We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

Metabolism provides a basis for using first principles of physics, chemistry, and biology to link the biology of individual organisms to the ecology of populations, communities, and ecosystems. Metabolic rate, the rate at which organisms take up, transform, and expend energy and materials, is the most fundamental biological rate. We have developed a quantitative theory for how metabolic rate varies with body size and temperature. Metabolic theory predicts how metabolic rate, by setting the rates of resource uptake from the environment and resource allocation to survival, growth, and reproduction, controls ecological processes at all levels of organization from individuals to the biosphere. Examples include: (1) life history attributes, including devel- opment rate, mortality rate, age at maturity, life span, and population growth rate; (2) population interactions, including carrying capacity, rates of competition and predation, and patterns of species diversity; and (3) ecosystem processes, including rates of biomass production and respiration and patterns of trophic dynamics. Data compiled from the ecological literature strongly support the theoretical predictions. Even- tually, metabolic theory may provide a conceptual foundation for much of ecology, just as genetic theory provides a foundation for much of evolutionary biology.

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Toward a metabolic theory of ecology

Significantly more carbon is stored in the world's soils--including peatlands, wetlands and permafrost--than is present in the atmosphere. Disagreement exists, however, regarding the effects of climate change on global soil carbon stocks. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Unravelling the feedback effect is particularly difficult, because the diverse soil organic compounds exhibit a wide range of kinetic properties, which determine the intrinsic temperature sensitivity of their decomposition. Moreover, several environmental constraints obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed 'apparent' temperature sensitivity, and these constraints may, themselves, be sensitive to climate.

/pdf/temperature-sensitivity-of-soil-carbon-decomposition-and-5ewzh1kof4.pdf

Temperature sensitivity of soil carbon decomposition and feedbacks to climate change

A series of experiments is presented investigating short term and long term changes of the nature of the response of rate of CO2 assimilation to intercellular p(CO2). The relationships between CO2 assimilation rate and biochemical components of leaf photosynthesis, such as ribulose-bisphosphate (RuP2) carboxylase-oxygenase activity and electron transport capacity are examined and related to current theory of CO2 assimilation in leaves of C3 species. It was found that the response of the rate of CO2 assimilation to irradiance, partial pressure of O2, p(O2), and temperature was different at low and high intercellular p(CO2), suggesting that CO2 assimilation rate is governed by different processes at low and high intercellular p(CO2). In longer term changes in CO2 assimilation rate, induced by different growth conditions, the initial slope of the response of CO2 assimilation rate to intercellular p(CO2) could be correlated to in vitro measurements of RuP2 carboxylase activity. Also, CO2 assimilation rate at high p(CO2) could be correlated to in vitro measurements of electron transport rate. These results are consistent with the hypothesis that CO2 assimilation rate is limited by the RuP2 saturated rate of the RuP2 carboxylase-oxygenase at low intercellular p(CO2) and by the rate allowed by RuP2 regeneration capacity at high intercellular p(CO2).

Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves.

This research is part of an effort that the ICT (Institute of Tropical Cultivation) has been doing for several years tending to develop superior genotypes of cocoa (Theobroma cacao L.). That is why this study aims to find tolerant or moderately tolerant cocoa genotypes and accessions to water stress with resistance to pests and diseases and high production and industrial quality. Twenty genotypes of cocoa seedlings were investigated, during the period of 6 months, in a soil with sandy-loam texture under nursery conditions, of controlled irrigation. A split plot design was used, with 40 treatments and 3 repetitions. In addition, daily data of the micro climatic characteristics (T °, HR) were taken, in which different indicators of variable were evaluated such as the stomatal conductance (CE) that is greatly influenced by the T ° and HR. The results obtained indicate that the genotypes that showed moderate tolerance to water stress were UNG - 77, UNG - 53, ICT - 1281 and ICT - 1112; the non-tolerant ones were PAS - 93, CEPEC - 2002, ICT - 2142, ICT - 1092, CP - 2005 - C10, TSH - 1188, CCN - 51, IMC - 67, PH - 17, AYP - 15, ICS - 6, BN - 34, ICT - 1506, PAS - 91, PH - 990 and ICS - 1. Under optimal conditions, the most outstanding genotype was ICS-1, both in plant height, number of leaves, and stomatal conductance, this being proof that this genotype develops excellently and stands out if it has the right conditions and water availability.

Stomatal conductance and photosynthesis

Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.

/pdf/a-biochemical-model-of-photosynthetic-co-2-assimilation-in-328htws229.pdf

A Biochemical Model of Photosynthetic CO 2 Assimilation in Leaves of C 3 Species

Photosynthesis is the incorporation of carbon, nitrogen, sulphur and other substances into plant tissue using light energy from the sun. Most of this energy is used for the reduction of carbon dioxide and, consequently, there is a large body of biochemical and biophysical information about photo synthetic carbon assimilation. In an ecophysiological context, we believe that most of today’s biochemical knowledge can be summarized in a few simple equations. These equations represent the rate of ribulose bisphosphate (RuP2)-saturated carboxylation, the ratio of photorespiration to carboxylation, and the rates of electron transport/photophosphorylation and of “dark” respiration in the light. There are many other processes that could potentially limit CO2 assimilation, but probably do so rarely in practice. Fundamentally this may be due to the expense, in terms of invested nitrogen, of the carboxylase and of thylakoid functioning. To reach our final simple equations we must first discuss the biochemical and biophysical structures — as they are understood at present — that finally reduce the vast number of potentially rate-limiting processes to the four or five listed above. A diagrammatic representation of these processes is given in Fig. 16.1.

Modelling of Photosynthetic Response to Environmental Conditions

CO2 reaches Rubisco by diffusion down a concentration gradient The gradient forms during photosynthesis as a result of restrictions to free movement imposed by stomata and leaf anatomy Recent techniques have enabled the internal conductance, gi, of leaves, that is, from sub-stomatal cavities to the sites of carboxylation, to be measured (1) A strong correlation has been found between gi and the rate of CO2 assimilation under high irradiance and ambient CO2 partial pressure, Ca (2,3,4) Unfortunately, the title of the paper by Lloyd et al (3), low conductances for CO, diffusion from stomata to the sites of carboxylation in leaves of woody species’ has been taken to mean that there is a correspondingly low partial pressure of CO2 in the chloroplast, which contributes to the low CO2 assimilation rate of sclerophyllous leaves The claim was repeated by Epron et al (5) ‘high internal resistances to CO2 transfer may account for the low net CO2 assimilation rates that often characterise tree leaves

https://www.ldeo.columbia.edu/~polissar/OrgGeochem/evans-and-von-caemmerer.pdf

Carbon Dioxide Diffusion inside Leaves

Despite mounting evidence showing that C4 plants can accumulate more biomass at elevated CO2 partial pressure (p(CO2)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C4 plants to elevated p(CO2) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO2) can stimulate the growth of both well-watered and waterstressed C4 plants. First, through enhanced leaf CO2 assimilation rates due to increased intercellular p(CO2). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO2 fixation in the bundle sheath or the presence of C3-like photosynthesis in young C4 leaves are unlikely explanations for the high CO2-responsiveness of C4 photosynthesis. The interactions between elevated p(CO2), leaf temperature and shoot water relations on the growth and photosynthesis of C4 plants are identified as key areas needing urgent research.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00609.x

S. von Caemmerer

Papers

A Biochemical Model of Photosynthetic CO 2 Assimilation in Leaves of C 3 Species

Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves.

Modelling of Photosynthetic Response to Environmental Conditions

Carbon Dioxide Diffusion inside Leaves

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment