Photosynthetic Recovery Research Articles

Rewatering after drought: Unravelling the drought thresholds and function recovery-limiting factors in maize leaves.

Drought and subsequent rewatering are common in agriculture, where recovery from mild droughts is easier than from severe ones. The specific drought threshold and factors limiting recovery are under-researched. This study subjected maize plants to varying drought degrees before rewatering, and measuring plant water status, gas exchange, hydraulic conductance, hormone levels, and cellular damage throughout. We discovered that stomatal reopening in plants was inhibited with leaf water potentials below about -1.7 MPa, hindering postdrought photosynthetic recovery. Neither hydraulic loss nor abscisic acid (ABA) content was the factor inhibited stomatal reopening on the second day following moderate drought stress and rewatering. But stomatal reopening was significantly correlated to the interaction between hydraulic signals and ABA content under severe drought. Extended drought led to leaf death at about -2.8 MPa or 57% relative water content, influenced by reduced rehydration capacity, not hydraulic failure. The lethal threshold remained relatively constant across leaf stages, but the recoverable safety margin (RSM), that is, the water potential difference between stomatal closure and recovery capacity loss, significantly decreased with leaf aging due to delayed stomatal closure during drought. Our findings indicate hydraulic failure alone does not cause maize leaf death, highlighting the importance of RSM in future research.

Red-light-dependent chlorophyll synthesis kindles photosynthetic recovery of chlorotic dormant cyanobacteria using a dark-operative enzyme

Chlorosis dormancy resulting from nitrogen starvation and its resuscitation upon available nitrogen contributes greatly to the fitness of cyanobacterial population under nitrogen-fluctuating environments. The reinstallation of the photosynthetic machinery is a key process for resuscitation from a chlorotic dormant state; however, the underlying regulatory mechanism is still elusive. Here, we reported that red light is essential for re-greening chlorotic Synechocystis sp. PCC 6803 (a non-diazotrophic cyanobacterium) after nitrogen supplement under weak light conditions. The expression of dark-operative protochlorophyllide reductase (DPOR) governed by the transcriptional factor RpaB was strikingly induced by red light in chlorotic cells, and its deficient mutant lost the capability of resuscitation from a dormant state, indicating DPOR catalyzing chlorophyll synthesis is a key step in the photosynthetic recovery of dormant cyanobacteria. Although light-dependent protochlorophyllide reductase is widely considered as a master switch in photomorphogenesis, this study unravels the primitive DPOR as a spark to activate the photosynthetic recovery of chlorotic dormant cyanobacteria. These findings provide new insight into the biological significance of DPOR in cyanobacteria and even some plants thriving in extreme environments.

Ocean deoxygenation dampens resistance of diatoms to ocean acidification in darkness

Respiratory activity in the oceans is declining due to the expansion of hypoxic zones and progressive deoxygenation, posing threats to marine organisms along with impacts of concurrent ocean acidification. Therefore, understanding the combined impacts of reduced pO2 and elevated pCO2 on marine primary producers is of considerable significance. Here, to simulate diatoms’ sinking into the aphotic zone of turbid coastal water, we exposed the diatoms Thalassiosira pseudonana and Thalassiosira weissflogii in darkness at 20°C to different levels of pO2 and pCO2 conditions for ~3 weeks, and monitored their biomass density, photosynthetic activity and dark respiration, and examined their recovery upon subsequent exposure to light at 20°C, simulating surface water conditions. Along with decreased cell abundance and size, measured photosynthetic capacity and dark respiration rates in these two diatoms both gradually decreased during the prolonged darkness. Reduced pO2 alone did not negatively affect the photosynthetic machinery in both the dark-survived diatom, and enhanced their subsequent recovery upon light exposure. Nevertheless, the combination of the elevated pCO2 and reduced pO2 (equivalent to hypoxia) led to the biomass loss by about 90% in T. pseudonana, and delayed the recovery of both species upon subsequent exposure to light, though it did not reduce the cell concentration of T. weissflogii during the elongated darkness. Our results suggest that reduced O2 availability diminishes the abilities of the diatoms to cope with the acidic stress associated with ocean acidification, and the expansion of hypoxic waters could delay the photosynthetic recovery of coastal diatoms when they are transported upwards through mixing from dark layers to sunlit waters.

