Light, temperature and nutrients as factors in photosynthetic adjustment to an elevated concentration of carbon dioxide

The short-term stimulation of the net rate of carbon dioxide exchange of leaves by elevated concentrations of CO₂ usually observed in C₃ plants sometimes does not persist. Experiments were conducted to test whether the patterns of response to the environment during growth were consistent with the hypotheses that photosynthetic adjustment to elevated CO₂ concentration is due to (1) feedback inhibition or (2) nutrient stress. Soybean [Glycine max (L.) Merr. cv. Williams] and sugar beet (Best vulgaris L. cv. Mono Hye-4) were grown from seed at 350 and 700 μl^? CO₂, at 20 and 25°C, at a photon flux density of 0.5 and 1.0 mmol m^?2 S^?1 and with three nutrient regimes until the third trifoliolate leaf of soybean or the sixth leaf of sugar beet had finished expanding. Net rates of CO₂ exchange of the most recently expanded leaves were then measured at both 350 and 700 μl 1^?1 CO₂. Plants grown at the elevated CO₂ concentration had net rates of leaf CO₂ exchange which were reduced by 33% in sugar beet and 23% in soybean when measured at 350 μl 1^?1 CO₂ and when averaged over all treatments. Negative photosynthetic adjustment to elevated CO₂ concentration was not greater at 20 than at 25°C, was not greater at a photon flux density of 1.0 than at 0.5 mmol m^?2 S^?1 and was not greater with limiting nutrients. Furthermore, in soybean, negative photosynthetic adjustment could be induced by a single night at elevated CO₂ concentration, with net rates of CO₂ exchange the next day equal to those of leaves of plants grown from seed at the elevated concentration of CO₂. These patterns do not support either the feedback-inhibition or the nutrient-stress hypothesis of photosynthetic adjustment to elevated concentrations of CO₂.     

T cell receptor specific apoptosis: an endogenous mechanism for T cell homeostasis and potential strategy for antigen modulation of disease

T lymphocytes can undergo an activation/proliferation response or an apoptotic response following T cell receptor engagement. The choice between these outcomes is dictated by the activation state of the T lymphocyte, the presence of interleukin-2 and the strength of the T cell receptor stimulus. Specifically, when quiescent cells encounter effectively presented antigen they are activated and begin to proliferate. In contrast, activated cells, moving through the cell cycle under the influence of IL-2, undergo apoptosis upon reencountering antigen. Both the tumour necrosis factor receptor and CD95 (FAS) are known to participate in mediating this cell death. Genetic defects in the molecules of the lymphocyte death pathway (CD95, FAS ligand, IL-2 receptor) lead to syndromes of autoimmunity and dysregulated lymphocyte homeostasis. An understanding of the principles of the autocrine feedback death model can provide the rationale basis for effective antigen specific modulation of T cell mediated disease processes.     

生猪供给系统反馈结构的定量分析

本文研究了浙江省生猪供给系统变量间的线性与非线性回归函数关系,确定了系统反馈结构．     

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract. Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.     

Skeletal muscle: A paradigm for testing principles of bioenergetics

Muscular activity converts chemical energy into useful work and metabolism restores muscle to its original state. This essay explores the organization and interactions of the regulatory system(s) which allow this energy balance to occur. The term energy balance is used in a biochemical rather than a thermodynamic sense—concerned not with deductions from the physical principles of thermodynamics, but rather with those enzymatic processes which nature evolved and which operate at remarkably fixed stoichiometry. Energy balance is a statement of conservation of energy put into biochemical observables.³¹P NMR spectroscopy is one of the most useful techniques for investigating these questions quantitatively under physiological conditionsin vivo. The author (1) describes the rules or principles of biochemical energy balance; (2) discusses sample results from human muscle to demonstrate its use in studying this class of questions; (3) presents a simple model of integrated cellular respiration to demonstrate its sufficiency to account for the main observations.     

Belowground positive and negative feedbacks on CO2 growth enhancement

In this paper we present a conceptual model of integrated plant-soil interactions which illustrates the importance of identifying the primary belowground feedbacks, both positive and negative, which can simultaneously affect plant growth responses to elevated CO₂. The primary negative feedbacks share the common feature of reducing the amount of nutrients available to plants. These negative feedbacks include increased litter C/N ratios, and therefore reduced mineralization rates, increased immobilization of available nutrients by a larger soil microbial pool, and increased storage of nutrients in plant biomass and detritus due to increases in net primary productivity (NPP). Most of the primary positive feedbacks share the common feature of being plant mediated feedbacks, the only exception being Zak et al.'s hypothesis that increased microbial biomass will be accompanied by increased mineralization rates. Plant nutrient uptake may be increased through alterations in root architecture, physiology, or mycorrhizal symbioses. Further, the increased C/N ratios of plant tissue mean that a given level of NPP can be achieved with a smaller supply of nitrogen.Identification of the net plant-soil feedbacks to enhanced productivity with elevated CO₂ are a critical first step for any ecosystem. It is necessary, however, that we first identify how universally applicable the results are from one study of one ecosystem before ecosystem models incorporate this information. The effect of elevated CO₂ on plant growth (including NPP, tissue quality, root architecture, mycorrhizal symbioses) can vary greatly for different species and environmental conditions. Therefore it is reasonable to expect that different ecosystems will show different patterns of interacting positive and negative feedbacks within the plant-soil system. This inter-ecosystem variability in the potential for long-term growth responses to rising CO₂ levels implies that we need to parameterize mechanistic models of the impact of elevated CO₂ on ecosystem productivity using a detailed understanding of each ecosystem of interest.     

