Effects of exogenous nitric oxide on antioxidation and DNA methylation of <Emphasis Type="Italic">Dendrobium huoshanense</Emphasis> grown under drought stress

Nitric oxide (NO) is an important biological messenger in plants, which has been implicated in response to abiotic stress. To study the effects of exogenous NO on drought menace, the tube seedlings of Dendrobium huoshanense were selected and treated with 10% polyethylene glycol (PEG-6000) to simulate drought stress. After application of sodium nitroprusside (SNP), the relative water content (RWC) and antioxidant enzyme activities were determined. As a result, plant treated with 50 μmol L⁻¹ of SNP maintained high level of RWC and lower content of malondialdehyde (MDA). Furthermore, the antioxidant enzyme activities were obviously enhanced. However, the higher concentration of SNP (100 μmol L⁻¹) enhanced the effects of drought stress for plant. For further analysis of the response mechanism to exogenous NO, the methylation-sensitive amplified polymorphism (MSAP) technique was used to investigate the changes of DNA methylation. When the seedlings of Dendrobium huoshanense were treated with 50 μmol L⁻¹ SNP containing 10% PEG-6000, levels of global DNA methylation of Dendrobium huoshanense were decreased. Nevertheless, the demethylation rate of methylated sites increased, accounting for 12.5% of total methylation sites. These results implied that some expressed genes were involved in the response process to drought stress triggered by NO in Dendrobium huoshanense.     

Physiological acclimation of two psammophytes to repeated soil drought and rewatering

To understand physiological acclimation of psammophyte to repeated soil drought and rewatering, two psammophytes (Setaria viridis and Digitaria ciliaris) were subjected to three cycles of soil drought and rewatering. The response process of leaf relative water content (RWC), membrane permeability, lipid peroxidation, gas exchange characteristics, antioxidant enzymes, soluble protein, and free proline was examined. Leaf RWC, the net photosynthesis rate, stomatal conductance, and water use efficiency decreased, while membrane permeability, lipid peroxidation, intercellular CO₂ concentration, soluble protein, and free proline increased during three soil drought periods for both psammophytes. These physiological characteristics were recovered to the control levels following rewatering for 4 days. However, activities of SOD, CAT, and POD were induced continuously under soil drought conditions, and remained higher than those in the control throughout the whole experiment period, which agrees with our hypothesis that drought hardening activates defensive systems of both psammophytes continuously. Decreasing level of leaf RWC and increasing levels of leaf membrane permeability and lipid peroxidation were suppressed with increasing the number of drought cycles, suggesting that drought hardening alleviates damages of both psammophytes and improves their drought tolerance and acclimation to soil drought conditions in the future. Additionally, the photosynthesis decreased more slowly in the subsequent drought cycles than in the first cycle, allowing both psammophytes to maximize assimilation in response to repeated soil drought conditions. Thus, both psammophytes acclimatize themselves to repeated soil drought.     

Photosynthesis of <Emphasis Type="Italic">Rehmannia glutinosa</Emphasis> subjected to drought stress is enhanced by choline chloride through alleviating lipid peroxidation and increasing proline accumulation

Rehmannia glutinosa seedlings were pretreated with choline chloride (CC) in concentrations of 0, 0.7, 2.1 and 3.5 mM, and then subjected to drought and rewatering treatment to study the effects of CC on the generation of reactive oxygen species (O₂⁻, H₂O₂), lipid peroxidation, proline accumulation, water status and photosynthesis. The results showed that pretreatment with CC alleviated the inhibition of SOD and APX activity caused by drought stress, and therefore, the rate of O₂⁻ production and H₂O₂ concentration were reduced and lipid peroxidation decreased in pretreated plants. CC pretreatment also accelerated accumulation of proline, maintained higher Ψw and RWC, deferred leaf water loss during drought stress and retarded the drop in proline concentration after rewatering. Consequently, drought-induced decreases in F_m/F₀, F_v/F_m, Φ_PS2, q_P, and A and increase in q_NP were inhibited and the recovery of photosynthesis after rewatering was quicker in pretreated plants. Although differences in F_v/F_m, Φ_PS2 and q_P between treatments were not significant, there was a general trend that the effects of CC increased with the rise of its concentrations. The data suggested that 2.1 mM of CC be suitable for alleviating lipid peroxidation, promoting proline accumulation, retarding leaf water loss and improving photosynthesis of R. glutinosa seedlings under drought stress.     

