Summary
A significant resistance to CO2 diffusion is imposed by mesophyll tissue inside leaves. Mesophyll resistance, rm (or its reciprocal, mesophyll conductance, gm), reduces the rate at which ...Rubisco can fix CO2, increasing the water and nitrogen costs of carbon acquisition. gm varies in proportion to the surface area of chloroplasts exposed to intercellular airspace per unit leaf area. It also depends on the thickness and effective porosity of the cell wall and the CO2 permeabilities of membranes. As no measurements exist for the effective porosity of mesophyll cell walls, and CO2 permeability values are too low to account for observed rates of CO2 assimilation, conclusions from modelling must be treated with caution. There is great variation in the mesophyll resistance per unit chloroplast area for a given cell wall thickness, which may reflect differences in effective porosity. While apparent gm can vary with CO2 and irradiance, the underlying conductance at the cellular level may remain unchanged. Dynamic changes in apparent gm arise for spatial reasons and because chloroplasts differ in their photosynthetic composition and operate in different light environments. Measurements of the temperature sensitivity of membrane CO2 permeability are urgently needed to explain variation in temperature responses of gm.
In a time when conservative politicians challenge the irrefutability of scientific findings such as climate change, it is more important than ever to understand the conflict at the heart of the ...“religion vs. science” debates unfolding in the public sphere. In this groundbreaking work, John H. Evans reveals that, with a few limited exceptions, even the most conservative religious Americans accept science’s ability to make factual claims about the world. However, many religious people take issue with the morality implicitly promoted by some forms of science. Using clear and engaging scholarship, Evans upends the prevailing notion that there is a fundamental conflict over the way that scientists and religious people make claims about nature and argues that only by properly understanding moral conflict between contemporary religion and science will we be able to contribute to a more productive interaction between these two great institutions.
Improving Photosynthesis Evans, John R.
Plant physiology,
08/2013, Letnik:
162, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Photosynthesis is the basis of plant growth, and improving photosynthesis can contribute toward greater food security in the coming decades as world population increases. Multiple targets have been ...identified that could be manipulated to increase crop photosynthesis. The most important target is Rubisco because it catalyses both carboxylation and oxygenation reactions and the majority of responses of photosynthesis to light, CO
2
, and temperature are reflected in its kinetic properties. Oxygenase activity can be reduced either by concentrating CO
2
around Rubisco or by modifying the kinetic properties of Rubisco. The C
4
photosynthetic pathway is a CO
2
-concentrating mechanism that generally enables C
4
plants to achieve greater efficiency in their use of light, nitrogen, and water than C
3
plants. To capitalize on these advantages, attempts have been made to engineer the C
4
pathway into C
3
rice (Oryza sativa). A simpler approach is to transfer bicarbonate transporters from cyanobacteria into chloroplasts and prevent CO
2
leakage. Recent technological breakthroughs now allow higher plant Rubisco to be engineered and assembled successfully in planta. Novel amino acid sequences can be introduced that have been impossible to reach via normal evolution, potentially enlarging the range of kinetic properties and breaking free from the constraints associated with covariation that have been observed between certain kinetic parameters. Capturing the promise of improved photosynthesis in greater yield potential will require continued efforts to improve carbon allocation within the plant as well as to maintain grain quality and resistance to disease and lodging.
The nitrogen cost of photosynthesis Evans, John R; Clarke, Victoria C
Journal of experimental botany,
01/2019, Letnik:
70, Številka:
1
Journal Article
Recenzirano
Odprti dostop
This review provides a generalized nitrogen budget for a C3 leaf cell, shows the use of nitrogen as a proxy for predicting photosynthetic capacity in ecosystem models and discusses the potential for ...improving crop photosynthesis from a nitrogen perspective.
