Decreased Electron Transport Rates Research Articles

Drought and heat stress interactions modify photorespiration and hydrogen peroxide content in Silver fir.

Photorespiration (PR) greatly reduces net carbon assimilation in trees (by c. 25%), but has received recent attention particular for its potential role in stress-signaling through the accumulation of hydrogen peroxide (H2O2), a stress signaling agent. Despite an increasing frequency of drought and heat events affecting forests worldwide, little is known about how concurrent abiotic stressors may interact to affect PR and subsequent H2O2 accumulation in trees. Here, we sought to identify how drought and a compounded one-day heat treatment individually and interactively affect PR (determined under variable O2) in Abies alba Mill. seedlings. Additionally, we quantified foliar H2O2 accumulation and enzymatic scavenging via peroxidase in relation to PR rates. We found drought stress to slightly increase PR (+5.2%) during mild-drought (12 days, Ψmd = -0.85 MPa), but ultimately to decrease PR (-13.6%) during severe-drought (26 days, Ψmd = -1.70 MPa) compared to the control, corresponding to increasing non-stomatal limitations of photosynthesis (i.e., decreased electron transport rate). The response of PR to heat stress was dependent on soil water availability as heat stress increased PR in control seedlings (+37.8%), but not in drought-stressed seedlings. Decreased PR during severe-drought corresponded to ~2x lower foliar H2O2 compared to the control. Despite increased PR under heat stress in control seedlings, foliar H2O2 decreased to near-zero likely due to enhanced scavenging as observed in ~2x greater peroxidase activity. Our results demonstrate that carbon loss to PR during drought stress can be highly dynamic, depending on the severity of soil dehydration. Additionally, increased PR under abiotic stress does not necessarily lead to accumulated H2O2, as tight regulation by scavenging enzymes instead minimize oxidative stress, reducing stress-signaling potential.

The Thermal Tolerances, Distributions, and Performances of Tropical Montane Tree Species

Due to global warming, many species will face greater risks of thermal stress, which can lead to changes to performance, abundance and/or geographic distributions. In plants, high temperatures above a species-specific critical thermal maximum will permanently damage photosystem II, leading to decreased electron transport rates, photosynthetic failure, and eventual leaf and plant death. Previous studies have shown that plant thermal tolerances vary with latitude, but little is known about how they change across smaller-scale thermal gradients (i.e., with elevation) or about how these thermal tolerances relate to species’ local performances and geographic distributions. In this study, we assess the maximum photosynthetic thermal tolerances (T50) of nearly 200 tropical tree species growing in 10 forest plots distributed across a >2500 meter elevation gradient (corresponding to a 17oC temperature gradient) in the northern Andes Mountains of Colombia. Using these data, we test the relationships between species’ thermal tolerances and 1) plot elevations and temperatures, 2) species’ large-scale geographic distributions, and 3) changes in species’ abundances through time within the plots. We found that species’ T50 do in fact decrease with plot elevation but significantly slower than the corresponding adiabatic lapse rate (-0.4 vs. -5.7oC km-1) and that there remains a large amount of unexplained variation in the thermal tolerances of co-occurring tree species. There was only a very weak association between species’ thermal tolerances and their large-scale geographic distributions and no significant relationships between species’ thermal tolerances and their changes in relative abundance through time. A potential explanation for these results is that thermal tolerances are adaptations to extreme leaf temperatures that can be decoupled from regional air temperatures due to microclimatic variations and differences in the species’ leaf thermoregulatory properties.

Response of Photosynthesis in Maize to Drought and Re-Watering

The mechanism by which photosynthetic adaptation occurs in maize (Zea mays L.) during both drought and the subsequent recovery after re-watering is currently unknown. To elucidate this mechanism, two maize cultivars (drought tolerant SD609 and drought sensitive SD902) were subjected to an eight-day drought, followed by re-watering for four days. The photosynthetic electron transport rates and related gene expression were measured in both cultivars. Compared to control plants, progressive drought stress significantly decreased electron transport rates at the donor and acceptor sides of PSI and PSII in SD902, and of PSII in SD609. Meanwhile, the expression of cab, psbP, psbA, psbD, petA, and petB genes involved in electron transport system were significantly down-regulated in both cultivars, particularly in the SD902. Moreover, while expression of psaA and psaB genes encoding PSI was down-regulated in SD902, there were no changes in SD609 during drought stress. After re-watering, measured fluorescence parameters and key photosynthesis-related gene expression levels returned to near control values in SD609, but not in SD902. This finding indicated that SD609 was characterized by reversible down-regulation of PSII, while in SD902, an impairment occurred in two photosystems, and such coordination between PSII and PSI contributed to drought tolerance in SD609. Overall, the higher drought tolerance and rapid recovery capability of SD609 were associated with more effective self-regulation of photochemical activities and photosynthesis-related gene expression, which appears to represent a critical adaptive mechanism to withstand and survive the rapidly changing climate.