Inhibition of nitric oxide synthase activity in cerebral cortical synaptosomes by nitric oxide donors: Evidence for feedback autoregulation

Despite evidence which supports a neurotransmitter-like role for nitric oxide (NO) in the CNS, relatively little is known regarding mechanisms which control NO formation within CNS neurons. In this study, isolated nerve endings (synaptosomes) from rat cerebral cortex were used to ascertain whether NO can autoregulate its own formation within neurons through feedback inhibition of the NO biosynthetic enzyme nitric oxide synthase (NOS). Under the conditions described here, N-nitro-l-arginine methyl ester-sensitive conversion ofl-[³H]arginine intol-[³H]citrulline (i.e., NOS activity) was found to be highly calcium-dependent and strongly inhibited (up to 60 percent) by NO donors, including sodium nitroprusside, hydroxylamine and nitroglycerin. The inhibitory effect of sodium nitroprusside was concentration-dependent (IC₅₀100 M) and prevented by the NO scavenger oxyhemoglobin.l-Citrulline, the other major end-product from NOS, had no apparent effect on synaptosomal NOS activity. Taken together, these results indicate that neuronal NOS can be inhibited by NO released from exogenous donors and, therefore, may be subject to end-product feedback inhibition by NO that is formed locally within neurons or released from proximal cells.     

Neutralization of radiation damping by selective feedback on a 400 MHz NMR spectrometer

Summary Radiation damping is a phenomenon well known among NMR spectroscopists of proteins as a source of undesirable features, especially in high-field and high-Q probe NMR. In this paper, we present an electronic neutralization network which dramatically reduces radiation damping. It detects the radiation field profile and feeds back into the probe an rf field with identical amplitude and opposite phase. Experimental results of a practical implementation carried out on a 400 MHz Bruker spectrometer are shown.     

Development of regulation mechanisms for SO42- influx in spring wheat roots

The development of SO₄^2- influx in roots and sulfur transport to shoots was followed in ³⁵S-tracer experiments for sulfur-deficient spring wheat (Triticum aestivum L. cv. Svenno) seedlings pretreated for various time periods (0–24 h) in nutrient solutions with SO₄^2-. Effects of the metabolic inhibitor 2,4-dinitrophenol (DNP) and the protein synthesis inhibitor cycloheximide (CH) on SO₄^2- influx were also evaluated. The SO₄^2- influx appears feedback-regulated by the internal sulfur level of the roots. Regulation may be achieved solely by a rapidly changed SO₄^2- carrier activity through an allosteric effect by the intracellular SO₄^2- concentration of the roots, followed first by induction of carrier synthesis and then by repression of carrier synthesis after transfer of the roots from SO₄^2--deficient nutrient solutions to solutions with SO₄^2-. A Hill plot of the partly sigmoidal relationship between SO₄^2- influx and intracellular sulfur concentration in the roots gave a Hill coefficient of -4.2, indicating negative cooperativity between a minimum number of four interacting allosteric binding sites for sulfur on each carrier entity. DNP-experiments showed that SO₄^2- influx was mainly metabolic, especially after short pretreatment in SO₄^2- at an external SO₄^2- concentration of 0.1 mM. Pretreatment with CH rapidly prevented new SO₄^2- carriers from being formed. Long CH pretreatment (24 h) and different SO₄^2- pretreatments reduced SO₄^2- influx below the non-metabolic level obtained by uptake experiments with DNP, indicating the existence of SO₄^2- carriers mediating passive SO₄^2- transport across the plasmalemma of the root cells. SO₄^2- influx was further decreased for the CH pretreated (24 h) plants by the presence of both CH and DNP in the experimental nutrient solution. This probably indicates the diffusive part of the non-metabolic SO₄^2- influx in the present experiments. Finally, it is suggested that there is a feedback signal between root and shoot, regulating sulfur transport upwards.     

Control of amino acid transport in the mammary gland of the pregnant mouse

The regulation of the uptake of the amino acid analog α-aminoisobutyric acid was studied in diced mammary glands from pregnant mice. Stimulation of uptake by insulin was not prevented by inhibitors of protein synthesis; protein synthesis inhibitors decreased uptake by 20%; this response occurred more promptly in insulintreated tissues. Elimination of extracellular amino acids led to a substantial increase in transport which was not abolished by inhibitors of protein synthesis. These results indicate that insulin does not increase amino acid transport in this system by altering synthesis and degradation of transport protein. They are consistent with a model in which the activity of the existing amino acid transport protein is subject to negative feedback regulation from the intracellular amino acid pool.