Estimation of electron density,effective atomic number and stopping power ratio using dual-layer computed tomography for radiotherapy treatment planning

PurposeAssess the accuracy for quantitative measurements of electron density relative to water (ρ_e/ρ_e,w), effective atomic number (Z_eff) and stopping power ratio relative to water (SPR_w) using a dual-layer computed tomography (DLCT) system.Methods and MaterialsA tissue characterization phantom was scanned using DLCT with varying scanning parameters (i.e., tube voltage, rotation time, CTDI_vol, and scanning mode) and different reference materials. Then, electron density ρ_e/ρ_e,w and atomic number Z_eff images were reconstructed, and their values were determined for each reference materials. Based on these two values, SPR_w was calculated. Finally, the percent error (PE) against the theoretical values was calculated for reference materials.ResultsSignificant linear relationships (p < 0.001) were observed between the measured and theoretical ρ_e/ρ_e,w (r = 1.000), Z_eff (r = 0.989) and SPR_w (r = 1.000) values. The PE for each reference material varied from –2.0 to 1.2% (mean, <0.1%) for electron density ρ_e/ρ_e,w, from –6.4 to 8.0% (mean, –2.0%) for atomic number Z_eff, and from –2.0 to 1.9% (mean, 0.3%) for stopping power ratio SPR_w. The mean PE of ρ_e/ρ_e,w (<0.1%), Z_eff (<–2.5%) and SPR_w (<0.4%) was verified across the variation of scanning parameters (p > 0.85).ConclusionsDLCT provides a reasonable accuracy in the measurements of ρ_e/ρ_e,w, Z_eff and SPR_w, and could enhance radiotherapy treatment planning and the subsequent outcomes.     

Solvent isotope effects on α-glucosidase

The solvent kinetic isotope effects (SKIE) on the yeast α-glucosidase-catalyzed hydrolysis of p-nitrophenyl and methyl-d-glucopyranoside were measured at 25 °C. With p-nitrophenyl-d-glucopyranoside (pNPG), the dependence of k_cat/K_m on pH (pD) revealed an unusually large (for glycohydrolases) solvent isotope effect on the pL-independent second-order rate constant, ^DOD(k_cat/K_m), of 1.9 (±0.3). The two pK_as characterizing the pH profile were increased in D₂O. The shift in pK_a2 of 0.6 units is typical of acids of comparable acidity (pK_a=6.5), but the increase in pK_a1 (=5.7) of 0.1 unit in going from H₂O to D₂O is unusually small. The initial velocities show substrate inhibition (K_is/K_m～200) with a small solvent isotope effect on the inhibition constant [^DODK_is=1.1 (±0.2)]. The solvent equilibrium isotope effects on the K_is for the competitive inhibitors d-glucose and α-methyl d-glucoside are somewhat higher [^DODK_i=1.5 (±0.1)]. Methyl glucoside is much less reactive than pNPG, with k_cat 230 times lower and k_cat/K_m 5×10⁴ times lower. The solvent isotope effect on k_cat for this substrate [=1.11 (±0. 02)] is lower than that for pNPG [=1.67 (±0.07)], consistent with more extensive proton transfer in the transition state for the deglucosylation step than for the glucosylation step.     

Isocitrate lyase from Neurospora crassa: pH dependence of catalysis and interaction with substrates and inhibitors.

The maximal velocity, V, for isocitrate cleavage by isocitrate lyase from Neurospora crassa is dependent on two dissociable groups with pK_a values of 6.1 and 8.6. A dissociable group with a pK_a of 8.5 on the enzyme-substrate complex affects the pK_m for isocitrate. The pK_i for homoisocitrate is affected in a like manner. The pH dependence of the pK_i's for succinate, a product of isocitrate cleavage, and the succinate analog maleate is similar to the pH dependence of the pK_m of isocitrate below pH 7.3, but is markedly different above this pH. Both the K_m for isocitrate and the K_i for succinate were dependent upon Mg²⁺ concentration. The pK_i for oxalate, an analog of glyoxylate which is also a product of isocitrate cleavage, is dependent on a group with a pK_a of 6.8 on the enzyme-inhibitor complex. The pH dependence of the pK_i for phosphoenolpyruvate, which binds to the succinate site, suggests that it is dependent on two dissociable groups, one on phosphoenolpyruvate and one, by analogy to the pK_m for isocitrate, on the enzyme-glyoxylate-inhibitor complex.     

