Root-derived bicarbonate assimilation in response to variable water deficit in <Emphasis Type="Italic">Camptotheca acuminate</Emphasis> seedlings

Water deficit is one of the key factors that limits the carbon (C) assimilation and productivity of plants. The effect of variable water deficit on recently root-derived bicarbonate assimilation in Camptotheca acuminate seedlings was investigated. Three-month-old seedlings were subjected to three water regimes, well-watered (WW), moderate stress (MS), and severe stress (SS) induced by polyethyleneglycol, in conjunction with relatively high (H) and low (L) natural ¹³C-abundance of NaHCO₃-labeled treatments in hydroponics for 14 days. The δ¹³C of the newly expanded leaves in H were generally more enriched in heavy isotopes than were those in L, indicative of the involvement of bicarbonate in aboveground tissues. The C isotope fractionation of newly expanded leaves relative to air (?¹³C_air-leaves) ranged from 17.78 to 21.78‰ among the treatments. The ?¹³C_air-leaves under the MS and SS treatments in H were both more negative than was that in L. A linear regression between Ci/Ca and ?¹³C_air-leaves in both L and H were different from the theoretical regression. On the basis of the two end-member mixing model, the proportion of fixed CO₂ supplied from bicarbonate contributing to the total photosynthetically inorganic C assimilation were 10.34, 20.05 and 16.60% under the WW, MS, and SS treatments, respectively. These results indicated that the increase in water deficit decreased the atmospheric CO₂ gain but triggered a compensatory use of bicarbonate in C. acuminate seedlings.     

Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂.     

Genetic engineering of polyamine metabolism changes <Emphasis Type="Italic">Medicago truncatula</Emphasis> responses to water deficit

In the current scenario of climate change and increasing water scarcity there is an increased need to combine research efforts for the development of abiotic stress resistant crops, specifically plants able to support water deficit (WD). Polyamines (PAs) have been described as being involved in the regulation of many physiological processes and a variety of stress responses in plants. Arginine decarboxylase (ADC) is considered a key enzyme of the polyamine (PA) biosynthetic pathway. In this study, a T₂ transgenic homozygous line of Medicago truncatula expressing the oat Adc under the control of CaMV 35S was obtained and was shown to have higher leaf accumulation of putrescine, spermidine and norspermidine compared to wild type plants. The photosynthetic parameters, leaf internal CO₂ concentration (Ci), net CO₂ assimilation rate (A), transpiration (E) and stomatal conductance (gs) of transformed and untransformed lines during WD and water deficit recovery experiments were measured by IRGA (infrared gas analyzer) and compared over time. Two light intensities were used, growth light intensity (391 μmol m^?2 s^?1) and saturating light intensity (1044 μmol m^?2 s^?1). Independently of the light intensity, and under WD, the transgenic line stood out with increased Ci, A, E and gs; suggesting a possible benefit of the augmented PAs under such disturbing environmental conditions. We showed that the constitutive expression of the oat Adc gene improve the physiological responses to WD and that WD recovered transgenic plants had higher seed yield, suggesting a possible benefit of PA metabolism manipulation in legumes.     

Interacting effects of elevated atmospheric CO<Subscript>2</Subscript> and hydrology on the growth and carbon sequestration of <Emphasis Type="Italic">Sphagnum</Emphasis> moss

