Response of photosynthesis and respiration to temperature under water deficit in two evergreen Nothofagus species

Respiration and photosynthesis were studied in two Nothofagus species with different drought tolerance in order to evaluate the effect of water deficit on foliar carbon balance and the possible role of the alternative pathway on respiratory adjustment. We propose that under severe water deficit the more drought‐tolerant species N. dombeyi is able to decrease its respiration more than the less drought‐tolerant species N. nitida, thus carbon gain could be maintained when photosynthesis is suppressed by drought. Dark respiration (R_d) and carbon assimilation under saturating light (A_sat) were evaluated under seasonal field conditions and during drying and re‐watering cycles under glasshouse. In addition, respiratory pathway changes were evaluated by oxygen isotope fractionation. In the field, N. dombeyi displayed greater light‐saturated photosynthetic capacity than N. nitida, but R_d did not differ between species during summer. In the glasshouse, N. dombeyi displayed an unchanged rate of R_d and increased carbon loss under severe water deficit. Nothofagus nitida displayed a more flexible respiratory response to water deficit, with a lower thermal sensitivity of respiration (decrease in Q₁₀) and a decrease in R_d. This contributed to maintaining leaf carbon balance during the water deficit period. Respiratory electron flow was mainly via the cytochrome pathway for both species and under all treatments, indicating no strong participation of alternative respiration. Our results suggest that under severe water stress, N. dombeyi could be more injured than N. nitida and that the lack of control in the carbon loss under prolonged periods of drought could be limiting for its survival.     

Dependence of carbon sequestration on the differential responses of ecosystem photosynthesis and respiration to rain pulses in a semiarid steppe

Precipitation pulses play an important role in regulating ecosystem carbon exchange and balance of semiarid steppe ecosystems. It has been predicted that the frequency of extreme rain events will increase in the future, especially in the arid and semiarid regions. We hypothesize that large rain pulses favor carbon sequestration, while small ones cause more carbon release in the semiarid steppes. To understand the potential response in carbon sequestration capacity of semiarid steppes to the changes in rain pulse size, we conducted a manipulative experiment with five simulated rain pulse sizes (0, 5, 10, 25, and 75 mm) in Inner Mongolia steppe. Our results showed that both gross ecosystem productivity (GEP) and ecosystem respiration (R_e) responded rapidly (within 24 h) to rain pulses and the initial response time was independent of pulse size. However, the time of peak GEP was 1–3 days later than that of R_e, which depended on pulse size. Larger pulses caused greater magnitude and longer duration of variations in GEP and R_e. Differences in the response time of microbes and plants to wetting events constrained the response pattern of heterotrophic (R_h) and autotrophic (R_a) components of R_e following a rain event. R_h contributed more to the increase of R_e in the early stage of rain pulse response, while R_a played an more important role later, and determined the duration of pulse response, especially for large rain events of >10 mm. The distinct responses of ecosystem photosynthesis and respiration to increasing pulse sizes led to a threshold in rain pulse size between 10 and 25 mm, above which post wetting responses favored carbon sequestration. The disproportionate increase of the primary productivity of higher plants, compared with those in the activities of microbial decomposers to larger pulse events suggests that the carbon sequestration capacity of Inner Mongolia steppes will be sensitive to changes in precipitation size distribution rather than just precipitation amount.     

Anaerobic respiration and antioxidant responses of Corythucha ciliata (Say) adults to heat-induced oxidative stress under laboratory and field conditions

High temperature often induces oxidative stress and antioxidant response in insects. This phenomenon has been well documented under controlled laboratory conditions, but whether it happens under fluctuating field conditions is largely unknown. In this study, we used an invasive lace bug (Corythucha ciliata) as a model species to compare the effects of controlled thermal treatments (2 h at 33–43 °C with 2 °C intervals in the laboratory) and naturally fluctuating thermal conditions (08:00–14:00 at 2-h intervals (29.7–37.2 °C) on a hot summer day in a field in Shanghai, China) on lipid peroxidation (malondialdehyde (MDA) was the marker) and anaerobic respiration (lactate dehydrogenase (LDH) was the marker), as well as superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione reductase (GR). The results show that MDA concentration increased significantly in response to heat stresses with similar trend in the laboratory and field. LDH activities did not significantly vary across temperatures in the laboratory-exposed individuals, but they significantly increased by rising temperature in the field. The activities or concentrations of SOD, CAT, GSH, and GR all significantly increased with increasing temperature in the two populations. These findings indicate that high temperature induces oxidative stress, resulting in high anaerobic respiration and antioxidant defenses in C. ciliata under both the laboratory and field conditions, which likely provide a defense mechanism against oxidative damage due to the accumulation of ROS.     

