Decrease In CO2 Fixation Research Articles

Reduction in chloroplastic ribulose-5-phosphate-3-epimerase decreases photosynthetic capacity in Arabidopsis

Chloroplast ribulose-5-phosphate-3-epimerase (RPE) is a critical enzyme involved in the Calvin-Benson cycle and oxidative pentose phosphate pathways in higher plants. Three Arabidopsis rpe mutants with reduced level of RPE were identified through their high NPQ (nonphotochemical quenching) phenotype upon illumination, and no significant difference of plant size was found between these rpe mutants and WT (wild type) plants under growth chamber conditions. A decrease in RPE expression to a certain extent leads to a decrease in CO2 fixation, V cmax and J max. Photosynthetic linear electron transport was partially inhibited and activity of ATP synthase was also decreased in the rpe mutants, but the levels of thylakoid protein complexes and other Calvin-Benson cycle enzymes in rpe mutants were not affected. These results demonstrate that some degree of reduction in RPE expression decreases carbon fixation in chloroplasts, which in turn feedback inhibits photosynthetic electron transport and ATP synthase activity due to the photosynthetic control. Taken together, this work provides evidence that RPE plays an important role in the Calvin-Benson cycle and influences the photosynthetic capacity of chloroplasts.

Mitigation of cadmium toxicity by zinc in juvenile cacao: Physiological, biochemical, molecular and micromorphological responses

Cadmium (Cd) is a trace metal without essential biological function due to its high toxicity to plants, animals and humans, even at low concentrations. On the other hand, Zn is an essential nutrient and plays important metabolic functions in plants. The study of the interaction between essential and a nonessential element may be important for understanding, analyzing and improving the defense strategies adapted by plants. The main objective of this work was to evaluate the mitigation of Cd toxicity by Zn in young plants of the CCN 51 cocoa genotype, grown in soil with different concentrations of Zn, Cd and Zn + Cd, through physiological, biochemical, molecular and micro-morphological responses. It was verified that high concentrations of Zn, Cd and Zn + Cd in the soil promoted alterations in the enzymatic and non-enzymatic antioxidative metabolism and expression of genes. This was demonstrated by increase in the activity of antioxidative enzymes, proline content and reduction in lipid peroxidation. Leaf gas exchange was affected at the highest soil Cd concentrations (0.4, 0.6 and 0.8 mmol Cd kg−1 soil) combined with different soil Zn concentrations (0.4, 0.8, 1.2 and 1.6 mmol kg-1 soil), resulting in a decrease in CO2 fixation. The higher concentration of soil Cd (0.8 mmol kg−1 soil), together with the intermediate concentrations of soil Zn + Cd (0.8 + 0.4 and 0.4 + 0.6 mmol kg−1 soil), promoted reduction of the thickness of the leaf mesophyll and, consequently, led to decrease of the leaf gas exchange. It was observed a hormesis effect due to high photosynthetic activity in low Cd concentration. The increase in Cd concentration in the soil altered the uptake of Cd and Zn by the roots of the CCN 51 cocoa genotype. The increase of Zn concentration in the soil promoted the decrease of the Cd uptake by the root system of the plants and thereby reduced the transport of Cd to the leaves. Part of Cd uptake by the plant's root system was immobilized in roots tissues, as a tolerance strategy, preventing that it was transported to the aerial part. The increase of Zn + Cd concentration in the soil did not influence the accumulation of Zn in the leaves of the young plants of the CCN 51 cocoa genotype.

Physiological, ultrastructural, biochemical and molecular responses of young cocoa plants to the toxicity of Cr (III) in soil

The objective of this study was to evaluate Cr toxicity in young plants of the CCN 51 Theobroma cacao genotype at different concentrations of Cr3+ in the soil (0, 100, 200, 400 and 600 mg kg−1) through physiological, ultrastructural, antioxidant and molecular changes. Doses of 400 and 600 mg Cr3+ kg−1 soil severely affected foliar gas exchange, promoted by damages in photosynthetic machinery evidenced by the decrease in CO2 fixation. Decreased expression of psbA and psbO genes, changes in enzymatic activity and lipid peroxidation also affected leaf gas exchange. A hormesis effect was observed at 100 mg Cr3+ kg−1 soil for the photosynthetic activity. As a metal exclusion response, the roots of the cocoa plants immobilized, on average, 75% of the total Cr absorbed. Ultrastructural changes in leaf mesophyll and roots, with destruction of mitochondria, plasmolysis and formation of vesicles, were related to the oxidative stress promoted by excess ROS. The activity of the antioxidant enzymes SOD, APX, GPX and CAT and the amino acid proline coincided with the greater expression of the sod cyt gene demonstrating synchronicity in the elimination of ROS. It was concluded, therefore, that the tolerance of the cocoa plants to the toxicity of Cr3+ depends on the concentration and time of exposure to the metal. Higher doses of Cr3+ in the soil promoted irreversible damage to the photosynthetic machinery and the cellular ultrastructure, interfering in the enzymatic and non-enzymatic systems related to oxidative stress and gene expression. However, the low mobility of the metal to the leaf is presented as a strategy of tolerance to Cr3+.

