Linear Electron Transport Rate Research Articles

Photosynthetic Traits of Quercus coccifera Green Fruits: A Comparison with Corresponding Leaves during Mediterranean Summer

Fruit photosynthesis occurs in an internal microenvironment seldom encountered by a leaf (hypoxic and extremely CO2-enriched) due to its metabolic and anatomical features. In this study, the anatomical and photosynthetic traits of fully exposed green fruits of Quercus coccifera L. were assessed during the period of fruit production (summer) and compared to their leaf counterparts. Our results indicate that leaf photosynthesis, transpiration and stomatal conductance drastically reduced during the summer drought, while they recovered significantly after the autumnal rainfalls. In acorns, gas exchange with the surrounding atmosphere is hindered by the complete absence of stomata; hence, credible CO2 uptake measurements could not be applied in the field. The linear electron transport rates (ETRs) in ambient air were similar in intact leaves and pericarps (i.e., when the physiological internal atmosphere of each tissue is maintained), while the leaf NPQ was significantly higher, indicating enhanced needs for harmless energy dissipation. The ETR measurements performed on leaf and pericarp discs at different CO2/O2 partial pressures in the supplied air mixture revealed that pericarps displayed significantly lower values at ambient gas levels, yet they increased by ~45% under high CO2/O2 ratios (i.e., at gas concentrations simulating the fruit’s interior). Concomitantly, NPQ declined gradually in both tissues as the CO2/O2 ratio increased, yet the decrease was more pronounced in pericarps. Furthermore, net CO2 assimilation rates for both leaf and pericarp segments were low in ambient air and increased almost equally at high CO2, while pericarps exhibited significantly higher respiration. It is suggested that during summer, when leaves suffer from photoinhibition, acorns could contribute to the overall carbon balance, through the re-assimilation of respiratory CO2, thereby reducing the reproductive cost.

Growth, Ecophysiological Responses, and Leaf Mineral Composition of Lettuce and Curly Endive in Hydroponic and Aquaponic Systems.

Against the backdrop of climate change, soil loss, and water scarcity, sustainable food production is a pivotal challenge for humanity. As the global population grows and urbanization intensifies, innovative agricultural methods are crucial to meet rising food demand, while mitigating environmental degradation. Hydroponic and aquaponic systems, has emerged as one of these solutions by minimizing land use, reducing water consumption, and enabling year-round crop production in urban areas. This study aimed at assessing the yield, ecophysiological performance, and nutritional content of Lactuca sativa L. and Cichorium endivia L. var. crispum grown in hydroponic and aquaponic floating raft systems, with Oreochromis niloticus L. integrated into the aquaponic system. Both species exhibited higher fresh biomass and canopy/root ratios in hydroponics compared to aquaponics. Additionally, hydroponics increased the leaf number in curly endive by 18%. Ecophysiological parameters, such as the leaf net photosynthesis rate, actual yield of PSII, and linear electron transport rate, were also higher in hydroponics for both species. However, the nutritional profiles varied between the two cultivation systems and between the two species. Given that standard fish feed often lacks sufficient potassium levels for optimal plant growth, potassium supplementation could be a viable strategy to enhance plant development in aquaponic systems. In conclusion, although aquaponic systems may demonstrate lower productivity compared to hydroponics, they offer a more sustainable and potentially healthier product with fewer harmful compounds due to the reduced use of synthetic fertilizers, pesticides, and the absence of chemical residue accumulation. However, careful system management and monitoring are crucial to minimize potential contaminants.

Stomatal and Non-Stomatal Leaf Responses during Two Sequential Water Stress Cycles in Young Coffea canephora Plants

Understanding the dynamics of physiological changes involved in the acclimation responses of plants after their exposure to repeated cycles of water stress is crucial to selecting resilient genotypes for regions with recurrent drought episodes. Under such background, we tried to respond to questions as: (1) Are there differences in the stomatal-related and non-stomatal responses during water stress cycles in different clones of Coffea canephora Pierre ex A. Froehner? (2) Do these C. canephora clones show a different response in each of the two sequential water stress events? (3) Is one previous drought stress event sufficient to induce a kind of “memory” in C. canephora? Seven-month-old plants of two clones (’3V’ and ‘A1’, previously characterized as deeper and lesser deep root growth, respectively) were maintained well-watered (WW) or fully withholding the irrigation, inducing soil water stress (WS) until the soil matric water potential (Ψmsoil) reached ≅ −0.5 MPa (−500 kPa) at a soil depth of 500 mm. Two sequential drought events (drought-1 and drought-2) attained this Ψmsoil after 19 days and were followed by soil rewatering until a complete recovery of leaf net CO2 assimilation rate (Anet) during the recovery-1 and recovery-2 events. The leaf gas exchange, chlorophyll a fluorescence, and leaf reflectance parameters were measured in six-day frequency, while the leaf anatomy was examined only at the end of the second drought cycle. In both drought events, the WS plants showed reduction in stomatal conductance and leaf transpiration. The reduction in internal CO2 diffusion was observed in the second drought cycle, expressed by increased thickness of spongy parenchyma in both clones. Those stomatal and anatomical traits impacted decreasing the Anet in both drought events. The ‘3V’ was less influenced by water stress than the ‘A1’ genotype in Anet, effective quantum yield in PSII photochemistry, photochemical quenching, linear electron transport rate, and photochemical reflectance index during the drought-1, but during the drought-2 event such an advantage disappeared. Such physiological genotype differences were supported by the medium xylem vessel area diminished only in ‘3V’ under WS. In both drought cycles, the recovery of all observed stomatal and non-stomatal responses was usually complete after 12 days of rewatering. The absence of photochemical impacts, namely in the maximum quantum yield of primary photochemical reactions, photosynthetic performance index, and density of reaction centers capable of QA reduction during the drought-2 event, might result from an acclimation response of the clones to WS. In the second drought cycle, the plants showed some improved responses to stress, suggesting “memory” effects as drought acclimation at a recurrent drought.

Natural variation of temperature acclimation of Arabidopsis thaliana.

Acclimation is a multigenic trait by which plants adjust photosynthesis and metabolism to cope with a changing environment. Here, natural variations of photosynthetic efficiency and acclimation of the central carbohydrate metabolism were analyzed in response to low and elevated temperatures. For this, 18 natural accessions of Arabidopsis thaliana, originating from Cape Verde Islands and Europe, were grown at 22°C before being exposed to 4°C and 34°C for cold and heat acclimation, respectively. Absolute amounts of carbohydrates were quantified together with their subcellular distribution across plastids, cytosol and vacuole. Linear electron transport rates (ETRs) were determined together with the maximum quantum efficiency of photosystem II (Fv/Fm) for all growth conditions and under temperature fluctuation. Under elevated temperature, ETR residuals under increasing photosynthetic photon flux densities significantly correlated with the degree of temperature fluctuation at the original habitat of accessions, indicating a geographical east/west gradient of photosynthetic acclimation capacities. Plastidial sucrose concentrations positively correlated with maximal ETRs under fluctuating temperature, indicating a stabilizing role within the chloroplast. Our findings revealed specific subcellular carbohydrate distributions that contribute differentially to the photosynthetic efficiency of natural Arabidopsis thaliana accessions across a longitudinal gradient. This sheds light on the relevance of subcellular metabolic regulation for photosynthetic performance in a fluctuating environment and supports the physiological interpretation of naturally occurring genetic variation of temperature tolerance and acclimation.

