Carbon Isotope Discrimination in Coffee Genotypes Grown under Limited Water Supply

Photosynthetic gas exchange, plant-water relations characteristics, and stable carbon isotope discrimination (Δ) were evaluated for five Coffea arabica L. genotypes growing under two soil moisture regimes in the field. The Δ of leaf tissue was strongly correlated (r = −0.95) with inherent water use efficiency (ratio of assimilation to stomatal conductance; A/g). The variation in inherent water use efficiency (WUE) among genotypes was 30% for plants irrigated weekly. The higher WUE exhibited by some of these plants resulted from reduced g rather than increased photosynthetic capacity at a given g. Withholding irrigation for 1 month caused Δ to decline substantially in expanding leaf tissue of all genotypes. A strong correlation (r = 0.92) was found between Δ and plant hydraulic efficiency estimated as the ratio of g to the diurnal range in leaf water potential (Ψ_l). The Δ values for plants irrigated weekly adequately predicted drought-induced changes in Δ (r = 0.99) and midday Ψ_l (r = 0.95). The results indicated that Δ might be used to evaluate several aspects of plant performance and response to specific environmental conditions, once suitable background physiological data have been gathered.     

Genetic variation for gas exchange rates in grain sorghum

Carbon assimilation rate (A) and stomatal conductance (g) are highly correlated. However, the slope of the A versus g relationship differs among species and environments resulting in differences in gas exchange efficiency which should reflect water use efficiency. The objective of this research was to determine the genetic variation for A and g in grain sorghum (Sorghum bicolor [L.] Moench.). Field experiments were conducted using 30 sorghum hybrids with four water supply treatments. A, g, and leaf water potential (Ψ_w) of individual leaves were monitored every 15 to 20 days. Significant genetic variation existed among the hybrids for A and g. Plant age and water supply also affected A and g as expected. When A was regressed on g for each hybrid, large and significant differences existed among the slopes, implying differences in intrinsic gas exchange efficiency. The regression analysis of A and g versus Ψ_w suggested that A was more sensitive than g to increasing water stress. Genetic differences in the rate of change in A as water stress increased were observed. Regression analysis was used to evaluate the individual hybrid response relative to other hybrids. Twofold difference in slopes existed for A. These results provide evidence for genetic variation in gas exchange rates which might directly contribute to whole plant water use efficiency and productivity.     

Correlation between the Carbon Isotope Discrimination in Leaf Starch and Sugars of C(3) Plants and the Ratio of Intercellular and Atmospheric Partial Pressures of Carbon Dioxide

Carbon isotope discrimination (Δ) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C₃ species (Populus nigra L. × P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble sugars were extracted for analysis of carbon isotope composition. The ratio of intercellular, p_i, and atmospheric, p_a, partial pressures of CO₂, was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with p_i/p_a in all species, as did Δ in leaf starch (r = 0.84). Starch was found to contain significantly more ¹³C than soluble sugar, and possible explanations are discussed. The strong correlation found between Δ and p_i/p_a suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of p_i/p_a weighted by CO₂ assimilation rate, over a day. Because p_i/p_a has a negative correlation with transpiration efficiency (mol CO₂/mol H₂O) of isolated plants, Δ in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C₃ species.     

Short-term changes in leaf carbon isotope discrimination in salt- and water-stressed c(4) grasses

Online carbon isotope discrimination (Δ) and leaf gas exchange measurements were made with control and salt-stressed Zea mays and Andropogon glomeratus, two NADP-ME type C₄ grasses. Linear relationships between Δ and p_i/p_a (the ratio of intercellular to atmospheric CO₂ partial pressure) were found for control plants which agreed well with theoretical models describing carbon isotope discrimination in C₄ plants. These data provided estimates of , the proportion of CO₂ fixed by phosphoenolpyruvate carboxylase which leaks out of the bundle sheath and the component of fractionation due to diffusion in air. Salt-stressed plants had wider variation in Δ for the same or less range in p_i/p_a. Additional work indicated Δ changed independently of p_i/p_a in both water- and salt-stressed plants, suggesting a possible diurnal change in as plant water status changed linked to a decrease in the activity of the C₃ photosynthetic pathway relative to C₄ pathway activity. The possible effect of stress-induced changes in on organic matter δ¹³ C of C₄ plants is apt to be most apparent in chronically stressed environments.     