Drought resistance and recovery in mature Bituminaria bituminosa var. albomarginata

Few studies have investigated the response of perennial legumes to drought stress (DS) and their ability, following rewatering, to regrow and restore photosynthetic activity. We examined these responses for two genotypes of drought‐tolerant tedera (Bituminaria bituminosa var. albomarginata) and one genotype of lucerne (Medicago sativa). Plants were grown outdoors in 1‐m deep PVC pots with a reconstructed field soil profile, regularly watered for 8 months (winter to mid‐summer), and then moved to a glasshouse where either watering was maintained or drought was imposed for up to 47 days, before rewatering for 28 days. Drought stress greatly decreased shoot dry matter (DM) production in both species. Lucerne plants showed severe leaf desiccation after 21 days of withholding water. Relative leaf water content (RWC = 42%) and midday leaf water potential (LWP = ?6.5 MPa) decreased in tedera in response to DS, whereas leaf angle (85°) and lateral root DM both increased. Proline and pinitol accumulated in tedera leaves during DS, and their concentration declined after rewatering. Nine days after rewatering, previously drought‐stressed tedera had similar RWC and LWP to well‐watered control plants. In tedera and lucerne, 28 days after rewatering, photosynthesis and stomatal conductance were greater than in the well‐watered controls. The lateral root DM for one tedera genotype decreased during the recovery phase but for lucerne, the lateral root DM did not change during either the drought or the recovery phases. Overall, the root systems in tedera showed greater plasticity in response to DS and rewatering than in lucerne. In conclusion, tedera and lucerne showed different physiological and morphological strategies to survive and recover from DS. Proline and soluble sugars may act as a carbon source for regrowth in tedera during recovery. In comparison with lucerne, tedera's more rapid recovery after rewatering should contribute to a greater aboveground DM yield under alternating dry and wet periods. Tedera genotypes are highly heterogeneous and selecting genotypes with enhanced concentrations of pinitol and proline could be a valuable tool to improve plant performance during DS and recovery.     

Physiochemical and antioxidant responses of the perennial xerophyte Capparis ovata Desf. to drought

Caper (Capparis ovata Desf.) is a perennial shrub (xerophyte) and drought resistant plant which is well adapted to Mediterranean Ecosystem. In the present study we investigated the plant growth, relative water content (RWC), chlorophyll fluorescence (F_V/F_M), lipid peroxidation (TBA-reactive substances content) as parameters indicative of oxidative stress and antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POX), catalase (CAT) and glutathione reductase (GR) in relation to the tolerance to polyethylene glycol mediated drought stress in C. ovata seedlings. For induction of drought stress, the 35 days seedlings were subjected to PEG 6000 of osmotic potential −0.81 MPa for 14 days. Lipid peroxidation increased in PEG stressed seedlings as compared to non-stressed seedlings of C. ovata during the experimental period. With regard to vegetative growth, PEG treatment caused decrease in shoot fresh and dry weights, RWC and F_V/F_M but decline was more prominent on day 14 of PEG treatment. Total activity of antioxidative enzymes SOD, APX, POX, CAT and GR were investigated in C. ovata seedlings under PEG mediated drought. Induced activities of SOD, CAT and POX enzymes were high and the rate of increment was higher in stressed seedling. APX activity increased on both days of PEG treatment, however, increase in GR activity was highest on day 14 of drought stress. We concluded that increased drought tolerance of C. ovata is correlated with diminishing oxidative injury by functioning of antioxidant system at higher rates under drought stress.     

Effect of Putrescine, 4-PU-30, and Abscisic Acid on Maize Plants Grown under Normal, Drought, and Rewatering Conditions