Effective/census population size ratio estimation: a compendium and appraisal

With an ecological-evolutionary perspective increasingly applied toward the conservation and management of endangered or exploited species, the genetic estimation of effective population size (N_e) has proliferated. Based on a comprehensive analysis of empirical literature from the past two decades, we asked: (i) how often do studies link N_e to the adult census population size (N)? (ii) To what extent is N_e correctly linked to N? (iii) How readily is uncertainty accounted for in both N_e and N when quantifying N_e/N ratios? and (iv) how frequently and to what degree might errors in the estimation of N_e or N affect inferences of N_e/N ratios? We found that only 20% of available N_e estimates (508 of 2617; 233 studies) explicitly attempted to link N_e and N; of these, only 31% (160 of 508) correctly linked N_e and N. Moreover, only 7% (41 of 508) of N_e/N ratios (correctly linked or not) reported confidence intervals for both N_e and N; for those cases where confidence intervals were reported for N_e only, 31% of N_e/N ratios overlapped with 1, of which more than half also reached below N_e/N = 0.01. Uncertainty in N_e/N ratios thus sometimes spanned at least two orders of magnitude. We conclude that the estimation of N_e/N ratios in natural populations could be significantly improved, discuss several options for doing so, and briefly outline some future research directions.     

Quantifying the electronic cis effect of phosphine, arsine and stibine ligands by use of rhodium(I) Vaska-type complexes

The cis effects of phosphine, arsine and stibine ligands have been evaluated by measuring the IR stretching frequency in dichloromethane of the carbonyl ligand in a series of Rh(I) Vaska-type complexes, trans-[RhCl(CO)(L)₂]. These data were correlated with those obtained by Tolman for the electronic trans influences in the [Ni(L)(CO)₃] complexes. The electronic contribution, χ_Fc, of ferrocenyl was determined as 0.8 from these plots by evaluating PPh₂Fc as ligand. In order to accommodate arsine and stibine ligands an additional correction term, to compensate for differences in the donor atom, was added to Tolman’s equation for calculation of the Tolman electronic parameter of phosphine ligands. In the resulting equation: ν(CO_Ni)=2056.1+∑_i=1³χ_i+C_L values for C_L of C_P=0, C_As=−1.5 and C_Sb=−3.1 are suggested for phosphine, arsine and stibine ligands, respectively. The crystal and molecular structures of trans-[RhCl(CO)(PPh₂Fc)₂] · 2C₆H₆, trans-[RhCl(CO){P(NMe₂)₃}₂] and trans-[RhCl(CO)(AsPh₃)₂] are reported. The Tolman cone angles for PPh₂Fc and P(NMe₂)₃ were determined as 169° and 166°, while the effective cone angles for PPh₂Fc, P(NMe₂)₃ and AsPh₃ were determined as 171°, 168° and 147°, respectively.     

The structural dependence of amino acid hydrophobicity parameters

Amino acid hydrophobicity parameters, G_hp log P (partition coefficient) values, free energies of solution, G_sol and hydration numbers, are well correlated by equations derived from the relationship O_X = Aα_X + Lσ_IX + Sν_X + I_iX + H₁n_HX + H₂n_nX + b₀ where O is the quantity correlated; X denotes the amino acid side chain; α is a polarizability parameter; σ_I, a localized electrical effect parameter; ν, a steric parameter; i, an indicator variable which accounts for an ionic X ; n_H and n_n the number of OH or NH bonds and of full nonbonding orbitals in X, respectively, and b₀ is the intercept. The equation is based on the assumption that Δ_hp log P and ΔG_sol are all functions of the difference in intermolecular forces between the amino acid and some medium, and the amino acid and water. The parameters were chosen to model the intermolecular forces of interest.Generally the most important factor is α_x. This is followed by ν, i, and n_H. Least important is σ_I. ΔG_sol depends on α, n_H and n_n. Hydration numbers depend on i, n_H and n_n. The hydrophobicity of amino acid side chains is the result of a preference for a nonpolar medium as a increases and for a polar medium as i, n_H and σ_I increase. It is quantitatively accounted for by the model, and no special “hydrophobic bond” need be involved. The results show that log P values for amino acids are composite quantities whose composition is variable.     

