Higher Carboxylation Efficiency Research Articles

Rubisco kinetic adaptations to extreme environments.

Photosynthetic and chemosynthetic extremophiles have evolved adaptations to thrive in challenging environments by finely adjusting their metabolic pathways through evolutionary processes. A prime adaptation target to allow autotrophy in extreme conditions is the enzyme Rubisco, which plays a central role in the conversion of inorganic to organic carbon. Here, we present an extensive compilation of Rubisco kinetic traits from a wide range of species of bacteria, archaea, algae, and plants, sorted by phylogenetic group, Rubisco type, and extremophile type. Our results show that Rubisco kinetics for the few extremophile organisms reported up to date are placed at the margins of the enzyme's natural variability. Form ID Rubisco from thermoacidophile rhodophytes and form IB Rubisco from halophile terrestrial plants exhibit higher specificity and affinity for CO2 than their non-extremophilic counterparts, as well as higher carboxylation efficiency, whereas form ID Rubisco from psychrophile organisms possess lower affinity for O2. Additionally, form IB Rubisco from thermophile cyanobacteria shows enhanced CO2 specificity when compared to form IB non-extremophilic cyanobacteria. Overall, these findings highlight the unique characteristics of extremophile Rubisco enzymes and provide useful clues to guide next explorations aimed at finding more efficient Rubiscos.

Salt-excluder rootstock improves physio-biochemical responses of grafted grapevine plants subjected to salinity stress

This study tests the hypothesis that a more salt-excluder rootstock can attenuate salt stress in grapevine plants by enhancing photosynthesis and providing ionic and oxidative protection. Plants of ‘BRS Vitória’ variety, grafted with the rootstocks IAC 313 (salt-excluder) and SO4, were subjected to salinity by adding NaCl (0, 50, and 100 mM) for 30 days. Plants with SO4 showed more severe salt toxicity symptoms in leaves and lower chlorophyll content under salinity. Conversely, plants with IAC 313 showed improved photosynthesis and stomatal conductance, along with higher carboxylation efficiency under salt compared to SO4. Under salinity, plants with SO4 showed higher losses of K+ in stems, roots, and petioles, as well as increased accumulation of Na+ in these organs, relative to IAC 313. Furthermore, plants with IAC 313 had lower leaf Na+ content under salinity and reduced leaf Cl− content at 50 mM NaCl, a response associated with a higher Na+ allocation in petioles of IAC 313. At 50 mM, IAC 313 exhibited better photochemical activity, as indicated by electron transport rate and non-photochemical quenching. However, at 100 mM, both rootstocks showed similar trends, suggesting that the photosynthetic restriction was primarily due to stomatal disturbances. Plants with IAC 313 showed better APX activity and ascorbate balance under salinity. IAC 313 showed more salt-resistance traits than SO4, although the growth was similarly affected in both rootstocks. This response could be due to the reduced time of salt treatment (30 days). In summary, our data indicate that IAC 313 rootstock possesses better salt tolerance traits than SO4.

The catalytic mechanism of direction-dependent interactions for 2,3-dihydroxybenzoate decarboxylase.