Abstract
Global food security depends on three main cereal crops (wheat, rice and maize) achieving and maintaining high yields, as well as increasing their future yields. Fundamental to the production of this biomass is photosynthesis. The process of photosynthesis involves a large number of proteins that together account for the majority of the nitrogen in leaves. As large amounts of nitrogen are removed in the harvested grain, this needs to be replaced either from synthetic fertilizer or biological nitrogen fixation. Knowledge about photosynthetic properties of leaves in natural ecosystems is also important, particularly when we consider the potential impacts of climate change. While the relationship between nitrogen and photosynthetic capacity of a leaf differs between species, leaf nitrogen content provides a useful way to incorporate photosynthesis into models of ecosystems and the terrestrial biosphere. This review provides a generalized nitrogen budget for a C3 leaf cell and discusses the potential for improving photosynthesis from a nitrogen perspective.
The temperature responses of mesophyll conductance (gₘ) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. ...Species included the evergreen trees Eucalyptus pauciflora and Quercus engelmannii; the tropical evergreen tree Lophostemon confertus; as well as the herbaceous species Nicotiana tabacum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Glycine max and Arabidopsis thaliana. Responses varied from a two‐ to threefold increase in mesophyll conductance between 15 and 40 °C observed for N. tabacum, G. hirsutum, G. max and E. pauciflora to almost no change in L. confertus and T. aestivum. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were Eucalyptus and Gossypium, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.
We show that silver(I) hexacyanocobaltate(III), Ag₃Co(CN)₆, exhibits positive and negative thermal expansion an order of magnitude greater than that seen in other crystalline materials. This ...framework material expands along one set of directions at a rate comparable to the most weakly bound solids known. By flexing like lattice fencing, the framework couples this to a contraction along a perpendicular direction. This gives negative thermal expansion that is 14 times larger than in ZrW₂O₈. Density functional theory calculations quantify both the low energy associated with this flexibility and the role of argentophilic (Ag⁺...Ag⁺) interactions. This study illustrates how the mechanical properties of a van der Waals solid might be engineered into a rigid, useable framework.
Powder diffraction is one of the most widely used analytical techniques for characterizing solid state materials. It can be used for phase or polymorph identification, quantitative analysis, cell ...parameter determination, or even full crystal structure analysis using the powerful Rietveld refinement method. As with much of modern crystallography, the software used for Rietveld refinement is frequently treated as a “black box” that produces often poorly understood outputs. This paper shows how it is possible for students to perform a full Rietveld refinement against experimental powder diffraction data from scratch using a simple spreadsheet like Excel. It starts by reviewing the basic ideas of least-squares fitting a straight line, develops these into fitting simple functions to peaks in simulated experimental data, and then combines these ideas with crystallographic equations to enable Rietveld refinement of the structure of an inorganic material (rutile, TiO2). At each stage, students can self-learn different fundamental aspects and pitfalls of data analysis that are widely reapplicable. The ideas can be taught as an online learning exercise or could be incorporated in a laboratory class where students collect and analyze their own experimental data.
The ethical debate about what is now called human gene editing (HGE) has gone on for more than 50 y. For nearly that entire time, there has been consensus that a moral divide exists between somatic ...and germline HGE. Conceptualizing this divide as a barrier on a slippery slope, in this paper, I first describe the slope, what makes it slippery, and describe strong barriers that arrest the slippage down to the dystopian bottom of pervasive eugenic enhancement. I then show how the somatic/germline barrier in the debate has been weakened to the level of ineffectiveness, with no replacement below. I examine a number of possible barriers on the slope below the somatic/germline barrier, most of which lack sufficient strength. With the exception of the minority of people in the HGE debate who see the eugenic society as utopia, the majority will need a barrier on the slope to stop the slide to dystopia.
ABSTRACT
The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed ...mesophyll conductance (gm). We investigated the temperature response of gm in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO2 in either 2 or 21% O2 with carbon isotope measurements using tuneable diode laser spectroscopy. The gm increased linearly with temperature in 2 or 21% O2. In 21% O2, isotope discrimination associated with gm decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO2 partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in gm with increasing leaf temperature resulted in the drawdown in CO2 partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.
A linear relationship between mesophyll conductance and temperature (15–40°C) was observed in tobacco leaves using carbon isotope discrimination measurements. Photorespiration made a significant contribution to the discrimination signal, contributing more than mesophyll conductance at 40°C. The increase in mesophyll conductance with increasing leaf temperature resulted in the drawdown in CO2 partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.
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