Seasonal variations in photosystem I compared with photosystem II of three alpine evergreen broad-leaf tree species

High salt-induced PSI-supercomplex is associated with high CEF and attenuation of state transitions

While PSI-driven cyclic electron flow (CEF) and assembly of thylakoid supercomplexes have been described in model organisms like Chlamydomonas reinhardtii, open questions remain regarding their contributions to survival under long-term stress. The Antarctic halophyte, C. priscuii UWO241 (UWO241), possesses constitutive high CEF rates and a stable PSI-supercomplex as a consequence of adaptation to permanent low temperatures and high salinity. To understand whether CEF represents a broader acclimation strategy to short- and long-term stress, we compared high salt acclimation between the halotolerant UWO241, the salt-sensitive model, C. reinhardtii, and a moderately halotolerant Antarctic green alga, C. sp. ICE-MDV (ICE-MDV). CEF was activated under high salt and associated with increased non-photochemical quenching in all three Chlamydomonas species. Furthermore, high salt-acclimated cells of either strain formed a PSI-supercomplex, while state transition capacity was attenuated. How the CEF-associated PSI-supercomplex interferes with state transition response is not yet known. We present a model for interaction between PSI-supercomplex formation, state transitions, and the important role of CEF for survival during long-term exposure to high salt.

Ethylene-induced improvement in photosynthetic performance of Zanthoxylum armatum under reoxygenation conditions

In this study, we evaluated the photosynthetic performance of Zanthoxylum armatum seedlings to test the tolerance to reoxygenation after waterlogging. The experiment included a control group without waterlogging (NW) and three reoxygenation groups with reoxygenation after 1day (WR1), 2days (WR2) and 3days (WR3). Seedlings were pretreated with concentrations of 0, 200 and 400μmolL-1 of ethylene. The results showed that reoxygenation after waterlogging for 1-3days decreased photosynthetic pigments content, enzymes activity, stomatal conductance (G s ), net photosynthetic rate (P n ), transpiration rate (T r ) and water-use efficiency (WUE). However, pretreatment with ethylene increased photosynthetic pigments content, enzymes activity and gas exchange parameters under both NW and WR3 treatments. The chlorophyll fluorescence results showed that the maximum quantum yield of PSII (F v /F m ) and actual photochemical efficiency of PSII (Φ PSII ) remained no significant changes under the NW and WR1 treatments, while they were significantly reduced with an increase in waterlogging days followed by reoxygenation under WR2 and WR3 treatments. Exogenous ethylene inhibited F v /F m and the non-photochemical quenching coefficient (NPQ), while enhanced Φ PSII and electron transfer efficiency (ETR) under WR2 treatments. Moreover, the accumulation of exogenous ethylene reduced photosynthetic ability. These findings provide insights into the role of ethylene in enhancing the tolerance of Z. armatum to reoxygenation stress, which could help mitigate the impact of continued climate change.

Seasonal Eco-Physiology Characteristics of Four Evergreen Rhododendron Species to the Subalpine Habitats

Four evergreen broadleaf Rhododendron spp. (Rhododendrons), namely, Rhododendron aganniphum, R. nyingchiense, R. wardii, and R. triflorum, occur in harsh subalpine habitats in the southwest Qinghai-Tibet Plateau (QTP), China. Considering that the four Rhododendrons cannot escape their unique environment, they must evolve a set of adaptations to survive, but the information is lacking. To uncover their physiological adaptation characteristics, in the present study, we monitored their physiological characteristics by determination of their seasonal variation in antioxidant enzyme activity, osmotic adjustment substrates, and carbohydrate contents, and their pigment content and photosynthetic efficiency. The results showed that superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) activities and proline content of four Rhododendrons had a significant difference in autumn and were insignificant in summer. Specifically, R. aganniphum had the maximum protective enzyme activity and proline content in winter as well as chl a, b, and car contents. The values of maximal quantum yield (Fv/Fm), photochemical efficiency (ΦPSII), and non-photochemical quenching (NPQ) of four Rhododendrons were significantly higher in summer than in other seasons. The lower qP indicated the four Rhododendrons were susceptible to photoinhibition. Overall, the four Rhododendrons had similar physical characteristics in subalpine habitats. The parameters of the maximum quantum yield of photosystem II (PSII), the actual quantum yield of PSII, the non-photochemical chlorophyll fluorescence quenching, and chlorophyll a content increased in summer. Meanwhile, the protective enzyme activity and total soluble sugar content, proline content, and carotenoid content increased in spring, autumn, and winter. These results suggested that the four Rhododendrons can adapt to subalpine habitats by heat dissipation to avoid the damage of excessive radiation during the warm season while scavenging reactive oxygen and increasing the intracellular fluid concentration to avoid damage caused by chilling temperatures during the cold seasons. These findings would provide a reference for the conservation and application of these valuable ornamental evergreen broadleaf Rhododendrons, and enrich theory of plant eco-physiology in the high altitudes of the QTP.

