Ichiro Terashima

In green leaves in vivo , photosystem I (PSI) was thought to be resistant to photoinhibition. In 1994, it was found that, in chilling-sensitive plants such as cucumber ( Cucumis sativus L.), exposing leaves to moderate constant light at temperatures below 10 °C induces PSI photoinhibition with little effect on PSII. Since the beginning of this century, PSI photoinhibition induced by fluctuating light (alternating high and low light, FL) or repetitive saturating pulses (rSPs) has been reported in many species. In this review, we compare the causes and consequences of PSI photoinhibition in cucumber at chilling temperatures with those of PSI photoinhibition induced by FL or rSPs. Changes in chloroplasts during these photoinhibitory treatments are followed, focusing on where and how reactive oxygen species are produced and which parts of PSI are damaged. We also discuss the roles of two pathways of cyclic electron flow around PSI, the NDH and PGR5/PGRL1 pathways, particularly in suppression of PSI photoinhibition. Lastly, we discuss problems with methods for measuring PSI parameters and future research directions from an ecophysiological viewpoint.

Legumes establish mutualistic symbiosis with nitrogen (N)‐fixing bacteria, which allows them to utilise atmospheric N ₂ . Because the maintenance of symbiosis requires abundant carbon (C) sources, legumes regulate the balance between carbon consumption and nitrogen acquisition by systemically controlling the nodule number through CLAVATA1 (CLV1)‐like receptors. In Lotus japonicus , the CLV1‐like receptor HYPERNODULATION ABERRANT ROOT FORMATION1 (HAR1) acts in shoots to regulate root nodulation and contributes to symbiotic C/N coordination. This raises the possibility that HAR1 may also influence plant growth and nitrogen utilisation beyond symbiotic nodulation. In this study, we showed that HAR1 plays a critical role in regulating nitrogen use to enhance growth under conditions of high nitrate availability, even in non‐symbiotic environments. Unlike the wild‐type, the har1 mutant failed to increase its growth in response to higher nitrate availability. This lack of growth response was associated with a lower rate of net biomass production per unit leaf area and a reduced capacity for biomass production per unit plant nitrogen. We further found that nitrate‐responsive TCA cycle‐related organic acids were higher in har1 leaves than in wild‐type leaves even under low nitrate conditions. Because the HAR1 mutation did not affect photosynthetic traits, we propose that HAR1 promotes growth under non‐symbiotic conditions by coordinating nitrogen utilisation with primary metabolism.

Slow stomatal opening after increases in irradiance constrains carbon gain under fluctuating light, yet stomatal kinetics remain an underexplored target for crop improvement. Here, we investigated Oryza sativa PROTON ATPASE TRANSLOCATION CONTROL 1 (OsPATROL1) , which encodes a Munc13-like protein implicated in stomatal regulation in Arabidopsis thaliana . OsPATROL1 overexpression had modest, condition-dependent effects on steady-state gas exchange and did not alter stomatal morphology or biochemical traits. In contrast, it consistently accelerated stomatal opening and photosynthetic induction, reducing the stomatal conductance time constant during induction by 41–43%. During 12 h of simulated natural fluctuating light, OsPATROL1-overexpressing plants maintained higher stomatal conductance and net CO₂ assimilation rate, increasing cumulative assimilation by 8–12% while maintaining their intrinsic water-use efficiency ( iWUE ). Under artificial fluctuating light, overexpression alleviated growth reductions relative to steady light. Under glasshouse conditions, total biomass increased by 34–44%, accompanied by greater tiller number, root biomass, bleeding sap rate, and leaf nitrogen content. Taken together, these results indicate that OsPATROL1 overexpression accelerates stomatal opening, enhances photosynthetic induction and daytime carbon gain without compromising iWUE , and is associated with greater growth.