The experiments were carried out with maize (Zea mays L.) seedlings, hybrid Kneja 530, grown hydroponically in a growth chamber. Twelve-day-old plants were foliar treated with putrescine, N¹-(2-chloro-4-pyridyl)-N²-phenylurea (4-PU-30), and abscisic acid (ABA) at concentrations of 10⁻⁵ m. Twenty-four hours later the plants were subjected to a water deficit program, induced by 15% polyethylene glycol (PEG; molecular weight, 6,000). Three days after drought stress half of the plants were transferred to nutrient solution for the next 3 days. The effects of the water shortage, rewatering, and plant growth regulator (PGR) treatment on the fresh and dry weights, leaf pigment content, proline level, relative water content (RWC), transpiration rate, activities of catalase and guaiacol peroxidase, hydrogen peroxide content, and level of the products of lipid peroxidation were studied. It was established that the application of PGRs alleviated to some extent the plant damage provoked by PEG stress. At the end of the water shortage program the plants treated with these PGRs possessed higher fresh weight than drought-subjected control seedlings. It was found also that putrescine increased the dry weight of plants. Under drought, the RWC and transpiration rate of seedlings declined, but PGR treatment reduced these effects. The accumulation of free proline, malondialdehyde, and hydrogen peroxide was prevented in PGR-treated plants compared with the water stress control. The results provided further information about the influence of putrescine, 4-PU-30, and ABA on maize plants grown under normal, drought, and rewatering conditions. Received September 25, 1997; accepted August 10, 1998     

接种两种固氮菌增强小麦幼苗抗渗透胁迫及生长能力

苗期是小麦(Triticum aestivum)物质和能量积累的关键时期, 苗期干旱影响小麦的后期群体建成。利用田菁茎瘤固氮根瘤菌(Azorhizobium caulinodans) ‘ORS571’与巴西固氮螺菌(Azospirillum brasilense) ‘Yu62’浸种侵染小麦和聚乙二醇(PEG)模拟渗透胁迫, 研究渗透胁迫下接菌小麦种子的发芽状况; 利用固氮菌涂抹小麦幼苗叶部, 测定PEG模拟渗透胁迫下小麦幼苗根体积、叶片相对含水量、脯氨酸含量及可溶性蛋白含量, 探究固氮菌增强小麦幼苗抗渗透胁迫的能力。结果表明, 接种混合固氮菌后在渗透胁迫下小麦种子的发芽率明显提高; 在渗透胁迫下叶部涂抹固氮菌小麦的根体积、叶片相对含水量、脯氨酸含量及可溶性蛋白含量明显升高, 表明接种固氮菌可提高小麦幼苗的抗渗透胁迫能力, 且混合固氮菌对小麦幼苗叶片的增强效果优于单一固氮菌。     

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.     

Evaluation of encapsulation stress and the effect of additives on viability and photosynthetic activity of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 encapsulated in silica gel

Stresses imposed on the cyanobacterium Synechocystis sp. PCC 6803 by various compounds present during silica sol–gel encapsulation, including salt, ethanol (EtOH), polyethylene glycol (PEG), glycerol, and glycine betaine, were investigated. Viability of encapsulated cells and photosynthetic activity of cells stressed by immediate (2 min) and 24-h exposure to the five stress-inducing compounds were monitored by pulse amplitude modulated fluorometry. Cells of Synechocystis sp. PCC 6803 readily survive encapsulation in both alkoxide-derived gels and gels from aqueous precursors and can remain active at least 8 weeks with slight degradation in PSII efficiency. Post-encapsulation survival was improved in gels containing no additive when compared with gels containing PEG or glycerol. Glycerol was shown to have a detrimental effect on Synechocystis sp. PCC 6803, reducing ϕPSII and F _v′/F _m′ by as much as 75%, possibly a result of disrupting excitation transfer between the phycobilisomes and photosystems. PEG was similarly deleterious, dramatically reducing the ability to carry out a state transition and adequately manage excitation energy distribution. EtOH stress also hindered state transitions, although less severely than PEG, and the cells were able to recover nearly all photosynthetic efficiency within 24 h after an initial drop. Betaine did not interfere with state transitions but did reduce quantum yield and photochemical quenching. Finally, Synechocystis sp. PCC 6803 was shown to recover from salt stress.     

Cyclic electron flow plays an important role in photoprotection for the resurrection plant <Emphasis Type="Italic">Paraboea</Emphasis><Emphasis Type="Italic">rufescens</Emphasis> under drought stress