Osmotic adjustment in sorghum: I. Mechanisms of diurnal osmotic potential changes

Osmotic adjustment, defined as a lowering of osmotic potential (ψ_π) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal ψ_π changes in sorghum (Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv AT×623 × RT×430) was grown in the field under variable water supplies. Water potential, ψ_π, and relative water content were measured diurnally on expanding and the uppermost fully expanded leaves before flowering and on fully expanded leaves during the grain-filling period. Diurnal changes in total osmotic potential (Δψ_π) in response to water stress was 1.1 megapascals before flowering and 1.4 megapascals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, passive concentration of solutes associated with dehydration accounted for 50% (0.55 megapascal) of the diurnal Δψ_π before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 megapascal) of the diurnal Δψ_π before flowering and 45% (0.63 megapascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (decreased turgid weight/dry weight) to diurnal Δψ_π was less than 8% at either growth stages. Water stress did not affect leaf tissue elasticity or partitioning of water between the symplasm and apoplasm.     

Heat capacity changes in RNA folding: application of perturbation theory to hammerhead ribozyme cold denaturation

In proteins, empirical correlations have shown that changes in heat capacity (ΔC_P) scale linearly with the hydrophobic surface area buried upon folding. The influence of ΔC_P on RNA folding has been widely overlooked and is poorly understood. In addition to considerations of solvent reorganization, electrostatic effects might contribute to ΔC_Ps of folding in polyanionic species such as RNAs. Here, we employ a perturbation method based on electrostatic theory to probe the hot and cold denaturation behavior of the hammerhead ribozyme. This treatment avoids much of the error associated with imposing two-state folding models on non-two-state systems. Ribozyme stability is perturbed across a matrix of solvent conditions by varying the concentration of NaCl and methanol co-solvent. Temperature-dependent unfolding is then monitored by circular dichroism spectroscopy. The resulting array of unfolding transitions can be used to calculate a ΔC_P of folding that accurately predicts the observed cold denaturation temperature. We confirm the accuracy of the calculated ΔC_P by using isothermal titration calorimetry, and also demonstrate a methanol-dependence of the ΔC_P. We weigh the strengths and limitations of this method for determining ΔC_P values. Finally, we discuss the data in light of the physical origins of the ΔC_Ps for RNA folding and consider their impact on biological function.     

DNA Barcode Identification of Podocarpaceae—The Second Largest Conifer Family

We have generated matK, rbcL, and nrITS2 DNA barcodes for 320 specimens representing all 18 extant genera of the conifer family Podocarpaceae. The sample includes 145 of the 198 recognized species. Comparative analyses of sequence quality and species discrimination were conducted on the 159 individuals from which all three markers were recovered (representing 15 genera and 97 species). The vast majority of sequences were of high quality (B ₃₀ = 0.596–0.989). Even the lowest quality sequences exceeded the minimum requirements of the BARCODE data standard. In the few instances that low quality sequences were generated, the responsible mechanism could not be discerned. There were no statistically significant differences in the discriminatory power of markers or marker combinations (p = 0.05). The discriminatory power of the barcode markers individually and in combination is low (56.7% of species at maximum). In some instances, species discrimination failed in spite of ostensibly useful variation being present (genotypes were shared among species), but in many cases there was simply an absence of sequence variation. Barcode gaps (maximum intraspecific p–distance > minimum interspecific p–distance) were observed in 50.5% of species when all three markers were considered simultaneously. The presence of a barcode gap was not predictive of discrimination success (p = 0.02) and there was no statistically significant difference in the frequency of barcode gaps among markers (p = 0.05). In addition, there was no correlation between number of individuals sampled per species and the presence of a barcode gap (p = 0.27).     

Trophic Discrimination Factors of Stable Carbon and Nitrogen Isotopes in Hair of Corn Fed Wild Boar

Stable isotope measurements are increasingly being used to gain insights into the nutritional ecology of many wildlife species and their role in ecosystem structure and function. Such studies require estimations of trophic discrimination factors (i.e. differences in the isotopic ratio between the consumer and its diet). Although trophic discrimination factors are tissue- and species- specific, researchers often rely on generalized, and fixed trophic discrimination factors that have not been experimentally derived. In this experimental study, captive wild boar (Sus scrofa) were fed a controlled diet of corn (Zea mays), a popular and increasingly dominant food source for wild boar in the Czech Republic and elsewhere in Europe, and trophic discrimination factors for stable carbon (Δ¹³C) and nitrogen (Δ¹⁵N) isotopes were determined from hair samples. The mean Δ¹³C and Δ¹⁵N in wild boar hair were –2.3 ‰ and +3.5 ‰, respectively. Also, in order to facilitate future derivations of isotopic measurements along wild boar hair, we calculated the average hair growth rate to be 1.1 mm d^-1. Our results serve as a baseline for interpreting isotopic patterns of free-ranging wild boar in current European agricultural landscapes. However, future research is needed in order to provide a broader understanding of the processes underlying the variation in trophic discrimination factors of carbon and nitrogen across of variety of diet types.     

Correlations between Carbon Isotope Discrimination and Leaf Conductance to Water Vapor in Common Beans

Carbon isotope discrimination (Δ) was measured in the field on 10 cultivars of common bean (Phaseolus vulgaris L.). There was substantial variation (more than 2‰) in leaf Δ values and these differences were maintained between vegetative and reproductive developmental stages. These bean lines also exhibited substantial differences in leaf conductance to water vapor, and again these differences were maintained across developmental stages. The differences in leaf conductance were positively correlated with Δ values, whether conductance was measured as total leaf conductance or as the individual conductances of either upper or lower leaf surfaces. The observed differences in leaf conductance were not associated with differences in stomatal density. There were small differences among bean lines in their leaf Kjeldahl nitrogen contents, which is interpreted as indicating that photosynthetic capacity among bean lines was similar. Thus, because Δ values and leaf conductance were positively correlated, these data suggested that there may have been differences among bean lines in the extent to which stomata limited photosynthetic gas exchange rates.     

Oxygen isotope values in bone carbonate and collagen are consistently offset for New World monkeys

Stable oxygen isotopes are increasingly used in ecological research. Here, I present oxygen isotope (δ¹⁸O) values for bone carbonate and collagen from howler monkeys (Alouatta palliata), spider monkeys (Ateles geoffroyi) and capuchins (Cebus capucinus) from three localities in Costa Rica. There are apparent differences in δ¹⁸O_carbonate and δ¹⁸O_collagen among species. Monkeys from moist forest have significantly lower isotope values than those from drier localities. Because patterns are similar for both substrates, discrimination (Δ) between δ¹⁸O_carbonate and δ¹⁸O_collagen is relatively consistent among species and localities (17.6 ± 0.9‰). Although this value is larger than that previously obtained for laboratory rats, consistency among species and localities suggests it can be used to compare δ¹⁸O_carbonate and δ¹⁸O_collagen for monkeys, and potentially other medium-bodied mammals. Establishing discrimination for oxygen between these substrates for wild monkeys provides a foundation for future environmental and ecological research on modern and ancient organisms.     

Stable Carbon Isotope Discrimination Is under Genetic Control in the C4 Species Maize with Several Genomic Regions Influencing Trait Expression

