Atmospheric CO2 concentration CO2] and global warming are undoubted facts. The increasing ground-level O3 concentration, which is and will be a serious threat of plant growth, is regarded as one of the important aspects among global changes. The increase in atmospheric CO2 and temperature is expected to be more in autumn and spring, and therefore winter wheat will be affected most by global environmental changes. Wheat is identified to be one of the most O3 sensitive crops, however, the weather during its grain filling stage is conducive to the formation of O3 via photochemical reaction. I firstly studied the growth and physiological responses of winter wheat cultivars released during the past 60 years in Northern China to doubled CO2] and 2oC warming; secondly, I compared the photosynthetic and yield responses of two winter wheat cultivars with distinct O3 sensitivity to short-term O3 stress at post-anthesis stage; thirdly, I studied the influences of mild drought during and after the O3 exposure on flag leaf photosynthesis and final yield; lastly, I studied the interactive effects of elevated O3 and warming during grain filling stage on flag leaf photosynthesis and final yield.
Current levels of Ozone (O3) in peri-urban areas in China are high enough to decrease crop growth and yield. However, little is known about the sensitivity to O3 in Chinese winter wheats and the magnitude of amelioration of O3-induced damage by rising atmospheric carbon dioxide (CO2) and drought. Four experiments were conducted to assess comparative sensitivity to ozone (O3) in Chinese winter wheats (i.e., 20 cultivars released between 1945 and 2004, and 9 winter wheat species including genome donors of modern wheat) and the magnitude of modification of that sensitivity by rising atmospheric CO2 and drought. Responses to O3, CO2 and drought were assessed by relative growth rate, allometric coefficient, chlorophyll fluorescence, levels of anti-oxidative activities, protein alteration, membrane lipid peroxidation, gas exchange, in vivo photosynthetic properties, leaf chlorophyll, dark respiration, biomass accumulation and grain yield.Elevated O3 significantly decreased relative growth rate of whole plant (RGR), shoot (RGRs) and root (RGRr). Exposure to O3 also decreased allometric coefficient (K), variable fluorescence (Fv), maximum photochemical efficiency (Fv/Fm), quantum yield (ФPSII), photochemical quenching (qP) and electron transport rate (ETR), but increased non-photochemical quenching (qN). Sensitivity to O3 in cultivars of winter wheat progressed with year of release and positively correlated with mean relative growth rate of control plants. Although recent O3 levels in the breeding sites increased remarkably, plant breeding failed to evolve O3 resistance in high-yielding recently released cultivars. Cultivars bred with hybridization appeared to be more sensitive to O3 than those bred with introduction and reselection. Breeding sites had no consistent effects on O3 resistance as determined by growth and photosynthetic physiology, indicating that genetic adaptation to O3 in these cultivars of winter wheat was uncorrelated with their ambient O3 exposures. Higher sensitivity to O3 was related to breeding of winter wheat with higher relative growth rate or higher photosynthetic capacity of cultivars, but not to O3 concentrations in breeding sites.Exposure to O3 significantly reduced levels of foliar ascorbate (AsA) and soluble protein, but increased peroxidase activity (POD) and malondialdehyde (MDA). Elevated O3 depressed light saturated net photosynthetic rate (Asat), stomatal conductance (gs) and total chlorophyll, while stimulated dark respiration (Rd) and intercellular CO2 concentration (Ci). O3 also reduced overall plant growth, but to a greater extent in root than in shoot biomass. There was significantly genotypic variation in potential sensitivity to O3 that did not correlate to observed O3 tolerance. Sensitivity to O3 in cultivars of winter wheat correlated with stomatal conductance and dark respiration in O3-exposed plants. O3-induced loss in photosynthetic rate was attributed primarily to impaired activity of mesophyll cells and loss of integrity of cellular membrane as evidenced by increased intercellular CO2 concentration and lipid peroxidation. Higher sensitivity to O3 in the more recently released cultivars was induced by higher stomatal conductance, larger reduction in anti-oxidative capacity and lower levels of dark respiration leading to higher oxidative damage to proteins and integrity of cellular membranes. Winter wheat