Environmental regulation of energetics of C4 photosynthesis

Many studies have been aimed at understanding the biochemistry and significance of C4 pathway since its discovery. It has become well established that this pathway enables the plant to photosynthesize at a higher rate than C3 photosynthesis under conditions of high temperatures and limited water supply. This is because of the CO2 concentrating mechanism (CCM) in C4 photosynthesis achieved by a series of anatomical and biochemical adaptations which allow the operation of two photosynthetic cycles, C4 and C3, across the outer mesophyll (MC) and inner bundle-sheath cells (BSC) to saturate Rubisco with CO2 in the BSC. CCM minimizes photorespiration where oxygen competes with CO2 uptake which consumes additional energy and decreases productivity. Most importantly, the activity rate of C4 photosynthesis is also higher under high light intensities thus it is not surprising to find C4 plants in open and arid habitats leading them to an agricultural and ecological importance. C4 photosynthesis is also traditionally grouped into three classical subtypes based on the major C4 acid decarboxylation step in the BSC: NADP malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and PEP carboxykinase (PCK). In view of the relative abundance of C4 plants in high light environments, it is interesting to ascertain what physiological factors associated with C4 photosynthesis might make it favourable in those environments. One reason is the differences in the maximum quantum yield for CO2 uptake (QY), which is the leaf-level ratio of photosynthetic carbon gain to absorbed photons. Under elevated temperature (>25oC) and ambient CO2 concentrations, previous studies showed that C3 species had lower QY than C4 species and among C4 subtypes, NAD-ME subtype had the lowest. Lower QY of C3 species is due to photorespiration, however, causes of differences in the QY among C4 subtypes are not clear. One reason for this variation is the efficiency of light energy conversion reactions (termed energetics) in MC and BSC which is associated with distinct composition, activity and/or efficiency of light-harvesting complexes (LHC) and electron transport components. Limited information is available explaining the connection between energetics under varying environmental conditions and QY among C4 subtypes. The main aim of this thesis was to investigate the plasticity of the components and activities of the light reactions of photosynthesis in representative species of C3, C3-C4, and the three subtypes of C4 species under environmental conditions that may affect the photosynthetic quantum yield such as low light and low CO2

Bimodality Phenomenon in Finite and Infinite Systems Within an Exactly Solvable Statistical Model

We present a few explicit counterexamples to the widely spread belief about an exclusive role of the bimodal nuclear fragment size distributions as the first order phase transition signal. In thermodynamic limit the bimodality may appear at the supercritical temperatures due to the negative values of the surface tension coefficient. Such a result is found within a novel exactly solvable formulation of the simplified statistical multifragmentation model based on the virial expansion for a system of the nuclear fragments of all sizes. The developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density. Its exact solution for finite volumes demonstrates the bimodal fragment size distribution right inside the finite volume analog of a gaseous phase. These counterexamples clearly demonstrate the pitfalls of Hill approach to phase transitions in finite systems.Comment: Talk given at the Helmholtz International Summer School "Physics of Heavy Quarks and Hadrons", held in Dubna, Russia, July 15-28, 201

The Flow Constraint Influence on the Properties of Nuclear Matter Critical Endpoint

We propose a novel family of equations of state for symmetric nuclear matter based on the induced surface tension concept for the hard-core repulsion. It is shown that having only four adjustable parameters the suggested equations of state can, simultaneously, reproduce not only the main properties of the nuclear matter ground state, but the proton flow constraint up its maximal particle number densities. Varying the model parameters we carefully examine the range of values of incompressibility constant of normal nuclear matter and its critical temperature which are consistent with the proton flow constraint. This analysis allows us to show that the physically most justified value of nuclear matter critical temperature is 15.5-18 MeV, the incompressibility constant is 270-315 MeV and the hard-core radius of nucleons is less than 0.4 fm.Comment: 8 pages, 3 figure