Thermotolerance Of Photosynthesis Research Articles

Monoterpene 'thermometer' of tropical forest-atmosphere response to climate warming.

Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis but elevated temperatures suppress this absorption and promote monoterpene emissions. Using 13 CO2 labeling, here we show that monoterpene emissions from tropical leaves derive from recent photosynthesis and demonstrate distinct temperature optima for five groups (Groups 1-5), potentially corresponding to different enzymatic temperature-dependent reaction mechanisms within β-ocimene synthases. As diurnal and seasonal leaf temperatures increased during the Amazonian 2015 El Niño event, leaf and landscape monoterpene emissions showed strong linear enrichments of β-ocimenes (+4.4% °C-1 ) at the expense of other monoterpene isomers. The observed inverse temperature response of α-pinene (-0.8% °C-1 ), typically assumed to be the dominant monoterpene with moderate reactivity, was not accurately simulated by current global emission models. Given that β-ocimenes are highly reactive with respect to both atmospheric and biological oxidants, the results suggest that highly reactive β-ocimenes may play important roles in the thermotolerance of photosynthesis by functioning as effective antioxidants within plants and as efficient atmospheric precursors of secondary organic aerosols. Thus, monoterpene composition may represent a new sensitive 'thermometer' of leaf oxidative stress and atmospheric reactivity, and therefore a new tool in future studies of warming impacts on tropical biosphere-atmosphere carbon-cycle feedbacks.

Water-stress-induced thermotolerance of photosynthesis in bean ( Phaseolus vulgaris L.) plants: The possible involvement of lipid composition and xanthophyll cycle pigments

A common feature in plants that are exposed to gradual stressful environmental factors is the development of a level of resistance to such constraints, which sometimes protects against different stressful environmental conditions. The aim of this study was to assess a possible cross-resistance between water stress and high-temperature stress and to gain a better knowledge regarding the physiological basis for heat resistance. The study was performed in two bean varieties; Orfeo INIA (OI) and Arroz Tuscola (AT) are a stress-resistant and stress-sensitive variety, respectively. An increased heat resistance in OI but not in AT results from water stress as revealed by the oxygen-evolution rate at 38 °C and the thermal threshold for non-reversible damage that was assessed by the temperature-dependent increases in basal fluorescence ( F 0); higher values of both parameters were observed in the leaves of water-stressed plants when compared to the control OI plants. The heat-shock proteins HSP70, HSP60 and HSP24 do not seem to be involved in the water-stress-induced resistance to high temperature because no difference in their contents was found between the water-stressed and control plants. The important features in the water-stressed OI plants, which can resist higher temperatures, are the maintenance of the xanthophyll pigment contents compared to the control plants and in contrast to the sensitive AT plants, an increase in phosphatidylglycerol and a reduction in the unsaturation level of the thylakoid fatty acids. The results from the comparative analyses of the xanthophyll, lipid and fatty acid compositions in the chloroplasts of well-watered and water-stressed AT and OI plants are discussed in terms of their possible involvement in conferring resistance to high temperature in water-stressed bean leaves.

Isoprene synthesis protects transgenic tobacco plants from oxidative stress

Isoprene emission represents a significant loss of carbon to those plant species that synthesize this highly volatile and reactive compound. As a tool for studying the role of isoprene in plant physiology and biochemistry, we developed transgenic tobacco plants capable of emitting isoprene in a similar manner to and at rates comparable to a naturally emitting species. Thermotolerance of photosynthesis against transient high-temperature episodes could only be observed in lines emitting high levels of isoprene; the effect was very mild and could only be identified over repetitive stress events. However, isoprene-emitting plants were highly resistant to ozone-induced oxidative damage compared with their non-emitting azygous controls. In ozone-treated plants, accumulation of toxic reactive oxygen species (ROS) was inhibited, and antioxidant levels were higher. Isoprene-emitting plants showed remarkably decreased foliar damage and higher rates of photosynthesis compared to non-emitting plants immediately following oxidative stress events. An inhibition of hydrogen peroxide accumulation in isoprene-emitting plants may stall the programmed cell death response which would otherwise lead to foliar necrosis. These results demonstrate that endogenously produced isoprene provides protection from oxidative damage.