In plants with C₄ photosynthesis, physiological mechanisms underlying variation in stable carbon isotope discrimination (Δ13C) are largely unknown, and genetic components influencing Δ13C have not been described. We analyzed a maize (Zea mays) introgression library derived from two elite parents to investigate whether Δ13C is under genetic control in this C₄ species. High-density genotyping with the Illumina MaizeSNP50 Bead Chip was used for a detailed structural characterization of 89 introgression lines. Phenotypic analyses were conducted in the field and in the greenhouse for kernel Δ13C as well as plant developmental and photosynthesis-related traits. Highly heritable significant genetic variation for Δ13C was detected under field and greenhouse conditions. For several introgression library lines, Δ13C values consistently differed from the recurrent parent within and across the two phenotyping platforms. Δ13C was significantly associated with 22 out of 164 analyzed genomic regions, indicating a complex genetic architecture of Δ13C. The five genomic regions with the largest effects were located on chromosomes 1, 2, 6, 7, and 9 and explained 55% of the phenotypic variation for Δ13C. Plant development stage had no effect on Δ13C expression, as phenotypic as well as genotypic correlations between Δ13C, flowering time, and plant height were not significant. To our knowledge, this is the first study demonstrating Δ13C to be under polygenic control in the C₄ species maize.During photosynthesis, plants use light energy to convert atmospheric CO₂ and water into carbohydrates. For the element carbon, there are two stable isotopes, ¹²C and ¹³C. Due to the physical and chemical properties of photosynthetic CO₂ fixation, plants are depleted in ¹³C compared with atmospheric CO₂. In C₃ plants, this discrimination of stable carbon isotopes (Δ13C) has long been used to detect genetic differences of water use efficiency and has been shown to be an accurate predictor for yield under drought (Rebetzke et al., 2002). As Δ13C is linearly related to the ratio of intercellular to atmospheric CO₂ partial pressure (Farquhar et al., 1982), stomatal closure under drought stress is associated with reduced Δ13C. For C₄ plants, our knowledge about the mechanisms underlying Δ13C and about its association with water use efficiency is much more limited. Differences in Δ13C between genotypes of C₄ species have been reported, among others, for sorghum (Sorghum bicolor; Hubick et al., 1990) and maize (Zea mays; Monneveux et al., 2007). However, comprehensive studies analyzing the inheritance of Δ13C have not been performed to date.In C₃ plants, the important steps of CO₂ uptake include the diffusion of atmospheric CO₂ through the boundary layer and the stomata. Subsequently, CO₂ is taken up by the cell and enters the chloroplast, where carboxylation by Rubisco takes place. During photosynthetic carbon fixation, the strongest fractionation of carbon isotopes occurs during the carboxylation reaction of Rubisco (Roeske and O’Leary, 1984). A theoretical model of Δ13C in C₃ photosynthesis has been described by Farquhar et al. (1982), in which Δ13C depends linearly on the ratio of intercellular to ambient partial pressure of CO₂ (p_i p_a−1), and thus provides an indication of stomatal conductance and photosynthetic capacity. Additionally, the model includes the dependency of Δ13C on the fractionation of carbon isotopes during CO₂ diffusion in the air and on the enzymatic properties of the Rubisco enzyme.For rice (Oryza sativa), tomato (Solanum lycopersicum), and wheat (Triticum aestivum), it has been shown that genetic variation for Δ13C is quantitative, genotype-by-environment interaction is small, and the trait heritability is high (Condon and Richards, 1992; Rebetzke et al., 2002; Comstock et al., 2005; Impa et al., 2005). Quantitative trait loci (QTL) for Δ13C have been mapped (Handley et al., 1994; Price et al., 2002; Rebetzke et al., 2008), and in the model plant Arabidopsis (Arabidopsis thaliana), four genes have been identified that are associated with Δ13C. Two are involved in stomatal patterning and thus influence stomatal conductance (Masle et al., 2005; Nilson and Assmann, 2010), and one of them influences photosynthetic capacity as well (Masle et al., 2005). One gene plays a role in cuticular wax composition and is also associated with stomatal