Legumes establish mutualistic symbiosis with nitrogen (N)-fixing bacteria, which allows them to utilize atmospheric N2. Because the maintenance of symbiosis requires abundant carbon (C) sources, legumes regulate the balance between carbon consumption and nitrogen acquisition by systemically controlling the nodule number through CLAVATA1 (CLV1)-like receptors. In Lotus japonicus , the CLV1-like receptor HYPERNODULATION ABERRANT ROOT FORMATION1 (HAR1) acts in shoots to regulate root nodulation and contributes to symbiotic C/N coordination. This raises the possibility that HAR1 may also influence plant growth and nitrogen utilization beyond symbiotic nodulation. In this study, we showed that HAR1 plays a critical role in regulating nitrogen use to enhance growth under conditions of high nitrate availability, even in non-symbiotic environments. Unlike the wild-type, the har1 mutant failed to increase its growth in response to higher nitrate availability. This lack of growth response was associated with a lower rate of net biomass production per unit leaf area and a reduced capacity for biomass production per unit plant nitrogen. We further found that nitrate-responsive TCA cycle-related organic acids were higher in har1 leaves than in wild-type leaves even under low nitrate conditions. Because the HAR1 mutation did not affect photosynthetic traits, we propose that HAR1 promotes growth under non-symbiotic conditions by coordinating nitrogen utilization with primary metabolism.

In indoor horticulture high planting densities often accelerate the senescence of lower leaves, increasing trimming frequency, reducing effective photosynthetic area, and raising labor costs. While spectral composition has been widely studied, the role of spectral bandwidth, particularly when peak wavelength is identical, remains poorly understood. Here, we used laser diodes (LDs) as narrow-band light sources to examine how spectral bandwidth influences photosynthesis, canopy architecture, leaf senescence, and plant growth under monochromatic blue light and combined red and blue (R+B) light in tobacco ( Nicotiana tabacum L. ‘Wisconsin-38’), lettuce ( Lactuca sativa L. ‘Red Fire’), and Arabidopsis thaliana (L.) Heynh. ‘Col-0’. Under monochromatic blue light, narrow-band LD blue (LD _B ) reduced CO□ assimilation rates and shoot dry weight compared with broad-band LED blue (LED _B ) across species. However, LED _B cultivation was accompanied by accelerated the senescence of lower leaves, a major limitation under dense planting. In contrast, plants grown under LD _B developed a more upright canopy architecture, characterized by larger leaf angles, which was associated with improved light penetration and delayed the senescence of lower leaves. When red and blue light were combined, narrow-band LD lighting (LD _R+B ) mitigated stress induced by 24-hour continuous illumination and promoted coordinated improvements in photosynthetic performance, leaf expansion, and canopy architecture. These integrated responses resulted in consistently higher shoot fresh weight and a healthier physiological state, as indicated by higher chlorophyll content and lower anthocyanin accumulation, compared with broad-band LED _R+B . Together, our results demonstrate that spectral bandwidth, even when peak wavelength is held constant, is a critical parameter shaping plant growth strategies. Precise control of bandwidth enables partial decoupling of traits traditionally linked in the classic ‘sun’ or ‘shade’ leaf syndromes, allowing independent regulation of canopy architecture, leaf senescence, and plant growth. This study highlights spectral bandwidth as a powerful yet underutilized lever for optimizing canopy health and yield in indoor horticulture.

Summary: Although the phenomenon of chloroplast movement was discovered a long time ago, the physiological role and the structural base for chloroplast movement remain ambiguous, largely due to uncertainties of methodologies. We derived a simple procedure to quantify chloroplast movement as probed by green light, based on leaf transmittance, reflectance, Chl concentration, and the apparent Chl extinction coefficient, and applied it across 10 woody species. The variation in chloroplast movement can be largely explained by differences in the width of cells in the first layer of the palisade mesophyll but not spongy cells. We further found that chloroplast movement in the palisade mesophyll is mainly driven by the lens effect of leaf epidermal cells. Chloroplast movement has little effect on photoprotection in terms of nonphotochemical quenching in 10 studied woody species. The finding of the linkage between chloroplast movement and the lens effect of leaf epidermal cells fills the knowledge gap of chloroplast movement and offers possibilities of working hypotheses regarding the physiological functions of chloroplast movement in future studies.