Reduction of PSII photosynthetic performance by low temperature is the reason for the growing inhibition of Kobresia pygmaea Clarke

AbstractKobresia pygmaea Clarke is mainly distributed in the alpine region from 3800 to 4500 m elevation on the Qinghai-Tibet Plateau. It is excellent in low-temperature tolerance and trampling resistance and plays an important role in maintaining the stability of regional ecological environments. Mainly based on the chlorophyll fluorescence imaging technique, using grass sods blocks with a size of 20 × 20 cm2 collected from K. pygmaea forb meadow, simulated a low-temperature treatment in cultural room with 8 °C/4 °C (day/night) at diurnal photoperiod, we analyzed the effects of low temperature on PSII photochemical efficiency and non-photochemical quenching, as well as the interaction between low temperature and light intensity. The results showed that low temperature could significantly decrease the maximum quantum efficiency of PSII photochemistry (Fv/Fm) and inhibit the leaf growth of K. pygmaea. The light response curve of PSII relative linear electron transport rate was reduced by low temperature, while it recovered to almost normal level after two days of removing adversity. Further experiments under steady-state actinic light confirmed that light intensity was the main factor affecting the PSII photochemical efficiency, and its reduction caused by the low temperature showed a certain degree of reversibility. The non-photochemical quenching (NPQ) was significantly affected by light intensity, and its fast and slow components (NPQf and NPQs) in NPQ accounted for 36% and 64% of NPQ, respectively. The decrease in NPQf and corresponding increase in NPQs slowly recovered after relieving from low-temperature condition. The relative inhibition of photosynthesis (Pinh) was significantly enhanced by low temperature and reduced after removal of adversity. Strong light intensity could intensify the adverse effect of low temperature on PSII operating efficiency Fq′/Fm′ and efficiency factor Fq′/Fv′, as well as NPQ. In conclusion, low temperature was one of the main reasons limiting the photosynthetic performance of K. pygmaea on the Qinghai-Tibet Plateau and could aggravate photosynthetic photoinhibition. The adversity of low temperature frequently occurring in the plant growing season on the Qinghai-Tibet Plateau was constituted of the main factor of the low productivity level in alpine meadow.

Vernalization Procedure of Tuberous Roots Affects Growth, Photosynthesis and Metabolic Profile of Ranunculus asiaticus L.

In Ranunculus asiaticus L., vernalization of propagation material is a common practice for the production scheduling of cut flowers, however little is known about the plant physiology and metabolism of this species as affected by cold treatments. We investigated the influence of two hybrids, MBO and MDR, and three preparation procedures of tuberous roots, only rehydration (control, C), and rehydration plus vernalization at 3.5 °C for 2 weeks (V2) and for 4 weeks (V4), on plant growth and flowering, leaf photosynthesis, and leaf metabolic profile in plants grown in pot in a cold greenhouse. Net photosynthesis (NP) was higher in MDR than in MBO. In the two genotypes, the NP did not change in V2 and increased in V4 compared to C in MBO, while was unaffected by vernalization in MDR. Quantum yield of PSII electron transport (ΦPSII), linear electron transport rate (ETR) and non-photochemical quenching (NPQ) did not differ in the two hybrids, whereas maximal PSII photochemical efficiency (Fv/Fm) was higher in MBO than in MDR. Fluorescence indexes were unaffected by the preparation procedure, except for ETR, which decreased in V2 compared to C and V4 in MDR. A significant interaction between genotype and preparation procedure was found in plant leaf area, which was reduced only in V4 in MBO, while decreased in both the vernalization procedures in MDR. In Control plants, flowering started in 65 days in MBO and 69 days in MDR. Compared to controls, both the vernalization treatments anticipated flowering in MDR, while they were detrimental or only slightly efficient in promoting flowering in MBO. Vernalization always reduced the quality of flower stems in both the hybrids.

Salt stress in plants and amelioration strategies: alleviation of agriculture and livelihood risks after the Covid-19 pandemic.

Agriculture sustains the livelihoods of over 2.5billion people worldwide. The growing nature of disasters, the systemic nature of risk, a more recent pandemic along with abiotic stress factors are endangering our entire food system. In these stressful environment, it is widely reprimanded that strategies should be encompassed to attain increased crop yield and economic returns which would alleviate food and nutritional scarcity in developing countries. To study the physiological responses to salt stress, Vigna radiata seedlings subjected to varying levels of salt stress (0, 25, 50, 100 and 200 mM NaCl) were evaluated by tracking changes in Chl a fluorescence, pigment content, free proline and carotenoids content by HPLC. The ability of plants to adapt to salt stress is related with the plasticity and resilience of photosynthesis. As salt concentration increased, chlorophyll fluorescence indices decreased and a reduction in the PSII linear electron transport rate was observed. Chlorophyll fluorescence parameters can be used for in vitro non-invasive monitoring of plants responses to salt stress. Overall, Vigna responded to salt stress by the changes in avoidance mechanism and protective systems. Chl fluorescence indices, enzymatic contents of POD, CAT and free proline were sensitive to salt stress. The study is significant to evaluate the tolerance mechanisms of plants to salt stress and may develop insights for breeding new salt-tolerant varieties.

