Inhibition Of CO2 Assimilation Research Articles

Effect of salt stress on photosynthesis and chlorophyll fluorescence in Medicago truncatula

In the present study, photosynthetic parameters including gas exchanges, pigment contents, and chlorophyll fluorescence, were compared in two contrasting local Medicago truncatula lines TN6.18 and TN8.20, in response to salt added to the nutrient solution. Plants were cultivated under symbiotic nitrogen fixation (SNF) after inoculation with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl), and grown in a controlled glasshouse. On one month old plants (with active SNF), salt treatment (75 mM NaCl) was gradually applied. Photosynthesis, assimilating pigments and chlorophyll fluorescence were monitored throughout the experiment during both short and long terms, compared to control (non-saline) conditions. A genotypic variation in salt tolerance was found; TN6.18 was the more sensitive to salinity. The relative tolerance of TN8.20 was concomitant with the highest photochemical quenching coefficient (qP) affecting the maximum quantum yield of PSII (Y); the real quantum yield (ɸexc) was the most affected in the sensitive line. Moreover, stomatal and PSII reaction centers activities differed clearly between the studied lines. We found that the effect of salinity on photosynthesis of M. truncatula was related to PSII activity reduction rather than to stomatal conductance limitation. Photosynthesis was reduced by the inhibition of CO2 assimilation caused by PSII damage. This was clearly estimated by the Y, ɸexc and especially by the quantum yield of electron transport of PSII (ΦPSII). Thus, on the basis of our results on the two local M. truncatula lines, we recommend the use of chlorophyll fluorescence as non-destructive screening method to discriminate susceptible and resistant legumes to salt stress.

Response of Chloroplast NAD(P)H Dehydrogenase-Mediated Cyclic Electron Flow to a Shortage or Lack in Ferredoxin-Quinone Oxidoreductase-Dependent Pathway in Rice Following Short-Term Heat Stress.

Cyclic electron flow (CEF) around photosystem I (PSI) can protect photosynthetic electron carriers under conditions of stromal over-reduction. The goal of the research reported in this paper was to investigate the responses of both PSI and photosystem II (PSII) to a short-term heat stress in two rice lines with different capacities of cyclic electron transfer, i.e., Q4149 with a high capacity (hcef) and C4023 with a low capacity (lcef). The absorbance change at 820 nm (ΔA820) was used here to assess the charge separation in the PSI reaction center (P700). The results obtained show that short-term heat stress abolishes the ferredoxin-quinone oxidoreductase (FQR)-dependent CEF in rice and accelerates the initial rate of P700+ re-reduction. The P700+ amplitude was slightly increased at a moderate heat-stress (35°C) because of a partial restriction of FQR but it was decreased following high heat-stress (42°C). Assessment of PSI and PSII activities shows that PSI is more susceptible to heat stress than PSII. Under high temperature, FQR-dependent CEF was completely removed and NDH-dependent CEF was up-regulated and strengthened to a higher extent in C4023 than in Q4149. Specifically, under normal growth temperature, hcef (Q4149) was characterized by higher FQR- and chloroplast NAD(P)H dehydrogenase (NDH)-dependent CEF rates than lcef (C4023). Following thermal stress, the activation of NDH-pathway was 130 and 10% for C4023 and Q4149, respectively. Thus, the NDH-dependent CEF may constitute the second layer of plant protection and defense against heat stress after the main route, i.e., FQR-dependent CEF, reaches its capacity. We discuss the possibility that under high heat stress, the NDH pathway serves as a safety valve to dissipate excess energy by cyclic photophosphorylation and overcome the stroma over-reduction following inhibition of CO2 assimilation and any shortage or lack in the FQR pathway. The potential role of the NDH-dependent pathway during the evolution of C4 photosynthesis is briefly discussed.

The higher sensitivity of PSI to ROS results in lower chilling–light tolerance of photosystems in young leaves of cucumber

The development of PSII tolerance to stress and photoprotection mechanisms during leaf growth has been widely studied, however, knowledge about PSI photoinhibition and interaction between PSI and PSII under stress during leaf growth is still lacking. This study showed that during the chilling–light treatment, the photoinhibitions of PSI and PSII were more severe in young leaves than in fully-expanded leaves of cucumber, but the inhibition of CO2 assimilation and the accumulation of reactive oxygen species (ROS) were similar in leaves at different development stages. During the chilling–light treatment, PSII photoinhibition was positive correlated to PSI photoinhibition in leaves, however, this correlation no longer existed in leaves pretreated with DCMU, an inhibitor of electron transport from PSII to PSI. Although the photoinhibitions of PSII and PSI in young leaves were more severe, the sensitivity of PSII to excitation pressure was lower in young leaves. The above results demonstrate that, the lower chilling–light tolerance of photosystems in young leaves was due to the higher sensitivity of PSI to ROS and the higher PSII excitation pressure caused by PSI photoinhibition in young leaves, rather than the difference of ROS content and sensitivity of PSII to excitation pressure between the young and fully-expanded leaves.