exhibited significant interspecies variation in the impacts of elevated O3 on photosynthesis and growth. Primitive cultivated species demonstrated the highest relative O3 tolerance followed by modern and wild showed the lowest in terms of growth and photosynthetic physiology. Among genome donors of modern wheat, Aegilops tauschii (DD) behaved as the most O3-sensitive followed by T. monococcum (AA) and Triticum turgidum ssp. durum (AABB) appeared to be the most O3-tolerant. Higher O3 sensitivity of modern wheat (AABBDD) was attributed to the increased O3 sensitivity of Aegilops tauschii (DD), but not to Triticum turgidum ssp. durum (AABB) during speciation.Elevated CO2 generally stimulated growth by increasing light saturated net assimilation rate (Asat), in vivo maximum rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax), light and CO2 saturated maximum net assimilation rate (Amax) of both old and modern winter wheat cultivars, which were contrasting sensitivity to O3. By contrast, O3 significantly reduced all physiological parameters. Although old and modern cultivars showed non-significant difference in response to elevated CO2, both cultivars showed sufficient protection from O3 damage by elevated CO2. Amelioration of O3-induced damage in Chinese winter wheat by elevated CO2 was found to be mediated through increased metabolic activity, but not O3 exclusion by partial stomatal closure.Water stress and ozone singly significantly reduced light saturated net assimilation rate (Asat) and stomatal conductance (gs). Drought significantly decreased in vivo maximum rate of carboxylation (Vcmax) and carboxylation efficiency (CE), but in vivo maximum rate of electron transport (Jmax) and respiration process (R, dark and light) were found to be non-significant. Elevated O3 caused significant reduction in vivo maximum rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax), respiration process (R) and caboxylation efficiency (CE) in winter wheat genotypes. Both drought and O3 reduced biomass accumulation and yield in wheat genotypes. Tetraploid wheat showed more sensitive to drought, but more resistant to O3 than hexaploid wheat. Drought caused significant reduction in O3 flux by partial stomatal closure in both wheats, but they showed remarkably different responses to elevated O3 and drought. Hexaploid wheat demonstrated approximately 16% yield and 50% harvest index protection against O3-induced damage, whereas tetraploid wheat showed no such protection either in physiological or yield performance under drought stress.
Hybrid necrosis is the premature gradual death of leaves in certain wheat hybrids. It is caused by two complementary genes, Ne1 and Ne2, when brought together in hybrid combination. Necrosis progressed from the tip to the base of the leaf and from mature to young leaves. Some severely necrotic F1 die at different stages of development without completing their life cycle which limit the parental choice for transfer of desirable traits as the F1 necrotic plants die without producing any seeds. Moreover, wheat hybrid necrosis may be a unique genetic system for studying the molecular mechanism of programmed cell death in plants. Although the hybrid necrosis in wheat was genetically known for many years, the precise molecular mechanism remains unclear. The study of wheat hybrid necrosis will helpful for overcoming the genetic barrier of hybrid necrosis and will providing the valuable information for understanding the mechanism of plant programmed cell death.In the present study, the study of wheat hybrid necrosis was carried out utilizing the high throughput proteomic approach. The winter wheat Pan555 (P) is an Ne2-carrier. Zheng891 (Z) is an Ne1-carrier. The F1 of the hybrid Pan555/Zheng891 (PZF1) exhibits hybrid necrosis in leaves. When the flag leaf (FL) just visible, leaf FL-1 (the 1st leaf below the FL) of the PZF1 plants is green totally; leaf FL-2 of the PZF1 plants began necrosis on the tips. The FL-2 leaves of hybrid which showed patches of necrosis lesions on T, showed small necrotic lesions sporadically on M and were as green as parents on B representing the different necrotic progress. The FL-1 and FL-2 leaves of the parents were normal. First, the protein expressions in the base, mid and tip segments of the FL-2 leaves of a hybrid, PZF1 and its parents, Pan555 and Zheng891, were analyzed. In total, 23 differentially displayed proteins in the 3 segments of hybrid were detected. The changes of those 23 proteins were not significant in the 3 segments of the two parents. 