conductance (Lü et al., 2012), whereas the fourth gene encodes a cellulose synthase subunit, and mutations in this gene lead to decreased Δ13C. Presumably, this is the result of a decreased cell turgor due to a decreased water transport capacity of the xylem (Liang et al., 2010).For C₄ plants, our knowledge about the genetic mechanisms and physiological processes underlying Δ13C is much more limited, due to the more complex mechanism of CO₂ fixation. The first carboxylation step in C₄ plants takes place in mesophyll cells, in which CO₂ is fixed by phosphoenolpyruvate carboxylase (PEPC). During this reaction, combined with the fractionation of carbon isotopes during HCO₃⁻ formation, carbon is actually enriched in ¹³C (Farquhar, 1983). The C₄ organic acid formed by PEPC is transported to the bundle sheath cells, where CO₂ is released to be fixed by Rubisco in the second step. However, a fraction of CO₂ released in the bundle sheath can diffuse back to the mesophyll cells. The proportion of carbon fixed by PEPC that subsequently leaks out of the bundle sheath cells is termed leakiness (ϕ) and reduces the opportunity of Rubisco to discriminate against ¹³C in C₄ plants. According to the theoretical model by Farquhar (1983), Δ13C and p_i p_a−1 are also linearly related in C₄ plants, but the regression slope is determined by ϕ. Consequently, there can be a positive or a negative correlation of Δ13C and p_i p_a−1 depending on ϕ (Hubick et al., 1990). Regarding the entire fixation process, discrimination against ¹³C in C₄ plants is not as strong as in C₃ plants, and so far there have been few studies reporting a genetic variation of Δ13C in C₄ plants. In sorghum, small but significant differences in Δ13C have been observed among 12 cultivars (Hubick et al., 1990), and similar to C₃ plants, Δ13C has been shown to be correlated with transpiration efficiency (Henderson et al., 1998). Additionally, it has been shown for maize and sugarcane (Saccharum officinarum) that stress conditions lead to an increase in Δ13C (Bowman et al., 1989; Meinzer et al., 1994; Ranjith et al., 1995; Buchmann et al., 1996). Experiments under drought and under well-watered conditions showed higher values for Δ13C in drought-tolerant maize hybrids than in susceptible checks (Monneveux et al., 2007).The use of Δ13C as an indirect trait in breeding for drought tolerance in C₄ species would be highly beneficial, given a stable trait expression and high heritability similar to that in C₃ plants. To assess whether Δ13C can also be used in C₄ plants as an indirect selection trait for drought-tolerant lines, it needs to be shown that Δ13C is under genetic control, although the physiology and molecular mechanisms of Δ13C are not yet fully understood. In this study, we used an introgression library (IL; Eshed and Zamir, 1994) derived from two elite parents to analyze the genetic variation in Δ13C under well-watered conditions. ILs have been successfully used in genetics to identify QTL for various qualitatively and quantitatively inherited traits. An IL is a defined set of nearly isogenic inbred lines derived from repeated backcrosses with one of the parents (recurrent parent [RP]) and marker-assisted selection for single fragments (Supplemental Fig. S1). Ideally, each IL line carries a single chromosome fragment of a donor parent (DP) in the genetic background of an RP. Taken together, the different segments cover the whole donor genome, allowing estimation of the effects of single donor fragments in an otherwise identical genetic background (Eshed and Zamir, 1994). The RP of the IL under investigation originates from southeastern Europe and is an elite inbred line of the maize dent pool. As DP, we chose an unrelated maize line representative of the European flint pool. The IL (IL_01–IL_89) was genotyped using the Illumina MaizeSNP50 Bead Chip (Ganal et al., 2011) carrying 56,110 single-nucleotide polymorphism (SNP) markers.Kernel Δ13C of 77 IL lines was measured in the field and in the greenhouse (Δ13C is genetically controlled in the C₄ species maize. Our specific goals were (1) to characterize the genetic architecture of Δ13C (i.e. to determine the number of genomic regions associated with Δ13C), (2) to localize genomic regions influencing Δ13C, and (3) to assess the extent to which genotypic variation in Δ13C might be the result of differences in plant development.