Cyclic electron flow (CEF) and ascorbate pathway activity provide constitutive photoprotection for the photopsychrophile, Chlamydomonas sp. UWO 241 (renamed Chlamydomonas priscuii)

Under environmental stress, plants and algae employ a variety of strategies to protect the photosynthetic apparatus and maintain photostasis. To date, most studies on stress acclimation have focused on model organisms which possess limited to no tolerance to stressful extremes. We studied the ability of the Antarctic alga Chlamydomonas sp. UWO 241 (UWO 241) to acclimate to low temperature, high salinity or high light. UWO 241 maintained robust growth and photosynthetic activity at levels of temperature (2 °C) and salinity (700 mM NaCl) which were nonpermissive for a mesophilic sister species, Chlamydomonas raudensis SAG 49.72 (SAG 49.72). Acclimation in the mesophile involved classic mechanisms, including downregulation of light harvesting and shifts in excitation energy between photosystem I and II. In contrast, UWO 241 exhibited high rates of PSI-driven cyclic electron flow (CEF) and a larger capacity for nonphotochemical quenching (NPQ). Furthermore, UWO 241 exhibited constitutively high activity of two key ascorbate cycle enzymes, ascorbate peroxidase and glutathione reductase and maintained a large ascorbate pool. These results matched the ability of the psychrophile to maintain low ROS under short-term photoinhibition conditions. We conclude that tight control over photostasis and ROS levels are essential for photosynthetic life to flourish in a native habitat of permanent photooxidative stress. We propose to rename this organism Chlamydomonas priscuii.

Photosynthetic regulation under fluctuating light at chilling temperature in evergreen and deciduous tree species

Plants usually experience fluctuating light conditions at chilling temperatures during the autumn season. We hypothesized that photosystem I (PSI) and PSII are more susceptible to photoinhibition under fluctuating light at chilling temperatures in deciduous species relative to evergreen species. We measured the photosynthetic performances under fluctuating light at 6 °C in two evergreen and two deciduous broadleaf tree species. Within the first 10 s after light increased at 6 °C, none of these species could generate an enough trans-thylakoid proton gradient. Meanwhile, PSI was highly oxidised in evergreen species but was highly reduced in deciduous species. This transient over-reduction of PSI in deciduous species was mainly caused by the higher electron flow from PSII. Furthermore, the deciduous species showed a significantly smaller violaxanthin pool and lower non-photochemical quenching under high light conditions at 6 °C, leading to more excess light energy could not be dissipated in PSII. Hence, we propose that fluctuating light combined with chilling temperature cause the over-reduction of photosynthetic electron chain in deciduous species.

Candidate Genes for the High-Altitude Adaptations of Two Mountain Pine Taxa

Mountain plants, challenged by vegetation time contractions and dynamic changes in environmental conditions, developed adaptations that help them to balance their growth, reproduction, survival, and regeneration. However, knowledge regarding the genetic basis of species adaptation to higher altitudes remain scarce for most plant species. Here, we attempted to identify such corresponding genomic regions of high evolutionary importance in two closely related European pines, Pinus mugo and P. uncinata, contrasting them with a reference lowland relative—P. sylvestris. We genotyped 438 samples at thousands of single nucleotide polymorphism (SNP) markers, tested their genetic differentiation and population structure followed by outlier detection and gene ontology annotations. Markers clearly differentiated the species and uncovered patterns of population structure in two of them. In P. uncinata three Pyrenean sites were grouped together, while two outlying populations constituted a separate cluster. In P. sylvestris, Spanish population appeared distinct from the remaining four European sites. Between mountain pines and the reference species, 35 candidate genes for altitude-dependent selection were identified, including such encoding proteins responsible for photosynthesis, photorespiration and cell redox homeostasis, regulation of transcription, and mRNA processing. In comparison between two mountain pines, 75 outlier SNPs were found in proteins involved mainly in the gene expression and metabolism.

Do all chlorophyll fluorescence emission wavelengths capture the spring recovery of photosynthesis in boreal evergreen foliage?

Chlorophyll a fluorescence (ChlF) is closely related to photosynthesis and can be measured remotely using multiple spectral features as solar-induced fluorescence (SIF). In boreal regions, SIF shows particular promise as an indicator of photosynthesis, in part because of the limited variation of seasonal light absorption in these ecosystems. Seasonal spectral changes in ChlF could yield new information on processes such as sustained nonphotochemical quenching (NPQ_S ) but also disrupt the relationship between SIF and photosynthesis. We followed ChlF and functional and biochemical properties of Pinus sylvestris needles during the photosynthetic spring recovery period to answer the following: (a) How ChlF spectra change over seasonal timescales? (b) How pigments, NPQ_S , and total photosynthetically active radiation (PAR) absorption drive changes of ChlF spectra? (c) Do all ChlF wavelengths track photosynthetic seasonality? We found seasonal ChlF variation in the red and far-red wavelengths, which was strongly correlated with NPQ_S , carotenoid content, and photosynthesis (enhanced in the red), but not with PAR absorption. Furthermore, a rapid decrease in red/far-red ChlF ratio occurred in response to a cold spell, potentially relating to the structural reorganization of the photosystems. We conclude that all current SIF retrieval features can track seasonal photosynthetic dynamics in boreal evergreens, but the full SIF spectra provides additional insight.

