variation of estimated parameters and artificially constrains parameter relationships to stronger shapes. These findings suggest the importance of quantifying g(m) for understanding in situ photosynthetic machinery functioning. We show that a nonzero resistance to CO2 movement in chloroplasts has small effects on estimated parameters. A non-linear function with gm as input is developed to convert the parameters estimated under an assumption of infinite gm to proper values. This function will facilitate gm representation in global carbon cycle models.

Overexpression of AtDREB1D transcription factor improves drought tolerance in soybean.

Drought is one of the major abiotic stresses that affect productivity in soybean (Glycine max L.) Several genes induced by drought stress include functional genes and regulatory transcription factors. The Arabidopsis thaliana DREB1D transcription factor driven by the constitutive and ABA-inducible promoters was introduced into soybean through Agrobacterium tumefaciens-mediated gene transfer. Several transgenic lines were generated and molecular analysis was performed to confirm transgene integration. Transgenic plants with an ABA-inducible promoter showed a 1.5- to two-fold increase of transgene expression under severe stress conditions. Under well-watered conditions, transgenic plants with constitutive and ABA-inducible promoters showed reduced total leaf area and shoot biomass compared to non-transgenic plants. No significant differences in root length or root biomass were observed between transgenic and non-transgenic plants under non-stress conditions. When subjected to gradual water deficit, transgenic plants maintained higher relative water content because the transgenic lines used water more slowly as a result of reduced total leaf area. This caused them to wilt slower than non-transgenic plants. Transgenic plants showed differential drought tolerance responses with a significantly higher survival rate compared to non-transgenic plants when subjected to comparable severe water-deficit conditions. Moreover, the transgenic plants also showed improved drought tolerance by maintaining 17-24 % greater leaf cell membrane stability compared to non-transgenic plants. The results demonstrate the feasibility of engineering soybean for enhanced drought tolerance by expressing stress-responsive genes.

Reliable estimation of biochemical parameters from C3 leaf photosynthesis-intercellular carbon dioxide response curves.

The Farquhar-von Caemmerer-Berry (FvCB) model of photosynthesis is a change-point model and structurally overparameterized for interpreting the response of leaf net assimilation (A) to intercellular CO2 concentration (Ci). The use of conventional fitting methods may lead not only to incorrect parameters but also several previously unrecognized consequences. For example, the relationships between key parameters may be fixed computationally and certain fits may be produced in which the estimated parameters result in contradictory identification of the limitation states of the data. Here we describe a new approach that is better suited to the FvCB model characteristics. It consists of four main steps: (1) enumeration of all possible distributions of limitation states; (2) fitting the FvCB model to each limitation state distribution by minimizing a distribution-wise cost function that has desirable properties for parameter estimation; (3) identification and correction of inadmissible fits; and (4) selection of the best fit from all possible limitation state distributions. The new approach implemented theoretical parameter resolvability with numerical procedures that maximally use the information content of the data. It was tested with model simulations, sampled A/Ci curves, and chlorophyll fluorescence measurements of different tree species. The new approach is accessible through the automated website leafweb.ornl.gov.

Applying a universal scaling model to vascular allometry in a single-stemmed, monopodially branching deciduous tree (Attim's model).

West, Brown and Enquist (1999a) modeled vascular plants as a continuously branching hierarchical network of connected links (basic structural units) that ends in a terminal unit, the leaf petiole, at the highest link order (WBE model). We applied the WBE model to study architecture and scaling between links of the water transport system from lateral roots to leafy lateral branches and petioles in Populus deltoides Bartr. ex Marsh. trees growing in an agroforestry system (open-grown trees) and in a dense plantation (stand-grown trees). The architecture of P. deltoides violates two WBE model assumptions: (1) the radii of links formed in a branching point are unequal; and (2) there is no terminal unit situated at the end of a hierarchical network, rather, petioles are situated at any link order greater than 1. Link cross sections were taken at various link orders and morphological levels in roots and shoots of open-grown trees and shoots of stand-grown trees. Scaling of link radii was area-preserving. From roots to branches, vessel diameters were scaled with link order in accordance with a 1/6-power, as predicted by the WBE model indicating general vessel tapering. However, analysis of the data at the morphological level showed that vessel radius decreased intermittently with morphological level rather than continuously between successive link orders. Estimation of total water conductive area in a link is based on conducting area and petiole radius in the WBE model. The estimation failed in P. deltoides, probably because petioles are not a terminal unit. Biomass of stand-grown trees scaled with stem basal radius according to the 3/8-power predicted by the WBE model. Thus, the WBE model adequately described vascular allometry and biomass at the whole-tree level in P. deltoides despite violation of Assumption 1, but failed in predictions where the leaf petiole was used as a terminal unit.

Influence of a drying cycle on post-drought xylem sap abscisic acid and stomatal responses in young temperate deciduous angiosperms.

