Photosystem I in low light-grown leaves of Alocasia odora, a shade-tolerant plant, is resistant to fluctuating light-induced photoinhibition

Excitation spillover from PSII to PSI measured in leaves at 77 K

Heterogeneous distribution of PSI and PSII in thick grana in shade chloroplasts is argued to hinder spillover of chlorophyll excitations from PSII to PSI. To examine this dogma, we measured fluorescence induction at 77 K at 690 nm (PSII) and 760 nm (mostly PSI) in the leaf discs of  Spinacia oleracea, Cucumis sativus, and shade-tolerant Alocasia odora, grown at high and low light, and quantified their spillover capacities. PSI fluorescence (FI) consists of the intrinsic PSI fluorescence (FIα) and fluorescence caused by excitations spilt over from PSII (FIβ). When FI and FII parameters between State 1 and State 2, induced by weak far-red and blue light, were compared, PSII maximum fluorescence (FIIm) and FIβ were greater, and FIα  was smaller in State 1; thereby, the spillover ratio, FIβ/(FIα + FIβ), was greater in State 1. When nonphotochemical quenching (NPQ) was induced, the spillover ratio decreased. Since analyses of Fv/Fmspectra tentatively suggested that ∼15% of  Fm at 760 nm was from PSII, all data were corrected accordingly. Even after the correction, the spillover ratio in  FIm  in State 1 ranged from 16% to 28%. The spillover ratios did not greatly differ between the species or growth light levels. Although extensive grana in low-light-grown plants would suggest that PSII and PSI are too separate for spillover, the ratios of nonappressed thylakoid membranes/total thylakoid membranes in A. odora chloroplasts were little affected by growth light and >40%. Spillover would occur efficiently in abundant nonappressed thylakoids and in the margins of appressed thylakoids.

Ecophysiological mechanisms and growth of Inga vera Willd. under different water and light availability

Light and water availability can impact plant survival and growth, making ecophysiological studies crucial for understanding their tolerance and to single and combined stresses. The aimed of this study was to investigate the physiological and growth responses of Inga vera Willd. plants induced by different water regimes and light intensities. Three water regimes were implemented based on substrate water retention capacity (WRC) - 50%, 75%, and 100%, along with shading levels (SH) - 0% (full sun), 30%, and 70%. Evaluations were conducted at 25 and 50 days after applying the water regimes, and during a recovery period of 30 days when all treatments were maintained at 75% of WRC. Photochemical efficiency, gas exchange, chlorophylls indices, growth, quality of the seedlings and content proline amino acid were assessed. Overall, I. vera plants showed greater sensitivity to increased exposure to light than to low water availability. The interaction of SH + WRC was beneficial for the gas exchange and chlorophylls indices characteristics under SH 70% + WRC 75-100% at 25 and 50 days, with higher results, greater plant growth and higher proline contents for leaves and roots under SH 30% and 70% + WRC 50%, 75% and 100% at 25 and 50 days. There was no recovery effect for seedlings grown in full sun. The plants grown under shade during the recovery period maintained their values for most of the characteristics evaluated. SH 30% + WRC 75% contributed to an increase in photosynthetic metabolism and, as a result, to the quality of the seedlings.

Tight relationship between two photosystems is robust in rice leaves under various nitrogen conditions

Leaf nitrogen (N) level affects not only photosynthetic CO₂ assimilation, but also two photosystems of the photosynthetic electron transport. The quantum yield of photosystem II [Y(II)] and the non-photochemical yield due to the donor side limitation of photosystem I [Y(ND)], which denotes the fraction of oxidized P700 (P700⁺) to total P700, oppositely change depending on leaf N level, and the negative correlation between these two parameters has been reported in leaves of plants cultivated at various N levels in growth chambers. Here, we aimed to clarify whether this correlation is maintained after short-term changes in leaf N level, and what parameters are the most responsive to the changes in leaf N level under field conditions. We cultivated rice varieties at two N fertilization levels in paddy fields, treated additional N fertilization to plants grown at low N, and measured parameters of two photosystems of mature leaves. In rice leaves under low N condition, the Y(ND) increased and the photosynthetic linear electron flow was suppressed. In this situation, the accumulation of P700⁺ can function as excess energy dissipation. After the N addition, both Y(ND) and Y(II) changed, and the negative correlation between them was maintained. We used a newly-developed device to assess the photosystems. This device detected the similar changes in Y(ND) after the N addition, and the negative correlation between Y(ND) and photosynthetic O₂ evolution rates was observed in plants under various N conditions. This study has provided strong field evidence that the Y(ND) largely changes depending on leaf N level, and that the Y(II) and Y(ND) are negatively correlated with each other irrespective of leaf N level, varieties and annual variation. The Y(ND) can stably monitor the leaf N status and the linear electron flow under field conditions.

Plants acclimate to Photosystem I photoinhibition by readjusting the photosynthetic machinery

Photosynthetic light reactions require strict regulation under dynamic environmental conditions. Still, depending on environmental constraints, photoinhibition of Photosystem (PSII) or PSI occurs frequently. Repair of photodamaged PSI, in sharp contrast to that of PSII, is extremely slow and leads to a functional imbalance between the photosystems. Slow PSI recovery prompted us to take advantage of the PSI-specific photoinhibition treatment and investigate whether the imbalance between functional PSII and PSI leads to acclimation of photosynthesis to PSI-limited conditions, either by short-term or long-term acclimation mechanisms as tested immediately after the photoinhibition treatment or after 24 h recovery in growth conditions, respectively. Short-term acclimation mechanisms were induced directly upon inhibition, including thylakoid protein phosphorylation that redirects excitation energy to PSI as well as changes in the feedback regulation of photosynthesis, which relaxed photosynthetic control and excitation energy quenching. Longer-term acclimation comprised reprogramming of the stromal redox system and an increase in ATP synthase and Cytochrome b₆ f abundance. Acclimation to PSI-limited conditions restored the CO₂ assimilation capacity of plants without major PSI repair. Response to PSI inhibition demonstrates that plants efficiently acclimate to changes occurring in the photosynthetic apparatus, which is likely a crucial component in plant acclimation to adverse environmental conditions.