The circadian night depression of photosynthesis analyzed in a herb, Pulmonaria vallarsae. Day/night quantitative relationships

Although many photosynthesis related processes are known to be controlled by the circadian system, consequent changes in photosynthetic activities are poorly understood. Photosynthesis was investigated during the daily cycle by chlorophyll fluorescence using a PAM fluorometer in Pulmonaria vallarsae subsp. apennina, an understory herb. A standard test consists of a light induction pretreatment followed by light response curve (LRC). Comparison of the major diagnostic parameters collected during day and night showed a nocturnal drop of photosynthetic responses, more evident in water-limited plants and consisting of: (i) strong reduction of flash-induced fluorescence peaks (FIP), maximum linear electron transport rate (Jmax, ETREM) and effective PSII quantum yield (ΦPSII); (ii) strong enhancement of nonphotochemical quenching (NPQ) and (iii) little or no change in photochemical quenching qP, maximum quantum yield of linear electron transport (Φ), and shape of LRC (θ). A remarkable feature of day/night LRCs at moderate to high irradiance was their linear-parallel course in double-reciprocal plots. Photosynthesis was also monitored in plants subjected to 2–3 days of continuous darkness (“long night”). In such conditions, plants exhibited high but declining peaks of photosynthetic activity during subjective days and a low, constant value with elevated NPQ during subjective night tests. The photosynthetic parameters recorded in subjective days in artificial darkness resembled those under natural day conditions. On the basis of the evidence, we suggest a circadian component and a biochemical feedback inhibition to explain the night depression of photosynthesis in P. vallarsae.

Inoculation with Arbuscular Mycorrhizal Fungi Alleviates the Adverse Effects of High Temperature in Soybean.

High temperature is foremost abiotic stress and there are inadequate studies explicating its impact on soybean. In this study, a pot experiment was done in a greenhouse maintained at a day/night temperature of 42/28 °C with a mean temperature of 35 °C to examine the effects of high temperature in soybean plants inoculated with and without arbuscular mycorrhizal fungi (AMF).Various parameters were taken in soybean plants treated with AMF (+) and AMF (−) such as growth analysis, chlorophyll content, canopy temperature, number of stomata, gas exchange, chlorophyll fluorescence, seed yield, and its attributes. It was observed that growth parameters like leaf area, stem height, root length, shoot and root dry biomass were increased in AMF (+) as compared to AMF (−) plants. Chlorophyll content, the number of stomata, photosynthesis rate, stomatal conductance, transpiration rate, and water use efficiency increased in AMF (+) as compared to AMF (−) plants. Chlorophyll fluorescence parameters such as Fv/Fm, Fv/Fo, PhiPSII, fluorescence area, performance index, photochemical quenching, linear electron transport rate, and active reaction centres density of PSII were also found to be enhanced in AMF (+) plants. However, canopy temperature, intercellular CO2, Fo/Fm, and non-photochemical quenching were higher in AMF (−) as compared to inoculated plants. An increase in growth and photosynthesis ultimately enhanced the seed yield and its attributes in AMF (+) as compared to AMF (−). Thus, AMF (+) plants have shown much better plant growth, photosynthesis parameters, and seed yield as compared to AMF (−) plants under high temperature. Thus, it is concluded that heat stress-induced damage to the structure and function of the photosynthetic apparatus was alleviated by AMF inoculum. Therefore, AMF can be used as a biofertilizer in alleviating the adverse effects of heat stress in soybean.