Analysis of changes of insect-plant ratio-improvement of key-factor analysis for evaluating bottom-up effects in insect population dynamics

A revised key-factor analysis was presented for analyzing the temporal changes in the ratio of insect absolute number to plant resource. Ten data sets for 5 insect species were then analyzed. In this key-factor analysis, the key factor is defined as the factor contributing highly to between-year variation inR _r , the log rate of the inter-year change of the insect-plant ratio. The yearly change of plant resource was handled as a separate factor, expressed byr _pl , log ratio of plant resource in yearn to plant resource in yearn+1. The following was revealed: 1) In 7 of the 10 data sets examined,r _pl influenced variations ofR _r ; in particular in 3 casesr _pl was the main key factor. 2) Generation-to-generation fluctuations of absolute insect densities showed density dependence in 4 cases, while those of insect-plant ratios, in 8 cases. 3) The Royama model or a linear model, explained well the relationship between log insect-plant ratio (X _r ) andR _r and the relationship betweenX _r and log yearly change rate of absolute insect density (R _abs ). However, in the 7 cases in whichr _pl was a critical factor for variations ofR _r , with, increase ofX _r ,R _r showed a steeper, decrease around the equilibrium point (the point for whichR _r is 0) thanR _abs . This occurred becauser _pl tended to be negatively correlated withX _r . Consequently, in two casesX _r fluctuated cyclicly or chaotically although without the changes in plant resource, fluctuations ofX _r would be damped oscillations approaching equilibrium.     

Inheritance and interactions of incompatibility alleles in the tetraploid sour cherry

Three progenies of sour cherry (Prunus cerasus) were analysed to correlate self-(in)compatibility status with S-RNase phenotype in this allotetraploid hybrid of sweet and ground cherry. Self-(in)compatibility was assessed in the field and by monitoring pollen tube growth after selfing. The S-RNase phenotypes were determined by isoelectric focusing of stylar proteins and staining for RNase activity and, for the parents, confirmed by PCR. Seedling phenotypes were generally consistent with disomic segregation of S-RNase alleles. The genetic arrangements of the parents were deduced to be ‘Köröser’ (self-incompatible) S ₁ S ₄ .S _B S _D , ‘Schattenmorelle’ (self-compatible) S ₆ S ₁₃ .S _B S _B , and clone 43.87 (self-compatible) S ₄ S ₁₃ .S _B S _B , where “.” separates the two homoeologous genomes. The presence of S ₄ and S ₆ alleles at the same locus led to self-incompatibility, whereas S ₁₃ and S _B at homoeologous loci led to self-compatibility. The failure of certain heteroallelic genotypes in the three crosses or in the self-incompatible seedlings indicates that S ₄ and S ₆ are dominant to S _B . However, the success of S ₁₃ S _B pollen on styles expressing corresponding S-RNases indicates competitive interaction or lack of pollen-S components. In general, the universal compatibility of S ₁₃ S _B pollen may explain the frequent occurrence of S ₁₃ and S _B together in sour cherry cultivars. Alleles S _B and S _D , that are presumed to derive from ground cherry, and S ₁₃ , presumably from sweet cherry, were sequenced. Our findings contribute to an understanding of inheritance of self-(in)compatibility, facilitate screening of progenies for self-compatibility and provide a basis for studying molecular interactions in heteroallelic pollen.     

四川柏木人工林林下植被生物量与林分结构的关系

森林结构与林下植被生物量的关系是森林持续经营与森林碳计量监测的科学基础,但一直缺乏必要的研究。以四川柏木(Cupressus funebris)人工林为研究对象,揭示林下植被生物量(Wu)、灌木生物量(Ws)和草本生物量(Wh)与林分结构的关系,并试图构建区域性林下植被生物量估测的混合模型。结果表明:(1)乔、灌、草群体共12个结构因子中,灌木群体的平均基径(Ds)、盖度(Cs)、高度(Hs)、体积(Vs)与林下植被生物量关系更紧密,在林下植被生物量模型构建中更有效;(2)多模型拟合与比较表明,柏木林Ws最佳估算模型为Ws=0.0005V1.0411s(R2a=0.762,P0.001,n=40),而Wu的最佳估算模型为ln Wu=0.0158Hs+0.0111Cs-0.5358(R2a=0.695,P0.001,n=40),但对于Wh未能获得较为理想的估算模型(R2a0.410,P0.01,n=40);(3)林分密度(Du)整合进入多元线性模型提高了林下植被生物量的估测精度,ln Wu=a+b Du+c Hs+d Cs(R2a=0.721,P0.001,n=40)。研究为区域性林下生物量估测模型构建提供了新论据。     

Self-Incompatibility in Petunia inflata: The Relationship between a Self-Incompatibility Locus F-Box Protein and Its Non-Self S-RNases

The highly polymorphic S (for self-incompatibility) locus regulates self-incompatibility in Petunia inflata; the S-RNase regulates pistil specificity, and multiple S-locus F-box (SLF) genes regulate pollen specificity. The collaborative non-self recognition model predicts that, for any S-haplotype, an unknown number of SLFs collectively recognize all non-self S-RNases to mediate their ubiquitination and degradation. Using a gain-of-function assay, we examined the relationships between S₂-SLF1 (for S₂-allelic product of Type-1 SLF) and four S-RNases. The results suggest that S₂-SLF1 interacts with S₇- and S₁₃-RNases, and the previously identified S₁- and S₃-RNases, but not with S₅- or S₁₁-RNase. An artificial microRNA expressed by the S₂-SLF1 promoter, but not by the vegetative cell-specific promoter, Late Anther Tomato 52, suppressed expression of S₂-SLF1 in S₂ pollen, suggesting that SLF1 is specific to the generative cell. The S₂ pollen with S₂-SLF1 suppressed was compatible with S₃-, S₅-, S₇-, S₁₁-, and S₁₃-carrying pistils, confirming that other SLF proteins are responsible for detoxifying S₅- and S₁₁-RNases and suggesting that S₂-SLF1 is not the only SLF in S₂ pollen that interacts with S₃-, S₇-, and S₁₃-RNases. Petunia may have evolved at least two types of SLF proteins to detoxify any non-self S-RNase to minimize the deleterious effects of mutation in any SLF.     

Challenge from the simple: some caveats in linearization of the Boyle-van't Hoff and Arrhenius plots

Some aspects of proper linearization of the Boyle-van’t Hoff (BVH) relationship for calculation of the osmotically inactive volume v_b, and Arrhenius plot (AP) for the activation energy E_a are discussed. It is shown that the commonly used determination of the slope and the intercept (v_b), which are presumed to be independent from each other, is invalid if the initial intracellular molality m₀ is known. Instead, the linear regression with only one independent parameter (v_b) or the Least Square Method (LSM) with v_b as the only fitting LSM parameter must be applied. The slope can then be calculated from the BVH relationship as the function of v_b. In case of unknown m₀ (for example, if cells are preloaded with trehalose, or electroporation caused ion leakage, etc.), it is considered as the second independent statistical parameter to be found. In this (and only) scenario, all three methods give the same results for v_b and m₀. AP can be linearized only for water hydraulic conductivity (L_p) and solute mobility (ω_s) while water and solute permeabilities P_w ≡ L_pRT and P_s ≡ ω_sRT cannot be linearized because they have pre-exponential factor (RT) that depends on the temperature T.     

Isocitrate lyase from Pseudomonas indigofera: pH dependence of catalysis and binding of substrates and inhibitors.

The maximal velocity, V, for isocitrate cleavage by isocitrate lysase from Pseudomonas indigofera was dependent on two dissociable groups (pK_a's of 6.9 and 8.6). The pH dependence of the pK_i for succinate, a product of isocitrate cleavage, implied that a dissociable group (pK_a of 6.0) on the enzyme functions in binding succinate. The pK_i's for maleate and itaconate (succinate analogs) were similarly pH dependent. The pK_i for oxalate, an analog of glyoxylate which is also a product of isocitrate cleavage, was pH independent. In contrast the pK_i's of the four-carbon dicarboxylic acid inhibitors, fumarate and meso-tartrate, both of which affect the glyoxylate site, were dependent on a dissociable group on the enzyme-inhibitor complex. Comparison of the pH dependence of the pK_m for isocitrate and the pK_i for succinate (and succinate analogs) indicated that the binding of isocitrate was dependent on an acidic dissociable group on the enzyme (pK_a of 5.8). The pH dependence of the pK_i for homoisocitrate was similar. In addition the K_i for succinate and K_m for isocitrate were dependent upon Mg²⁺ concentration. Inhibition by phosphoenolpyruvate, which binds to the succinate site and may regulate isocitrate lyase from P. indigofera, was twice as pH dependent as that for succinate. Two dissociable groups, one on the enzyme (pK_a of 5.8) and one on phosphoenolpyruvate (pK_a of 6.35), contributed to the pH dependence observed with phosphoenolpyruvate.     

Diffusion and sedimentation interaction parameters for measuring the second virial coefficient and their utility as predictors of protein aggregation

The concentration-dependence of the diffusion and sedimentation coefficients (k_D and k_s, respectively) of a protein can be used to determine the second virial coefficient (B₂), a parameter valuable in predicting protein-protein interactions. Accurate measurement of B₂ under physiologically and pharmaceutically relevant conditions, however, requires independent measurement of k_D and k_s via orthogonal techniques. We demonstrate this by utilizing sedimentation velocity (SV) and dynamic light scattering (DLS) to analyze solutions of hen-egg white lysozyme (HEWL) and a monoclonal antibody (mAb1) in different salt solutions. The accuracy of the SV-DLS method was established by comparing measured and literature B₂ values for HEWL. In contrast to the assumptions necessary for determining k_D and k_s via SV alone, k_D and ks were of comparable magnitudes, and solution conditions were noted for both HEWL and mAb1 under which 1), k_D and k_s assumed opposite signs; and 2), k_D ≥ k_s. Further, we demonstrate the utility of k_D and k_s as qualitative predictors of protein aggregation through agitation and accelerated stability studies. Aggregation of mAb1 correlated well with B₂, k_D, and k_s, thus establishing the potential for k_D to serve as a high-throughput predictor of protein aggregation.     

Variability of Reaction Kinetics for Ribulose-1,5-bisphosphate Carboxylase in a Barley Population

The photosynthetic enzyme ribulose bisphosphate carboxylase-oxygenase [EC 4.1.1.39] (RuBPCase) plays a key role in the carbon reduction system of plants. In this study, we determined the kinetic variability of RuBPCase among 46 varieties of Hordeum vulgare L. at two ages. The V_max CO₂ and K_m CO₂ of RuBPCase was determined for each cultivar. Varietal differences were found in K_m CO₂ and V_max CO₂ for one and four genotypes, respectively. One variety exhibited atypical behavior in both K_m and V_max. A comparison of varieties and age showed a significant interaction between these factors for K_m but not for V_max. These data indicate the presence of kinetic variability in RuBPCase within the H. vulgare population and perhaps between plant ages.     

Cytogenetic characterization of ten araneomorph spiders (Araneae): Karyotypes and meiotic features

Karyotypes, sex chromosome systems and meiotic characteristics are reported for ten spider species belonging to the families Gnaphosidae, Philodromidae, Salticidae, Oxyopidae and Sicariidae by using standard Giemsa staining. The male diploid numbers (2n) and sex chromosome systems are as follows: Berinda hakani 2n = 22 (X₁X₂), Berinda ensigera 2n = 22 (X₁X₂), Trachyzelotes lyonneti 2n = 22 (X₁X₂), Trachyzelotes malkini 2n = 22 (X₁X₂), Zelotes caucasius 2n = 22 (X₁X₂) (Gnaphosidae); Thanatus pictus 2n = 28 (X₁ X₂), Tibellus macellus 2n = 24 (X₁ X₂) (Philodromidae); Neon reticulatus 2n = 21 (X₀) (Salticidae); Peucetia virescens 2n = 28 (X₁X₂) (Oxyopidae) and Loxosceles rufescens 2n = 21 (X₁ X₂Y) (Sicariidae). All species have monoarmed chromosomes with the exception of L. rufescens that has biarmed (metacentric and submetacentric) chromosomes. The obtained data are the first results for the genera Berinda, Trachyzelotes and Neon. Additionally, with the exception of L. rufescens, all species are being chromosomally analyzed for the first time.     