Adaptive responses to fescue toxicosis under thermoneutral and heat stress conditions

This study determined the potential for short-term adaptation to fescue toxicosis and heat stress in rats. Male CD outbred rats (n=24) were implanted with temperature transmitters (Respironics, Bend, OR) to measure core temperature (Tc) and general activity. All rats were initially fed diets with ground, uninfected tall fescue seed (E−) and exposed to 21 °C (thermoneutral, TN) to establish baseline values. In Period 1, all groups were maintained at TN for 7 days, with one group fed a diet containing ground, endophyte-infected tall fescue seed (E+, approximately 165 μg ergovaline/kg BW/d) and two groups fed E− diet. Ergovaline is thought to be the primary toxin responsible for many symptoms associated with fescue toxicosis. Period 1 was followed by 7 days at 31 °C (heat stress, HS, Period 2) on the same diets. All animals were fed E− diet during the second 7 day of HS (Period 3). In the final 7 day (Period 4), E+ diet was returned to the original group and fed to one of the previously E− groups, with the third group remaining on E− diet. A 40% decrease in FI occurred with E+ treatment at TN (P<0.05), with a comparable BW reduction (P<0.05) after 4 day. Both responses worsened during HS. Treatment with E+ in Period 4 indicated that FI and BW had not adapted to fescue toxicosis. A reduction in daily Tc occurred with E+ treatment at TN (P<0.05) followed by hyperthermia during the initial stage of HS (P<0.05). Although feed intake and growth rate showed no change over time, there was a reduction in fescue toxicosis-induced hyperthermia in the heat with repeat treatment. Conditioning animals to fescue toxicosis and heat stress prior to exposure may be beneficial in reducing impacts on thermal status of the animal.     

Experimental and simulated light responses of photosynthesis in leaves of three tree species under different soil water conditions

Water deficit is one of the major limiting factors in vegetation recovery and restoration in loess, hilly-gully regions of China. The light responses of photosynthesis in leaves of two-year old Prunus sibirica L., Hippophae rhamnoides L., and Pinus tabulaeformis Carr. under various soil water contents were studied using the CIRAS-2 portable photosynthesis system. Light-response curves and photosynthetic parameters were analyzed and fitted using the rectangular hyperbola model, the exponential model, the nonrectangular hyperbola model, and the modified rectangular hyperbola model. Under high light, photosynthetic rate (P _N) and stomatal conductance (g _s) were steady and photoinhibition was not significant, when the relative soil water content (RWC) varied from 56.3–80.9%, 47.9–82.9%, and 33.4–92.6% for P. sibirica, H. rhamnoides, and P. tabulaeformis, respectively. The light-response curves of P _N, the light compensation point (LCP), and the dark respiration rate (R _D) were well fitted using the above four models. The nonrectangular hyperbola was the best model in fitting the data; the modified rectangular hyperbola model was the second, and the rectangular hyperbola model was the poorest one. When RWC was higher or lower than the optimal range, the obvious photoinhibition and significant decrease in P _N with increasing photosynthetic photon flux density (PPFD) were observed in all three species under high light. The light saturation point (LSP) and apparent quantum yield also decreased significantly, when the upper limit of PPFD was 200 μmol m^?2 s^?1. Under these circumstances, only the modified rectangular hyperbola model was able to fit well the curves of the light response, LCP, LSP, R _D, and light-saturated P _N.     

Arbuscular mycorrhizal fungi influence growth, osmotic adjustment and photosynthesis of citrus under well-watered and water stress conditions

The influence of arbuscular mycorrhizal (AM) fungus Glomus versiforme on plant growth, osmotic adjustment and photosynthesis of tangerine (Citrus tangerine) were studied in potted culture under well-watered and water stress conditions. Seven-day-old seedlings of tangerine were transferred to pots containing Glomus versiforme or non-AMF. After 97 days, half of the seedlings were subject to water stress and the rest were well-watered for 80 days. AM colonization significantly stimulated plant growth and biomass regardless of water status. The soluble sugar of leaves and roots, the soluble starch of leaves, the total non-structural carbohydrates (NSC) of leaves and roots, and the Mg(2+) of leaves were higher in AM seedlings than those in corresponding non-AM seedlings. The levels of K(+) and Ca(2+) in leaves and roots were higher in AM seedlings than those in non-AM seedlings, but differences were only significant under water stress conditions. Moreover, AM colonization increased the distributed proportions of soluble sugar and NSC to roots. However, the proline was lower in AM seedlings compared with that in non-AM seedlings. AM seedlings had higher leaf water potential (Psi), transpiration rates (E), photosynthetic rates (Pn), stomatal conductance (g(s)), relative water content (RWC), and lower leaf temperature (Lt) than corresponding non-AM seedlings. This research also suggested that AM colonization improved the osmotic adjustment originating not from proline but from NSC, K(+), Ca(2+) and Mg(2+), resulting in the enhancement of drought tolerance.     