Strawberry (Fragaria ×ananassa Duch.) Growth and Productivity as Affected by Temperature

Thermotolerance of photosynthesis and productivity in `Chandler' and `Sweet Charlie' strawberry plants (Fragaria ×ananassa Duch.) exposed to three temperature regimes was studied. Net CO2 assimilation rate (A), variable chlorophyll fluorescence (Fv), efficiency of photosystem II (Fv/Fm), relative chlorophyll content, plant growth, and fruit yield and quality were measured. High temperature (40 °C day/35 °C night) was more detrimental to photosynthesis and productivity than the moderate or low temperature (30/25 or 20/15 °C). Net CO2 assimilation rate in both cultivars was markedly reduced by 40/35 °C, although there was slight decline in `Sweet Charlie' at 30/25 °C. `Chandler' maintained significantly higher A rates than `Sweet Charlie' for at least three weeks of heat stress, indicating that `Chandler' might tolerate longer exposure to high temperature. In parallel to the decrease in A rate, intercellular CO2 concentration (Ci) and instantaneous water use efficiency (WUE) were significantly decreased at high temperature. `Chandler' leaves were cooler and transpired more than `Sweet Charlie' leaves, suggesting that each cultivar adopted different heat resistance mechanisms at 40/35 °C. There were changes in Fv and Fv/Fm with increasing temperature, indicating irreversible damage to photosystem II at 40/35 °C might have occurred. The trend of reduction in stomatal conductance (gS) in both cultivars at high temperature did not coincide with the reduction in A rates. Decline in A rates at high temperature was more related to changes in Fv/Fm than to gS activity. The optimal temperature for vegetative growth was 30/25 °C. Reduction in A rate at high temperature resulted in reduction in total leaf area (LA), shoot, root, and leaf biomasses. Strawberry roots were more responsive than shoot growth to temperatures above 20/15 °C. Fruit yield for `Chandler' was higher at 20/15 °C than at 30/25 °C, suggesting that `Chandler' might have a higher source-to-sink relationship at 20/15 °C than at 30/25 °C. Fruit skin color was temperature dependent only for `Chandler'. A quadratic relationship between flower development and duration of exposure to 30/25 °C for both cultivars was observed; more than two weeks of 30/25 °C can be detrimental to flower development. Regardless of the cultivar and duration of exposure, 40/35 °C was the temperature regime most detrimental to fruit set.

Effect of activase level and isoform on the thermotolerance of photosynthesis in Arabidopsis

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation decreases under moderate heat stress. This decrease is caused by an impairment of activase function, which is exacerbated by faster rates of Rubisco deactivation at elevated temperatures. To determine if stromal oxidation causes inhibition of activase, transgenic Arabidopsis plants expressing suboptimal amounts of either the redox-regulated 46 kDa alpha- or non-redox regulated 43 kDa beta-isoform of activase were examined. Photosynthesis, as measured by gas exchange and chlorophyll fluorescence, and Rubisco activation were inhibited to a much greater extent by moderately high temperatures in the two transgenic lines expressing suboptimal levels of the individual isoforms of activase compared with wild-type plants or transgenic plants expressing levels of the beta-isoform sufficient for wild-type rates of photosynthesis. Net photosynthesis and Rubisco activation in transgenic plants expressing suboptimal amounts of the beta-isoform of activase from the Antarctic hairgrass were even more sensitive to inhibition by moderate heat stress than in the transgenic plants containing Arabidopsis activase. The results demonstrate that photosynthesis exhibits a similar sensitivity to inhibition by moderately high temperature in plants expressing either of the two different isoforms of activase. Thus, impairment of activase function under heat stress is not caused by oxidation of the redox-sensitive sulphydryls of the alpha-isoform of activase. Instead, the results are consistent with thermal denaturation of activase under moderate heat stress, the effects of which on Rubisco activation would be enhanced when activase levels are suboptimal for photosynthesis.

Contrasting effects of elevated carbon dioxide concentration and temperature on Rubisco activity, chlorophyll fluorescence, needle ultrastructure and secondary metabolites in conifer seedlings.