Photosynthetic pigments and photochemical efficiency of precocious dwarf cashew (<i>Anacardium occidentale</i> L.) under salt stress and potassium fertilization

Cashew cultivation is an activity of great socioeconomic relevance, especially for agriculture in the semi-arid region of Northeast Brazil, standing out as an option to generate jobs and income. Thus, the present study aimed to evaluate the photosynthetic pigments and photochemical efficiency of precocious dwarf cashew under salt stress and potassium fertilization. The study was conducted using a Regolithic Neosol with sandy loam texture, adopting a randomized block design in 5×2 factorial arrangement, which corresponded to five levels of electrical conductivity of irrigation water—ECw (0.4; 1.2; 2.0; 2.8 and 3.6 dS m-1) associated with two doses of potassium fertilization—KD (100 and 150% of recommendation), with three replicates and two plants per plot. Increasing water salinity inhibited chlorophyll synthesis and decreased electron transport rate and quantum yield of non-regulated energy dissipation in precocious dwarf cashew, at 50 days after sowing. There was significant interaction for chlorophyll a and b contents and the effects of salt stress were intensified by potassium doses on the chlorophyll b content of precocious dwarf cashew. Potassium doses of 100 and 150% of recommendation (150 and 225 mg K2O kg-1 of soil) do not mitigate the effect of salt stress on photosynthetic pigment synthesis and photochemical efficiency of cashew plants in the rootstock formation phase.

Response of photosynthetic performance, water relations and osmotic adjustment to salinity acclimation in two wheat cultivars

Experiment was conducted to identify the impacts of the salinity acclimation process on the photosynthetic efficiency, osmotic adjustment, membrane integrity, and yield components in two wheat cultivars differing in their salinity tolerance. The design of the experiment was factorial randomized block, where genotype is factor 1 and acclimation treatments represent factor 2. Genotypes were grown from emergence to 30 days after sowing (DAS) by irrigating with tap water [electrical conductivity (EC) of 0.776 dS m−1]. Thereafter, both the genotypes were divided into two groups and exposed to either irrigation with sublethal level of salinity EC of 2.09 or 3.76 dS m−1 for 21 days. At booting stage (65 DAS), both groups were subjected to lethal level of salinity stress EC of 12 dS m−1 for 21 days, followed by irrigation with tap water till maturity. Non-acclimated plants were irrigated with tap water from emergence to 65 days, then directly irrigated with lethal level of salinity for 21 days, followed by irrigation with tap water till maturity. The control plants were continuously irrigated with tap water from emergence until maturity. The non-acclimated plants had decreased electron transport rates at the donor and acceptor side of PSII and PSI in Giza 168, and decreased electron transport rates at PSII acceptor side in Sakha 8 compared to control plants. In both genotypes, the non-acclimated plants had decreased chlorophyll a, b, carotenoid, proline and total soluble sugar concentration, relative water content, membrane stability index, yield and yield components compared with acclimated plants. While, osmotic potential and lipid peroxidation showed an opposite trend. Overall, acclimation treatment (EC of 2.09 dS m−1) during vegetative stage alleviated the inhibitory effects of lethal level of salinity stress at booting stage through enhanced photosynthetic efficiency and osmotic adjustment, resulting in increased membrane integrity, biomass production and grain yield than in non-acclimated plants.

Enhanced leaf photosynthesis as a target to increase grain yield: insights from transgenic rice lines with variable Rieske FeS protein content in the cytochrome b6 /f complex.

Although photosynthesis is the most important source for biomass and grain yield, a lack of correlation between photosynthesis and plant yield among different genotypes of various crop species has been frequently observed. Such observations contribute to the ongoing debate whether enhancing leaf photosynthesis can improve yield potential. Here, transgenic rice plants that contain variable amounts of the Rieske FeS protein in the cytochrome (cyt) b6 /f complex between 10 and 100% of wild-type levels have been used to investigate the effect of reductions of these proteins on photosynthesis, plant growth and yield. Reductions of the cyt b6 /f complex did not affect the electron transport rates through photosystem I but decreased electron transport rates through photosystem II, leading to concomitant decreases in CO2 assimilation rates. There was a strong control of plant growth and grain yield by the rate of leaf photosynthesis, leading to the conclusion that enhancing photosynthesis at the single-leaf level would be a useful target for improving crop productivity and yield both via conventional breeding and biotechnology. The data here also suggest that changing photosynthetic electron transport rates via manipulation of the cyt b6 /f complex could be a potential target for enhancing photosynthetic capacity in higher plants.