F(ST) and Q(ST) under neutrality

A commonly used test for natural selection has been to compare population differentiation for neutral molecular loci estimated by F_ST and for the additive genetic component of quantitative traits estimated by Q_ST. Past analytical and empirical studies have led to the conclusion that when averaged over replicate evolutionary histories, Q_ST = F_ST under neutrality. We used analytical and simulation techniques to study the impact of stochastic fluctuation among replicate outcomes of an evolutionary process, or the evolutionary variance, of Q_ST and F_ST for a neutral quantitative trait determined by n unlinked diallelic loci with additive gene action. We studied analytical models of two scenarios. In one, a pair of demes has recently been formed through subdivision of a panmictic population; in the other, a pair of demes has been evolving in allopatry for a long time. A rigorous analysis of these two models showed that in general, it is not necessarily true that mean Q_ST = F_ST (across evolutionary replicates) for a neutral, additive quantitative trait. In addition, we used finite-island model simulations to show there is a strong positive correlation between Q_ST and the difference Q_ST − F_ST because the evolutionary variance of Q_ST is much larger than that of F_ST. If traits with relatively large Q_ST values are preferentially sampled for study, the difference between Q_ST and F_ST will also be large and positive because of this correlation. Many recent studies have used tests of the null hypothesis Q_ST = F_ST to identify diversifying or uniform selection among subpopulations for quantitative traits. Our findings suggest that the distributions of Q_ST and F_ST under the null hypothesis of neutrality will depend on species-specific biology such as the number of subpopulations and the history of subpopulation divergence. In addition, the manner in which researchers select quantitative traits for study may introduce bias into the tests. As a result, researchers must be cautious before concluding that selection is occurring when Q_ST ≠ F_ST.     

A test of the thermal coadaptation hypothesis with ultimate measures of fitness in flour beetles

Whole-organism performance of ectotherms depends on body temperature, which is tightly linked to environmental temperatures. Individuals attempting to optimize fitness must thus select appropriate temperatures. The thermal coadaptation hypothesis posits that T_o for traits closely linked to fitness should match temperatures selected by a species (T_set) and should coevolve with T_set. T_o may mismatch T_set if the thermal reaction norm for fitness is asymmetric. In this study, we examined six traits related to fitness in red and in confused flour beetles (Tribolium castaneum and T. confusum, respectively), including longevity, lifetime reproductive success, reproductive rate, and development time at four temperatures between 23 and 32 °C. For reproductive traits, T_o matched T_set whereas for longevity T_o was lower than T_set. Tribolium species have a strongly r-selected life history strategy, therefore reproductive traits are likely more tightly linked to fitness than longevity due to high predation rates at early life stages. We therefore provide support for the thermal coadaptation hypothesis for reproductive traits that are tightly linked to fitness. Our results highlight the importance of knowing the relationships of traits to fitness when studying thermal physiology.     

Rapid Biomass Quality Determination of Corn Stover Using Near-Infrared Reflectance Spectroscopy

Near-infrared reflectance spectroscopy (NIRS) has been used extensively in the forage industry for rapid measurement of forage constituents and could be useful for determining quality of biomass feedstocks at the point of delivery. In previous work, we developed an assay that partitions feedstock carbohydrates based on their availability to be converted to fermentable sugars, including non-structural carbohydrates (C _N), biochemically available carbohydrates (C _B) with an associated first-order availability rate constant (k _B), and unavailable carbohydrates (C _U ). Additional quality parameters measured included neutral detergent lignin (NDL), total available carbohydrates (C _A), and total carbohydrates (C _T). We evaluated the variability of biomass quality parameters in a set of corn stover samples and developed calibration equations for determining parameter values using NIRS. Fifty-two corn stover samples harvested in Iowa and Wisconsin in 2005 and 2006 were analyzed using a high-throughput assay for determining feedstock quality for biochemical conversion. Non-structural carbohydrates ranged from 84 to 155?g?kg^?1 dry matter (DM); C _B ranged from 354 to 557?g?kg^?1 DM; k _B ranged from 0.199 to 0.330?h^?1; C _A ranged from 463 to 699?g?kg^?1 DM, and NDL ranged from 32 to 74?g?kg^?1 DM. Significant differences (P?<?0.0001) among samples were observed for all parameters, except k _B. Near-infrared reflectance spectroscopy calibration equations were developed for C _N, C _B, C _A, C _U , C _T, and NDL. It was not possible to generate a meaningful calibration equation for k _B. There is significant variability within the corn stover population for several key quality-related carbohydrate and lignin constituents which can be predicted reliably using NIRS.     