Differential responses of production and respiration to temperature and moisture drive the carbon balance across a climatic gradient in New Mexico

Southwestern North America faces an imminent transition to a warmer, more arid climate, and it is critical to understand how these changes will affect the carbon balance of southwest ecosystems. In order to test our hypothesis that differential responses of production and respiration to temperature and moisture shape the carbon balance across a range of spatio‐temporal scales, we quantified net ecosystem exchange (NEE) of CO₂ and carbon storage across the New Mexico Elevational Gradient, which consists of six eddy‐covariance sites representing biomes ranging from desert to subalpine conifer forest. Within sites, hotter and drier conditions were associated with an increasing advantage of respiration relative to production such that daily carbon uptake peaked at intermediate temperatures – with carbon release often occurring on the hottest days – and increased with soil moisture. Across sites, biotic adaptations modified but did not override the dominant effects of climate. Carbon uptake increased with decreasing temperature and increasing precipitation across the elevational gradient; NEE ranged from a source of ～30 g C m^?2 yr^?1 in the desert grassland to a sink of ～350 g C m^?2 yr^?1 in the subalpine conifer forest. Total aboveground carbon storage increased dramatically with elevation, ranging from 186 g C m^?2 in the desert grassland to 26 600 g C m^?2 in the subalpine conifer forest. These results make sense in the context of global patterns in NEE and biomass storage, and support that increasing temperature and decreasing moisture shift the carbon balance of ecosystems in favor of respiration, such that the potential for ecosystems to sequester and store carbon is reduced under hot and/or dry conditions. This implies that projected climate change will trigger a substantial net release of carbon in these New Mexico ecosystems (～3 Gt CO₂ statewide by the end of the century), thereby acting as a positive feedback to climate change.     

Photochemical responses and oxidative stress in two clones of Coffea canephora under water deficit conditions

The effects of water deficit on photochemical parameters and activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), as well as, cellular damages were investigated in two clones of Coffea canephora differing in drought tolerance. After 6 days without irrigation, predawn leaf water potential fell to −3.0 MPa that was accompanied by the suppression of net photosynthesis in both clones. The variable to maximum chlorophyll fluorescence ratio remained unchanged regardless of the imposed treatments. Both clones showed a similar decline (about 25%) in the photochemical quenching coefficient, but only the drought-sensitive clone exhibited an enhancement (31%) of thermal deactivation under water deficit conditions. The quantum yield of electron transport decreased similarly in both genotypes. Under drought conditions, activities of SOD, CAT and APX increased to a greater extent in the drought-tolerant clone than in the drought-sensitive one. This seemed to be matched with higher protection against oxidative stress, as judged from the lower levels of lipid peroxidation and electrolyte leakage in the drought-tolerant clone. Thus, the ability to increase the antioxidant system activity in order to limit cellular damages might be an important attribute linked to the drought tolerance in C. canephora.     

Unchanged carbon balance driven by equivalent responses of production and respiration to climate change in a mixed‐grass prairie

Responses of grassland carbon (C) cycling to climate change and land use remain a major uncertainty in model prediction of future climate. To explore the impacts of global change on ecosystem C fluxes and the consequent changes in C storage, we have conducted a field experiment with warming (+3 °C), altered precipitation (doubled and halved), and annual clipping at the end of growing seasons in a mixed‐grass prairie in Oklahoma, USA, from 2009 to 2013. Results showed that although ecosystem respiration (ER) and gross primary production (GPP) negatively responded to warming, net ecosystem exchange of CO₂ (NEE) did not significantly change under warming. Doubled precipitation stimulated and halved precipitation suppressed ER and GPP equivalently, with the net outcome being unchanged in NEE. These results indicate that warming and altered precipitation do not necessarily have profound impacts on ecosystem C storage. In addition, we found that clipping enhanced NEE due to a stronger positive response of GPP compared to ER, indicating that clipping could potentially be an effective land practice that could increase C storage. No significant interactions between warming, altered precipitation, and clipping were observed. Meanwhile, we found that belowground net primary production (BNPP) in general was sensitive to climate change and land use though no significant changes were found in NPP across treatments. Moreover, negative correlations of the ER/GPP ratio with soil temperature and moisture did not differ across treatments, highlighting the roles of abiotic factors in mediating ecosystem C fluxes in this grassland. Importantly, our results suggest that belowground C cycling (e.g., BNPP) could respond to climate change with no alterations in ecosystem C storage in the same period.     