POSSIBLE ROLE OF MANNITOL AS AN OXYGEN RADICAL SCAVENGER IN OLIVE

POSSIBLE ROLE OF MANNITOL AS AN OXYGEN RADICAL SCAVENGER IN OLIVE

Analysis of inhibition of photosynthesis due to water stress in the C3 species Hordeum vulgare and Vicia faba: Electron transport, CO2 fixation and carboxylation capacity

A C3 monocot, Hordeum vulgare and C3 dicot, Vicia faba, were studied to evaluate the mechanism of inhibition of photosynthesis due to water stress. The net rate of CO2 fixation (A) and transpiration (E) were measured by gas exchange, while the true rate of O2 evolution (J O2) was calculated from chlorophyll fluorescence analysis through the stress cycle (10 to 11 days). With the development of water stress, the decrease in A was more pronounced than the decrease in J O2 resulting in an increased ratio of Photosystem II activity per CO2 fixed which is indicative of an increase in photorespiration due to a decrease in supply of CO2 to Rubisco. Analyses of changes in the J O2 A ratios versus that of CO2 limited photosynthesis in well watered plants, and RuBP pool/RuBP binding sites on Rubisco and RuBP activity, indicate a decreased supply of CO2 to Rubisco under both mild and severe stress is primarily responsible for the decrease in CO2 fixation. In the early stages of stress, the decrease in C i (intercellular CO2) due to stomatal closure can account for the decrease in photosynthesis. Under more severe stress, CO2 supply to Rubisco, calculated from analysis of electron flow and CO2 exchange, continued to decrease. However, C i, calculated from analysis of transpiration and CO2 exchange, either remained constant or increased which may be due to either a decrease in mesophyll conductance or an overestimation of C i by this method due to patchiness in conductance of CO2 to the intercellular space. When plants were rewatered after photosynthesis had dropped to 10-30% of the original rate, both species showed near full recovery within two to four days.

Effect of long‐term H2S fumigation on photosynthesis in spinach. Correlation between CO2 fixation and chlorophyll a fluorescence

Spinach plunts (Spinacia oleracea L. cv. Monosa) were exposed to air with and without 0.25 μl l‐1 H2S. Effects of H2S exposure for up to 18 days on photosynthesis, dark respiration and on chlorophyll a fluorescence were studied. Dark respiration was not affected by H2S fumigation. Photosynthetic CO2 fixation decreased linearly with time in both control and fumigated plants. The rate of decrease in CO2 fixation was faster in the fumigated plants; after 14 days of exposure the fumigated plants showed a decrease in CO2 fixation of 23%äs compared with the control plants. The H2S‐induced decrease in CO2 fixation was accompanied by a decrease in quenching of the chlorophyll fluorescence. The most characteristic change in chlorophyll fluorescence was a decreased difference between maximum and steady‐state fluorescence [(P‐T)/P), suggesting a reduced efficiency in the use of photochemical energy in photosynthesis. Differences in CO2 fixation were more pronounced whcn measured at high light intensity; the maximum rate of CO2 fixation at light saturation decreased significantly with time in the H2S‐exposed plants; after 14 days of H2S exposure a decrease of more than 70% was noted. The decrease in CO2 fixation could not be attributed to a decreased chlorophyll content; on the contrary, chlorophyll content even slightly increased during fumigation. The initial increase in CO2 fixation rate with increasing light intensity was also reduced by prolonged H2S fumigation, indicating an effect of H2S fumigation on photosynthetic electron transport.Finally, the phytotoxicity of H2S is discusscd in relation to the H2S‐induced changes in photosynthetic CO2 fixation and chlorophyll a fluorescence, and the effect of H2S on leaf development observed in earlier studies.

Effect of high light intensities on oxygen evolution and the light activation of NADP-malate dehydrogenase in intact spinach chloroplasts

The factors limiting the photosynthetic carbon metabolism of intact spinach (Spinacia oleracea L.) chloroplasts after a high-light pretreatment have been studied. Photosynthetic CO2 fixation was decreased and became more sensitive to the inhibitory effect of the cyclic-electron-flow inhibitor, antimycin A. Depending on the extent of photoinhibition, changing the balance of linear to cyclic electron flow by adding oxaloacetate and antimycin A either did not relieve, or partially relieved the photoinhibitory effect. The decrease in CO2 fixation appeared to be the consequence of either a limitation by photosystem-II activity (in the case of moderate inhibition) or, at least partially an unfavourable balance between the linear and cyclic electron flows (in the case of strong inhibition). The light activation of NADP-malate dehydrogenase (EC 1.1.1.82) was decreased only in the presence of CO2, i.e. when there was strong competition for reducing power; otherwise, it was unaffected by photoinhibitory treatments, in accordance with its low energy requirement.

The Effect of Canopy Wetting on Plant Water Status, CO2 Fixation, Ion Content and Growth Rate of ‘Baccara’ Roses

AbstractThe effect of canopy wetting of rose (Rosa hybrida) plants on transpiration rate, stomatal resistance, plant water potential, ion concentration in leaves, and several growth parameters was investigated. Plants were grown in a greenhouse and their canopies were exposed to wetting cycles during the daytime. Canopy wetting decreased transpiration rate and increased diffusive resistance to water vapor transfer. Plant water potential was only slightly increased by wetting, mainly in the spring.Canopy wetting did not prevent the decrease in CO2 fixation, which took place before midday when plant water potential dropped below – 10 bars during a rise in radiation intensity. The concentrations of Cl− and Ca2+ decreased in wetted leaves, while the concentration of Na+ was increased and other cations were hardly affected. Canopy wetting inhibited growth rate of rose shoots and flower formation, and increased the average flower weight.