Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: Vcmax, J, TPU, Rd and gm [maximum carboxylation rate allowed by ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO2 assimilation (A) and [CO2] in the intercellular airspaces of the leaf (Ci) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO2 partial pressure at the sites of carboxylation, Cc, (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25°C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant.
The conductance for CO2 diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO2 diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO2 in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO2 partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.The photosynthetic fixation of CO2 occurs at the enzyme Rubisco that is at the end of a complex diffusion path. The partial pressure of CO2 drops across any part of the pathway that has a low conductance. Therefore, any low-conductance component of the diffusion path poses a limitation to photosynthesis whenever CO2 is not saturating. The conductance to diffusion of CO2 in photosynthesizing leaves is commonly divided into three components: boundary layer, stomatal, and mesophyll conductances (9). Mesophyll conductance has sometimes been defined in such a way that it includes biochemical factors, but we use it here in the more restricted sense of a physical diffusion phenomenon. These conductances may be subdivided further or lumped together in various ways (21), but the simple three-part formulation serves our purpose. '
The capacity for isoprene emission evolved many times in plants, probably as a mechanism for coping with heat flecks. It also confers tolerance of reactive oxygen species. It is an example of isoprenoids enhancing membrane function, although the mechanism is likely to be different from that of sterols. Understanding the regulation of isoprene emission is advancing rapidly now that the pathway that provides the substrate is known.
The effect of long-term (weeks to months) CO2 enhancement on (a) the gas-exchange characteristics, (b) the content and activation state of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) leaf nitrogen, chlorophyll, and dry weight per area were studied in five C3 species (Chenopodium album, Phaseolus vulgaris, Solanum tuberosum, Solanum melongena, and Brassica oleracea) grown at CO2 partial pressures of 300 or 900 to 1000 microbars. Long-term exposure to elevated CO2 affected the CO2 response of photosynthesis in one of three ways: (a) the initial slope of the CO2 response was unaffected, but the photosynthetic rate at high CO2 increased (S. tuberosum); (b) the initial slope decreased but the C02-saturated rate of photosynthesis was little affected (C. album, P. vulgaris); (c) both the initial slope and the C02-saturated rate of photosynthesis decreased (B. oleracea, S. melongena). In all five species, growth at high CO2 increased the extent to which photosynthesis was stimulated following a decrease in the partial pressure of 02 or an increase in measurement CO2 above 600 microbars. This stimulation indicates that a limitation on photosynthesis by the capacity to regenerate orthophosphate was reduced or absent after acclimation to high CO2. Leaf nitrogen per area either increased (S. tuberosum, S. melongena) or was little changed by CO2 enhancement. The content of rubisco was lower in only two of the five species, yet its activation state was 19% to 48% lower in all five species following longterm exposure to high CO2. These results indicate that during growth in C02-enriched air, leaf rubisco content remains in excess of that required to support the observed photosynthetic rates.During short-term (minutes to hours) exposure to elevated partial pressure of C02, the rate of light-saturated CO2 assimilation (A2) in many C3 plants is primarily limited by the capacity to regenerate Pi from phosphorylated photosynthetic intermediates (10, 17,22,23,26). A Pi-regeneration limitation of photosynthesis is characterized by a lack of sensitivity ofA to changes in ambient partial pressure of CO2 and/ or 02 (10,
Nonmethane hydrocarbons are ubiquitous trace atmospheric constituents yet they control the oxidation capacity of the atmosphere. Both anthropogenic and biogenic processes contribute to the release of hydrocarbons to the atmosphere. In this manuscript, the state of the science concerning biosynthesis, transport, and chemical transformation of hydrocarbons emitted by the terrestrial biosphere is reviewed. In particular, the focus is on isoprene, monoterpenes, and oxygenated hydrocarbons. The generated science during the last 10 years is reviewed to explain and quantify hydrocarbon emissions from vegetation and to discern impacts of biogenic hydrocarbons on local and regional atmospheric chemistry. Furthermore, the physiological and environmental processes controlling biosynthesis and production of hydrocarbon compounds are reported on. Many advances have been made on measurement and modeling approaches developed to quantify hydrocarbon emissions from leaves and forest ecosystems. A synthesis of the atmospheric chemistry of biogenic hydrocarbons and their role in the formation of oxidants and aerosols is presented. The integration of biogenic hydrocarbon kinetics and atmospheric physics into mathematical modeling systems is examined to assess the contribution of biogenic hydrocarbons to the formation of oxidants and aerosols, thereby allowing us to study their impacts on the earth's climate system and to develop strategies to reduce oxidant precursors in affected regions. 1538
Plants resist infection and herbivory with innate immune responses that are often associated with reduced growth. Despite the importance of growth-defense tradeoffs in shaping plant productivity in natural and agricultural ecosystems, the molecular mechanisms that link growth and immunity are poorly understood. Here, we demonstrate that growth-defense tradeoffs mediated by the hormone jasmonate are uncoupled in an Arabidopsis mutant (jazQ phyB) lacking a quintet of Jasmonate ZIM-domain transcriptional repressors and the photoreceptor phyB. Analysis of epistatic interactions between jazQ and phyB reveal that growth inhibition associated with enhanced anti-insect resistance is likely not caused by diversion of photoassimilates from growth to defense but rather by a conserved transcriptional network that is hardwired to attenuate growth upon activation of jasmonate signalling. The ability to unlock growth-defense tradeoffs through relief of transcription repression provides an approach to assemble functional plant traits in new and potentially useful ways.
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