Benzoic acid decarboxylases offer an elegant alternative to CO2 fixation by reverse reaction-carboxylation, which is named the bio-Kolbe-Schmitt reaction, but they are unfavorable to carboxylation. Enhancing the carboxylation efficiency of reversible benzoic acid decarboxylases is restricted by the unexplained carboxylation mechanisms. The direction of reversible enzyme catalytic reactions depends on whether catalytic residues at the active center of the enzyme are protonated, which is subjected by the pH. Therefore, the forward and reverse reactions could be separated at different pH values. Reversible 2,3-dihydroxybenzoate acid decarboxylase undergoes decarboxylation at pH 5.0 and carboxylation at pH 8.6. However, it is unknown whether the interaction of enzymes with substrates and products in the forward and reverse reactions can be exploited to improve the catalytic activity of reversible enzymes in the unfavorable direction. Here, we identify a V-shaped tunnel of 2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae (2,3-DHBD_Ao) through which the substrate travels in the enzyme, and demonstrate that the side chain conformation of a tyrosine residue controls the entry and exit of substrate/product during reversible reactions. Together with the kinetic studies of the mutants, it is clarified that interactions between substrate/product traveling through the enzyme tunnel in 2,3-DHBD_Ao are direction-dependent. These results enrich the understanding of the interactions of substrates/products with macromolecular reversible enzymes in different reaction directions, thereby demonstrating a possible path for engineering decarboxylases with higher carboxylation efficiency. KEY POINTS: • The residue Trp23 of 2,3-DHBD_Ao served as a switch to control the entry and exit of catechol • A V-shaped tunnel of 2,3-DHBD_Ao for decarboxylation and carboxylation reactions was identified • The results provide a promising strategy for engineering decarboxylases with direction-dependent residues inside the substrate/product traveling tunnel of the enzyme.

Drought-triggered leaf transcriptional responses disclose key molecular pathways underlying leaf water use efficiency in sugarcane (Saccharum spp.).

Drought is a major constraint to sugarcane (Saccharum spp.) production and improving the water use efficiency (WUE) is a critical trait for the sustainability of this bioenergy crop. The molecular mechanism underlying WUE remains underexplored in sugarcane. Here, we investigated the drought-triggered physiological and transcriptional responses of two sugarcane cultivars contrasting for drought tolerance, 'IACSP97-7065' (sensitive) and 'IACSP94-2094' (tolerant). After 21 days without irrigation (DWI), only 'IACSP94-2094' exhibited superior WUE and instantaneous carboxylation efficiency, with the net CO2 assimilation being less impacted when compared with 'IACSP97-7065'. RNA-seq of sugarcane leaves at 21 DWI revealed a total of 1,585 differentially expressed genes (DEGs) for both genotypes, among which 'IACSP94-2094' showed 617 (38.9%) exclusive transcripts (212 up- and 405 down-regulated). Functional enrichment analyses of these unique DEGs revealed several relevant biological processes, such as photosynthesis, transcription factors, signal transduction, solute transport, and redox homeostasis. The better drought-responsiveness of 'IACSP94-2094' suggested signaling cascades that foster transcriptional regulation of genes implicated in the Calvin cycle and transport of water and carbon dioxide, which are expected to support the high WUE and carboxylation efficiency observed for this genotype under water deficit. Moreover, the robust antioxidant system of the drought-tolerant genotype might serve as a molecular shield against the drought-associated overproduction of reactive oxygen species. This study provides relevant data that may be used to develop novel strategies for sugarcane breeding programs and to understand the genetic basis of drought tolerance and WUE improvement of sugarcane.

Discovery of a readily heterologously expressed Rubisco from the deep sea with potential for CO2 capture

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the key CO2-fixing enzyme in photosynthesis, is notorious for its low carboxylation. We report a highly active and assembly-competent Form II Rubisco from the endosymbiont of a deep-sea tubeworm Riftia pachyptila (RPE Rubisco), which shows a 50.5% higher carboxylation efficiency than that of a high functioning Rubisco from Synechococcus sp. PCC7002 (7002 Rubisco). It is a simpler hexamer with three pairs of large subunit homodimers around a central threefold symmetry axis. Compared with 7002 Rubisco, it showed a 3.6-fold higher carbon capture efficiency in vivo using a designed CO2 capture model. The simple structure, high carboxylation efficiency, easy heterologous soluble expression/assembly make RPE Rubisco a ready-to-deploy enzyme for CO2 capture that does not require complex co-expression of chaperones. The chemosynthetic CO2 fixation machinery of chemolithoautotrophs, CO2-fixing endosymbionts, may be more efficient than previously realized with great potential for next-generation microbial CO2 sequestration platforms.

Leaf mass area determines water use efficiency through its influence on carbon gain in rice mutants.