Mitochondrial alternative oxidase pathway acts as an electron sink during photosynthetic induction in Rumex K-1 leaves

Chloroplast PSII photochemical efficiency is upregulated more rapidly than CO2 assimilation during photosynthesis induction, suggesting the existence of other electron sinks than that of CO2 assimilation. We hypothesized that the mitochondrial alternative oxidase (AOX) pathway could be such a sink. Inhibition of the AOX restricted light activation of the malate-oxaloacetate shuttle and caused an excessive reduction of PSI acceptor side and substantial accumulation of QA-, hindering the photosynthetic linear electron transport rate (ETR) and leading to an imbalance between light energy absorption and exploitation during photosynthetic induction. ETR limitation also restricted the formation of thylakoid pH gradient, evidenced by a decreased de-epoxidation of the xanthophyll cycle, thus preventing nonphotochemical quenching. Delayed CO2 assimilation due to thylakoid pH gradient restriction was partially reversed by exogenous ATP application. The AOX pathway acts as a photosynthetic electron sink, protecting the photosynthetic apparatus against photoinhibition and accelerating the induction of CO2 assimilation during photosynthetic induction in Rumex K-1 leaves.

Lowering medium pH improves tolerance of tomato (Lycopersicon esculentum) plants to long-term salinity exposure

Although salinity stress adversely affects plant function, manipulation of the rhizosphere may alleviate those negative impacts. We examined whether adjustment of rhizosphere pH, unadjusted control (∼pH 8.5–9), pH 5.5, and pH 4.5 would mitigate adverse effects of salinity on tomato plants (cv. Mobil) in hydroponics. Plants were evaluated based on the leaf chlorophyll parameters, plant survival, leaf water relations, and yield. Chlorophyll fluorescence parameters were lowest at 28 days after salinity onset (DAS). The maximum PSII quantum yield (Fv’/Fm’) and operating efficiency (ΦPSII) were recorded in plants grown in pH 4.5 at 56 DAS. At 28 DAS, the linear electron transport rate (J) was decreased by 10 and 13%, in control and pH 5.5, respectively, compared with the day zero. The fraction of photons used in photochemistry (%P) was suppressed at 28 DAS, but %P was the greatest under 4.5 pH at 56 DAS. Stomatal conductance and leaf osmotic potential (ΨO ) were negatively correlated. Plants grown at pH 5.5 had the greatest fresh fruit weight and plant dry weight compared with the other pH levels. Although salinity adversely affected plant performance, lowering rhizosphere pH alleviated the adverse impacts of salinity. It seems that the tomato variety ‘Mobil’ used to measure leaf chlorophyll fluorescence parameters in this study had a salinity tolerance to which was enhanced at more acidic pH.

Photosynthetic, anatomical and biochemical responses of olive tree (Olea europaea) cultivars under water stress

The effect of water deficit on photochemical parameters, hydraulic properties, phytochemical composition and radical scavenging activity were studied on four Olea europaea cultivars (Chetoui, Oueslati, Jarboui and Meski). After 60 days without irrigation, maximum quantum yield of photosystem II (Fv/Fm), linear electron transport rate (ETR) and quantum efficiency of PSII electron transport (ΦPSII) decreased in all cultivars. A significant decrease was observed for Jarboui and even more for Meski cultivars, reaching 0.57 and 0.24 for Fv/Fm, 1.41 and 0.65 for ETR, 0.51 and 0.23 for ΦPSII, respectively, Furthermore, water stress induced an increase in xylem vessel frequency, providing a greater security of xylem sap conduction under drought conditions. Chetoui had the highest total phenolic (37.4 mg GAE/g extract) and flavonoid (13.25 mg QE/g extract) contents under water stress. It had also the best antiradical activity by both DPPH and ABTS methods with antiradical activity of respectively 80.05% and 81.99%. Chetoui cultivar can be considered as the most tolerant one and we consider this cultivar very promising for cultivation in semi-arid areas.