Table I.

Overview of the experiments and experimental designs

THEORY AND MEASUREMENT OF VISUAL MECHANISMS : III. ΔI AS A FUNCTION OF AREA,INTENSITY, AND WAVE-LENGTH,FOR MONOCULAR AND BINOCULAR STIMULATION

Measurements of ΔI as a function of retinal area illuminated have been obtained at various levels of standard intensity I ₁, using "white" light and light of three modal wave-lengths (λ465, 525, 680), for monocular stimulation and for simultaneous excitation of the two eyes ("binocular"), using several methods of varying (rectangular) area and retinal location, with control of exposure time. For data homogeneous with respect to method of presentation, log ΔI_m = -Z log A + C, where ΔI = Ĩ ₂ – I ₁, A is area illuminated, and C is a terminal constant (= log ΔI_m for A = 1 unit) depending on the units in which ΔI and A are expressed, and upon I ₁. The equation is readily deduced on dimensional grounds, without reference to specific theories of the nature of ΔI or of retinal area in terms of its excitable units. Z is independent of the units of I and A. Experimentally it is found to be the same for monocular and binocular excitations, as is to be expected. Also as is expected it is not independent of λ, and it is markedly influenced by the scheme according to which A is varied; it depends directly upon the rate at which potentially excitable elements are added when A is made to increase. For simultaneous excitation of the two eyes (when of very nearly equivalent excitability), ΔĪ_B is less than for stimulation of either eye alone, at all levels of I ₁, A, λ. The mean ratio (ΔĪ_L + ΔĪ_R)/2 to ΔI^B was 1.38. For white light, doubling A on one retina reduces ΔI_m in the ratio 1.21, or a little less than for binocular presentation under the same conditions. These facts are consistent with the view that the properties of ΔI are quantitatively determined by events central to the retina. The measure σ_1ΔI of organic variation in discrimination of intensities and ΔI_m are found to be in simple proportion, independent of I ₁, A, λ (and exposure time). Variability (σ_1ΔI) is not a function of the mode of presentation, save that it may be slightly higher when both retinas are excited, and its magnitude (for a given level of ΔI_m) is independent of the law according to which the adjustable intensity I ₂ is instrumentally controlled.     

Diversification of substrate specificities in teleostei Fads2: characterization of Δ4 and Δ6Δ5 desaturases of Chirostoma estor

Currently existing data show that the capability for long-chain PUFA (LC-PUFA) biosynthesis in teleost fish is more diverse than in other vertebrates. Such diversity has been primarily linked to the subfunctionalization that teleostei fatty acyl desaturase (Fads)2 desaturases have undergone during evolution. We previously showed that Chirostoma estor, one of the few representatives of freshwater atherinopsids, had the ability for LC-PUFA biosynthesis from C₁₈ PUFA precursors, in agreement with this species having unusually high contents of DHA. The particular ancestry and pattern of LC-PUFA biosynthesis activity of C. estor make this species an excellent model for study to gain further insight into LC-PUFA biosynthetic abilities among teleosts. The present study aimed to characterize cDNA sequences encoding fatty acyl elongases and desaturases, key genes involved in the LC-PUFA biosynthesis. Results show that C. estor expresses an elongase of very long-chain FA (Elovl)5 elongase and two Fads2 desaturases displaying Δ4 and Δ6/Δ5 specificities, thus allowing us to conclude that these three genes cover all the enzymatic abilities required for LC-PUFA biosynthesis from C₁₈ PUFA. In addition, the specificities of the C. estor Fads2 enabled us to propose potential evolutionary patterns and mechanisms for subfunctionalization of Fads2 among fish lineages.     

Harnessing Genetic Variation in Leaf Angle to Increase Productivity of Sorghum bicolor

The efficiency with which a plant intercepts solar radiation is determined primarily by its architecture. Understanding the genetic regulation of plant architecture and how changes in architecture affect performance can be used to improve plant productivity. Leaf inclination angle, the angle at which a leaf emerges with respect to the stem, is a feature of plant architecture that influences how a plant canopy intercepts solar radiation. Here we identify extensive genetic variation for leaf inclination angle in the crop plant Sorghum bicolor, a C4 grass species used for the production of grain, forage, and bioenergy. Multiple genetic loci that regulate leaf inclination angle were identified in recombinant inbred line populations of grain and bioenergy sorghum. Alleles of sorghum dwarf-3, a gene encoding a P-glycoprotein involved in polar auxin transport, are shown to change leaf inclination angle by up to 34° (0.59 rad). The impact of heritable variation in leaf inclination angle on light interception in sorghum canopies was assessed using functional-structural plant models and field experiments. Smaller leaf inclination angles caused solar radiation to penetrate deeper into the canopy, and the resulting redistribution of light is predicted to increase the biomass yield potential of bioenergy sorghum by at least 3%. These results show that sorghum leaf angle is a heritable trait regulated by multiple loci and that genetic variation in leaf angle can be used to modify plant architecture to improve sorghum crop performance.     