Saving water and enhancing rice productivity are consensually the most important research goals globally. While increasing canopy cover would enhance growth rates by higher photosynthetic carbon gain, an accompanied increase in transpiration would have a negative impact on saving water as well as for sustainability under water-limited conditions. Increased water use efficiency (WUE) by virtue of higher carbon assimilatory capacity can significantly circumvent this trade-off. Here, we report leaf mass area (LMA) has an important canopy architecture trait which when combined with superior carboxylation efficiency (CE) would achieve higher water productivity in rice. A set of 130 ethyl methanesulfonate induced mutants of an upland cultivar Nagina-22 (N22), was screened for leaf morphological traits leading to the identification of mutants differing in LMA. The wild-type, N22, along with a selected low-LMA (380-4-3) and two high-LMA mutants (392-9-1 and 457-1-3), all with comparable total leaf area, were raised under well-watered (100% Field Capacity (FC)) and water-limited (60% FC) conditions. Low Δ13 C and a higher RuBisCO content in high-LMA mutants indicated higher carboxylation efficiency, leading to increased carbon gain. Single parent backcross populations developed by crossing high and the low-LMA mutants with N22, separately, were screened for LMA, Δ13 C and growth traits. Comparison of dry matter accumulation per unit leaf area among the progenies differing in LMA and Δ13 C reiterated the association of LMA with CE. Results illustrated that high-LMA when combined with higher CE (low Δ13 C) lead to increased WUE and growth rates.

Combining ability, heritability and genotypic relations of different physiological traits in cacao hybrids.

Selecting parents and evaluating progenies is a very important step in breeding programs and involves approaches such as understanding the initial stages of growth and characterizing the variability among genotypes for different parameters, such as physiological, growth, biomass partitioning and nutrient translocation to the aerial part. In these cases, facilitating tools can be used to understand the involved gene dynamics, such as diallel crosses and genetic and phenotypic correlations. Our main hypothesis is that the contrasting phenotypes of these parental genotypes of cocoa used are due to genetic factors, and progenies derived from crosses of these parental genotypes are useful for breeding programs related to plant architecture, physiological parameters and translocation of mineral nutrients. We aimed to evaluate the combining abilities in progenies of cacao (Theobroma cacao L) originating from contrasting parents for canopy vigor. Emphasis was given to the evaluation of morphological and physiological parameters and the phenotypic and genotypic correlations to understand the dynamics of the action of the genes involved, as well as in expression profile from genes of gibberellins biosynthesis pathway in the parents. Fifteen F1 progenies were obtained from crosses of six clones (IMC 67, P4B, PUCALA, SCA 6, SCA 24 and SJ 02) that were evaluated in a randomized complete block design with four replicates of 12 plants per progeny, in a balanced half table diallel scheme. It is possible to identify and select plants and progenies of low, medium and high height, as there is expressive genetic variability for the evaluated parameters, some of these on higher additive effects, others on larger nonadditive effects and others under a balance of these effects. Most physiological parameters evaluated show that for selection of plants with the desired performance, no complex breeding methods would be necessary due to the high and medium heritability observed. Strong genetic components were observed from many of the correlations, which indicate the possibility to formulate selection indices for multi-traits, such as dwarfism or semidwarfism, tolerance to increase of leaf sodium concentrations and maintenance of the photosynthetic apparatus integrity under these conditions. Additionally, plants with higher carbon fixation, better water use, higher carboxylation efficiency and greater magnesium accumulation in leaves can be selected.

Effect of waterlogging on photosynthetic and biochemical parameters in pigeonpea

We studied the effect of waterlogging stress on photosynthetic and biochemical parameters in pigeonpea [Cajanus cajan (L.) Mill sp.] genotypes viz. ICPL-84023 (waterlogging resistant) and MAL-18 (waterlogging susceptible). Plants at early growth stage (20 days) were subjected to stress by keeping the pots in water filled containers. Stress was imposed for six days. Waterlogging reduced carbon exchange rate, stomatal conductance, intercellular CO2 concentration and transpiration rate. Photosynthesis was limited by stomatal as well as nonstomatal components under waterlogging. Reduction in chlorophyll, starch and increase in alcohol dehydrogenase activity along with the membrane injury were observed under waterlogging. High carboxylation efficiency, more chlorophyll content, starch availability, alcohol dehydrogenase activity, and membrane stability were associated with better survival of ICPL-84023 under waterlogging.

Effects of arbuscular mycorrhizal fungi on photosynthesis, carbon content, and calorific value of black locust seedlings

Arbuscular mycorrhizal fungi (AMF) form symbioses with many plants. Black locust (Robinia pseudoacacia L.) is an important energy tree species that can associate with AMF. We investigated the effects of AMF (Rhizophagus irregularis and Glomus versiforme) on the growth, gas exchange, chlorophyll (Chl) fluorescence, carbon content, and calorific value of black locust seedlings in the greenhouse. The total biomass of the arbuscular mycorrhizal (AM) seedlings was 4 times greater than that of the nonmycorrhizal (NM) seedlings. AMF greatly promoted the photosynthesis of black locust seedlings. AM seedlings had a significantly greater leaf area, higher carboxylation efficiency, Chl content, and net photosynthetic rate (P N) than NM seedlings. AMF also significantly increased the effective photochemical efficiency of PSII and significantly enhanced the carbon content and calorific value of black locust seedlings. Seedlings inoculated with G. versiforme had the largest leaf area and highest biomass, Chl content, P N, and calorific value.

Impacts of leafroll‐associated viruses (GLRaV‐1 and ‐3) on the physiology of the Portuguese grapevine cultivar ‘Touriga Nacional’ growing under field conditions

AbstractThe impact of mixed infection of grapevine leafroll‐associated virus 1 and 3 (GLRaV‐1&‐3) on physiological performance of the Portuguese grapevine variety ‘Touriga Nacional’ was evaluated during 3 years with the main purpose of understanding the drastic reduction in yield. Overall, gas exchange was negatively affected in leaves with these leafroll virus infections. Particularly at ripeness stage, the reduction in stomatal conductance (gs) was higher than in net CO2 assimilation rate (A), leading to higher intrinsic water use efficiency (A/gs) in infected leaves. However, the decrease in gs and A were not a consequence of the decrease in bulk water potential, as the water index/normalised difference vegetation index ratio suggested similar magnitude for both treatments. The maximum quantum efficiency of photosystem II was unaffected by GLRaV‐1&‐3, whereas quantum effective efficiency of PSII, apparent electron transport rate and photochemical quenching significantly decreased in infected leaves and these was paralleled by a significant increase of non‐photochemical quenching. Relative to carbon metabolism, the analyses of the net CO2 assimilation rate/photosynthetic photon flux density (A/PPFD) and net CO2 assimilation rate/internal CO2 concentration (A/Ci) curves revealed that virus infection had a negative effect on light saturated rate of CO2 fixation at high irradiances and carboxylation efficiency but, in contrast, apparent quantum yield of CO2 fixation was significantly higher. Meanwhile, the presence of GLRaV‐1&‐3 resulted in a marked decrease in photosynthetic pigments, soluble sugars and soluble proteins contents, while starch and anthocyanins were significantly improved. N, P, Ca, S and Fe leaf concentrations significantly decreased, while K, Mg, B, Cu, Zn and Mn were unaffected by these two leafroll virus species. Infected plants showed a significant decrease in yield, mainly due to a lower cluster weight. These results emphasised the important role of GLRaV‐1&‐3 as a biotic stress for the grapevine physiology and consequently to yield attributes.

Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis from drought stress in a C3perennial grass species

Stomatal closure and metabolic impairment under drought stress limits photosynthesis. The objective of this study was to determine major stomatal and metabolic factors involved in photosynthetic responses to drought and recovery upon re-watering in a C(3) perennial grass species, Kentucky bluegrass (Poa pratensis L.). Two genotypes differing in drought resistance, 'Midnight' (tolerant) and 'Brilliant' (sensitive), were subjected to drought stress for 15 days and then re-watered for 10 days in growth chambers. Single-leaf net photosynthetic rate (A), stomatal conductance (g(s)) and transpiration rate (Tr) decreased during drought, with a less rapid decline in 'Midnight' than in 'Brilliant'. Photochemical efficiency, Rubisco activity and activation state declined during drought, but were significantly higher in 'Midnight' than in 'Brilliant'. The relationship between A and internal leaf CO(2) concentration (A/Ci curve) during drought and re-watering was analyzed to estimate the relative influence of stomatal and non-stomatal components on photosynthesis. Stomatal limitation (Ls %), non-stomatal limitation (Lns %), CO(2) compensation point (CP) and dark respiration (Rd) increased with stress duration in both genotypes, but to a lesser extent in 'Midnight'. Maximum CO(2) assimilation rate (A(max)), carboxylation efficiency (CE) and mesophyll conductance (g(m)) declined, but 'Midnight' had significantly higher levels of A(max), CE and g(m) than 'Brilliant'. Maximum carboxylation rate of Rubisco (V(cmax)) and ribulose-1,5-bisphospate (RuBP) regeneration capacity mediated by maximum electron transport rate (J(max)) decreased from moderate to severe drought stress in both genotypes, but to a greater extent in 'Brilliant' than in 'Midnight'. After re-watering, RWC restored to about 90% of the control levels in both genotypes, whereas A, g(s), Tr and Fv/Fm was only partially recovered, with a higher recovery level in 'Midnight' than in 'Brilliant'. Rubisco activity and activation state restored to the control level after re-watering, with more rapid increase in 'Midnight' than in 'Brilliant'. The values of Ls, Lns, CP and Rd declined, and A(max), CE, V(cmax), J(max) and g(m) increased after re-watering, with more rapid change in all parameters in 'Midnight' than in 'Brilliant'. These results indicated that the maintenance of higher A and A(max) under drought stress in drought-tolerant Kentucky bluegrass could be attributed to higher Rubico activation state, higher CE and less stomatal limitation. The ability to resume metabolic activity (A(max), CE, Fv/Fm and Rubisco) was observed in the drought-tolerant genotype and is the most likely cause for the increased recuperative ability of photosynthesis. Incomplete recovery of photosynthesis upon re-watering could be attributable to lasting stomatal limitations caused by severe drought damage in both genotypes. Promoting rapid stomatal recovery from drought stress may be critical for plants to resume full photosynthetic capacity in C(3) perennial grass species.

Light-saturated photosynthetic rate in high-nitrogen rice (Oryza sativa L.) leaves is related to chloroplastic CO2 concentration

To identify the effect of nitrogen (N) nutrition on photosynthetic efficiency and mesophyll conductance of rice seedlings (Oryza sativa L., cv. 'Shanyou 63' hybrid indica China), hydroponic experiments with different concentrations of N were conducted in a greenhouse. Although leaf N concentration on a dry mass basis increased with increasing supply of N, no significant differences in seedling biomass were observed. A higher light-saturated CO(2) assimilation rate (A) with a high concentration of supplied N was associated with a higher carboxylation efficiency (CE), but not a higher apparent quantum yield (alpha). Based on classic photosynthetic models, both the Rubisco content and the ribulose bisphosphate (RuBP) regeneration rate were sufficient for light-saturated photosynthesis in rice seedlings; the estimated chloroplastic CO(2) concentration (C(c)) and mesophyll conductance (g(m)) demonstrated that a low C(c) was the ultimate limiting factor to photosynthetic efficiency with a higher N supply. Due to a greater chloroplast size (i.e. a shorter distance to the plasma membrane) with a higher supply of N, the CO(2) transport resistance in the liquid phase (g(liq)) in high-N leaves was lower than that in low-N leaves, which resulted in higher g(m) and C(c) in high-N leaves. Although CE(A/Ci) was higher with a high supply of N, there were no differences in CE(A/Cc) between plants grown with different concentrations of N, indicating that the carboxylation capacity of Rubisco between plants grown at different N concentrations was constant. The enhanced photosynthetic rate with supply of a high N concentration was attributed to a higher CO(2) concentration in the chloroplasts, related to a higher mesophyll conductance due to an increased chloroplast size.