Reactive oxygen species mediated extracellular polymeric substances production assisting the recovery of Thalassiosira pseudonana from polystyrene micro and nanoplastics exposure

As emerging pollutants in the aquatic environments, micro- and nano-plastics (MNPs) aroused widespread environmental concerns for their potential threats to the ecological health. Previous research has proved that microalgae growth could recover from the MNPs toxicities, in which the extracellular polymeric substances (EPS) might play the key role. In order to comprehensively investigate the recovery process of microalgae from MNPs stress and the effecting mechanisms of EPS therein, this study conducted a series of experiments by employing two sizes (0.1 and 1 μm) of polystyrene (PS) MNPs and the marine model diatom Thalassiosira pseudonana during 14 days. The results indicated: the pigments accumulations and photosynthetic recovery of T. pseudonana under MPs exposure showed in the early stage (4–5 days), while the elevation of reactive oxygen species (ROS) and EPS contents lasted longer time period (7–8 days). EPS was aggregated with MNPs particles and microalgal cells, corresponding to the increased settlement rates. More increase of soluble (SL)-EPS contents was found than bound (B)-EPS under MNPs exposure, in which the increase of the protein proportion and humic acid-like substances in SL-EPS was found, thus facilitating aggregates formation. ROS was the signaling molecule mediating the overproduction of EPS. The transcriptional results further proved the enhanced EPS biosynthesis on the molecular level. Therefore, this study elucidated the recovery pattern of microalgae from MNPs stress and linked “ROS-EPS production changes-aggregation formation” together during the growth recovery process, with important scientific and environmental significance.

Variation in xylem vulnerability to cavitation shapes the photosynthetic legacy of drought.

Increased drought conditions impact tree health, negatively disrupting plant water transport which, in turn, affects plant growth and survival. Persistent drought legacy effects have been documented in many diverse ecosystems, yet we still lack a mechanistic understanding of the physiological processes limiting tree recovery after drought. Tackling this question, we exposed saplings of a common Australian evergreen tree (Eucalyptus viminalis) to a cycle of drought and rewatering, seeking evidence for a link between the spread of xylem cavitation within the crown and the degree of photosynthetic recovery postdrought. Individual leaves experiencing >35% vein cavitation quickly died but this did not translate to a rapid overall canopy damage. Rather, whole canopies showed a gradual decline in mean postdrought gas exchange rates as water stress increased. This gradual loss of canopy function postdrought was due to a significant variation in cavitation vulnerability of leaves within canopies leading to diversity in the capacity of leaves within a single crown to recover function after drought. These results from the evergreen E. viminalis emphasise the importance of within-crown variation in xylem vulnerability as a central character regulating the dynamics of canopy death and the severity of drought legacy through time.

The magnitude and pace of photosynthetic recovery after wildfire in California ecosystems

Wildfire modifies the short- and long-term exchange of carbon between terrestrial ecosystems and the atmosphere, with impacts on ecosystem services such as carbon uptake. Dry western US forests historically experienced low-intensity, frequent fires, with patches across the landscape occupying different points in the fire-recovery trajectory. Contemporary perturbations, such as recent severe fires in California, could shift the historic stand-age distribution and impact the legacy of carbon uptake on the landscape. Here, we combine flux measurements of gross primary production (GPP) and chronosequence analysis using satellite remote sensing to investigate how the last century of fires in California impacted the dynamics of ecosystem carbon uptake on the fire-affected landscape. A GPP recovery trajectory curve of more than five thousand fires in forest ecosystems since 1919 indicated that fire reduced GPP by [Formula: see text] g C m[Formula: see text] y[Formula: see text]([Formula: see text]) in the first year after fire, with average recovery to prefire conditions after [Formula: see text] y. The largest fires in forested ecosystems reduced GPP by [Formula: see text] g C m[Formula: see text] y[Formula: see text] (n = 401) and took more than two decades to recover. Recent increases in fire severity and recovery time have led to nearly [Formula: see text] MMT CO[Formula: see text] (3-y rolling mean) in cumulative forgone carbon uptake due to the legacy of fires on the landscape, complicating the challenge of maintaining California's natural and working lands as a net carbon sink. Understanding these changes is paramount to weighing the costs and benefits associated with fuels management and ecosystem management for climate change mitigation.

Mitochondrial alternative NADH dehydrogenases NDA1 and NDA2 promote survival of reoxygenation stress in Arabidopsis by safeguarding photosynthesis and limiting ROS generation.