Fast chlorophyll a fluorescence induction (OJIP) phenotyping of chlorophyll-deficient wheat suggests that an enlarged acceptor pool size of Photosystem I helps compensate for a deregulated photosynthetic electron flow

The wheat lines affected by a decrease in the leaf chlorophyll content typically experience a biomass loss. A known major problem of the chlorophyll-deficient wheat mutants is their limited prevention of Photosystem I (PSI) over-reduction brought about by an insufficient cyclic electron flow, potentially exposing them to a higher sensitivity to light fluctuations. However, the resistance of some mutant lines against fluctuating light suggests the occurrence of regulatory processes compensating for the defect in cyclic electron flow. In this study, a phenotyping approach based on fast chlorophyll a fluorescence induction (OJIP transient), corroborated by P700 redox kinetics, was applied to a collection of chlorophyll-deficient wheat lines, grown under continuous or fluctuating light. Quantitative parameters calculated from the OJIP transient are considered informative about Photosystem II (PSII) functional antenna size and photochemistry, as well as the functioning of the entire photosynthetic electron transport chain. The mutants tended to recover a wild-type-like chlorophyll content, and mature plants could hardly be distinguished based on their effective PSII antenna size. Nevertheless, specific OJIP-derived parameters were strongly correlated with the phenotype severity, in particular the amplitude of the I-P phase and the I-P/J-P amplitude ratio, which are indicative of a more capacitive pool of PSI final electron acceptors (ferredoxin and ferredoxin-NADP⁺ oxidoreductase, FNR). We propose that the enlargement of such pool of electron carriers is a compensatory response operating at the acceptor side of PSI to alleviate potentially harmful over-reduced states of PSI. Our results also suggest that, in chlorophyll-deficient mutants, higher F_V /F_M cannot prove a superior PSII photochemistry and wider I-P phase is not indicative of a higher relative content of PSI.

Adaptive Responses of Common and Hybrid Bermudagrasses to Shade Stress Associated With Changes in Morphology, Photosynthesis, and Secondary Metabolites

Alteration of ploidy in one particular plant species often influences their environmental adaptation. Warm-season bermudagrass is widely used as forage, turfgrass, and ground-cover plant for ecological remediation, but exhibits low shade tolerance. Adaptive responses to shade stress between triploid hybrid bermudagrass cultivars ["Tifdwarf" (TD), "Tifsport" (TS), and "Tifway" (TW)] and tetraploid common bermudagrass cultivar "Chuanxi" (CX) were studied based on changes in phenotype, photosynthesis, and secondary metabolites in leaves and stems. Shade stress (250 luminance, 30 days) significantly decreased stem diameter and stem internode length, but did not affect the leaf width of four cultivars. Leaf length of CX, TD, or TW showed no change in response to shade stress, whereas shade stress significantly elongated the leaf length of TS. The CX and the TS exhibited significantly higher total chlorophyll (Chl), Chl a, carotenoid contents, photosynthetic parameters [PSII photochemical efficiency (Fv/Fm), transpiration rate, and stomatal conductance] in leaves than the TW and the TD under shade stress. The CX also showed a significantly higher performance index on absorption basis (PIABS) in leaf and net photosynthetic rate (Pn) in leaf and stem than the other three cultivars under shade stress. In addition, the TS maintained higher proantho cyanidims content than the TW and the TD after 30 days of shade stress. Current results showed that tetraploid CX exhibited significantly higher shade tolerance than triploid TD, TS, and TW mainly by maintaining higher effective photosynthetic leaf area, photosynthetic performance of PSI and PSII (Pn and Fv/Fm), and photosynthetic pigments as well as lower Chl a/b ratio for absorption, transformation, and efficient use of light energy under shade stress. For differential responses to shade stress among three triploid cultivars, an increase in leaf length and maintenance of higher Fv/Fm, gas exchange, water use efficiency, carotenoid, and proanthocyanidin contents in leaves could be better morphological and physiological adaptations of TS to shade than other hybrid cultivars (TD and TW).

Differential Response of the Photosynthetic Machinery to Fluctuating Light in Mature and Young Leaves of Dendrobium officinale

A key component of photosynthetic electron transport chain, photosystem I (PSI), is susceptible to the fluctuating light (FL) in angiosperms. Cyclic electron flow (CEF) around PSI and water-water cycle (WWC) are both used by the epiphytic orchid Dendrobium officinale to protect PSI under FL. This study examined whether the ontogenetic stage of leaf has an impact on the photoprotective mechanisms dealing with FL. Thus, chlorophyll fluorescence and P700 signals under FL were measured in D. officinale young and mature leaves. Upon transition from dark to actinic light, a rapid re-oxidation of P700 was observed in mature leaves but disappeared in young leaves, indicating that WWC existed in mature leaves but was lacking in young leaves. After shifting from low to high light, PSI over-reduction was clearly missing in mature leaves. By comparison, young leaves showed a transient PSI over-reduction within the first 30 s, which was accompanied with highly activation of CEF. Therefore, the effect of FL on PSI redox state depends on the leaf ontogenetic stage. In mature leaves, WWC is employed to avoid PSI over-reduction. In young leaves, CEF around PSI is enhanced to compensate for the lack of WWC and thus to prevent an uncontrolled PSI over-reduction induced by FL.