Effects of Salinity on Stomatal Conductance, Photosynthetic Capacity, and Carbon Isotope Discrimination of Salt-Tolerant (Gossypium hirsutum L.) and Salt-Sensitive (Phaseolus vulgaris L.) C(3) Non-Halophytes

The effects of salinity on growth, stomatal conductance, photosynthetic capacity, and carbon isotope discrimination (Δ) of Gossypium hirsutum L. and Phaseolus vulgaris L. were evaluated. Plants were grown at different NaCl concentrations from 10 days old until mature reproductive structures were formed. Plant growth and leaf area development were strongly reduced by salinity, in both cotton and bean. Stomatal conductance also was reduced by salinity. The Δ always declined with increasing external salinity concentration, indicating that stomatal limitation of photosynthesis was increased. In cotton plant dry matter, Δ correlated with the ratio of intercellular to atmospheric CO₂ partial pressures (p_l/p_a) calculated by gas exchange. This correlation was not clear in bean plants, although Δ showed a more pronounced salt induced decline in bean than in cotton. Possible effects of heterogeneity of stomatal aperture and consequent overestimation of p_l as determined from gas exchange could explain these results. Significant differences of Δ between leaf and seed material were observed in cotton and bean. This suggests different patterns of carbon allocation between leaves and seeds. The photon yield of O₂ evolution determined at rate-limiting photosynthetic photon flux density was insensitive to salinity in both species analyzed. The light- and CO₂-saturated rate of CO₂ uptake and O₂ evolution showed a salt induced decline in both species. Possible explanations of this observation are discussed. O₂ hypersensitivity was observed in salt stressed cotton plants. These results clearly demonstrate that the effect of salinity on assimilation rate was mostly due to the reduction of stomatal conductance, and that calculation of p_l may be overestimated in salt stressed plants, because of heterogeneity of stomatal aperture over the leaf surface.     

Biogeography of the Purple Nonsulfur Bacterium Rhodopseudomonas palustris

The biogeography of the purple nonsulfur bacterium Rhodopseudomonas palustris on a local scale was investigated. Thirty clones of phototrophic bacteria were isolated from each of five unevenly spaced sampling locations in freshwater marsh sediments along a linear 10-m transect, and a total of 150 clones were characterized by BOX-PCR genomic DNA fingerprinting. Cluster analysis of 150 genomic fingerprints yielded 26 distinct genotypes, and 106 clones constituted four major genotypes that were repeatedly isolated. Representatives of these four major genotypes were tentatively identified as R. palustris based on phylogentic analyses of 16S rRNA gene sequences. The differences in the genomic fingerprint patterns among the four major genotypes were accompanied by differences in phenotypic characteristics. These phenotypic differences included differences in the kinetics of carbon source use, suggesting that there may be functional differences with possible ecological significance among these clonal linages. Morisita-Horn similarity coefficients (C_MH), which were used to compare the numbers of common genotypes found at pairs of sampling locations, showed that there was substantial similarity between locations that were 1 cm apart (C_MH, ≥0.95) but there was almost no similarity between locations that were ≥9 m apart (C_MH, ≤0.25). These calculations showed there was a gradual decrease in similarity among the five locations as a function of distance and that clones of R. palustris were lognormally distributed along the linear 10-m transect. These data indicate that natural populations of R. palustris are assemblages of genetically distinct ecotypes and that the distribution of each ecotype is patchy.     

How yield relates to ash content, ��13C and ��18O in maize grown under different water regimes

Background and Aims

Stable isotopes have proved a valuable phenotyping tool when breeding for yield potential and drought adaptation; however, the cost and technical skills involved in isotope analysis limit its large-scale application in breeding programmes. This is particularly so for Δ¹⁸O despite the potential relevance of this trait in C₄ crops. The accumulation of minerals (measured as ash content) has been proposed as an inexpensive way to evaluate drought adaptation and yield in C₃ cereals, but little is known of the usefulness of this measure in C₄ cereals such as maize (Zea mays). The present study investigates how yield relates to ash content, Δ¹³C and Δ¹⁸O, and evaluates the use of ash content as an alternative or complementary criterion to stable isotopes in assessing yield potential and drought resistance in maize.