Plant submergence stress is a growing problem for global agriculture. During desubmergence, rising O2 concentrations meet a highly reduced mitochondrial electron transport chain (mETC) in the cells. This combination favors the generation of reactive oxygen species (ROS) by the mitochondria, which at excess can cause damage. The cellular mechanisms underpinning the management of reoxygenation stress are not fully understood. We investigated the role of alternative NADH dehydrogenases (NDs), as components of the alternative mETC in Arabidopsis, in anoxia-reoxygenation stress management. Simultaneous loss of the matrix-facing NDs, NDA1 and NDA2, decreased seedling survival after reoxygenation, while overexpression increased survival. The absence of NDAs led to reduced maximum potential quantum efficiency of photosystem II linking the alternative mETC to photosynthetic function in the chloroplast. NDA1 and NDA2 were induced upon reoxygenation, and transcriptional activation of NDA1 was controlled by the transcription factors ANAC016 and ANAC017 that bind to the mitochondrial dysfunction motif (MDM) in the NDA1 promoter. The absence of NDA1 and NDA2 did not alter recovery of cytosolic ATP levels and NADH : NAD+ ratio at reoxygenation. Rather, the absence of NDAs led to elevated ROS production, while their overexpression limited ROS. Our observations indicate that the control of ROS formation by the alternative mETC is important for photosynthetic recovery and for seedling survival of anoxia-reoxygenation stress.

Physiological response and photosynthetic recovery to an extreme drought: Evidence from plants in a dry-hot valley savanna of Southwest China

The frequency of extreme drought events has been rising worldwide, but due to its unpredictability, how plants will respond remains poorly understood. Here, we aimed to characterize how the hydraulics and photosynthesis of savanna plants respond to extreme drought, and tested whether they can subsequently recover photosynthesis after drought. There was an extreme drought in 2019 in Southwest (SW) China. We investigated photosynthetic gas exchange, leaf-, stem-, and whole-shoot hydraulic conductance of 18 plant species with diverse leaf habits (deciduous, semi-deciduous and evergreen) and growth forms (tree and shrub) from a dry-hot valley savanna in SW China for three rainy seasons from 2019 to 2021. We also compared photosynthetic gas exchange to those of a regular year (2014). We found that leaf stomatal and hydraulic conductance and maximum photosynthetic rate were significantly lower during the drought in 2019 than in the wetter years. In 2019, all studied plants maintained stomatal conductance at their minimum level observed, which could be related to high vapor pressure deficits (VPD, >2 kPa). However, no significant difference in stem and shoot hydraulic conductance was detected across years. The reductions in leaf hydraulic conductance and stomatal regulation under extreme drought might help keep the stem hydraulic function. Stomatal conductance and photosynthesis after drought (2020 and 2021) showed comparable or even higher values compared to that of 2014, suggesting high recovery of photosynthetic gas exchange. In addition, the response of hydraulic and photosynthetic traits to extreme drought was convergent across leaf habits and growth forms. Our results will help better understand the physiological mechanism underlying the response of savanna ecosystems to climate change.

Photosynthetic recovery in drought-rehydrated grapevines is associated with high demand from the sinks, maximizing the fruit-oriented performance.

To understand how grapevine sinks compete with each other during water stress and subsequent rehydration, carbon (C) allocation patterns in drought-rehydrated vines (REC) at the beginning of fruit ripening were compared with control vines maintained under drought (WS) or fully irrigated (WW). In the 30 days following rehydration, the quantity and distribution of newly fixed C between leaves, roots and fruits was evaluated through 13 CO2 pulse-labeling and stable isotope ratio mass spectrometry. REC plants diverted the same percentage of fixed C towards the berries as the WS plants, although the percentage was higher than that of WW plants. Net photosynthesis (measured simultaneously with root respiration in a multichamber system for analysis of gas exchange above- and below-ground) was approximately two-fold greater in REC compared to WS treatment, and comparable or even higher than in WW plants. Maximizing C assimilation and delivery in REC plants led to a significantly higher amount of newly fixed C compared to both control treatments, already 2 days after rehydration in root, and 2 days later in the berries, in line with the expression of genes responsible for sugar metabolism. In REC plants, the increase in C assimilation was able to support the requests of the sinks during fruit ripening, without affecting the reserves, as was the case in WS. These mechanisms clarify what is experienced in fruit crops, when occasional rain or irrigation events are more effective in determining sugar delivery towards fruits, rather than constant and satisfactory water availabilities.