CO₂ and inorganic nutrient enrichment affect the performance of a calcifying green alga and its noncalcifying epiphyte.

Ocean acidification studies in the past decade have greatly improved our knowledge of how calcifying organisms respond to increased surface ocean CO2 levels. It has become evident that, for many organisms, nutrient availability is an important factor that influences their physiological responses and competitive interactions with other species. Therefore, we tested how simulated ocean acidification and eutrophication (nitrate and phosphate enrichment) interact to affect the physiology and ecology of a calcifying chlorophyte macroalga (Halimeda opuntia (L.) J.V. Lamouroux) and its common noncalcifying epiphyte (Dictyota sp.) in a 4-week fully crossed multifactorial experiment. Inorganic nutrient enrichment (+NP) had a strong influence on all responses measured with the exception of net calcification. Elevated CO2 alone significantly decreased electron transport rates of the photosynthetic apparatus and resulted in phosphorus limitation in both species, but had no effect on oxygen production or respiration. The combination of CO2 and +NP significantly increased electron transport rates in both species. While +NP alone stimulated H. opuntia growth rates, Dictyota growth was significantly stimulated by nutrient enrichment only at elevated CO2, which led to the highest biomass ratios of Dictyota to Halimeda. Our results suggest that inorganic nutrient enrichment alone stimulates several aspects of H. opuntia physiology, but nutrient enrichment at a CO2 concentration predicted for the end of the century benefits Dictyota sp. and hinders its calcifying basibiont H. opuntia.

Changes in carbon and nitrogen allocation, growth and grain yield induced by arbuscular mycorrhizal fungi in wheat (Triticum aestivum L.) subjected to a period of water deficit

Drought is a major abiotic factor limiting agricultural crop production. One of the effective ways to increase drought resistance in plants could be to optimize the exploitation of symbiosis with arbuscular mycorrhizal fungi (AMF). Hypothesizing that alleviation of water deficits by AMF in wheat will help maintain photosynthetic carbon-use, we studied the role of AMF on gas-exchange, light-use efficiencies, carbon/nitrogen ratios and growth and yield parameters in the contrasting wheat (Triticum aestivum L.) cultivars ‘Vinjett’ and ‘1110’ grown with/without AMF symbiosis. Water deficits applied at the floret initiation stage significantly decreased rates of photosynthetic carbon gain, transpiration and stomatal conductance in the two wheat cultivars. AMF increased the rates of photosynthesis, transpiration and stomatal conductance under drought conditions. Water deficits decreased electron transport rate and increased non-photochemical quenching (NPQ) in ‘1110’ but not in ‘Vinjett’. With AMF, nitrogen concentrations increased in roots of both cultivars, but decreased in grains of ‘Vinjett’ and in side-tiller grains of ‘1110’ regardless of water status. With water deficits, AMF colonization increased plant height in both cultivars. AMF also increased biomass and grain yield in ‘1110’ but not in ‘Vinjett’. The results showed that the improvements in growth and yield were the results of AMF-mediated increases in photosynthesis during drought stress and that the alleviating effect of AMF depended on the wheat cultivar.