Comparison of changes in water status and photosynthetic parameters in wild type and abscisic acid-deficient sitiens mutant of tomato (Solanum lycopersicum cv. Rheinlands Ruhm) exposed to sublethal and lethal salt stress

Abscisic acid (ABA) regulates many salt stress-related processes of plants such as water balance, osmotic stress tolerance and photosynthesis. In this study we investigated the responses of wild type (WT) and the ABA-deficient sitiens mutant of tomato (Solanum lycopersicum cv. Rheinlands Ruhm) to sublethal and lethal salt stress elicited by 100 mM and 250 mM NaCl, respectively. Sitiens mutants displayed much higher decrease in water potential, stomatal conductance and net CO2 assimilation rate under high salinity, especially at lethal salt stress, than the WT. However, ABA deficiency in sitiens caused more severe osmotic stress and more moderate ionic stress, higher K+/Na+ ratio, in leaf tissues of plants exposed to salt stress. The higher salt concentration caused irreversible damage to Photosystem II (PSII) reaction centres, severe reduction in the linear photosynthetic electron transport rate and in the effective quantum yields of PSII and PSI in sitiens plants. The cyclic electron transport (CET) around PSI, which is an effective defence mechanism against the damage caused by photoinhibition in PSI, decreased in sitiens mutants, while WT plants were able to increase CET under salt stress. This suggests that the activation of CET needs active ABA synthesis and/or signalling. In spite of ABA deficiency, proline accumulation could alleviate the stress injury at sublethal salt stress in the mutants but its accumulation was not sufficient at lethal salt stress.

Photosynthetic efficiency of bottle gourd [Lagenaria siceraria (Molina) Standl.] under drought stress

Neglected and underutilized crops such as bottle gourd [Lagenaria siceraria (Molina) Standl.] are associated with considerable drought tolerance. The objective of this study was to determine variation in the level of drought tolerance among bottle gourd landraces based on photosynthetic efficiency and to identify unique genotypes for breeding. Photosynthetic efficiency of 12 selected bottle gourd landraces were evaluated under controlled environment under drought-stressed (DS) and non-stressed (NS) conditions. A significant genotype × water regime interaction (P < 0.05) was observed for minimum fluorescence (Fm′), maximum quantum efficiency of PS II photochemistry (Fv′/Fm′), the effective quantum efficiency of PS II photochemistry (ΦPSII), photochemical quenching (qP), non-photochemical quenching (qN), linear electron transport rate (LETR), non-cyclic electron transport rate (NCETR), proportion of open PS II reaction centers (1 − qP), photo-inhibition of PS II reaction centers Y (NO), the fraction of photon energy trapped by “open” PS II reaction centers and utilized in PS II photochemistry (P), the portion of the absorbed photon energy that was thermally dissipated (D), rate of photochemistry (RPC) and rate of heat dissipation (RHD) suggesting variation in genotypic response under NS and DS conditions. Principal component analysis under NS and DS conditions identified two principal components (PC’s) which accounted for a total variance of 90 and 96%, respectively. Under DS condition, minimum fluorescence (F0′), Fm′, Fv′/Fm′, qN, NCETR, D, and RHD correlated with PC1 which accounted for 56% of total variation. ΦPSII, qP, LETR, P, RPC and 1 − qP correlated with PC2 which accounted for 40% of total variation. The PC biplot identified landraces BG-27, BG-58 and BG-78 as the most drought tolerant characterized by high values for LETR, qP, P, ΦPSII and RPC under drought stress condition. These landraces could be a useful genetic resource in the development of bottle gourd genotypes with enhanced drought tolerance.