23 differentially displayed protein spots were analyzed by MALDI-TOF-MS, and 18 were identified. The protein expressions in FL-1 and FL-2 hybrid leaves as well as the corresponding two leaves of its parents were also analyzed. There were no detectable and reproducible proteins differentially expressed in FL-1 and FL-2 leaves of the parents. Twenty down-regulated and 19 up-regulated proteins were differentially expressed in FL-2 leaf in comparison with FL-1 leaf. These proteins were analyzed by mass spectrometry and 26 were identified. According to the possible functions and the express changes of the differentially expressed proteins, the possible roles of these proteins were discussed.In comparison with the base segment, S-adenosyl homocysteine hydrolase was down-regulated significantly in the mid and had no significant change between the mid and the tip keeping in very low abundance; AdoMet synthase 3 as well as methionine synthase 1 were up-regulated significantly in the tip. The imbalance of those 3 enzymes of methylation cycle may accelerate cell aging in different pathways. The down-regulation of uroporphyrinogen decarboxylase in hybrid FL-2 leaf suggested the accumulation of uroporphyrin (Ogen) III. Lipoxygenases was up-regulated in FL-2 leaf of hybrid. Reactive oxygen species generated by photooxidized uroporphyrin (Ogen) and/or lipoxygenases caused an oxidative environment in cells undergoing necrosis. In this oxidative environment lipid peroxidation may be initiated by lipoxygenases singly or by ROS and lipoxygenases together, leading to loss of membrane integrity and finally to the development of wheat hybrid leaf necrosis. The significantly up-regulation of many defensive proteins in tip and/or mid as well as in FL-2 hybrid leaf in comparison with FL-1 leaf suggested that the activation of the anti cell death system may be an intrinsic stress response to the cell damages related to ROS, lipid peroxidation and the enzymes of methylation cycle. However, the abnormal expressions of the energy related proteins in the tip and/or mid segments as well as in the FL-2 leaf in comparison with FL-1 leaf may aggravate the further cell death by interfering with the energy cycle processes; the down-regulation of some defensive proteins, protein synthesis related proteins as well as ssDNA-binding protein in FL-2 hybrid leaf in comparison with FL-1 suggested that the abilities of resistance, protein synthesis and DNA repair etc were decreased in the leaf cell during the progress of hybrid necrosis. The normal metabolism was disturbed in many aspects and the cell died at last. This study hinted vaguely that hybrid necrosis in wheat may be an event of epigenetics. We surmised that our data may provide a starting point for further functional studies to examine the possible methylation Apoptosis in addition to the oxidative Apoptosis.
Depletion of the stratospheric ozone layer, mainly due to the anthropogenic production of chlorofluorocarbons, leads to the enhanced solar UV-B radiation reaching the earth’s surface. To date, numerous studies have investigated on the effects of UV-B radiation on the plants, as well as the combined effects of UV-B radiation and other environmental factors are developed widely. However, few studies have focused on the interaction of UV-B radiation and temperature, especially low temperature. The over-winter plants in Northern hemisphere often encounter the double stresses with UV-B and low temperature simultaneously or subsequently from late autumn to early spring. Therefore, it is very necessary to study the responses and its mechanisms of plants under the interaction of UV-B and low temperature. In the present study, winter wheat (Triticum aestivum) was used as the materials for two independent experiments. Firstly, the seedlings of winter wheat at a chamber temperature of 20/16°C were exposed to UV-B radiation with daily biologically effective UV-B radiation (UV-BBE) at low (4.2 kJ m-2 d-1, LUVB) and high (7.0 kJ m-2 d-1, HUVB) levels and then subjected to freezing stress and recovered to growth temperature. The variations of freezing tolerance and antioxidant system were determined after UV-B radiation and freezing-recovery to investigate the UV-B induced cross acclimation of freezing tolerance and its physiological mechanisms. Secondly, the seedlings at different chamber temperatures of 25/20°C and 10/5°C were exposed to UV-B radiation with UV-BBE at low (4.2 kJ m-2 d-1, LUVB) and super-higher (7.0 kJ m-2 d-1, HUVB) levels. The relative growth rates, photosynthesis, chlorophyll fluorescence, xanthophyll cycle