华南地区3种具有不同入侵性的近缘植物对低温胁迫的敏感性

Chilling is an important constraint factor to the growing development and geographic distribution of plant species.Ipomoea cairica( L.) Sweet is one of the most notorious invasive weeds in South China,while it seems to be not invasive outside South China. Ipomoea nil( L.) Choisy( slightly invasive or non-invasive) and Ipomoea triloba L.( non-invasive)are two of the I. cairica's closely related species that are also originated from the tropical America. In the present study,we compared the chilling sensitivities of these three vine plants by testing their ecophysiological responses to low-temperature treatments( 15 ℃,10 ℃) in a hydroponic experiment,so as to illustrate the potential connections between the chilling sensitivity and invasiveness. We hypothesized that( i) the three alien plants were all sensitive to chilling stress and( ii)the chilling sensitivity of the non-invasive( or slightly invasive) plants I. triloba and I. nil were higher than that of the serious invasive species I. cairica.Cuttings of I. cairica,I. nil and I. triloba collected from selected habitats in South China were planted in water to let the root grow for one week,and then cultivated in Hoagland nutrient solution for 4 weeks. Similarly sized plants of each species were placed in the light incubators with different temperatures( 28 ℃,15 ℃,10 ℃) for 8 days. The chlorophyll fluorescence technique was used to monitor the photosynthetic apparatus of these Ipomoea plants every other day. After harvest the shoot biomass and root length of new growth for all plants were calculated. The accumulation of H 2 O 2, malodialdehyde( MDA),proline and soluble carbohydrate in the leaves,as well as the root metabolic activity and membrane integrity were also tested as indications of physiological performance after different temperature treatments. The chlorophyll fluorescence results showed that photosystem Ⅱ( PS Ⅱ) of all three Ipomoea species was partly damaged by chilling( 15 ℃,resulted in increased initial fluorescence level( F 0),but decreased electron transport rate( ETR) and maximum photochemical efficiency( F v / F m)),especially for I. triloba,which always was most sensitive. There was also higher non-photochemical quenching( NPQ) under chilling stress in all three species,indicating increased thermal dissipation in PS Ⅱ. Compared to the control( 28 ℃),all plants under chilling stress accumulated significantly more proline and soluble carbohydrate( P 0. 01),both of which could improve their chilling tolerance. There were significantly less shoot biomass and root length of new growth,and higher levels of cell membrane integrity in roots under the 15 ℃ and 10 ℃ treatment for all three species( P0. 05),and that should be partly attributed to the accumulation of H 2 O 2 and MDA,which were observed in all of the plants under chilling stress. These results clearly showed that at 15 ℃ there had been significant stress effects in these Ipomoea species. The first hypothesis of this research,i. e. the three species have high chilling sensitivities,was thus supported. As for the other aspect,when comparing the response intensities of the three Ipomoea species to chilling treatment,it is found that at the 10 ℃ treatment only I. cairica has continued to grow,while no growth was observed in I. nil and I. triloba during the periods of chilling stress,indicating that I. cairica has the highest chilling tolerances among the three species. The second hypothesis of this study was thus also supported. Besides,among the three Ipomoea species,I. triloba seems to be the most sensitive one to chilling as it usually had the highest physiological response intensities to chilling stress. The results from this study implied that the comparatively higher chilling tolerance of I. cairica in comparison to I. nil and I. triloba should be one of the reasons that in South China it is more invasive than the latter two species. The chilling sensitivity is an important key factor to the invasiveness of alien plants. Besides,the relatively high chilling sensitivity of I. cairica should be one of the most important reasons that it has not invaded other areas than South China. Therefore,it is necessary to pay more attention to the potential range expansion of I. cairica under the background of global warming.

Gas Exchange of Five Warm‐Season Grain Legumes and their Susceptibility to Heat Stress

AbstractObjective of this study was to compare the heat stress performance of four pulses from dry and hot areas (mungbeans, limabeans, and teparybeans and cowpeas) with that of soybeans. Two experiments were conducted in growth chambers, and data were pooled because results of both experiments were similar. Plants were raised up to flowering at 24/17 °C (day/night) and were then either exposed to these temperatures until maturity or stressed with 33/24 °C for 2 weeks starting at day 1 or 15 after onset of flowering (early vs. late stress). Before, during and after these stress intervals, gas exchange of representative upper leaves was examined; additionally, immediate effects of increasing leaf temperatures from 24 to 32 or 40 °C on chlorophyll fluorescence were assessed. Without heat stress rates of photosynthesis (Pn), and of transpiration (TR), stomatal and mesophyll conductance (gs, gm) and intrinsic transpiration efficiency (iTE) differed significantly among the five crops at each date. However, because of crop‐specific time‐courses ranking among unstressed crops was instable with time, so values were integrated or averaged over time. This procedure revealed high Pn potentials in mung‐ and teparybeans and high iTE values in limabeans compared to the other crops. Heat stress lowered Pn and gs considerably, but increased TR in all five crops. Relative lowering of Pn during heat stress displayed a crop‐specific pattern with limabeans being least susceptible to both early and late heat stress, while cowpeas were highly susceptible to early stress. Effects on Pn were mainly attributable to lowering of gs and only in part to gm. The latter was supported by very small changes (<10 %) of various chlorophyll fluorescence signals shortly after raising leaf temperature to 32 °C in all species. However, in limabeans, a decreased electron transport rate (e‐rate, −18 %) and an increased non‐photochemical quenching (QN, +16 %) pointed to an adaptive mechanism to avoid oxidative strains under heat. Leaf temperatures of 40 °C immediately provoked stronger changes in all fluorescence signals than 32 °C; substantial damages at 40 °C were indicated by effective quantum yield, photochemical quenching and ratio of fluorescence decrease in mungbeans and low ones in cowpeas and soybeans. Nevertheless, some adaptive responses of e‐rates and QN were observed in all crops and were most expressed in limabeans.