Carbon assimilation, 𝛅13C and water relations of Elaeagnus angustifolia grown at two groundwater depths in the Minqin desert, China

We investigated the physiological responses of Elaeagnus angustifolia to variation in groundwater depth. Elaeagnus angustifolia seedlings were grown in the Minqin desert in lysimeters supplied with underground water at the soil depth of 1.40 m and 3.40 m. Results showed that constant access to groundwater allowed plants supplied with water at the lower soil layer to meet their water requirement and, consequently, they were not affected by water stress. There were no differences in A max (the net CO2 assimilation rate under conditions of photosynthetically photon flux density and CO2 saturation), J max (maximum rate of electron transport) and stomatal conductance between the two underground water treatments. However, plants with deeper groundwater had a significantly higher V cmax (i.e. a higher carboxylation efficiency of Rubisco) and mesophyll conductance resulting in increased photosynthesis measured at the CO2 growth condition (A) and, consequently higher intrinsic water-use efficiency (WUE). However, respiration was also increased in plants grown with deeper groundwater. This may have offset the increased A and led to a similar long-term WUE, as expressed by carbon isotope discrimination (δ13C), between the two ground water treatments. In the present study, we also found a high foliage nitrogen concentration in the E. angustifolia plants (3.75% on average), that may be very significant ecologically in improving soil properties. The physiological traits of E. angustifolia found in this study confirm that the use of perennial phreatophytic, nitrogen fixing species has significant potential to positively impact soil fertility and carbon sequestration under environmental conditions found in the Minqin desert.

Photosynthetic properties of C4 plants growing in an African savanna/wetland mosaic

Photosynthesis rates and photosynthesis-leaf nutrient relationships were analysed in nine tropical grass and sedge species growing in three different ecosystems: a rain-fed grassland, a seasonal floodplain, and a permanent swamp, located along a hydrological gradient in the Okavango Delta, Botswana. These investigations were conducted during the rainy season, at a time of the year when differences in growth conditions between the sites were relatively uniform. At the permanent swamp, the largest variations were found for area-based leaf nitrogen contents, from 20 mmol m(-2) to 140 mmol m(-2), nitrogen use efficiencies (NUE), from 0.2 mmol (C) mol(-1) (N) s(-1) to 2.0 mmol (C) mol(-1) (N) s(-1), and specific leaf areas (SLA), from 50 cm(2) g(-1) to 400 cm(2) g(-1). For the vegetation growing at the rain-fed grassland, the highest leaf gas exchange rates, high leaf nutrient levels, a low ratio of intercellular to ambient CO(2) concentration, and high carboxylation efficiency were found. Taken together, these observations indicate a very efficient growth strategy that is required for survival and reproduction during the relatively brief period of water availability. The overall lowest values of light-saturated photosynthesis (A(sat)) were observed at the seasonal floodplain; around 25 micromol m(-2) s(-1) and 30 micromol m(-2) s(-1). To place these observations into the broader context of functional leaf trait analysis, relationships of photosynthesis rates, specific leaf area, and foliar nutrient levels were plotted, in the same way as was done for previously published 'scaling relationships' that are based largely on C(3) plants, noting the differences in the analyses between this study and the previous study. The within- and across-species variation in both A(sat) and SLA appeared better predicted by foliar phosphorus content (dry mass or area basis) rather than by foliar nitrogen concentrations, possibly because the availability of phosphorus is even more critical than the availability of nitrogen in the studied relatively oligotrophic ecosystems.