pigments, antioxidant system, freezing tolerance and phenolic compounds were determined at the beginning and end of UV-B radiation to investigate the UV-B effects on the growth, photosynthesis and freezing tolerance in the winter wheat at different temperatures. The main results are listed as follows:1.LUVB treatment significantly decreased the half lethal low temperature (LT50) values in the seedlings at 20/16°C and 25/20°C. The LT50 values were significantly decreased in the HUVB seedlings at 20/16°C but did not change in the SHUVB seedlings at 25/20°C. Nevertheless, both LUVB and SHUVB seedlings at 10/5°C had higher LT50 values compared with the control. The results showed the UV-B induced cross acclimation that suitable UV-B level increase the freezing tolerance in the seedlings at higher temperatures of 20/16°C and 25/20°C. However, the freezing tolerance was impaired in the UV-B treated seedlings at low growth temperature with 10/5°C.2.After 6 h recovery from the freezing stress at -6°C for 6 h, activities of catalase (CAT), guaiacol peroxidase (GPX), glutathione reductase (GR) and the reduced-to-oxidized ratios of glutathione (GSH/GSSG) increased in both LUVB and HUVB leaves compared with the control. Moreover, the concentrations of thiobarbituric acid reactive substances (TBARS) in UV-B pretreated seedlings were lower than the control. After UV-B exposure, the H2O2 concentrations were significantly increased in the UV-B treated seedlings, but decreased in these seedlings after recovery from freezing stress.These results indicated that UV-B induced cross tolerance appears to involve the antioxidant systems. Moreover, H2O2 may act as a signal molecule to trigger the cross tolerance with antioxidant systems.3.There were significant lower relative growth rate (RGR), net photosynthesis rate (Pn), maximal quantum yield of photosystem II (Fv/Fm), actual quantum yield of PS II electron transport ((F΄m−Fs)/F΄m), and photochemical quenching (qP) in the SHUVB treatment at 25/20°C and both UV-B treatments at 10/5°C, but no change in LUVB at 25/20°C. UV-B radiation did not alter intercellular CO2 concentration (Ci) at both temperatures. The results demonstrated that UV-B radiation inhibited growth and photosynthesis in winter wheat seedlings by the enhanced intensity and lower temperature. Moreover, the decreased leaf Pn in the UV-B treated seedlings was not due to stomatal limitation but was mainly associated with low PS II photochemistry efficiency.4.UV-B radiation increased the size of violaxanthin (V) but inhibited the transformation to zeaxanthin (Z) in the xanthophyll cycle. Compared with the control, the de-epoxidation state of xanthophyll cycle pigments (DEPS), and non-photochemical quenching (NPQ) significantly did not change in the LUVB seedlings and decreased in the SHUVB seedlings at 25/20°C and 10/5°C. In the present study, the xanthophyll cycle did not dissipate excess excitation energy as heat for photoprotection in the UV-B treated seedlings at both temperatures.5.UV-B radiation increased the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), together with the reduced-to-oxidized ratios of ascorbate (AsA/DHA) and GSH/GSSG in the seedlings at 25/20°C. However, UV-B radiation only increased the activities of SOD and CAT, but failed to change APX activity and AsA/DHA, even decreased GR activity and GSH/GSSG in the seedlings at 10/5°C. The results indicated that antioxidant systems were up-regulated in the UV-B treated seedlings at the higher temperature, but were impaired seriously in those seedlings at low temperature, indicating the different expression of antioxidant systems under different temperatures. Low temperature impaired the function of photoprotection mechanism with metabolic pathways.6.The phenolic compounds were significantly increased in the UV-B treated seedlings at both growth temperatures, particularly under the combined UV-B radiation and low temperature. The results suggested that the phenolic compounds participated in the protection of UV-B treated seedlings at both temperatures especially 10/5°C. 7.TBARS concentrations were significantly increased only in the SHUVB seedlings at 25/20°C, but in the both UV-B treated seedlings at 10/5°C, compared with the control respectively. The results indicated that low temperature aggravated the oxidative stress in the UV-B treated seedlings than those at high temperature. The accumulated phenolic compounds and up-expressed antioxidant enzymes could not rescue the seedlings at 10/5°C from the UV-B induced damage compared with those at high temperature.