Soil applied silicon and manganese combined with foliar application of 5-aminolevulinic acid mediate photosynthetic recovery in Cd-stressed Salvia miltiorrhiza by regulating Cd-transporter genes.

Salvia miltiorrhiza is an important medicinal plant that experiences significant growth and biomass losses when cultivated on cadmium (Cd) contaminated soils. High Cd accumulation in plant tissues also increases the risk of metal entry into the food chain. In this study, we proposed that Cd accumulation in S. miltiorrhiza can be restricted through plant growth regulators and nutrient management. Therefore, S. miltiorrhiza seedlings were transplanted into mixed nutrient soil for two weeks, then treated with 30 mg kg-1 CdCl2, 200 mg kg-1 Na2SiO3·9H2O, and 100 mg kg-1 MnSO4, and simultaneously sprayed with 10 mg L-1 ALA on the leaves one week later. This study showed that elevated Cd accumulation significantly reduced plant growth and biomass. This growth inhibition damaged photosynthetic machinery and impaired carbon assimilation. In contrast, 5-aminolevulinic acid (ALA) significantly promoted the biomass of S. miltiorrhiza, and the dry weight of plants treated with ALA combined with manganese (Mn)/silicon (Si) increased by 42% and 55% as compared with Cd+Mn and Cd+Si treatments. Exogenously applied ALA and Si/Mn significantly activated antioxidant enzymes and promoted the growth recovery of S. miltiorrhiza. Further, exogenous ALA also reduced the Cd concentration in S. miltiorrhiza, especially when combined with Si. Compared with the Cd+Si treatment, the Cd+Si+ALA treatment reduced the Cd concentration in roots and leaves by 59% and 60%, respectively. Gene expression analysis suggested that ALA and Si significantly up-regulated genes associated with Cd transport. Other genes related to heavy metal tolerance mechanisms are also regulated to cope with heavy metal stress. These results indicated that the combined action of ALA and Si/Mn could reduce Cd-toxicity by increasing chlorophyll content and changing oxidative stress and can also affect Cd accumulation by regulating gene expression.

Different survival strategies involve carbon translocation rather than de novo C assimilation under complete submergence in rice plant.

This study investigated the effect of transient submergence on the recovery of photosynthetic activity and translocation of photosynthate in IR67520 (Sub1A genotype) and IR72442 (non-Sub1A genotype) using 13C-labeled tracer, coupled with some photosynthetic physiological assessments. Plant growth, photosynthetic capacity, and photosynthetic recovery were studied by treating the two rice genotypes without or completely submerged for 7days in transparent acrylic tanks filled with water to a depth of 80cm, followed by 7days of reaeration. Results revealed that the IR67520 was able to obtain new carbon source for assimilation during at 7days of recovery periods. The IR72442 genotype partitioned 13C to the newly developed upper leaves more than the IR67520 genotype did. This was due to its inability to obtain CO2 from other source during post submergence. Recovery of chlorophyll content, ability to retain higher biomass, and ability to grow faster at 7days of recovery periods also indicated the ability of Sub1A genotype to reactivate its photosynthetic capacity.

Nitric oxide reduces the yield loss of waterlogged cotton by enhancing post-stress compensatory growth