Comparative growth analyses of panicum species with differing rates of photorespiration

Panicum milioides, a naturally occurring species with reduced photorespiration, P. bisulcatum, a C₃ species, and P. miliaceum, a C₄ species, were grown for 4 weeks at altered pO₂ and pCO₂ and several vegetative growth parameters were determined at weekly intervals. Compared to a pO₂ of 10%, a greater O₂ inhibition of the relative growth rate and dry matter production was observed for P. bisulcatum than for P. milioides at both 21% and 40% O₂, whereas little effect of O₂ was noted for P. miliaceum. Similarly, exposures to elevated pCO₂ of 500 and 1000 μ1 CO₂/liter resulted in a greater stimulation of vegetative growth for P. bisulcatum than for P. milioides, with little effect on P. miliaceum. The CO₂ compensation concentration of P. milioides was less than that of P. bisulcatum over a pO₂ range of 5 to 40%. At 5% O₂, the compensation concentration was relatively O₂-insensitive, whereas above 5% it increased with increasing pO₂. It is concluded that P. milioides represents the first well documented example of a C₃ plant with reduced photorespiration, based on both leaf CO₂ exchange parameters and growth analyses of dry matter production.     

THE KINETICS OF PENETRATION : I. EQUATIONS FOR THE ENTRANCE OF ELECTROLYTES

When the only solute present is a weak acid, HA, which penetrates as molecules only into a living cell according to a curve of the first order and eventually reaches a true equilibrium we may regard the rate of increase of molecules inside as See PDF for Equation where P_M is the permeability of the protoplasm to molecules, M_o, denotes the external and M_i the internal concentration of molecules, A_i denotes the internal concentration of the anion A^- and See PDF for Equation (It is assumed that the activity coefficients equal 1.) Putting P_MF_M = V_M, the apparent velocity constant of the process, we have See PDF for Equation where e denotes the concentration at equilibrium. Then See PDF for Equation where t is time. The corresponding equation when ions alone enter is See PDF for Equation. where K is the dissociation constant of HA, P_A is the permeability of the protoplasm to the ion pair H⁺ + A^-, and A_ie denotes the internal concentration of A_i at equilibrium. Putting P_AKF_M = V_A, the apparent velocity constant of the process, we have See PDF for Equation and See PDF for Equation When both ions and molecules of HA enter together we have See PDF for Equation where S_i = M_i + A_i and S_ie is the value of S_i at equilibrium. Then See PDF for Equation V_M, V_A, and V_MA depend on F_M and hence on the internal pH value but are independent of the external pH value except as it affects the internal pH value. When the ion pair Na⁺ + A^- penetrates and Na_i = BA_i, we have See PDF for Equation and See PDF for Equation where P _NaA is the permeability of the protoplasm to the ion pair Na⁺ + A^-, Na_o and Na_i are the external and internal concentrations of Na⁺, See PDF for Equation, and V _Na is the apparent velocity constant of the process. Equations are also given for the penetration of: (1) molecules of HA and the ion pair Na⁺ + A^-, (2) the ion pairs H⁺ + A^- and Na⁺ + A^-, (3) molecules of HA and the ion pairs Na⁺ + A^- and H⁺ + A^-. (4) The penetration of molecules of HA together with those of a weak base ZOH. (5) Exchange of ions of the same sign. When a weak electrolyte HA is the only solute present we cannot decide whether molecules alone or molecules and ions enter by comparing the velocity constants at different pH values, since in both cases they will behave alike, remaining constant if F_M is constant and falling off with increase of external pH value if F_M falls off. But if a salt (e.g., NaA) is the only substance penetrating the velocity constant will increase with increase of external pH value: if molecules of HA and the ions of a salt NaA. penetrate together the velocity constant may increase or decrease while the internal pH value rises. The initial rate See PDF for Equation (i.e., the rate when M_i = 0 and A_i = 0) falls off with increase of external pH value if HA alone is present and penetrates as molecules or as ions (or in both forms). But if a salt (e.g., NaA) penetrates the initial rate may in some cases decrease and then increase as the external pH value increases. At equilibrium the value of M_i equals that of M_o (no matter whether molecules alone penetrate, or ions alone, or both together). If the total external concentration (S_o = M_o + A_o) be kept constant a decrease in the external pH value will increase the value of M_o and make a corresponding increase in the rate of entrance and in the value at equilibrium no matter whether molecules alone penetrate, or ions alone, or both together. What is here said of weak acids holds with suitable modifications for weak bases and for amphoteric electrolytes and may also be applied to strong electrolytes.