Soil application of P can mitigate the copper toxicity in grapevine: physiological implications

Abstract The continuous use of copper-based fungicides in viticulture to contrast fungal diseases may result into an accumulation of Cu in the topsoil, reaching toxicity levels for plants and soil biota. A possible strategy to mitigate the negative effects of excess of Cu could involve the formation of insoluble molecules by the application of P to the soil. The aim of this study was to evaluate the effectiveness P soil application as a strategy to alleviate the toxic effect of excess of Cu on potted grapevines plants, with particular emphasis on the physiological response of plants. Micropropagated plants of 1103 Paulsen rootstock were grown according to a factorial experimental design with two factors: Cu (4 levels: 0, 100, 200, 300 mg kg−1 dw) and P (2 levels: 0 and 100 mg kg−1 dw) and three replicates. Net photosynthesis, efficiency of PSII photochemistry and linear electron transport rate decreased as soil Cu concentration increased. The addition of P increased net photosynthesis, improved the efficiency of PSII photochemistry and linear electron transport rate, but decreased stomatal conductance. As a consequence, shoot and root biomass declined with the increase of Cu concentration in soil, while the increment of P tackled this decline and improved nutritional status. CuEDTA extractable fraction in soil was increased by both the supply of Cu and P; this because the addition of P in combination with the highest Cu soil concentration determined a decrease of soil pH. In conclusion, the addition of P to the soil mitigated Cu excess toxicity symptoms in grape through a preservation of photosynthetic apparatus efficiency and an improvement of nutrient uptake.

A Mathematical Model of Photosynthetic Electron Transport in Response to the Light Spectrum Based on Excitation Energy Distributed to Photosystems.

To enable us to analyze more systematically the effects of the spectral distribution of light (i.e. light quality) on photosynthetic electron transport, we propose a simple mathematical model which describes electron transport reactions under light-limited conditions based on the excitation energy distributed to the photosystems. The model assumes that the rate-limiting photosystem performs the photochemical reaction at its maximum yield, while the yield in the other photosystem is passively down-regulated to equalize the rates of linear electron transport through the photosystems. Using intact cucumber leaves, we tested the model by comparing actual and estimated photosynthetic parameters under several combinations of photon flux densities of red and far-red lights (R and FR, respectively). Simultaneously provided R and FR yielded greater gross photosynthetic rates than the sums of the rates under only R and only FR, which is known as the 'enhancement effect'. The present model reproduced these non-additive increases in the gross photosynthetic rates in response to supplemental FR to R and provided more accurate estimates than an existing method that did not take the enhancement effect into account (root mean square errors: 0.11 and 0.21 μmol m-2 s-1, respectively). Using the present model, the photon flux density of the supplemental FR which gives the changing point of rate-limiting photosystem and the photochemical yields of the non-rate-limiting photosystems were estimated reasonably well. The present study has therefore formulated a simplified quantitative electron transport model in response to the light spectrum based on generally accepted concepts and demonstrated its validity experimentally.

Physiological validation of photochemical reflectance index (PRI) as a photosynthetic parameter using Arabidopsis thaliana mutants

Non-photochemical quenching (NPQ) is the most important photoprotective system in higher plants. NPQ can be divided into several steps according to the timescale of relaxation of chlorophyll fluorescence after reaching a steady state (i.e., the fast phase, qE; middle phase, qZ or qT; and slow phase, qI). The dissipation of excess energy as heat during the xanthophyll cycle, a large component of NPQ, is detectable during the fast to middle phase (sec to min). Although thermal dissipation is primarily investigated using indirect methods such as chlorophyll a fluorescence measurements, such analyses require dark adaptation or the application of a saturating pulse during measurement, making it difficult to continuously monitor this process. Here, we designed an unconventional technique for real-time monitoring of changes in thylakoid lumen pH (as reflected by changes in xanthophyll pigment content) based on the photochemical reflectance index (PRI), which we estimated by measuring light-driven leaf reflectance at 531 nm. We analyzed two Arabidopsis thaliana mutants, npq1 (unable to convert violaxanthin to zeaxanthin due to inhibited violaxanthin de-epoxidase [VDE] activity) and npq4 (lacking PsbS protein), to uncover the regulator of the PRI. The PRI was variable in wild-type and npq4 plants, but not in npq1, indicating that the PRI is related to xanthophyll cycle-dependent thermal energy quenching (qZ) rather than the linear electron transport rate or NPQ. In situ lumen pH substitution using a pH-controlled buffer solution caused a shift in PRI. These results suggest that the PRI reflects only xanthophyll cycle conversion and is therefore a useful parameter for monitoring thylakoid lumen pH (reflecting VDE activity) in vivo.