Waterlogging stress hampers cotton growth and development, ultimately causing yield loss. Nitric oxide (NO) was previously found to alleviate waterlogging damage to cotton plants, but it is unclear whether NO can enhance compensatory growth after waterlogging relief. Herein, field-grown cotton ( Gossypium hirsutum L.) plants at peak flowering were subjected to a 10-day waterlogging stress treatment, and the effects of foliar spraying of sodium nitroprusside (SNP)—a NO donor—during waterlogging were evaluated. The NO concentrations in the plant tissues, plant biomass, and related physiological events at 10 d post-waterlogging relief, as well as the seedcotton yield and yield components at harvest, were measured over two consecutive years. Non-waterlogged and waterlogged cotton exhibited 38.5% and 48.3% increased plant biomass at 10 d post-waterlogging relief, respectively, compared with that on the day of stress relief, indicating that significant compensatory growth occurred following stress relief. Foliar spraying cotton with SNP during waterlogging not only increased the NO concentration (40–183%) before stress relief, but also increased the NO concentration (30–75%) after stress relief compared with no SNP application. As the NO concentration increased, plant biomass at 10 d post-waterlogging, as well as biological yield, boll density, and seedcotton yield of waterlogged cotton at harvest, were increased by 54.4%, 8.0%, 7.5%, and 7.6% under foliar SNP spraying compared to no SNP spraying, indicating that NO promoted compensatory growth and reduced the yield loss of waterlogged cotton. Correlation analysis showed that yield loss reduction was positively correlated with compensatory growth (R 2 = 0.83 ** ). Foliar spraying of SNP decreased the malondialdehyde content and the activities of alcohol dehydrogenase and pyruvate decarboxylase in waterlogged cotton at 10-d post stress relief by 15.2%, 42.4%, and 38.2%, but increased the contents of auxin (IAA), gibberellic acid (GA), and chlorophyll and the photosynthetic rate by 28.6%, 59.6%, 12.1%, and 6.4%, respectively, and decreased the abscisic acid (ABA) and ethylene contents by 12.5% and 8.5%, respectively, compared to no SNP spraying. In conclusion, foliar spraying of SNP increased the content of NO in waterlogged cotton plants even at 10-d post stress. Hormonal rebalance was realized through increased IAA and GA and reduced ABA and ethylene due to the elevated NO content, which promoted photosynthetic recovery and compensatory growth after stress relief, ultimately reducing yield loss. The overall results indicated that enhancing the compensatory growth of cotton after waterlogging is an important mechanism by which NO mitigates waterlogging stress. • Nitric oxide enhanced the compensatory growth of waterlogged cotton and reduced the yield loss. • Enhanced compensatory growth by nitric oxide was achieved through physiological adjustments after waterlogging relief. • The reduction of yield loss was correlated with the compensatory growth of waterlogged cotton. • Enhanced compensatory growth by NO is an important attribute to the yield loss reduction under waterlogging.

Adjustments in physiological and morphological traits suggest drought-induced competitive release of some California plants.

Drought and competition affect how morphological and physiological traits are expressed in plants. California plants were previously found to respond less negatively to resource limitation compared to invasive counterparts. In a glasshouse in Santa Cruz, CA, USA, we exposed five native California C3 grassland species to episodic drought and competition (via five locally invasive species). We hypothesized that leaf morphology would be more affected by competition, and leaf photosynthetic gas exchange more so by drought, consistent with optimal partitioning and environmental filter theories. We expected that traits would exhibit trade‐offs along a spectrum for resource conservatism versus acquisition. Bromus carinatus had greater photosynthetic recovery, while Diplacus aurantiacus had lower percent loss of net assimilation (PLA) and intrinsic water‐use efficiency (iWUE) during drought and competition simultaneously compared to just drought. Stipa pulchra and Sidalcea malviflora gas exchange was unaffected by drought, and leaf morphology exhibited drought‐related adjustments. Lupinus nanus exhibited trait adjustments for competition but not drought. Functional traits sorted onto two principal components related to trade‐offs for resource conservatism versus acquisition, and for above‐ versus belowground allocation. In summary, morphological traits were affected by competition and drought, whereas physiological traits, like leaf gas exchange, were primarily affected by drought. The grassland plants we studied showed diverse responses to drought and competition with trait trade‐offs related to resource conservatism versus acquisition, and for above‐ versus belowground allocation consistent with optimal partitioning and environmental filter theories. Diplacus aurantiacus experienced competitive release based on greater iWUE and lower PLA when facing drought and competition.

Impact of heat stress of varying severity on papaya (Carica papaya) leaves: Major changes in stress volatile signatures, but surprisingly small enhancements of total emissions

Tropical plants are considered to be highly heat stress resistant, but heat waves of different frequency and duration induced by global climate change can exceed the plant heat stress tolerance in the tropics. To gain insight into the overall heat resistance, leaves of papaya were exposed to mild (37 and 41 °C), moderate (46 °C) and severe (49 °C) heat stress (applied for 5 min), and plant recovery was monitored for 48 h upon return to pre-stress conditions. Leaf photosynthetic characteristics were reduced immediately after stress in all cases, but photosynthesis rate almost fully recovered after mild heat treatment, and partly recovered in 48 h after moderate stress, while no recovery was observed under severe stress. Constitutive BVOC emissions dominated by long-chained aldehydes, and glucosinolate breakdown products, attained the emission maxima at 46 °C. Acute heat stress led only to a minor induction of BVOC emissions, and the greatest share of induced emissions was observed in leaves exposed to 46 °C. Induced emissions were dominated by oxygenated monoterpenes, indicating an increase in leaf oxidative status. Our results demonstrate that papaya could withstand moderately severe heat stress of up to 46 °C, but this level of stress will have negative impacts on plant carbon gain due to delayed photosynthetic recovery after heat stress. Although volatile profiles sensitively responded to heat stress, the magnitude and degree of elicitation were surprisingly small throughout the recovery, suggesting that papaya is relatively heat resistant and possess a suite of heat-protective traits. • Papaya photosynthetic recovery was rapid under mild heat stress, delayed under moderate stress and absent under severe stress. • Heat stress of different severity had only moderate effects on volatile emissions, but the composition of emitted volatiles was strongly altered. • Volatile emission blend rather than the absolute amount characterizes the impact of heat stress on leaves of papaya.