Photosynthetic responses to chromosome doubling in relation to leaf anatomy in Lonicera japonica subjected to water stress.

Gas exchange, chlorophyll fluorescence, and contents of some metabolites in two Japanese honeysuckle (Lonicera japonica Thunb.) cultivars, Damaohua (2n = 2x) and Jiufengyihao (2n = 4x), were compared with explore the function of chromosome doubling under water stress conditions. Water stress significantly decreased net photosynthesis rate, stomatal conductance, and transpiration rate of both cultivars. It also decreased electron transport rate, effective quantum yield of Photosystem II, photochemical quenching, and starch content, but increased non-photochemical quenching and contents of total soluble sugars, proline, and malondialdehyde. However, the tetraploid cultivar showed higher resistance to water stress than the diploid, as indicated by the fact that gas exchange, chlorophyll fluorescence, and metabolites were less affected for the tetraploid than the diploid. Moreover, the tetraploid recovered more quickly than the diploid after re-watering. Morphological and anatomical analysis further revealed that the tetraploid possessed less whole plant leaf area, higher leaf mass per unit area, thicker epidermis (both upper and lower) and palisade tissue, as well as denser pubescence. All of those specialised structures caused by chromosome doubling might lead to greater capacity in coping with drought stress. Our findings suggest that the effect of chromosome doubling on drought resistance in L. japonica could attribute to the improvement of structure and photosynthesis-related traits.

Minor Antenna Proteins CP24 and CP26 Affect the Interactions between Photosystem II Subunits and the Electron Transport Rate in Grana Membranes of Arabidopsis

We investigated the function of chlorophyll a/b binding antenna proteins Chlorophyll Protein 26 (CP26) and CP24 in light harvesting and regulation of photosynthesis by isolating Arabidopsis thaliana knockout lines that completely lacked one or both of these proteins. All three mutant lines had a decreased efficiency of energy transfer from trimeric light-harvesting complex II (LHCII) to the reaction center of photosystem II (PSII) due to the physical disconnection of LHCII from PSII and formation of PSII reaction center depleted domains in grana partitions. Photosynthesis was affected in plants lacking CP24 but not in plants lacking CP26: the former mutant had decreased electron transport rates, a lower DeltapH gradient across the grana membranes, reduced capacity for nonphotochemical quenching, and limited growth. Furthermore, the PSII particles of these plants were organized in unusual two-dimensional arrays in the grana membranes. Surprisingly, overall electron transport, nonphotochemical quenching, and growth of the double mutant were restored to wild type. Fluorescence induction kinetics and electron transport measurements at selected steps of the photosynthetic chain suggested that limitation in electron transport was due to restricted electron transport between Q(A) and Q(B), which retards plastoquinone diffusion. We conclude that CP24 absence alters PSII organization and consequently limits plastoquinone diffusion.

Virus-like particles associated with Lyngbya majuscula (Cyanophyta; Oscillatoriacea) bloom decline in Moreton Bay, Australia

Expansive blooms of the toxic cyanobacterium Lyngbya majuscula were observed in 2 shallow water regions of Moreton Bay, Australia. The rapid bloom decline (8 to <1 km 2 in <7 d) prompted an investigation of the role of cyanophage viruses in the ecophysiology of L. majuscula. Virus-like particles produced by decaying L. majuscula were observed using electron microscopy. The virus-like particles were similar in morphology to viruses in the genus Cyanostyloviridae. The effect of viruses on L. majuscula photosynthesis was investigated by: (1) creating a virus concentrate using tangential-flow ultrafiltration of seawater surrounding L. majuscula; (2) inoculating L. majus- cula with the concentrate; and (3) measuring photosynthetic response using a pulse-amplitude mod- ulated fluorometer. Virus concentrate addition resulted in decreased initial fluorescence, decreased photochemical efficiency and decreased electron transport rate in rapid light curves after 5 d. Viruses present within L. majuscula filaments may play an important role in the bloom dynamics of this eco- logically important cyanobacterium.