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were grown for 50 days in growth chambers in an ambient or twice ambient carbon dioxide concentration ([CO2]) at a day/night temperature of 19/12 degrees C or 23/16 degrees C. Although elevated [CO2] (EC) had only slight effects on the growth parameters measured, elevated temperature (ET) increased above ground dry mass of both species. Among treatments, biomass accumulation of both species was greatest in the combined EC + ET treatment. The EC treatment induced thylakoid swelling and increased numbers of plastoglobuli observed in Scots pine needles. Although EC had little effect on Rubisco protein or N concentration of needles, ET had a large effect on N-containing compounds and enhanced N allocation from 1-year-old needles. Terpenoids were more responsive to EC and ET than total phenolics. Generally, terpene concentrations were reduced by EC and increased by ET. Increased terpenoid concentrations in response to ET might be associated with thermotolerance of photosynthesis. In Norway spruce, EC decreased total phenolic concentrations in needles, probably as a result of increased growth. We conclude that, in seedlings of these boreal species, the effects of elevated [CO2] on the studied parameters were small compared with the effects of elevated temperature.

Isoprene increases thermotolerance of fosmidomycin-fed leaves.

Isoprene is synthesized and emitted in large amounts by a number of plant species, especially oak (Quercus sp.) and aspen (Populus sp.) trees. It has been suggested that isoprene improves thermotolerance by helping photosynthesis cope with high temperature. However, the evidence for the thermotolerance hypothesis is indirect and one of three methods used to support this hypothesis has recently been called into question. More direct evidence required new methods of controlling endogenous isoprene. An inhibitor of the deoxyxylulose 5-phosphate pathway, the alternative pathway to the mevalonic acid pathway and the pathway by which isoprene is made, is now available. Fosmidomycin eliminates isoprene emission without affecting photosynthesis for several hours after feeding to detached leaves. Photosynthesis of fosmidomycin-fed leaves recovered less following a 2-min high-temperature treatment at 46 degrees C than did photosynthesis of leaves fed water or fosmidomycin-fed leaves in air supplemented with isoprene. Photosynthesis of Phaseolus vulgaris leaves, which do not make isoprene, exhibited increased thermotolerance when isoprene was supplied in the airstream flowing over the leaf. Other short-chain alkenes also improved thermotolerance, whereas alkanes reduced thermotolerance. It is concluded that thermotolerance of photosynthesis is a substantial benefit to plants that make isoprene and that this benefit explains why plants make isoprene. The effect may be a general hydrocarbon effect and related to the double bonds in the isoprene molecule.

Evaluation of Heat and Flood Tolerance among Diverse Taxa of Hollies (Ilex)

Temperature sensitivity of net photosynthesis (PSN), dark respiration, and chlorophyll fluorescence was evaluated among three taxa of hollies including I. aquifolium, I. cornuta, and I. rugosa. Variations in foliar heat tolerance among these species were expressed as differential temperature responses for PSN. Temperature optima for PSN was 22.0, 26.3 and 27.9 umol·m–2·s–1 for I. rugosa, I. cornuta, and I. aquifolium, respectively. Differences in temperature optima for PSN and thermotolerance of PSN appeared to result from a combination of stomatal and nonstomatal limitations. At 40°C, potential photosynthetic capacity, measured under saturating CO2, was 4.1, 9.4, and 14.8 μmol·m–2·s–1 for I. rugosa, I. aquifolium, and I. cornuta, respectively. Based on these results, I. rugosa was identified as the most heat-sensitive species followed by I. aquifolium then I. cornuta. Comparative tolerance to root-zone inundation was evaluated among 14 holly taxa. Following 8 weeks of flooding, four of the taxa: I. cornuta `Burfordii', I. × `Nellie R. Stevens', I. cassine, and I. × attenuata `Foster's #2' performed remarkably well during and after flooding with photosynthetic rates > 40% of the controls, root ratings >75% of the controls, <5% of the foliage showing deterioration, and 100% survival. Conversely, I. crenata `Convexa', Ilex × meserveae `Blue Princess', I. rugosa and I. aquifolium `Sparkler' did not tolerate flooding well as indicated by severely depressed photosynthetic rates, deterioration of foliage and roots, and decreased survival. The remaining taxa were intermediate.0