• Post-drought patterns of water relations and gas exchange were studied in relation to xylem sap abscisic acid (ABA) concentration during recovery for young plants of five woody species. The potential role of xylem sap [ABA] in these responses was the object of study. • Potted plants were allowed to deplete soil water and then were rewatered. At peak drought and during recovery, predawn and midday leaf water potential (Ψl ), stomatal conductance (gs ), and xylem sap [ABA] were measured. • Water potentials recovered rapidly after rewatering but stomatal re-opening was delayed. Xylem sap [ABA] was elevated early in recovery and might have affected stomatal opening, but after 1 d at high soil water content [ABA] in recovering plants was equal to or lower than in control plants. Stomata appeared to be more sensitive to xylem sap [ABA] in recovering than droughted plants. • Xylem sap [ABA] may play some role in delayed recovery of stomatal opening after drought, but may not completely explain the responses.

Response of liquid flow resistance to soil drying in seedlings of four deciduous angiosperms.

Soil-leaf resistance to liquid water flow (R) in moist and drying soil was compared in three-month-old seedlings of two drought tolerant (white [Quercus alba L.], post oak [Q. stellata Wangenh.]) and two drought sensitive forest species (sugar maple [Acer saccharum Marsh.], black walnut [Juglans nigra L.]). At high soil moisture (Ψs≥-0.3 MPa), R was higher in J. nigra than in the other species, and as soil water was depleted R increased most in this species. In contrast, the lowest resistance at all levels of soil moisture was observed in Q. stellata. At Ψs of -1.5 MPa, R of drought-sensitive J. nigra and A. saccharum was about twice as high as that of the two drought-tolerant Quercus species. The difference in R between the two Quercus species was much smaller than that between this pair and the other two species. These differences among species in flow resistance may be attributable to: 1) variation in the balance between root surface area and leaf area, 2) variation in the inherent absorption capacity of the root systems and in xylem water conducting systems or 3) differences in root permeability, shrinkage and mortality in severely stressed seedlings. As the soil dried, seedlings of all species exhibited pronounced reductions in transpiration rate, which prevented development of large water potential gradients between leaves and the soil. Reduction in transpiration in J. nigra was especially pronounced, resulting in a decrease in the soil-to-leaf water potential gradient in dry soil despite high flow resistance. The observed differences among species in flow resistance are correlated with natural distribution patterns.

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of young plants of four temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid (ABA) concentration ([ABA]) and stomatal aperture were compared in drought-sensitive black walnut (Juglans nigra L.) and black willow (Salix nigra Marsh.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.). Strong correlations among reduction in predawn water potential, increase in xylem sap [ABA] and stomatal closure were observed in all species. Stomatal response was more highly correlated with xylem [ABA] than with ABA flux. Xylem sap pH and ion concentrations appeared not to play a major role in the stomatal response of these species. Stomata were more sensitive to relative changes in [ABA] in drought-sensitive black walnut and black willow than in sugar maple and white oak. In the early stages of drought, increased [ABA] in the xylem sap of black walnut and black willow was probably of root origin and provided a signal to the shoot of the water status of the roots. In sugar maple and white oak, leaf water potential declined with the onset of stomatal closure, so that stomatal closure also may have occurred in response to the change in leaf water potential.

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.

Acute drought stress and plant age effects on alkamide and phenolic acid content in purple coneflower roots.

The effects of acute periods of drought stress on dry weight, and alkamide and phenolic acid content in purple coneflower [Echinacea purpurea (L.) Moench, Asteraceae] roots are described. Plants subjected to brief drought stress periods for two seasons during the initial flowering stage (D-F2) produced fall-harvested roots with significantly greater cichoric acid concentration (mg/g) than corresponding well-watered controls of the same age (C-2). Total alkamide, including the tetraenoic acid isomers, and chlorogenic acid concentrations from fall-harvested roots were largely unaffected by drought stress, regardless of when the stress occurred developmentally. The alkamide concentration in three-year roots was significantly less than that in two-year roots, with an average decrease of 50.5 %. Conversely, total phenolic acids increased an average of 67.1 % for all treatments from two to three years of age. Root dry weight increased significantly by an average of 70.0 % for all drought-stressed plants from two to three years of age, compared to an increase of 35.2 % for well-watered controls. The results suggest that controlled drought stress can stimulate increased root dry weight and root cichoric acid content, and that root age is the predominant factor determining overall phytochemical content variation.

Effect of acute drought stress and time of harvest on phytochemistry and dry weight of St. John's wort leaves and flowers.

The phytochemistry and dry weight of cultivated St. John's wort are significantly influenced by acute drought stress and time of harvest. In this study, plants subjected to brief drought stress during both flower and seed development periods exhibited increased concentrations in 8 of the 10 phytochemicals examined in this study, including hypericin, pseudohypericin, chlorogenic acid, rutin, hyperoside, isoquercitrin, quercitrin, and quercetin. Increases ranged from 5% to 36% (hyperoside and rutin, respectively). Conversely, the concentrations of hyperforin and adhyperforin in flowers were decreased by an average of 10% in drought-stressed plants as compared to well-watered control plants. Acute drought stress decreased flower dry weight significantly during both drydown periods, although vegetative parameters (height, leaf dry weight and stem dry weight) were not adversely affected. While acute drought stress significantly altered the chemical yield in the leaves and flowers (phytochemical content x harvested dry weight), the time of harvest was the predominant factor determining phytochemical concentration in the organs of H. perforatum.