Physiological aspects of tolerance in <Emphasis Type="Italic">Atriplex</Emphasis><Emphasis Type="Italic">halimus</Emphasis> L. to NaCl and drought

Forty-day-old seedlings of Atriplex halimus were treated either with NaCl (50, 300 and 550 mM) for the subsequent 30 days or with 15% PEG for the subsequent 10 days. As much as 50 mM of NaCl significantly increased shoot fresh and dry weight and height; nevertheless, 300 or 550 mM NaCl seemed to have no effect. On the other hand, these growth parameters were not affected by drought after 3 or 6 days, but were reduced after 10 days. The gas exchange parameters (photosynthetic rate, stomatal conductance and transpiration rate) were increased by 50 mM NaCl, but decreased by 300 and 550 mM. These parameters were decreased in response to drought only after 10 days of withholding water. In contrast to Na⁺, K⁺ was significantly decreased by NaCl but not by drought. The time course effect revealed that phosphoenol pyruvate carboxylase (PEPC) protein was doubled in response to NaCl after 1 and 5 h and continued thereafter, higher than control, while drought had no significant effect. Rubisco seemed unchanged by NaCl or drought. It could be concluded that the decrease in fresh weight might be attributed to the decrease in water content. