High growth temperatures and high soil nitrogen do not alter differences in CO2 assimilation between invasive Phalaris arundinacea (reed canarygrass) and Carex stricta (tussock sedge).

PREMISE OF THE STUDY: Global change in temperature and soil nitrogen availability could affect plant community composition, potentially giving an advantage to invasive species compared to native species. We addressed how high temperatures affected CO2 assimilation parameters for invasive Phalaris arundinacea and a sedge, Carex stricta, it displaces, in natural and controlled environments. METHODS: Photosynthetic parameters were measured in a wetland in Indiana, USA during the abnormally warm year of 2012. In a growth chamber, photosynthetic parameters were measured on the plants grown under three levels of nitrogen and exposed to optimum temperatures followed by 2012-like summer conditions and then hot temperatures with an autumn-like photoperiod. KEY RESULTS: In the wetland, C. stricta exhibited signs of midsummer leaf senescence, whereas P. arundinacea maintained CO2 assimilation at ambient pCO2 (Aamb ) through mid-October. In the chamber, 2012-like conditions reduced Aamb for both species through reductions in maximum carboxylation (Vcmax ) and electron transport (Jmax ) without further change during subsequent hot, autumn-like conditions, whereas the quantum efficiency of carbon assimilation (qe) declined throughout the experiment. However, P. arundinacea had higher values of Aamb , Jmax , and qe than C. stricta. A general, the positive effect of increasing nitrogen availability occurred for photosynthetic processes for both species in hot conditions. CONCLUSIONS: Our data suggest that C. stricta is more susceptible to excessive light stress than P. arundinacea during hot, sunny periods, leading to leaf senescence. Field confirmation of this idea is needed, but frequent heat waves should favor P. arundinacea over C. stricta with or without eutrophication.

Plasticity of nitrogen allocation in the leaves of the invasive wetland grass, Phalaris arundinacea and co-occurring Carex species determines the photosynthetic sensitivity to nitrogen availability.

Phalaris arundinacea displaces the slower-growing, native sedge, Carex stricta, where nitrogen availability is high. Our aim was to address whether morphological and physiological traits associated with carbon gain for P. arundinacea and C. stricta responded to nitrogen supply differently and if the species exhibited different degrees of plasticity in these traits. The plants were grown in gravel and provided modified Hoagland's solution containing four nitrogen concentrations from 0.15 to 15 mM for 6 to 7 weeks. Supplied nitrogen affected the leaf nitrogen content to the same degree for both species. Increasing supplied nitrogen strongly increased CO2 assimilation (A), photosynthetic nitrogen use efficiency (PNUE), and respiration for P. arundinacea but had only a small effect on these parameters for C. stricta. Relative to growth at 15 mM nitrogen, growth at 0.15 mM for young leaves decreased carboxylation capacity and efficiency and the capacity for electron transport for P. arundinacea and a larger, stouter Carex species, Carex lacustris, by 53 to 70% but only 20 to 24% for C. stricta. Leaf nitrogen decreased approximately 50% for all species, but vacuolar nitrate did not decrease for P. arundinacea and C. stricta, suggesting that it does not serve as a nitrogen reserve for use during nitrogen deprivation in these species. After 4 months of nitrogen deprivation, P. arundinacea doubled A in 12 days after being supplied 15 mM nitrogen, whereas A for C. stricta increased only 22%. We propose that one factor linking P. arundinacea abundance to nitrogen availability involves this species' plastic response of carbon gain to nitrogen supply. C. stricta appears to be adapted to tolerate low nitrogen availability but cannot respond as rapidly and extensively as P. arundinacea when nitrogen supply is high.

Constitutively overexpressing a tomato fructokinase gene (LeFRK1) in cotton (Gossypium hirsutum L. cv. Coker 312) positively affects plant vegetative growth, boll number and seed cotton yield.

Increasing fructokinase (FRK) activity in cotton (Gossypium hirsutum L.) plants may reduce fructose inhibition of sucrose synthase (Sus) and lead to improved fibre yield and quality. Cotton was transformed with a tomato (Solanum lycopersicum L.) fructokinase gene (LeFRK1) under the control of the CMV 35S promoter. In a greenhouse, the LeFRK1 plants had increased fibre and leaf FRK activity over nonexpressing nulls, but not improved fibre length and strength. Compared with the nulls, LeFRK1 plants yielded 13-100% more seed-cotton mass per boll and more bolls per plant, and therefore more seed cotton and fibre yield per plant. The enhanced yield was related to a greater seed number per boll for LeFRK1 plants. Photosynthetic rates were not appreciably different among genotypes. However, more area per leaf and leaf number (in some instances) for LeFRK1 plants than for nulls enhanced the capacity for C gain. Larger leaf areas for LeFRK1 plants were associated with larger stem diameters. Lower sucrose levels in developing leaves of LeFRK1 plants suggest that LeFRK1 overexpression leads to improved in vivo Sus activity in developing leaves and possibly in developing seeds. The improvement in yield for LeFRK1 plants may also be the result of improvements in photosynthate supply as a consequence of greater leaf area.

Expression of AtSAP5 in cotton up-regulates putative stress-responsive genes and improves the tolerance to rapidly developing water deficit and moderate heat stress.

The regulation of gene expression is a key factor in plant acclimation to stress, and it is thought that manipulation of the expression of critical stress-responsive genes should ultimately provide increased protection against abiotic stress. The aim of this study was to test the hypothesis that the ectopic expression of the AtSAP5 (AT3G12630) gene in transgenic cotton (Gossypium hirsutum, cv. Coker 312) will improve tolerance to drought and heat stress by up-regulating the expression of endogenous stress-responsive genes. The SAP5 gene is a member of the stress-associated family of genes that encode proteins containing A20/AN1 zinc finger domains. Under non-stressful conditions, cotton plants that expressed the AtSAP5 gene showed elevated expression of at least four genes normally induced during water deficit or heat stress. The rate of net CO(2) assimilation A for three of four transgenic lines tested was less sensitive to rapidly developing water deficit over 4d than untransformed wild-type plants, but the recovery of A following drought was not significantly affected. The enhanced protection of photosynthesis during drought was determined to be primarily at the biochemical level, since the extent of stomatal closure was not significantly different for all genotypes. Expression of AtSAP5 resulted in the complete protection of photosystem (PS) II complexes from photodamage at mid-day after 4 d of drought, whereas wild-type plants experienced a 20% decline in active photosystem II (PSII) complexes. In addition, enhanced protection of seedling growth and leaf viability was associated with the expression of AtSAP5. Since A for the transgenic plants was significantly more heat tolerant than A for wild-type plants, we conclude that ectopic expression of SAP genes is a potentially viable approach to improving carbon gain and productivity for cotton grown in semi-arid regions with severe drought and heat stress.

Greater seasonal carbon gain across a broad temperature range contributes to the invasive potential of Phalaris arundinacea (Poaceae; reed canary grass) over the native sedge Carex stricta (Cyperaceae).

PREMISE OF THE STUDY: Most invasive plants grow faster and produce more biomass than the species that they displace, but physiological mechanisms leading to invasive success are poorly understood. To foster novel control approaches, our goal was to determine whether the grass Phalaris arundinacea possessed superior physiological strategies that contributed to its success over native sedges. METHODS: Data for spring, summer, and autumn diel gas-exchange, leaf morphology, and nitrogen content for plants of P. arundinacea and Carex stricta in water-saturated, drained, and periodically flooded sites in northern Indiana, USA, were compared with similar data for plants in a greenhouse. KEY RESULTS: Phalaris arundinacea had higher maximum CO(2) assimilation (A) across a broad range of temperatures, greater summer/autumn net carbon gain, higher water use efficiencies, larger leaf areas per shoot, and higher specific leaf areas than did C. stricta. Species differences in gas-exchange data were similar in the greenhouse. However, long-term flooding reduced A for P. arundinacea. Greater declines in leaf A and nitrogen content from July to October compared to P. arundinacea were suggestive of earlier leaf senescence for C. stricta. CONCLUSIONS: We propose that superior daily and seasonal carbon gain, especially during rhizome carbohydrate storage in the summer and autumn, contribute to the success of invasive P. arundinacea over C. stricta. This advantage may be enhanced by frequent summer/autumn heat waves. The poor performance of P. arundinacea during long-term flooding is consistent with C. stricta's dominance in water-saturated soil, implying that water management strategies could be crucial to controlling P. arundinacea.

The differential response of photosynthesis to high temperature for a boreal and temperate Populus species relates to differences in Rubisco activation and Rubisco activase properties.

Significant inhibition of photosynthesis occurs at temperatures only a few degrees (

Transgenic cotton over-producing spinach sucrose phosphate synthase showed enhanced leaf sucrose synthesis and improved fiber quality under controlled environmental conditions.

Prior data indicated that enhanced availability of sucrose, a major product of photosynthesis in source leaves and the carbon source for secondary wall cellulose synthesis in fiber sinks, might improve fiber quality under abiotic stress conditions. To test this hypothesis, a family of transgenic cotton plants (Gossypium hirsutum cv. Coker 312 elite) was produced that over-expressed spinach sucrose-phosphate synthase (SPS) because of its role in regulation of sucrose synthesis in photosynthetic and heterotrophic tissues. A family of 12 independent transgenic lines was characterized in terms of foreign gene insertion, expression of spinach SPS, production of spinach SPS protein, and development of enhanced extractable V (max) SPS activity in leaf and fiber. Lines with the highest V (max) SPS activity were further characterized in terms of carbon partitioning and fiber quality compared to wild-type and transgenic null controls. Leaves of transgenic SPS over-expressing lines showed higher sucrose:starch ratio and partitioning of (14)C to sucrose in preference to starch. In two growth chamber experiments with cool nights, ambient CO(2) concentration, and limited light below the canopy, the transgenic line with the highest SPS activity in leaf and fiber had higher fiber micronaire and maturity ratio associated with greater thickness of the cellulosic secondary wall.

The Arabidopsis ascorbate peroxidase 3 is a peroxisomal membrane-bound antioxidant enzyme and is dispensable for Arabidopsis growth and development.

Ascorbate peroxidase (APX) exists as several isoforms that are found in various compartments in plant cells. The cytosolic and chloroplast APXs appear to play important roles in antioxidation metabolism in plant cells, yet the function of peroxisomal APX is not well studied. In this study, the localization of a putative peroxisomal membrane-bound ascorbate peroxidase, APX3 from Arabidopsis, was confirmed by studying the green fluorescent protein (GFP)-APX3 fusion protein in transgenic plants. GFP-APX3 was found to co-localize with a reporter protein that was targeted to peroxisomes by the peroxisomal targeting signal 1. The function of APX3 in Arabidopsis was investigated by analysing an APX3 knockout mutant under normal and several stress conditions. It was found that loss of function in APX3 does not affect Arabidopsis growth and development, suggesting that APX3 may not be an important antioxidant enzyme in Arabidopsis, at least under the conditions that were tested, or the function of APX3 could be compensated by other antioxidant enzymes in plant cells.

The role of antioxidant enzymes in photoprotection.

The enzymatic component of the antioxidant system is discussed as one of the defensive mechanisms providing protection against excessive light absorption in plants. We present an analysis of attempts to improve stress tolerance by means of the creation of transgenic plants with elevated antioxidant enzyme activities and conclude that the effect of such transgenic manipulation strongly depends on the manner in which the stress is imposed. The following factors may diminish the differences in photosynthetic performance between transgenic plants and wild type under field conditions: effective functioning of the thermal dissipation mechanisms providing a primary line of defense against excessive light, long-term adjustments of the antioxidant system and other photoprotective mechanisms, the relatively low level of control over electron transport exerted by the Water-Water cycle, especially under warm conditions, and a decrease in the content of the transgenic product during leaf aging.

Compensation for PSII photoinactivation by regulated non-photochemical dissipation influences the impact of photoinactivation on electron transport and CO2 assimilation.

The extent to which PSII photoinactivation affects electron transport (PhiPSII) and CO2 assimilation remains controversial, in part because it frequently occurs alongside inactivation of other components of photosynthesis, such as PSI. By manipulating conditions (darkness versus low light) after a high light/low temperature treatment, we examined the influence of different levels of PSII inactivation at the same level of PSI inactivation on PhiPSII and CO2 assimilation for Arabidopsis. Furthermore, we compared PhiPSII at high light and optimum temperature for wild-type Arabidopsis and a mutant (npq4-1) with impaired capacities for energy dissipation. Levels of PSII inactivation typical of natural conditions (< 50%) were not associated with decreases in PhiPSII and CO2 assimilation at photon flux densities (PFDs) above 150 micromol m(-2) s(-1). At higher PFDs, the light energy being absorbed was in excess of the energy that could be utilized by downstream processes. Arabidopsis plants downregulate PSII activity to dissipate such excess in accordance with the level of PSII photoinactivation that also serves to dissipate absorbed energy. Therefore, the overall levels of non-photochemical dissipation and the efficiency of photochemistry were not affected by PSII inactivation at high PFD. Under low PFD conditions, such compensation is not necessary, because the amount of light energy absorbed is not in excess of that needed for photochemistry, and inactive PSII complexes are dissipating energy. We conclude that moderate photoinactivation of PSII complexes will only affect plant performance when periods of high PFD are followed by periods of low PFD.

Expression of an Arabidopsis vacuolar sodium/proton antiporter gene in cotton improves photosynthetic performance under salt conditions and increases fiber yield in the field.

Drought and salinity are two major limiting factors in crop productivity. One way to reduce crop loss caused by drought and salinity is to increase the solute concentration in the vacuoles of plant cells. The accumulation of sodium ions inside the vacuoles provides a 2-fold advantage: (i) reducing the toxic levels of sodium in cytosol; and (ii) increasing the vacuolar osmotic potential with the concomitant generation of a more negative water potential that favors water uptake by the cell and better tissue water retention under high soil salinity. The success of this approach was demonstrated in several plants, where the overexpression of the Arabidopsis gene AtNHX1 that encodes a vacuolar sodium/proton antiporter resulted in higher plant salt tolerance. Overexpression of AtNHX1 increases sodium uptake in vacuoles, which leads to increased vacuolar solute concentration and therefore higher salt tolerance in transgenic plants. In an effort to engineer cotton for higher drought and salt tolerance, we created transgenic cotton plants expressing AtNHX1. These AtNHX1-expressing cotton plants generated more biomass and produced more fibers when grown in the presence of 200 mM NaCl in greenhouse conditions. The increased fiber yield was probably due to better photosynthetic performance and higher nitrogen assimilation rates observed in the AtNHX1-expressing cotton plants as compared with wild-type cotton plants under saline conditions. Furthermore, the field-grown AtNHX1-expressing cotton plants produced more fibers with better quality, indicating that AtNHX1 can indeed be used for improving salt stress tolerance in cotton.

Long-term night chilling of cotton (Gossypium hirsutum) does not result in reduced CO2 assimilation.

The aim of this study was to characterise the response of CO2 assimilation (A) of cotton (Gossypium hirsutum L.) to short- and long-term exposures to night chilling. We hypothesised that short-term exposures to night chilling would induce reductions in gs and, therefore, A during the following days, while growth of cotton plants for several weeks in cool night conditions would cause elevated leaf carbohydrate content, leading to the down-regulation of the capacity for A. Transferring warm-grown seedlings of wild type cotton, transgenic cotton with elevated sucrose-phosphate synthase activity (SPS+) that might produce and export more sucrose from the leaf, and a segregating null to cool nights (9°C minimum) for 1 or 2 d caused a small reduction in A (12%) and gs (21-50%) measured at 28°C. Internal CO2 did not change, suggesting some biochemical restriction of A along with a gs restriction. After 30 d, new leaves that developed in cool nights exhibited acclimation of A and partial acclimation of gs. Despite the elevated leaf carbohydrate content when plants were grown to maturity with night chilling, no reduction in A, gs, carboxylation capacity, electron transport capacity, or triose-phosphate utilisation capacity occurred. Instead, growth in cool nights tended to retard the diminishing of photosynthetic parameters and gs for aging stem and subtending leaves. However, elevated SPS activity did not affect any photosynthetic parameters. Therefore, when cotton that is well fertilised with nitrogen is grown with continuous night chilling, photosynthesis should not be negatively affected. However, an occasional exposure to cool nights could result in a small reduction in A and gs for leaves that have developed in warm night conditions.

Transgenic overproduction of glutathione reductase does not protect cotton, Gossypium hirsutum (Malvaceae), from photoinhibition during growth under chilling conditions.

In some studies, tissues from plants that have been genetically transformed to overproduce antioxidant enzymes sustain less damage when abruptly exposed to short-term chilling in the laboratory. However, few studies have examined the performance of transgenic plants during longer-term growth under chilling conditions. We compared growth of transgenic cotton that overproduces glutathione reductase (GR+; ∼40-fold overproduction) to growth of the wild type in a controlled environment chamber as leaf temperature was lowered from 28° to 14°C over 9 d and for a subsequent 9-d period at 14°C. In wild-type and GR+ cotton, chilling temperatures resulted in decreased dark-adapted F(v)/F(m) (the ratio of variable to maximal fluorescence; a measure of maximum photosystem II quantum yield) and mid-light period photosystem II quantum yield, coupled with increased 1 - q(P) (a nonlinear estimate of the reduction state of the primary quinone acceptor of photosystem II). The capacity for photosynthetic oxygen evolution decreased during the first portion of the chilling exposure, but recovered slightly during the second half. At no point during the chilling exposure did the performance of GR+ plants differ significantly from that of wild-type plants in any of the above parameters. The absence of an effect of GR overproduction under longer-term chilling may be explained, in part, by the fact that wild-type cotton acclimated to chilling by upregulating native GR activity.

Elevated chloroplastic glutathione reductase activities decrease chilling-induced photoinhibition by increasing rates of photochemistry, but not thermal energy dissipation, in transgenic cotton.

The effect of the overproduction of glutathione reductase (GR+) in cotton (Gossypium hirsutum L. cv.Coker 312) chloroplasts on the response of photosynthetic parameters to chilling in the light was examined. After 180 min at 10°C and 500 µmol photons m-2 s-1 in the chamber of an oxygen electrode, leaf discs of GR+ plants exhibited lower levels of sustained PSII photoinhibition than leaf discs of wild-type plants. No genotypic differences in thermal energy dissipation, leaf pigment composition, or the dynamics of xanthophyll cycle de-epoxidation were observed. The rate of induction and steady-state levels of photochemistry were greater for GR+ in comparison to wild-type plants. Enhanced photochemistry in GR+ plants could not be attributed to higher rates of CO2 assimilation at 10°C. Although GR overproduction afforded some increased protection against PSI photoinactivation, suggesting improved scavenging of reactive oxygen species, higher PSI activities could not completely explain the greater rates of photochemistry. Pools of glutathione and ascorbate were significantly more reduced in GR+ plants. Increased demand for reducing power to maintain these constituents in the reduced state may contribute to the higher rates of photochemistry observed in GR+ plants.

Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes.

The aim of this study was to determine whether increases in stromal superoxide dismutase (SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) via transformation could reduce photosystem (PS) II photoinhibition at low temperature for cotton (Gossypium hirsutum L.) plants and to determine by what mechanism this protection may be realized. During 3-h exposures of lincomycin-treated leaf discs to 10 degrees C and a photon flux density of 500 &mgr;mol m-2 s-1, all transgenic plants exhibited significantly greater PSII activity and O2 evolution than did wild-type plants. Also, the rate constant of PSII photoinactivation was significantly lower for all transgenic plants than for wild-type plants. No significant differences existed between genotypes in non-photochemical quenching of chlorophyll a fluorescence and the regulated component of the thermal dissipation of excitation energy. The relationship between changes in variable to maximum chlorophyll fluorescence (Fv/Fm) and the time-dependent averaged excessive light exposure was similar for all genotypes. This observation excluded the possibility that differences in PSII photodamage were due to improvements in the direct protection of PSII from active oxygen by antioxidant enzyme overproduction. Similar decreases in Fv/Fm during the stress treatment for all genotypes when leaves were pre-treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the effect of overproduction involved events downstream of PSII in the electron transfer pathway. Since all transgenic plants exhibited a significantly higher photochemical quenching of chlorophyll fluorescence during the chilling treatment, we concluded that, under the conditions used in this study, the enhancement of the protection of PSII from photodamage by increasing the stromal antioxidant enzyme activity in cotton leaves was due to the maintenance of a higher rate of electron transport and, consequently, a lower reduction state of QA.