Optimizing leaf photosynthesis and root water and mineral uptake in crops during drought is crucial for enhancing agricultural productivity under climate change. Although plasma membrane H + -ATPase plays a key role in plant physiological processes, its overexpression alone does not consistently improve growth. While PROTON ATPASE TRANSLOCATION CONTROL 1 (PATROL1) regulates H + -ATPase translocation in response to various environmental stimuli in leaves, its function in roots remains largely unknown. Here, we show that H + -ATPase was coimmunoprecipitated with PATROL1 in roots of Arabidopsis thaliana. Under hyperosmotic stress, PATROL1 overexpression line had significantly greater root length and lateral root numbers than wild type (WT) and knockout lines. Micrografting between WT and PATROL1 knockout or overexpression lines showed that PATROL1 is indispensable in both shoots and roots, indicating that root uptake and leaf photosynthesis are simultaneous limiting factors for plant growth under soil water deficit. Compared with the WT, PATROL1 overexpression in whole plants resulted in a 41% increase in shoot dry weight and a 43% increase in shoot nitrogen content under drought conditions. These findings highlight the potential of H + -ATPase regulation in both roots and shoots as a new strategy to improve plant productivity, particularly under drought conditions.

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 77K 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 (FI_α) and fluorescence caused by excitations spilt over from PSII (FI_β). When FI and FII parameters between State 1 and State 2, induced by weak far-red and blue light, were compared, PSII maximum fluorescence (FII_m) and FI_β were greater, and FI_α was smaller in State 1 and thereby the spillover ratio, FI_β/(FI_α+FI_β), was greater in State 1. When non-photochemical quenching (NPQ) was induced, the spillover ratio decreased. Since analyses of F_v/F_m spectra tentatively suggested that about 15% of F_m at 760 nm was from PSII, all data were corrected accordingly. Even after the correction, the spillover ratio in FI_m 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 separated for spillover, the ratios of non-appressed thylakoid membranes/total thylakoid membranes in A. odora chloroplasts were little affected by growth light and more than 40%. Abundant non-appressed thylakoids and margins of appressed-thylakoids would enable efficient spillover. (250 words)

Optimizing leaf photosynthesis and root water and mineral uptake in crops during drought is crucial for enhancing agricultural productivity under climate change. Although plasma membrane H ⁺ -ATPase plays a key role in plant physiological processes, its overexpression alone does not consistently improve growth. While PROTON ATPASE TRANSLOCATION CONTROL 1 (PATROL1) regulates H ⁺ -ATPase translocation in response to various environmental stimuli in leaves, its function in roots remains largely unknown. Here we show that H ⁺ -ATPase was coimmunoprecipitated with PATROL1 in roots of Arabidopsis thaliana . Micrografting between wild type and PATROL1 knockout or overexpression lines showed that PATROL1 are indispensable in both shoots and roots, indicating that root uptake and leaf photosynthesis are simultaneous limiting factors for plant growth under drought conditions. PATROL1 overexpression in whole plants resulted in a 41% increase above wild type in shoot dry weight under drought conditions. These findings highlight the potential of H ⁺ -ATPase regulation in roots as a new strategy to improve plant productivity, particularly under drought conditions.

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 77K at 690 nm (PSII) and 760 nm (mainly 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 ( F I) consists of the intrinsic PSI fluorescence ( F I _α ) and fluorescence caused by excitations spilt over from PSII ( F I _β ). When F I and F II parameters between State 1 and State 2, induced by weak far-red and blue light, were compared, PSII maximum fluorescence ( F II _m ) and F I _β were greater, and F I _α was smaller in State 1 and thereby the spillover ratio, F I _β /( F I _α +F I _β ) or F I _β / F I _m , was greater in State 1. Since the leftover F II _m was found to be about 10% of total F _m at 760 nm, all the data were corrected. Even after the correction, the spillover ratio in F I _m in State 1 ranged from 21 to 32%, and the spillover ratios were comparable irrespective of growth light conditions. Although extensive grana in low light grown plants would suggest that PSII and PSI are too separated for spillover, in A. odora chloroplasts, the ratio of non-appressed thylakoid membranes/total thylakoid membranes was little affected by growth light and more than 40%. Abundant non-appressed thylakoids would contribute to efficient spillover.