Internal water balance of barley under soil moisture stress

Leaf water potential, leaf relative water content, and relative transpiration of barley were determined daily under greenhouse conditions at 3 growth stages: tillering to boot, boot to heading, and heading to maturity. The leaf moisture characteristic curve (relative water content versus leaf water potential) was the same for leaves of the same age growing in the same environment for the first 2 stages of growth, but shifted at the heading to maturity stage to higher leaf relative water content for a given leaf water potential. Growth chamber experiments showed that the leaf moisture characteristic curve was not the same for plants growing in different environments.

Relative transpiration data indicated that barley stomates closed at a water potential of about −22 bars at the 3 stages studied.

The water potential was measured for all the leaves on barley to determine the variation of water potential with leaf position. Leaf water potential increased basipetally with plant leaf position. In soil with a moisture content near field capacity a difference of about 16.5 bars was observed between the top and bottom leaves on the same plant, while in soil with a moisture content near the permanent wilting point the difference was only 5.6 bars between the same leaf positions.

     

Initial water deficit effects on Lupinus albus photosynthetic performance, carbon metabolism, and hormonal balance: metabolic reorganization prior to early stress responses

The early (2-4 d) effects of slowly imposed soil water deficit on Lupinus albus photosynthetic performance, carbon metabolism, and hormonal balance in different organs (leaf blade, stem stele, stem cortex, and root) were evaluated on 23-d-old plants (growth chamber assay). Our work shows that several metabolic adjustments occurred prior to alteration of the plant water status, implying that water deficit is perceived before the change in plant water status. The slow, progressive decline in soil water content started to be visible 3 d after withholding water (3 DAW). The earliest plant changes were associated with organ-specific metabolic responses (particularly in the leaves) and with leaf conductance and only later with plant water status and photosynthetic rate (4 DAW) or photosynthetic capacity (according to the Farquhar model; 6 DAW). Principal component analysis (PCA) of the physiological parameters, the carbohydrate and the hormone levels and their relative values, as well as leaf water-soluble metabolites full scan data (LC-MS/MS), showed separation of the different sampling dates. At 6 DAW classically described stress responses are observed, with plant water status, ABA level, and root hormonal balance contributing to the separation of these samples. Discrimination of earlier stress stages (3 and 4 DAW) is only achieved when the relative levels of indole-3-acetic acid (IAA), cytokinins (Cks), and carbon metabolism (glucose, sucrose, raffinose, and starch levels) are taken into account. Our working hypothesis is that, in addition to single responses (e.g. ABA increase), the combined alterations in hormone and carbohydrate levels play an important role in the stress response mechanism. Response to more advanced stress appears to be associated with a combination of cumulative changes, occurring in several plant organs. The carbohydrate and hormonal balance in the leaf (IAA to bioactive-Cks; soluble sugars to IAA and starch to IAA; relative abundances of the different soluble sugars) flag the initial responses to the slight decrease in soil water availability (10-15% decrease). Further alterations in sucrose to ABA and in raffinose to ABA relative values (in all organs) indicate that soil water availability continues to decrease. Such alterations when associated with changes in the root hormone balance indicate that the stress response is initiated. It is concluded that metabolic balance (e.g. IAA/bioactive Cks, carbohydrates/IAA, sucrose/ABA, raffinose/ABA, ABA/IAA) is relevant in triggering adjustment mechanisms.     