The effects of desiccation and salinity gradients in the PSII photochemical efficiency of a subtidal brown alga, Saccharina japonica (Laminariales) from Hokkaido, Japan

SUMMARYWe determined the response of photochemical efficiency to desiccation and salinity in an edible brown alga, Saccharina japonica (var. japonica), using a pulse amplitude modulation chlorophyll fluorometer. Natural populations of this species in Muroran, Hokkaido, Japan are most of the time immersed in the subtidal zone; however, those at the uppermost part of the sublittoral are sometimes emersed during low tide. In the laboratory experiment, the alga under aerial exposure up to 8 h under dim light at 20°C and 50% humidity showed that the effective quantum yields (ΔF/Fm′) of Photosystem II quickly declined after more than 45 min of emersion; ΔF/Fm′ also failed to recover to the initial level even after 1 day of rehydration in seawater. Under emersion, the ΔF/Fm′ values were relatively stable and above 0.5 when the absolute water content (AWC, %) was greater than 50%; however, it declined as the AWC decreased. Once the algae were dehydrated to an AWC of less than 50%, their ΔF/Fm′ did not return to the initial level despite subsequent reimmersion in seawater, suggesting the low ability of photosynthetic recovery from dehydration. Results of in situ measurements in Muroran showed that the ΔF/Fm′ of S. japonica declined during the tidal exposure; however, it recovered after they were again immersed in seawater due to the rising tide. Furthermore, S. japonica showed a stenohaline photosynthetic response between 20 and 40 psu, as their ΔF/Fm′ values were found to be unchanged in these salinities after 5 days of laboratory culture; this suggests the seaweed’s photosynthetic tolerance and/or acclimation within such a relatively narrow range, which might be one of the traits that preclude its occurrence in the intertidal and brackish waters. Indeed, desiccation risk and fluctuating salinity are important limiting factors that influence the stability of kelp beds for sustainable utilization in the regional fishery including mariculture of this species.

Comparative transcriptome and physiological analyses reveal key factors in the tolerance of peach rootstocks to iron deficiency chlorosis.

The online version contains supplementary material available at 10.1007/s13205-021-03046-6.

Photochemical changes during rehydration of an intertidal cyanobacterial mat exposed to variations in salinity and light intensity.

This study focuses on a Lyngbya cf. aestuarii dominated mat community from the intertidal zone of the Laguna Ojo de Liebre, Baja California Sur. In this environment, the mat is desiccated for several days between spring tides. While the mats were desiccated, photosynthetic activity was absent but recovered rapidly (~3 h) upon rehydration. It has been shown previously that the rate of photosynthetic recovery is dependent on both light intensity and salinity. In the current study, photosynthetic recovery was measured based on chlorophyll a fluorescence using pulse amplitude modulated (PAM) fluorometry. Upon the addition of water, photosystem II (PSII) complexes recovered the capacity for reaction centre excitation. However, these functional centres were initially closed. Respiratory activity early in recovery probably reduced the plastoquinone pool through the shared use of part of the photosynthetic transport chain, thus temporarily blocking electron transport downstream of PSII. The time that PSII complexes remained closed increased with light intensities above saturation. This condition is potentially damaging to the cyanobacteria since the exposure of closed PSII centres to high light intensities can lead to the production of singlet oxygen. After this initial lag period, PSII centres opened rapidly indicating an increase in the flow of electrons from PSII to PSI. The rate of photosynthetic recovery appeared to be limited primarily by the relatively slow return of functional PSII. Photosynthetic recovery rates were slower in salinities greater than those that naturally occur in the intertidal zone.

Production of Betacyanins in Transgenic Nicotiana tabacum Increases Tolerance to Salinity.