Resurrection plants could survive severe drought stress, but the underlying mechanism for protecting their photosynthetic apparatus against drought stress is unclear. Cyclic electron flow (CEF) has been documented as a crucial mechanism for photoprotection in Arabidopsis and tobacco. We hypothesized that CEF plays an important role in protecting photosystem I (PSI) and photosystem II (PSII) against drought stress for resurrection plants. To address this hypothesis, the effects of mild drought stress on light energy distribution in PSII and P700 redox state were examined in a resurrection plant Paraboea rufescens. Cyclic electron flow was not activated below the photosynthetic photon flux density (PPFD) of 400 μmol m⁻² s⁻¹ in leaves without drought stress. However, CEF was activated under low light in leaves with mild drought stress, and the effective quantum yield of PSII significantly decreased. Meanwhile, non-photochemical quenching (NPQ) was significantly stimulated not only under high light but also under low light. Compared with the control, the fraction of overall P700 that cannot be oxidized in a given state (PSI acceptor side limitation) under high light was maintained at low level of 0.1 in leaves with water deficit, indicating that the over-reduction of the PSI acceptor side was prevented by the significant stimulation of CEF. Furthermore, methyl viologen could significantly increase the PSII photo-inhibition induced by high light compared with chloramphenicol. These results suggested that CEF is an important mechanism for protecting PSI and PSII from drought stress in resurrection plants.     

Selenium improves recovery of wheat seedlings at rewatering after drought stress

Rewatering after drought is beneficial to plants subjected to moderate drought stress, and selenium (Se) could increase the tolerance of plants to stressful environment. The role of Se in rewatering of drought-treated wheat seedlings (Triticum aestivum L., cv Hengmai5229) was studied. The objective was to elucidate whether Se could improve recovery of wheat seedlings at rewatering after drought stress. Drought stress induced a significant reduction in growth parameters, total chlorophyll and soluble protein contents, and increased the rate of superoxide radical (O ₂ ^·? ) production, MDA content, and the activities of peroxidase, catalase (CAT), and superoxide dismutase in wheat seedlings. Rewatering after drought did not significantly affect biomass accumulation of seedlings over drought treatment, although it decreased the rate of O ₂ ^·? production and MDA content. However, the combined treatment of rewatering and Se evidently promoted biomass accumulation of seedlings over drought treatment and rewatering alone; and the rate of O ₂ ^·? production, MDA content, soluble protein content and CAT activity were recovered to the control values. This indicates that Se improved recovery of wheat seedlings at rewatering after drought stress.     

Genotypic variation in drought stress response and subsequent recovery of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Three wheat (Triticum aestivum L.) genotypes, Sadovo, Katya and Prelom, with different tolerance to drought were comparatively evaluated in terms of leaf respiratory responses to progressing dehydration and consecutive rewatering. Under drought stress, the respiration of all varieties gradually decreased, as the drought-tolerant Katya showed the most pronounced decline at earlier stages of dehydration. When water stress intensified, this genotype gave relatively stable respiration rates compared with the drought-sensitive varieties. Additionally, dehydrated Katya leaves displayed lower stomatal conductance and higher photosynthesis values, which resulted in greater water use efficiency during the dehydration period. Combination of drought stress and short-term changes in leaf temperature also induced genotype-specific response that differed from the response to drought only. Over the whole temperature range, the leaves of Katya exposed to dehydration for 14 days, showed higher respiration rates compared to the drought-sensitive varieties. The sensitive varieties maintained higher respiration rates under control conditions and mild dehydration, and very low rates under severe drought. In Katya, respiration and photosynthesis were fully restored from the stress within the first day of rewatering. The drought-sensitive genotypes displayed a considerably slower recovering capacity. The results are discussed in terms of possible physiological mechanisms underlying plant tolerance to drought.     

Photosynthesis and PSII functionality of drought-resistant and drought-sensitive weeping lovegrass plants

World areas subject to drought are expected to increase under conditions of climate change. The purpose of this study is to clarify the response of grass species that can grow and produce under water stress. Therefore leaf photosynthesis, chlorophyll fluorescence and pigment content response to water stress were studied in two varieties of the C₄ grass Eragrostis curvula. Two-year-old plants of cv Ermelo and Consol were grown in plastic pots. Drought stress was imposed by withholding irrigation for 15 days and then rewatering for 5 days. During drought relative water content (RWC) decreased 65% in cv Ermelo, while lower reductions of RWC were observed in cv Consol. During the experiment in cv Ermelo increasing drought stress severity caused large decreases in photosynthetic rates, maximal PSII photochemical efficiency (F_V/F_M) and leaf pigment content. Cv Consol showed small variations in these parameters. Compared to cv Consol, after 15 days of drought, effective PSII quantum yield (Φ_II) was significantly lower in cv Ermelo. Reductions of Φ_II were related to significant reductions of open PSII energy capture efficiency (F′_V/F′_M). Photosynthetic response to increasing PPFD levels and to internal CO₂ concentration (C_i) were reduced by drought in cv Ermelo. Compared to well-watered control plants and to cv Consol, drought stressed plants of cv Ermelo showed also reductions of the initial slope of photosynthetic response to C_i and in the photosynthetic rate measured at saturating C_i. Moreover stomatal conductance (g) of both cvs decreased during drought. However, g was lower in drought stressed plants of cv Consol than in cv Ermelo. Water stress caused large reductions in leaf chlorophyll and carotenoid content in cv Ermelo, and small reductions in cv Consol. In drought-sensitive cv Ermelo water stress reduced the capabilities to down regulate PSII functionality through thermal energy dissipation. Results suggest that drought resistance of cv Consol, can be attributed to a higher water use efficiency.     