Sodium‐calcium interactions with growth, water, and photosynthetic parameters in salt‐treated beans

AbstractCalcium (Ca2+) amelioration of the plant's growth response to salinity depends on genetic factors. In this work, supplemental Ca2+ did not improve growth in Phaseolus vulgaris L. cv. Contender under high‐saline conditions and negatively affected several physiological parameters in nonsalinized plants. The response to supplemental Ca2+ was examined using plants grown in 25% modified Hoagland solution at different Na+ : Ca2+ ratios. In control plants (1 mM Ca2+; 1 mM Na+) surplus Ca2+ (4 or 10 mM) was associated with stomatal closure, decrease of hydraulic conductivity, sap flow, leaf specific dry weight, leaf K+ and leaf Mg2+ concentrations, and inhibition of CO2 assimilation. Leaf water content was enhanced, while water‐use efficiency and dry matter were unaffected during the 15 d experimental period. The Ca2+ effect was not cation‐specific since similar results were found in plants supplied with high external Mg2+ or with a combination of Ca2+ and Mg2+. Relative to control plants, salinization (50 and 100 mM NaCl) caused a decrease in dry matter, hydraulic conductivity, sap flow, leaf Mg2+ activity, and inhibition of stomatal opening and CO2 assimilation. However, NaCl (50 and 100 mM NaCl) enhanced leaf K+ concentration and water‐use efficiency. At 100 mM NaCl, leaf water content also significantly increased. Supplemental Ca2+ had no amelioration effect on the salt‐stress response of this bean cultivar. In contrast, the 50 mM–NaCl treatment improved stomatal conductance and CO2‐assimilation rate in plants exposed to the highest Ca2+ concentration (10 mM). Phaseolus vulgaris is classified as a very NaCl‐sensitive species. The similarities in the effects caused by supplemental Ca2+, supplemental Mg2+, and NaCl salinity suggest that P. vulgaris cv. Contender has a high non‐ion‐specific salt sensitivity. On the other hand, the improvement in gas‐exchange parameters in Ca2+‐supplemented plants by high NaCl could be the result of specific Na+‐triggered responses, such as an increase in the concentration of K+ in the leaves.

Dark chilling imposes metabolic restrictions on photosynthesis in soybean

ABSTRACTDark chilling inhibited photosynthesis in two soybean [Glycine max (L.) Merr.] cultivars (Fiskeby V and Maple Arrow). The inhibition of CO2 assimilation was characterized by a simultaneous decrease in stomatal conductance (Gs) and intercellular CO2 concentration (Ci) in Maple Arrow, whereas a similar decrease in Gs in Fiskeby V occurred without any change in Ci. Dark chilling had little effect on total ribulose‐1, 5‐bisphosphate carboxylase/oxygenase (Rubisco) activity, Rubisco protein content or Rubisco activation state in the subsequent light period. Chilling reduced the abundance of the nocturnal Rubisco inhibitor, 2‐carboxyarabinitol 1‐phosphate, only in Fiskeby V. The abundance of Rubisco small subunit transcripts was enhanced in both cultivars as a result of dark chilling. Dark chilling decreased the maximal extractable activities and activation states of stromal fructose‐1,6‐bisphosphatase (FBPase) and NADP‐malate dehydrogenase but had no effect on sucrose phosphate synthase or leaf sucrose and starch contents. It is concluded that dark chilling‐induced limitations on CO2 assimilation are predominantly due to metabolic restrictions rather than to direct effects on electron transport reactions and that stromal FBPase is particularly susceptible to dark chilling.

Oxygen reduction in the Mehler reaction is insufficient to protect photosystems I and II of leaves against photoinactivation

The relative roles of assimilatory and photorespiratory electron flows on one side and of the Mehler‐peroxidase pathway on the other side in sustaining electron transport and providing protection against photoinhibition were investigated in leaves of spinach (Spinacia oleracea L.) and sunflower (Helianthus annuus L.). After inhibiting photosynthesis and photorespiration of intact leaves by either HCN or glycolaldehyde, light‐dependent linear electron transport was decreased by more than 90% at a photon flux density of 800 µmol m−2 s−1. Remaining electron transport exhibited characteristics of the Mehler reaction. Nonphotochemical quenching of chlorophyll fluorescence increased after inhibition of CO2 assimilation and photorespiration indicating effective dissipation of excess excitation energy. Nevertheless, appreciable photoinactivation was observed under these conditions not only of photosystem II but also of photosystem I. This damage was oxygen‐dependent. It was much reduced or absent when the oxygen concentration of the atmosphere was reduced from 21 to 1%.