Although red betalain pigments (betacyanins) have been associated with salinity tolerance in some halophytes like Disphyma australe, efforts to determine whether they have a causal role and the underlying mechanisms have been hampered by a lack of a model system. To address this, we engineered betalain-producing Nicotiana tabacum, by the introduction of three betalain biosynthetic genes. The plants were violet-red due to the accumulation of three betacyanins: betanin, isobetanin, and betanidin. Under salt stress, betacyanic seedlings had increased survivability and leaves of mature plants had higher photochemical quantum yields of photosystem II (Fv/Fm) and faster photosynthetic recovery after saturating light treatment. Under salt stress, compared to controls betacyanic leaf disks had no loss of carotenoids, a slower rate of chlorophyll degradation, and higher Fv/Fm values. Furthermore, simulation of betacyanin pigmentation by using a red filter cover improved Fv/Fm value of green tissue under salt stress. Our results confirm a direct causal role of betacyanins in plant salinity tolerance and indicate a key mechanism is photoprotection. A role in delaying leaf senescence was also indicated, and the enhanced antioxidant capability of the betacyanic leaves suggested a potential contribution to scavenging reactive oxygen species. The study can inform the development of novel biotechnological approaches to improving agricultural productivity in saline-affected areas.

The effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from a native distributional range in Japan

We determined the effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from its native distributional range in Japan by using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–irradiance curves at three temperatures (8, 20, and 28°C) showed that the maximum net photosynthetic rates (NPmax) and saturation irradiance were highest at 28°C. Gross photosynthesis determined at 8–36°C (every four increments) and 300 μmol photons m−2 s−1 showed that the maximum gross photosynthetic rate (GPmax) occurred at 19.5°C ( $$ {T}_{\mathrm{opt}}^{\mathrm{GP}} $$ ), which is consistent with the seawater temperature at its peaked abundance in Japan. The maximum quantum yields (Fv/Fm) of photosystem II (PSII) during the 72-h temperature exposures were above 0.60 at 8–28°C but dropped at higher temperatures. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m−2 s−1 at three temperatures showed remarkable decline in the effective quantum yields (ΔF/Fm') of PSII under high irradiance at 8°C only; the Fv/Fm measured after 12-h dark acclimation also did not recover to initial values, signifying its sensitivity to photoinhibition at 8°C. Furthermore, the alga exhibited tolerance to 2 h of desiccation with 80% of water loss from the thallus, and ΔF/Fm' recovered after 24 h of rehydration in seawater, suggesting potential of photosynthetic recovery of this alga at such low hydration threshold. In conclusion, the adaptation of S. muticum to relatively high irradiance, to broad range of temperature (8–28°C), and to desiccation explains its potentially high invasive capacity.

Effects of calcium ion on the colony formation, growth, and photosynthesis in the edible cyanobacterium Nostoc sphaeroides

Calcium ions are crucial for the growth and development of photosynthetic organisms, whereas its eco-physiological significance on the edible cyanobacterium Nostoc sphaeroides remains unclear. The percentage of spherical microcolonies from N. sphaeroides vegetative filaments was up to 88–93% after 18 days at higher Ca2+ levels (0.25, 1, and 4 mM), but only about 51% at lower Ca2+ levels (0.01 and 0.06 mM). When cultured at higher Ca2+ levels, N. sphaeroides specific growth rates based on the chlorophyll a content and absolute growth rates based on the dry weight significantly increased by 13–18% and 23–30%, respectively, compared to those at lower Ca2+ levels. The colonies cultured at higher Ca2+ levels exhibited 13–20% increase in light-limited photosynthetic efficiency, 28–42% increase in the maximum electron transport rate and 57–95% increase in the light-saturated photosynthetic rate relative to those at lower Ca2+ levels. The higher Ca2+-treated colonies were more tolerant to high light than lower Ca2+-treated colonies and more rapid recovery of photoinhibited colonies induced by the subsequent dim light was also observed in higher Ca2+-treated colonies. We further found the photosynthetic recovery of photoinhibited colonies required the influx of apoplastic Ca2+ and the maintenance of photosynthetic activity for desiccated colonies during rehydration was also extracellular Ca2+-dependent. These results indicated that higher levels of Ca2+ promoted colony formation, growth, and photosynthesis of N. sphaeroides, suggesting available calcium ions in natural environment may be an important factor affecting its productivity and distribution.