Photosynthetic responses of apricot (Prunus armeniaca L.) to photosynthetic photon flux density, leaf temperature, and CO<sub>2</sub> concentration

Two cultivars (Katy and Erhuacao) of apricot (Prunus armeniaca L.) were evaluated under open-field and solar-heated greenhouse conditions in northwest China, to determine the effect of photosynthetic photon flux density (PPFD), leaf temperature, and CO2 concentration on the net photosynthetic rate (P N). In greenhouse, Katy registered 28.3 µmol m−2 s−1 for compensation irradiance and 823 µmol m−2 s−1 for saturation irradiance, which were 73 and 117 % of those required by Erhuacao, respectively. The optimum temperatures for cvs. Katy and Erhuacao were 25 and 35 °C in open-field and 22 and 30 °C in greenhouse, respectively. At optimal temperatures, P N of the field-grown Katy was 16.5 µmol m−2 s−1, 21 % less than for a greenhouse-grown apricot. Both cultivars responded positively to CO2 concentrations below the CO2 saturation concentration, whereas Katy exhibited greater P N (18 %) and higher carboxylation efficiency (91 %) than Erhuacao at optimal CO2 concentration. Both cultivars exhibited greater photosynthesis in solar-heated greenhouses than in open-field, but Katy performed better than Erhuacao under greenhouse conditions.

Mapping of QTLs Controlling Carbon Isotope Discrimination in the Photosynthetic System using Recombinant Inbred Lines Derived from a Cross between Two Different Rice(Oryza sativa L.) Cultivars

Carbon isotope discrimination (Ċ) occuring in the process of photosynthesis, shows variation among rice (Oryza sativa L.) cultivars. Elucidation of specific traits associated with the extent of this discrimination under irrigated conditions may be useful to improve photosynthetic ability in rice plants. We measured leaf photosynthesis and Ċ in Milyang 23 and Akihikari, and conducted quantitative trait loci (QTL) analysis on Ċ at heading stage using a population of 126 recombinant inbred lines (RILs), derived from a cross between the two cultivars. While the two parental cultivars showed a similar Ċ, the RILs showed a wide variation in Ċ including transgressive segregation. Seven QTLs were detected for Ċ; four on chromosomes 2 (two regions), 7, and 11 were those for Ċ that is increased by the Milyang 23 allele, whereas the other three on chromosomes 1, 2, and 6 were those for Ċ that is increased by the Akihikari allele. These results suggest that 13C in Milyang 23 may be discriminated through a photosynthetic process different from that in Akihikari. Milyang 23 showed a higher stomatal conductance and a higher ratio of intercellular to ambient CO2 concentration (Ci/Ca), while Akihikari showed a higher carboxylation efficiency but a lower Ci/Ca. According to the theory that a higher Ci/Ca leads to a higher Ċ, the QTLs for Ċ that is increased by the Milyang 23 allele might be related to a higher stomatal conductance. However, the theory provided no persuasive factors to explain the QTLs for Ċ that is increased by the Akihikari allele. Plausible factors associated with these QTLs are discussed.

Seasonal changes in the physiology of shade leaves of Acer saccharum

Summary Changes in the physiology of shade leaves of sugar maple trees (Acer saccharum Marsh.) in eastern Massachusetts were monitored throughout several growing seasons. In the late spring, following leaf expansion, the sugar maple leaves showed high photosynthetic rates and a high carboxylation efficiency which increased only slightly into the early part of summer. In contrast, pigment and protein concentrations increased sharply from late spring to mid summer. The average photosynthetic rates remained fairly high during the summer and started to decrease in early September. This decrease was accompanied by decreases in leaf pigment, protein and nitrogen concentrations. In the last few weeks before leaf abscission photosynthesis was not limited by stomatal conductance as indicated by high internal CO2 concentrations. However, photosynthesis may have been CO2 limited at times during the summer. Low quantum yields of carbon fixation and low photosynthetic rates on some days during the summer indicated temporary stress. Experiments revealed that maple leaves responded to short-term stress by temporarily downregulating PSII. In general, leaves exhibited high Fv/Fm values throughout most of the year. Sustained photoinhibition as indicated by reduced Fv/Fm was only observed during the last few weeks before leaf abscission. Photochemical quenching remained high throughout the fall, indicating that the efficiency of excitation energy capture by open PSII reaction centers was the major reason for the decrease in photochemical efficiency of PSII during the last few weeks before the leaves were shed. At the same time the non-photochemical quenching increased throughout the fall and was only low in extremely yellow leaves.