Photorespiration is the major alternative electron sink under high light in alpine evergreen sclerophyllous Rhododendron species

Owing to the high leaf mass per area, alpine evergreen sclerophyllous Rhododendron have low values of mesophyll conductance (g_m). The resulting low chloroplast CO₂ concentration aggravates photorespiration, which requires a higher ATP/NADPH ratio. However, the significance of photorespiration and underlying mechanisms of energy balance in these species are little known. In this study, eight alpine evergreen sclerophyllous Rhododendron species grown in a common garden were tested for their g_m, electron flow to photorespiration, and energy balancing. Under saturating light, g_m was the most limiting factor for net photosynthesis (A_N) in all species, and the species differences in A_N were primarily driven by g_m rather than stomatal conductance. The total electron flow through photosystem II (ETRII) nearly equaled the electron transport required for Rubisco carboxylation and oxygenation. Furthermore, blocking electron flow to photosystem I with appropriate inhibitors showed that electron flow to plastic terminal oxidase was not observed. As a result, these studied species showed little alternative electron flow mediated by water-water cycle or plastic terminal oxidase. By comparison, the ratio of electron transport consumed by photorespiration to ETRII (J_PR/ETRII), ranging from 43%∼55%, was negatively correlated to A_N and g_m. Furthermore, the increased ATP production required by enhanced photorespiration was regulated by cyclic electron flow around photosystem I. These results indicate that photorespiration is the major electron sink for dissipation of excess excitation energy in the alpine evergreen sclerophyllous Rhododendron species. The coordination of g_m, photorespiration and cyclic electron flow is important for sustaining leaf photosynthesis.

Stimulation of cyclic electron flow around photosystem I upon a sudden transition from low to high light in two angiosperms Arabidopsis thaliana and Bletilla striata

In angiosperms, cyclic electron flow (CEF) around photosystem I (PSI) is more important for photoprotection under fluctuating light than under constant light. However, the underlying mechanism is not well known. In the present study, we measured the CEF activity, P700 redox state and electrochromic shift signal upon a sudden transition from low to high light in wild-type plants of Arabidopsis thaliana and Bletilla striata (Orchidaceae). Within the first 20 s after transition from low to high light, P700 was highly reduced in both species, which was accompanied with a sufficient proton gradient (ΔpH) across the thylakoid membranes. Meanwhile, the level of CEF activation was elevated. After transition from low to high light for 60 s, the plants generated an optimal ΔpH. Under such condition, PSI was highly oxidized and the level of CEF activation decreased to the steady state. Furthermore, the CEF activation was positively correlated to the P700 reduction ratio. These results indicated that upon a sudden transition from low to high light, the insufficient ΔpH led to the over-reduction of PSI electron carriers, which in turn stimulated the CEF around PSI. This transient stimulation of CEF not only favored the rapid ΔpH formation but also accepted electrons from PSI, thus protecting PSI at donor and acceptor sides. These findings provide new insights into the important role of CEF in regulation of photosynthesis under fluctuating light.

Chloroplastic ATP Synthase Alleviates Photoinhibition of Photosystem I in Tobacco Illuminated at Chilling Temperature

Chloroplastic ATP synthase plays a significant role in the regulation of proton motive force (pmf) and proton gradient (ΔpH) across the thylakoid membranes. However, the regulation of chloroplastic ATP synthase at chilling temperature and its role in photoprotection are little known. In our present study, we examined the chlorophyll fluorescence, P700 signal, and electrochromic shift signal at 25°C, and 6°C in tobacco (Nicotiana tabacum L. cv. Samsun). Although photosynthetic electron flow through both PSI and PSII were severely inhibited at 6°C, non-photochemical quenching and P700 oxidation ratio were largely increased. During the photosynthetic induction under high light, the formation of pmf at 6°C was similar to that at 25°C. However, the ΔpH was significantly higher at 6°C, owing to the decreased activity of chloroplastic ATP synthase (g H +). During illumination at 6°C and high light, a high ΔpH made PSI to be highly oxidized, preventing PSI from photoinhibition. These results indicate that the down-regulation of g H + is critical to the buildup of ΔpH at low temperature, adjusting the redox state of PSI, and thus preventing photodamage to PSI. Our findings highlight the importance of chloroplastic ATP synthase in photoprotection at chilling temperature.