Inverse determination of heterotrophic soil respiration response to temperature and water content under field conditions

Heterotrophic soil respiration is an important flux within the global carbon cycle. Exact knowledge of the response functions for soil temperature and soil water content is crucial for a reliable prediction of soil carbon turnover. The classical statistical approach for the in situ determination of the temperature response (Q₁₀ or activation energy) of field soil respiration has been criticised for neglecting confounding factors, such as spatial and temporal changes in soil water content and soil organic matter. The aim of this paper is to evaluate an alternative method to estimate the temperature and soil water content response of heterotrophic soil respiration. The new method relies on inverse parameter estimation using a 1-dimensional CO₂ transport and carbon turnover model. Inversion results showed that different formulations of the temperature response function resulted in estimated response factors that hardly deviated over the entire range of soil water content and for temperature below 25°C. For higher temperatures, the temperature response was highly uncertain due to the infrequent occurrence of soil temperatures above 25°C. The temperature sensitivity obtained using inverse modelling was within the range of temperature sensitivities estimated from statistical processing of the data. It was concluded that inverse parameter estimation is a promising tool for the determination of the temperature and soil water content response of soil respiration. Future synthetic model studies should investigate to what extent the inverse modelling approach can disentangle confounding factors that typically affect statistical estimates of the sensitivity of soil respiration to temperature and soil water content.     

Growth and developmental responses of potato to osmotic stress under in vitro conditions

The effect of mannitol on different genotypes of potato was studied in callus and plantlet culture. In vitro responses of five potato genotypes with well-known field behaviour to water deficit were analysed. After a 4-week-long cultivation on media containing mannitol up to 0.8 M, different morpho-physiological parameters were determined and statistically analysed. The useful concentration of mannitol for in vitro screening the osmotic tolerance of different genotypes depended on the type of culture; it was 0.4 M in plantlet-test and 0.8 M in callus-test. In callus-test the relative increase of callus mass was a useful parameter for determination of osmotic tolerance of genotypes at cellular level. In plantlet culture, stress index calculated from the rate of surviving in vitro shoots, number and length of roots per surviving explant and the rate of rooted explants were applicable to determine three groups according to the tolerant, medium tolerant and sensitive categories in agreement with the field behaviour of these genotypes. Under in vitro stress conditions we were able to distinguish the examined genotypes with different drought tolerance.     

Comparison of responses to CO2 under the conditions of natural respiration and controlled artificial pulmonary ventilation

The relationship between the sensation of lack of air caused by the addition of carbon dioxide to inhaled air and minute pulmonary ventilation was studied in eight healthy male volunteers. Under the conditions of both natural respiration and controlled artificial ventilation of the lungs, ventilation increased in proportion to the increase in the partial pressure of CO₂ in the end-tidal exhaled air. The individual differences in the ventilation responses to CO₂ under the conditions of controlled ventilation and natural ventilation are regarded as a property of voluntary control.     

番茄叶片光合作用对快速水分胁迫的响应

采用聚乙二醇(PEG-6000)溶液控制番茄根际水势和叶片离体的方式设置了水分胁迫处理,测算了光合诱导过程中净光合速率、暗呼吸速率和CO2补偿点等光合参数的变化.结果表明:在1000μmol·m-2·s-1光诱导下,水分胁迫处理的番茄叶片净光合速率(Pn)达到最大值所需时间缩短为对照的1/3,气孔导度(gs)快速增大为对照的1.5倍.水分胁迫处理的番茄叶片光饱和点(LSP)比对照降低了65％～85％,而光补偿点(LCP)比对照增加了75％ ～100％,缩小了番茄叶片利用光能的有效范围.水分胁迫处理的番茄叶片最大光合能力(Amax)低于对照40％以上,暗呼吸速率(Rd)增大了约45％.可见,快速水分胁迫处理使番茄叶片气孔迅速开放,光合诱导初始阶段消失.水分胁迫导致植物利用光能的效率和潜力降低是植物生产力下降的重要原因,而气孔调节是番茄适应快速水分胁迫的重要生理机制.     

Jasmonic acid interacts with abscisic acid to regulate plant responses to water stress conditions

Phytohormones are key players in signaling environmental stress conditions. Hormone profiling together with proline accumulation were studied in leaves and roots of different mutant lines of Arabidopsis. Regulation of proline accumulation in this system seems complex and JA-deficient (jar1-1) and JA-insensitive (jai1) lines accumulating high levels of proline despite their very low ABA levels seems to discard an ABA-dependent response. However, the pattern of proline accumulation in jai1 seedlings parallels that of ABA. Under stress conditions, there is an opposite pattern of ABA accumulation in roots of jar1-1/coi1-16 (in which ABA only slightly increase) and jai1 (in which ABA increase is even higher than in WT plants). This also makes JA-ABA crosstalk complex and discards any lineal pathway that could explain this hormonal interaction.