Clonal Differences in Photosynthesis in Hevea Brasiliensis Müll. Arg.

The objective of the present investigation was to examine the extent of variations in single leaf net photosynthetic rate (PN) and its relative dependence on stomatal conductance (gs) and the mesophyll capacity to fix carbon in 12 clones of the natural rubber plant. There were significant variations in PN measured at low and saturating photon flux density (PFD); the extent of variation was larger at low than at saturating PFD. The compensation irradiance (CI) and apparent quantum yield of CO2 assimilation (φc) calculated from the PN/PFD response curves showed significant variations among the clones. PN at low irradiance was positively correlated with φc. Thus a clone with large PN at low irradiance, high φc, and low CI may tolerate shade better and thus produce a high tree stand per hectare. A strong positive correlation existed between PN saturated with radiant energy (Psat) and carboxylation efficiency (CE) estimated from the response curves of PN on intercellular CO2 concentration (Ci), but gs showed a poor correlation with Psat High CO2 compensation concentration (Γ) led to low CE in Hevea clones. A clone with large Psat, high CE, low gs, and low Γ is the one in which photosynthesis is more dependent on the mesophyll factors than stomata. Such a clone may produce relatively high biomass and maintain high water use efficiency.

Characterization of rbcS genes in the fern Pteris vittata and their photoregulation.

The fern Pteris vittata L. belongs to the evolutionarily highest group of vascular plants that still maintains a free-living gametophytic stage. The two-dimensional gametophytes developed under blue light exhibit higher CO2 fixation efficiency and different ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit (SSU) composition when compared to the red-induced filamentous gametophytes (H. Eilenberg et al., 1991, Plant Physiol 95: 298-304). To unravel the correlation between SSU structural differences and light regulation, two rbcS genes and two additional partial cDNAs were characterized. Fern rbcS genes resemble those of higher plants in their promoter light-regulatory elements (LREs) and intron number and positions. However, the primary structure of the fern mature SSUs displays much higher divergency within the gene family. This structural variability was correlated with differential steady-state mRNA levels under red and blue light. Genes rbcS-1 and-4 and 4-to 6-fold higher transcript levels in red light while rbcS-2 and-3 contribute relatively more to the blue rbcS mRNA levels. Five of the 12 amino acids that differ between rbcS-2 and-4 affect hydrophobicity and might play a crucial role in determining the efficiency of CO2 fixation. Dendrograms of Rubisco SSUs and LSUs indicate early divergence of the fern types from the rest of the vascular plants. However, prominent higher-plant-like Rubisco features such as high carboxylation efficiency, promoter LREs and exon-intron structure, suggest that molecular specialization of the higher-plant Rubisco prototype occurred earlier than the emergence of ferns.

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO2 enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO2 concentrations of 350 or 1 000 μmol CO2 mol‐1. At each CO2 concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO2 mol‐1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO2 mol‐1 although actual net CO2 assimilation (A) was higher at the higher ambient CO2 concentration due to increased intercellular CO2 concentrations (Ci resulting from CO2 enrichment. Thus, plants grown at 1 000 μmol CO2 mol‐1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO2 mol‐1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO2 mol‐1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO2 enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO2 mol‐1 there was generally no effect of root chamber size on plant dry weight.