Peatlands are a critical carbon store comprising 30% of the Earth’s terrestrial soil carbon. Sphagnum mosses comprise up to 90% of peat in the northern hemisphere but impacts of climate change on Sphagnum mosses are poorly understood, limiting development of sustainable peatland management and restoration. This study investigates the effects of elevated atmospheric CO₂ (eCO₂) (800 ppm) and hydrology on the growth of Sphagnum fallax, Sphagnum capillifolium and Sphagnum papillosum and greenhouse gas fluxes from moss–peat mesocosms. Elevated CO₂ levels increased Sphagnum height and dry weight but the magnitude of the response differed among species. The most responsive species, S. fallax, yielded the most biomass compared to S. papillosum and S. capillifolium. Water levels and the CO₂ treatment were found to interact, with the highest water level (1 cm below the surface) seeing the largest increase in dry weight under eCO₂ compared to ambient (400 ppm) concentrations. Initially, CO₂ flux rates were similar between CO₂ treatments. After week 9 there was a consistent three-fold increase of the CO₂ sink strength under eCO₂. At the end of the experiment, S. papillosum and S. fallax were greater sinks of CO₂ than S. capillifolium and the ? 7 cm water level treatment showed the strongest CO₂ sink strength. The mesocosms were net sources of CH₄ but the source strength varied with species, specifically S. fallax produced more CH₄ than S. papillosum and S. capillifolium. Our findings demonstrate the importance of species selection on the outcomes of peatland restoration with regards to Sphagnum’s growth and GHG exchange.     

Photosynthetic characteristics and nitrogen allocation in the black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) grown in a FACE system

Key message

The black locust is adapted to elevated [CO ₂ ] through changes in nitrogen allocation characteristics in leaves.

Abstract

The black locust (Robinia pseudoacacia L.) is an invasive woody legume within Japan. This prolific species has a high photosynthetic rate and growth rate, and undergoes symbiosis with N₂-fixing micro-organisms. To determine the effect of elevated CO₂ concentration [CO₂] on its photosynthetic characteristics, we studied the chlorophyll (Chl) and leaf nitrogen (N) content, and the leaf structure and N allocation patterns in the leaves and acetylene reduction activity after four growing seasons, in R. pseudoacacia. Our specimens were grown at ambient [CO₂] (370 μmol mol^?1) and at elevated [CO₂] (500 μmol mol^?1), using a free air CO₂ enrichment (FACE) system. Net photosynthetic rate at growth [CO₂] (A _growth) and acetylene reduction activity were significantly higher, but maximum carboxylation rate of RuBisCo (V _cmax), maximum rate of electron transport driving RUBP regeneration (J _max), net photosynthetic rate under enhanced CO₂ concentration and light saturation (A _max), the N concentration in leaf, and in leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) content were significantly lower grown at elevated [CO₂] than at ambient [CO₂]. We also found that RuBisCo/N were less at elevated [CO₂], whereas Chl/N increased significantly. Allocation characteristics from N in leaves to photosynthetic proteins, N_L (Light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and other proteins also changed. When R. pseudoacacia was grown at elevated [CO₂], the N allocation to RuBisCo (N_R) decreased to a greater extent but N_L and N remaining increased relative to specimens grown at ambient [CO₂]. We suggest that N remobilization from RuBisCo is more efficient than from proteins of electron transport (N_E), and from N_L. These physiological responses of the black locust are significant as being an adaptation strategy to global environmental changes.

     

Changes in leaf epicuticular wax,gas exchange and biochemistry metabolism between <Emphasis Type="Italic">Jatropha mollissima</Emphasis> and <Emphasis Type="Italic">Jatropha curcas</Emphasis> under semi-arid conditions

Jatropha curcas and Jatropha mollissima plants were evaluated under conditions of high (HSM) and low (LSM) soil moisture in a semi-arid environment, as changes in the content and concentration of epicuticular wax and the leaf metabolism which could have a relationship with drought tolerance. Besides epicuticular wax, gas exchange, antioxidant system and biochemical parameters of the photosynthetic metabolism were measured. The epicuticular wax content increased only in J. mollissima leaves 95 % under LSM, when compared with HSM conditions. Therefore, J. curcas invested less in the production of long-chain n-alkanes than did J. mollissima under LSM conditions. J. mollissima plants showed the highest CO₂ assimilation rate during the HSM period compared to J. curcas. Both species showed high stability in some leaf biochemistry products, highlighting the highest sugar content, free amino acids, total soluble protein, and photosynthetic pigments in the leaves of J. mollissima plants under both of the soil moisture conditions. Moreover, the stability and performance of the different parameters, such as morphologic variables, seem to allow J. mollissima plants to tolerate semi-arid conditions.     

Impact of elevated CO<Subscript>2</Subscript> in <Emphasis Type="Italic">Casuarina equisetifolia</Emphasis> rooted stem cuttings inoculated with <Emphasis Type="Italic">Frankia</Emphasis>

Impact of different levels of elevated CO ₂ on the activity of Frankia (Nitrogen-fixing actinomycete) in Casuarina equisetifolia rooted stem cuttings has been studied to understand the relationship between C. equisetifolia, Frankia and CO₂. The stem cuttings of C. equietifolia were collected and treated with 2000 ppm of Indole Butyric Acid (IBA) for rooting. Thus vegetative propagated rooted stem cuttings of C. equisetifolia were inoculated with Frankia and placed in the Open top chambers (OTC) with elevated CO₂ facilities. These planting stocks were maintained in the OTC for 12 months under different levels of elevated CO₂ (ambient control, 600 ppm, 900 ppm). After 12 months, the nodule numbers, bio mass, growth, and photosynthesis of C. equisetifolia rooted stem cuttings inoculated with Frankia were improved under 600 ppm of CO₂. The rooted stem cuttings of C. equisetifolia inoculated with Frankia showed a higher number of nodules under 900 ppm of CO₂ and cuttings without Frankia inoculation exhibited poor growth. Tissue Nitrogen (N) content was also higher under 900 ppm of CO₂ than ambient control and 600 ppm levels. The photosynthetic rate was higher (17.8 μ mol CO₂ m^?2 s^?1) in 900 ppm of CO₂ than in 600 ppm (13.2 μ mol CO₂ m^?2 s^?1) and ambient control (8.3 μ mol CO₂ m^?2 s^?1). This study showed that Frankia can improve growth, N fixation and photosynthesis of C. equietifolia rooted stem cuttings under extreme elevated CO₂ level conditions (900 ppm).     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Molecular docking of <Emphasis Type="Italic">Glycine max</Emphasis> and <Emphasis Type="Italic">Medicago truncatula</Emphasis> ureases with urea; bioinformatics approaches

Urease (EC 3.5.1.5) is a nickel-dependent metalloenzyme catalyzing the hydrolysis of urea into ammonia and carbon dioxide. It is present in many bacteria, fungi, yeasts and plants. Most species, with few exceptions, use nickel metalloenzyme urease to hydrolyze urea, which is one of the commonly used nitrogen fertilizer in plant growth thus its enzymatic hydrolysis possesses vital importance in agricultural practices. Considering the essentiality and importance of urea and urease activity in most plants, this study aimed to comparatively investigate the ureases of two important legume species such as Glycine max (soybean) and Medicago truncatula (barrel medic) from Fabaceae family. With additional plant species, primary and secondary structures of 37 plant ureases were comparatively analyzed using various bioinformatics tools. A structure based phylogeny was constructed using predicted 3D models of G. max and M. truncatula, whose crystallographic structures are not available, along with three additional solved urease structures from Canavalia ensiformis (PDB: 4GY7), Bacillus pasteurii (PDB: 4UBP) and Klebsiella aerogenes (PDB: 1FWJ). In addition, urease structures of these species were docked with urea to analyze the binding affinities, interacting amino acids and atom distances in urease-urea complexes. Furthermore, mutable amino acids which could potentially affect the protein active site, stability and flexibility as well as overall protein stability were analyzed in urease structures of G. max and M. truncatula. Plant ureases demonstrated similar physico-chemical properties with 833–878 amino acid residues and 89.39–90.91 kDa molecular weight with mainly acidic (5.15–6.10 pI) nature. Four protein domain structures such as urease gamma, urease beta, urease alpha and amidohydro 1 characterized the plant ureases. Secondary structure of plant ureases also demonstrated conserved protein architecture, with predominantly α-helix and random coil structures. In structure-based phylogeny, plant ureases from G. max, M. truncatula and C. ensiformis were clearly diverged from bacterial ureases of B. pasteurii and K. aerogenes. Glu, Thr, His and Gly were commonly found as interacting residues in most urease-urea docking complexes while Glu was available in all docked structures. Besides, Ala and Arg residues, which are reported in active-site architecture of plant and bacterial ureases were present in G. max urea-urease complex but not present in others. Moreover, Arg435 and Arg437 in M. truncatula and G. max, respectively were identified as highly mutable hotspot residues residing in amidohydro 1 domain of enzyme. In addition, a number of stabilizing residues were predicted upon mutation of these hotspot residues however Cys and Thr made strong implications since they were also found in codon-aligned sequences as substitutions of hotspot residues. Comparative analyses of primary sequence and secondary structure in 37 different plants demonstrated quite conserved natures of ureases in plant kingdom. Structure-based phylogeny indicated the presence of a possible prokaryote-eukaryote split and implicated the subjection of bacterial ureases to heavy selection in prokaryotic evolution compared to plants. Urea-urease docking complexes suggested that different species could share common interacting residues as well as may have some other uncommon residues at species-dependent way. In silico mutation analyses identified mutable amino acids, which were predicted to reside in catalytic site of enzyme therefore mutagenesis at these sites seemed to have adverse effects on enzyme efficiency or function. This study findings will become valuable preliminary resource for future studies to further understand the primary, secondary and tertiary structures of urease sequences in plants as well as it will provide insights about various binding features of urea-urease complexes.     

Utilization of gaseous emissions from a methanol plant for cultivation of <Emphasis Type="Italic">Acutodesmus obliquus</Emphasis>

This study aimed to culture the green alga Acutodesmus obliquus utilizing the gaseous emissions containing a high concentration of CO₂ (99.13 %) from a methanol plant and study the tolerance of microalgae. The effect of CO₂ concentration, aeration rate, inoculum concentration, intermittent sparging, and nitrogen sources on the growth of A. obliquus was examined. Acutodesmus obliquus also was cultivated in a 500-L pilot outdoor tubular photobioreactor (OTP) to advance the laboratory scale system to outdoor scale-up applications. The results showed that A. obliquus could tolerate high CO₂ concentrations of 50 %, and a maximum biomass of 0.935 g L^?1 (dry weight) was achieved at 20 % CO₂. An aeration rate of 500 mL min^?1, inoculum concentration (optical density at 680 nm [OD₆₈₀]?=?0.3), and intermittent sparging of 10 min per 2 h enhanced growth to the optimum and influenced culture pH and photosynthesis. Urea as a nitrogen source was shown to be more beneficial to cell growth. A urea concentration of 0.3 g L^?1 and an N/P ratio of 15 led to maximum biomass accumulation thus enhancing the gaseous emission utilization efficiency. In conclusion, this work demonstrated that gaseous emissions containing high concentration of CO₂ from a methanol plant could be directly introduced into A. obliquus cultures and that A. obliquus was suitable well for large-scale outdoor cultivation in a tubular photobiorecator.     

Epiphytic bacterial communities of the alga <Emphasis Type="Italic">Fucus vesiculosus</Emphasis> in oil-contaminated water areas of the Barents Sea

Taxonomic compositions of epiphytic bacterial communities in water areas differing in levels of oil pollution were revealed. In total, 82 bacterial genera belonging to 16 classes and 11 phyla were detected. All detected representatives of epiphytic bacterial communities belonged to the phyla Actinobacteria, Bacteroidetes, Planctomycetes, Proteobacteria, Verrucomicrobia, Acidobacteria, Cyanobacteria, Firmicutes, and Fusobacteria and candidate division TM7. The ratio of the phyla in the communities varied depending on the levels of oil pollution. New data on taxonomic composition of uncultivated epiphytic bacterial communities of Fucus vesiculosus were obtained.     

Simple generalisation of a mesophyll resistance model for various intracellular arrangements of chloroplasts and mitochondria in C<Subscript>3</Subscript> leaves

The classical definition of mesophyll conductance (g _m) represents an apparent parameter (g _m,app) as it places (photo)respired CO₂ at the same compartment where the carboxylation by Rubisco takes place. Recently, Tholen and co-workers developed a framework, in which g _m better describes a physical diffusional parameter (g _m,dif). They partitioned mesophyll resistance (r _m,dif = 1/g _m,dif) into two components, cell wall and plasmalemma resistance (r _wp) and chloroplast resistance (r _ch), and showed that g _m,app is sensitive to the ratio of photorespiratory (F) and respiratory (R _d) CO₂ release to net CO₂ uptake (A): g _m,app = g _m,dif/[1?+?ω(F?+?R _d)/A], where ω is the fraction of r _ch in r _m,dif. We herein extend the framework further by considering various scenarios for the intracellular arrangement of chloroplasts and mitochondria. We show that the formula of Tholen et al. implies either that mitochondria, where (photo)respired CO₂ is released, locate between the plasmalemma and the chloroplast continuum or that CO₂ in the cytosol is completely mixed. However, the model of Tholen et al. is still valid if ω is replaced by ω(1?σ), where σ is the fraction of (photo)respired CO₂ that experiences r _ch (in addition to r _wp and stomatal resistance) if this CO₂ is to escape from being refixed. Therefore, responses of g _m,app to (F?+?R _d)/A lie somewhere between no sensitivity in the classical method (σ =1) and high sensitivity in the model of Tholen et al. (σ =0).     

Photosynthetic behavior of Spanish Arbequina and Italian Maurino olive (<Emphasis Type="Italic">Olea europaea</Emphasis> L.) cultivars under super-intensive grove conditions

The study was carried out in a four-year-old super-high density olive grove in Central Italy to compare leaf gas exchanges of Spanish Arbequina and Italian Maurino olive cultivars. Overall, from mid July to mid November, Maurino had a slightly higher maximum light-saturated net photosynthetic rate (P _Nmax) than Arbequina. The lowest and the highest P _Nmax values were recorded at the end of July and in mid November, respectively. Current-season leaves showed similar or slightly higher P _Nmax values than one-year-old leaves. During the day Maurino always had slightly higher values or values similar to Arbequina, with the highest P _Nmax being in the morning. Maurino had similar or higher dark respiration rate (R _D) values compared to Arbequina. During the day, in both cultivars the R _D was lower at 9:00 than in the afternoon. The pattern of the photosynthetic irradiance-response curve was similar in the two genotypes, but the apparent quantum yield (Y _Q) was higher in Maurino. In both cultivars intercellular CO₂ concentration (C _i) tended to increase when P _Nmax decreased. The increase in C _i corresponded to a decrease in stomatal conductance (g _s). The transpiration rate (E) increased from mid July to the beginning of August, then decreased in September and increased again in November. Particularly in the morning, the current-season leaves showed similar or slightly higher E values than the one-year-old leaves. During the day, in both cultivars and at both leaf ages, E was higher in the afternoon. No effects on leaf gas exchanges due to the presence or absence of fruit on the shoot were found. Overall, there was satisfactory physiological adaptation for Arbequina to the conditions of Central Italy and for Maurino to the superintensive grove conditions.     

<Emphasis Type="Italic">Artemisia frigida</Emphasis> and <Emphasis Type="Italic">Stipa krylovii</Emphasis>, two dominant species in Inner Mongolia steppe,differed in their responses to elevated atmospheric CO<Subscript>2</Subscript> concentration

Aims

Despite extensive studies on effects of elevated CO₂ concentration ([CO₂]_e) on plant growth, few studies have investigated the responses of native grassland plant species to [CO₂]_e in terms of nutrient acquisition.

Methods

The effects of [CO₂]_e (769 ± 23 ppm) on Artemisia frigida and Stipa krylovii, two dominant species in Inner Mongolia steppe were investigated by growing them for 7 weeks in Open-Top Chambers (OTC).

Results

Exposure to [CO₂]_e enhanced shoot and root growth of A. frigida and S. krylovii. Elevated [CO₂] increased photosynthetic rates (Pn) by 34 % in A. frigida but decreased Pn by 52 % in S. krylovii. Moreover, root-secreted acid phosphatase activity in A. frigida was stimulated by [CO₂]_e, while exudation of malate from roots of S. krylovii was suppressed by [CO₂]_e. Exposure to [CO₂]_e led to a decrease in P concentration in shoots and roots of A. frigida and S. krylovii, but total amount of P accumulated in shoots and roots of both species was increased by [CO₂]_e.

Conclusions

The two dominant species in temperate steppes differed in their responses to [CO₂]_e, such that A. frigida was more adapted to [CO₂]_e than S. krylovii under low availability of soil P.

     

Photosynthetic acclimation and leaf traits of <Emphasis Type="Italic">Stipa bungeana</Emphasis> in response to elevated CO<Subscript>2</Subscript> under five different watering conditions

Although plant performance under elevated CO₂ (EC) and drought has been extensively studied, little is known about the leaf traits and photosynthetic performance of Stipa bungeana under EC and a water deficiency gradient. In order to investigate the effects of EC, watering, and their combination, S. bungeana seedlings were exposed to two CO₂ regimes (ambient, CA: 390 ppm; elevated, EC: 550 ppm) and five levels of watering (?30%, ?15%, control, +15%, +30%) from 1 June to 31 August in 2011, where the control water level was 240 mm. Gas exchange and leaf traits were measured after 90-d treatments. Gas-exchange characteristics, measured at the growth CA, indicated that EC significantly decreased the net photosynthetic rate (P _N), water-use efficiency, nitrogen concentration based on mass, chlorophyll and malondialdehyde (MDA) content, while increased stomatal conductance (g _s), intercellular CO₂ concentration (C _i), dark respiration, photorespiration, carbon concentration based on mass, C/N ratio, and leaf water potential. Compared to the effect of EC, watering showed an opposite trend only in case of P _N. The combination of both factors showed little influence on these physiological indicators, except for g _s, C _i, and MDA content. Photosynthetic acclimation to EC was attributed to the N limitation, C sink/source imbalance, and the decline of photosynthetic activity. The watering regulated photosynthesis through both stomatal and nonstomatal mechanisms. Our study also revealed that the effects of EC on photosynthesis were larger than those on respiration and did not compensate for the adverse effects of drought, suggesting that a future warm and dry climate might be unfavorable to S. bungeana. However, the depression of the growth of S. bungeana caused by EC was time-dependent at a smaller temporal scale.     

Water use efficiency in the drought-stressed sorghum and maize in relation to expression of aquaporin genes

Zea mays L. is less tolerant to drought than Sorghum bicolor L. In the present study, we investigated the response of both plants to drought stress applied under field conditions by withholding water for 10 d. The plant growth in terms of shoot fresh and dry masses was more severely reduced in maize than in sorghum, consistently with reduction of leaf relative water content. Gas exchange was also more inhibited by drought in maize than in sorghum. The water use efficiency (WUE) of maize fluctuated during the day and in response to the drought stress. In contrast, sorghum was able to maintain a largely constant WUE during the day in the well-watered plants as well as in the stressed ones. Studying the expression of four aquaporin genes (PIP1;5, PIP1;6, PIP2;3, and TIP1;2) revealed that PIP1;5 in leaves and PIP2;3 in roots were highly responsive to drought in sorghum but not in maize, where they might have supported a greater water transport. The expression pattern of PIP1;6 suggests its possible role in CO₂ transport in control but not droughty leaves of both the plants. TIP1;2 seemed to contribute to water transport in leaves of the control but not droughty plants. We conclude that PIP1;5 and PIP2;3 may have a prominent role in drought tolerance and maintenance of WUE in sorghum plants.     

Effects of nitrogen forms and supply modes on colony formation in <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis>

Microcystis blooms can lead to a decline in water quality and ecological damage, and pose risks to human health. Therefore, studies on the mechanisms of Microcystis colony formation and bloom occurrence are of great significance for the aquatic ecosystem. In this study, Microcystis aeruginosa was cultured with nitrate, ammonium, or urea as the nitrogen source in the medium to investigate the effects of nitrogen forms on colony formation. Nitrogen was added as a single dose or in multiple doses to determine the effect of the nitrogen supply modes on colony formation. Compared with urea, nitrate significantly stimulated the growth of M. aeruginosa while ammonium inhibited growth. Among the three nitrogen forms, ammonium resulted in the highest concentrations of total dissolved nitrogen (TDN). Colonies larger than 10 μm were significantly promoted in the ammonium treatment. Cells were generally smaller in the nitrate treatment than in the ammonium and urea treatments. The extracellular polysaccharide (EPS) contents were lower in the nitrate and urea treatments than in the ammonium treatments. Within the same nitrogen form, there was little difference in growth and colony formation between the single-dose and multiple-dose treatments. Our results demonstrated that ammonium significantly promoted M. aeruginosa colony formation, and that the nitrogen supply mode did not affect colony formation in M. aeruginosa.     

Expression on roots and contribution to maize phytostimulation of 1-aminocyclopropane-1-decarboxylate deaminase gene <Emphasis Type="Italic">acdS</Emphasis> in <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> F113

Aims

The plant-beneficial bacterium Pseudomonas fluorescens F113 harbours an acdS gene, which enables deamination of 1-aminocyclopropane-1-carboxylate. The impact of abiotic and biotic factors on the expression of this gene was assessed, as well as the plant-beneficial properties of F113 under different soil moistures.

Methods

An acdS-egfp biosensor was constructed in F113, validated in vitro and used to analyse, by microscopy, its expression on roots of Zea mays comparatively to Beta vulgaris. An acdS mutant was constructed and compared with the wild-type to characterize plant-beneficial effects of F113 on maize lines EP1 and FV2, under well-watered and water deficit conditions.

Results

Different patterns of root colonization and acdS expression were observed according to plant genotype. acdS rhizoplane expression was higher on Beta vulgaris, and on maize line FV2 and hybrid PR37Y15 than on maize line EP1 and teosinte. Strain F113 but not its acdS mutant promoted root growth of EP1 under well-watered conditions and germination of FV2 under water deficit conditions.

Conclusions

Maize lines differed in their ability to induce acdS expression and to respond to P. fluorescens F113. The maize line leading to higher acdS expression, FV2, was the one benefiting from inoculation under water deficit.

     

Aquaporin gene expression and physiological responses of <Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L. to the mycorrhizal fungus <Emphasis Type="Italic">Rhizophagus irregularis</Emphasis> and drought stress

The influence of arbuscular mycorrhiza (AM) and drought stress on aquaporin (AQP) gene expression, water status, and photosynthesis was investigated in black locust (Robinia pseudoacacia L.). Seedlings were grown in potted soil inoculated without or with the AM fungus Rhizophagus irregularis, under well-watered and drought stress conditions. Six full-length AQP complementary DNAs (cDNAs) were isolated from Robinia pseudoacacia, named RpTIP1;1, RpTIP1;3, RpTIP2;1, RpPIP1;1, RpPIP1;3, and RpPIP2;1. A phylogenetic analysis of deduced amino acid sequences demonstrated that putative proteins coded by these RpAQP genes belong to the water channel protein family. Expression analysis revealed higher RpPIP expression in roots while RpTIP expression was higher in leaves, except for RpTIP1;3. AM symbiosis regulated host plant AQPs, and the expression of RpAQP genes in mycorrhizal plants depended on soil water condition and plant tissue. Positive effects were observed for plant physiological parameters in AM plants, which had higher dry mass and lower water saturation deficit and electrolyte leakage than non-AM plants. Rhizophagus irregularis inoculation also slightly increased leaf net photosynthetic rate and stomatal conductance under well-watered and drought stress conditions. These findings suggest that AM symbiosis can enhance the drought tolerance in Robinia pseudoacacia plants by regulating the expression of RpAQP genes, and by improving plant biomass, tissue water status, and leaf photosynthesis in host seedlings.