Relationship between osmotic stress and polyamines conjugated to the deoxyribonucleic acid-protein in wheat seedling roots

The contents of covalently conjugated polyamines (CC-PAs) and noncovalently conjugated polyamines (NCC-PAs) to deoxyribonucleic acid-protein (DNP) isolated from wheat (Triticum aestivum L.) seedling roots under osmotic stress were detected. Results showed that after osmotic stress treatment for 7 d, the levels in NCC-spermine (NCC-Spm) and NCC-spermidine (NCC-Spd) of drought-tolerant Yumai No. 18 cv. increased more markedly than that of drought-sensitive Yangmai No. 9 cv., while the NCC-putrescine (NCC-Put) could not be statistically detected in two cultivars. Exogenous Spm treatment alleviated osmotic stress injury to Yangmai No. 9 cv. seedlings, coupled with marked increases of NCC-Spm and NCC-Spd levels of this cultivar. Under PEG osmotic stress, the concomitant treatment of drought-tolerant Yumai No. 18 cv.seedlings with methylglyoxyl-bis (guanylhydrazone) (MGBG), an inhibitor of S-adenosylmethionine decarboxylase (SAMDC), aggravated osmotic stress injury to this cultivar, coupled with market decreases of the NCC-Spm and NCC-Spd levels. The levels in CC-Put and CC-Spd of drought-tolerant Yumai No. 18 cv. increased more markedly than that of drought-sensitive Yangmai No. 9 cv. Under osmotic stress. The treatment of drought-tolerant Yumai No. 18 cv. seedlings with phenanthrolin (o-Phen), an inhibitor of transglutaminase (TGase), aggravated osmotic stress injury to this cultivar, coupled with a reduction of sum contents of CC-Put+CC-Spd. These results suggested that NCC-Spm and NCC-Spd, together with CC-Put and CC-Spd, in DNP of roots could enhance tolerance of the wheat seedlings to osmotic stress.     

Influence of enhanced temperature on photosynthesis,photooxidative damage,and antioxidant strategies in <Emphasis Type="Italic">Ceratonia siliqua</Emphasis> L. seedlings subjected to water deficit and rewatering

Predicted future climatic changes for the Mediterranean region give additional importance to the study of photooxidative stress in local economic species subjected to combined drought and high-temperature conditions. Under this context, the impact of these stresses on photosynthesis, energy partitioning, and membrane lipids, as well as the potential ability to attenuate oxidative damage, were investigated in Ceratonia siliqua L. Two thermal regimes (LT: 25/18°C; HT: 32/21°C) and three soil water conditions (control, water stress, and rewetting) were considered. HT exacerbated the adverse effects of water shortage on photosynthetic rates (P _N) and PSII function. The decrease in P _N was 33% at LT whereas at HT it was 84%. In spite of this, the electron transport rate (ETR) was not affected, which points to an increased allocation of reductants to sinks other than CO₂ assimilation. Under LT conditions, water stress had no significant effects on yield of PSII photochemistry (Φ_PSII) and yields of regulated (Φ_NPQ) and nonregulated (Φ_NO) energy dissipation. Conversely, drought induced a significant decrease of Φ_PSII and a concomitant increase of Φ_NO in HT plants, thereby favouring the overproduction of reactive oxygen species (ROS). Moreover, signs of lipid peroxidation damage were detected in HT plants, in which drought caused an increase of 40% in malondialdehyde (MDA) content. Concurrently, a marked increase in proline content was observed, while the activities of catalase (CAT) and ascorbate peroxidase (APX) were unaffected. Despite the generation of a moderate oxidative stress response, C. siliqua revealed a great capability for photosynthetic recovery 36 h after rewatering, which suggests that the species can cope with predicted climate change.