Methods

A set of tropical maize hybrids developed by CIMMYT were subjected to different water availabilities, in order to induce water stress during the reproductive stages under field conditions. Ash content and Δ¹³C were determined in leaves and kernels. In addition, Δ¹⁸O was measured in kernels.

Key Results

Water regime significantly affected yield, ash content and stable isotopes. The results revealed a close relationship between ash content in leaves and the traits informing about plant water status. Ash content in kernels appeared to reflect differences in sink–source balance. Genotypic variation in grain yield was mainly explained by the combination of ash content and Δ¹⁸O, whilst Δ¹³C did not explain a significant percentage of such variation.

Conclusions

Ash content in leaves and kernels proved a useful alternative or complementary criterion to Δ¹⁸O in kernels for assessing yield performance in maize grown under drought conditions.     

The measurement of the intrinsic alkaline Bohr effect of various human haemoglobins by isoelectric focusing

We have used isoelectric focusing to measure the differences between the pI values of various normal and mutant human haemoglobins when completely deoxygenated and when fully liganded with CO. It was assumed that the ΔpI(deox.–ox.) values might correspond quantitatively to the intrinsic alkaline Bohr effect, as most of the anionic cofactors of the haemoglobin molecule are `stripped' off during the electrophoretic process. In haemoglobins known to exhibit a normal Bohr coefficient (ΔlogP₅₀/ΔpH) in solutions, the ΔpI(deox.–ox.) values are lower the higher their respective pI(ox.) values. This indicates that for any particular haemoglobin the ΔpI(deox.–ox.) value accounts for the difference in surface charges at the pH of its pI value. This was confirmed by measuring, by the direct-titration technique, the difference in pH of deoxy and fully liganded haemoglobin A₀ (α₂β₂) solutions in conditions approximating those of the isoelectric focusing, i.e. at 5°C and very low concentration of KCl. The variation of the ΔpH(deox.–ox.) curve as a function of pH (ox.) was similar to the isoelectric-focusing curve relating the variation of ΔpI(deox.–ox.) versus pI(ox.) in various haemoglobins with Bohr factor identical with that of haemoglobin A₀. In haemoglobin A₀ the ΔpI(deox.–ox.) value is 0.17 pH unit, which corresponds to a difference of 1.20 positive charges between the oxy and deoxy states of the tetrameric haemoglobin. This value compares favourably with the values of the intrinsic Bohr effect estimated in back-titration experiments. The ΔpI(deox.–ox.) values of mutant or chemically modified haemoglobins carrying an abnormality at the N- or C-terminus of the α-chains are decreased by 30% compared with the ΔpI value measured in haemoglobin A₀. When the C-terminus of the β-chains is altered, as in Hb Nancy (α₂β^{Tyr-145→Asp}₂), we observed a 70% decrease in the ΔpI value compared with that measured in haemoglobin A₀. These values are in close agreement with the estimated respective roles of the two major Bohr groups, Val-1α and His-146β, at the origin of the intrinsic alkaline Bohr effect [Kilmartin, Fogg, Luzzana & Rossi-Bernardi (1973) J. Biol. Chem. 248, 7039–7043; Perutz, Kilmartin, Nishikura, Fogg, Butler & Rollema (1980) J. Mol. Biol. 138, 649–670]. In other mutant haemoglobins it is demonstrated also that the ΔpI(deox.–ox.) value may be decreased or even suppressed when the substitution affects residues involved in the stability of the tetramer. These results support the interpretation proposed by Perutz, Kilmartin, Nishikura, Fogg, Butler & Rollema [(1980), J. Mol. Biol. 138, 649–670] for the mechanism of the alkaline Bohr effect, and also indicate that the transition between the two quaternary configurations is a prerequisite for the full expression of the alkaline Bohr effect.