Optimising the linear electron transport rate measured by chlorophyll a fluorescence to empirically match the gross rate of oxygen evolution in white light: towards improved estimation of the cyclic electron flux around photosystem I in leaves

The cyclic electron flux (CEF) around photosystem I (PSI) was discovered in isolated chloroplasts more than six decades ago, but its quantification has been hampered by the absence of net formation of a product or net consumption of a substrate. We estimated in vivo CEF in leaves as the difference (ΔFlux) between the total electron flux through PSI (ETR1) measured by a near infrared signal, and the linear electron flux through both photosystems by optimised measurement of chlorophyll a fluorescence (LEFfl). Chlorophyll fluorescence was excited by modulated green light from a light-emitting diode at an optimal average irradiance, and the fluorescence was detected at wavelengths >710nm. In this way, LEFfl matched the gross rate of oxygen evolution multiplied by 4 (LEFO2) in broad-spectrum white actinic irradiance up to half (spinach, poplar and rice) or one third (cotton) of full sunlight irradiance. This technique of estimating CEF can be applied to leaves attached to a plant.

Calibration matters: On the procedure of using the chlorophyll fluorescence method to estimate mesophyll conductance

Estimates of mesophyll conductance (gm), when calculated from chlorophyll fluorescence, are uncertain, especially when the photosystem II (PSII) operating efficiency is measured from the traditional single saturation pulse methodology. The multiphase flash method has recently been recommended to replace the single saturation pulse method, allowing a more reliable estimation of gm. Also, many researchers still directly use the PSII operating efficiency to derive linear electron transport rate J (that is required to estimate gm), without appropriate calibration using measurements under non-photorespiratory conditions. Here we demonstrate for tomato and rice that (i) using the multiphase flash method did not yield realistic estimates of gm if no calibration was conducted; and (ii) using the single saturation pulse method still gave reasonable estimates of gm when calibration based on the non-photorespiratory measurements was properly conducted. Therefore, conducting calibration based on data under non-photorespiratory conditions was indispensable for a reliable estimation of gm, regardless whether the multiphase flash or the single saturation pulse method was used for measuring the PSII operating efficiency. Other issues related to the procedure of using the chlorophyll fluorescence method to estimate gm were discussed.

Photochemical Acclimation of Three Contrasting Species to Different Light Levels: Implications for Optimizing Supplemental Lighting

Photosynthetic responses to light are dependent on light intensity, vary among species, and can be affected by acclimation to different light environments (e.g., light intensity, spectrum, and photoperiod). Understanding how these factors affect photochemistry is important for improving supplemental lighting efficiency in controlled-environment agriculture. We used chlorophyll fluorescence to determine photochemical light response curves of three horticultural crops with contrasting light requirements [sweetpotato (Ipomea batatas), lettuce (Lactuca sativa), and pothos (Epipremnum aureum)]. We also quantified how these responses were affected by acclimation to three shading treatments-full sun, 44% shade, and 75% shade. The quantum yield of photosystem II (ΦPSII), a measure of photochemical efficiency, decreased exponentially with increasing photosynthetic photon flux (PPF) in all three species. By contrast, linear electron transport rate (ETR) increased asymptotically with increasing PPF. Within each shading level, the high-light-adapted species sweetpotato used high light more efficiently for electron transport than light-intermediate lettuce and shade-tolerant pothos. Within a species, plants acclimated to high light (full sun) tended to have higher ΦPSII and ETR than those acclimated to low light (44% or 75% shade). Nonphotochemical quenching (NPQ) (an indicator of the amount of absorbed light energy that is dissipated as heat) was upregulated with increasing PPF; faster upregulation was observed in pothos as well as in plants grown under 75% shade. Our results have implications for supplemental lighting: supplemental light is used more efficiently and results in a greater increase in ETR when provided at low ambient PPF. In addition, high-light-adapted crops and crops grown under relatively high ambient light can use supplemental light more efficiently than low-light-adapted crops or those grown under low ambient light.