Inhibition of photosynthesis, acidification and stimulation of zeaxanthin formation in leaves by sulfur dioxide and reversal of these effects

Leaves of Pelargonium zonale L. and Spinacia oleracea L. were fumigated with high concentrations of SO2 for very short periods of time with the aim of first producing acute symptoms of damage and then observing repair. The response of different photosynthetic parameters to SO2 was monitored during and after fumigation. The following results were obtained: (1) Inhibition of CO2 assimilation in the light was accompanied by increased reduction of the quinone acceptor, QA, of photosystem II and by increased oxidation of the electrondonor pigment P700 of photosystem I. Increased control of photosystem II activity in the SO2-inhibited state was also indicated by increased light scattering and by increased non-photochemical quenching of chlorophyll fluorescence. Both are indicators of chloroplast energization. Apparently, SO2 did not decrease but rather increased energization of the chloroplast thylakoid system by light. (2) Accumulation of dihydroxyacetone phosphate, fructose-1,6-phosphate and ribulose-1,5-phosphate and a decrease of 3-phosphoglycerate and hexosephosphate indicated that SO2 inhibited enzymes of the Calvin cycle. (3) Stimulated postillumination CO2 evolution suggested that when photosynthesis declined respiration increased to provide energy for repair reactions. (4) Increased leaf absorbance at 505 nm indicated increased stimulation of zeaxanthin formation in thylakoid membranes under the influence of SO2. A similar increase in 505-nm absorbance could be induced by high concentrations of CO2. In darkened leaves, SO2 did not produce changes in 505-nm absorbance. (5) While zeaxanthin formation was stimulated, changes in the fluorescence of the pH-indicating dye pyranine, which had been fed to the leaves, indicated acidification of the cytoplasm of leaf cells by SO2. Maximum acid production by SO2 required light. In contrast, cytoplasmic acidification of leaf cells by CO2 was similar in the light and in the dark. (6) Since zeaxanthin formation is known to depend on the acidification of the thylakoid lumen, SO2-dependent zeaxanthin formation indicated SO2-dependent acidification of the thylakoid lumen as the indirect result of cytoplasmic acidification by SO2. (7) Inhibition of photosynthesis and other effects of SO2 were fully reversible in the light. Detoxification of SO2 and reactivation of the photosynthetic apparatus were slow or absent in the dark. Light had a dual effect on the action of SO2. Transiently, it first increased the extent of inhibition of assimilation, but, finally, it reversed inhibition. Sulfur dioxide was inhibitory as a consequence of the chemical reactivity of its hydration products rather than as a result of cellular acidification by the produced acid. The initial acidification was followed by an appreciable alkalisation demonstrating the action of the pH-stat mechanism. (8) The data are discussed in relation to SO2 toxicity under field conditions when plants are chronically exposed to polluted air.

The effects of oil spill chemicals on transpiration, CO2 exchange, and cuticular structure in Salix interior

The effects of three oil spill chemicals (Corexit 9600, 9550, and 7664) on cuticular structure and function in the sandbar willow Salix interior were assessed from direct observations of the leaf surface, using scanning electron microscopy, and from measurements of water loss through transpiration. Rates of CO2 exchange in the light and dark were also measured. Although the Corexits coated the leaf surfaces, wax plates (crystals) associated with the cuticle were not visibly altered. The dispersants did not increase rates of evaporative water loss. Rates of net CO2 assimilation in the light, however, were reduced by 54, 63, and 94% 1 h after contact with Corexit 7664, 9550, and 9600, respectively. Measurements of surface contact angles and observations on the movement of dye–dispersant mixtures indicated that the rapid inhibition of CO2 assimilation resulted from the spontaneous infiltration of stomata by the dispersants and direct action on the internal tissues of the leaf. Rates of dark respiration were initially unaffected but decreased after 1 day. Further reductions in rates of CO2 exchange were observed over the first 4 days (associated with extensive leaf chlorosis and necrosis); then the rates increased following new tissue growth. Although the dispersants are potent contact poisons, damage to the protective cuticle in Salix interior does not appear to be a major contributing factor to their toxicity.

Inhibition of Photosynthesis by Bentazon in Intact Plants and Isolated Cells in Relation to the pH

Abstract Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4 (3H)-one 2,2-dioxide] causes an increase in the chlorophyll fluorescence in the leaves of mustard plants [Smapis alba (L.) ] as a result of its in­hibitory effect on the photosynthetic electron transport. The rate of this fluorescence induction depends on the pH of the suspension medium. As the concentration of hydrogen ions diminishes, the maximum fluorescence that indicates a total inhibition of photosynthesis is reached at a later time. In the case of single cells isolated from the leaves of soybean plants [Glycine max (L.) Mer “Harosoy”], bentazon inhibits photosynthetic CO1, assimilation. The amount of inhibition is depen­dent on the pH of the suspension medium. The uptake of CO2 is inhibited more at pH 6 than at pH 7 or 8. Bentazon is more readily absorbed by cells at pH 6 than at pH 7 or 8. The inhibition of CO2 assimilation is proportional to the amount of bentazon taken up.

Ribulose diphosphate carboxylase regulates soybean photorespiration.

THE inhibition of CO2 assimilation caused by oxygen in the leaves of higher plants which fix CO2 through ribulose diphosphate carboxylase has been well documented1–5; oxygen inhibits photosynthesis and stimulates photorespiration1,2. Equation (l)1,2 describes the light-saturated photosynthesis of soybeans and other photorespiratory species in the linear portion of the CO2 response curve.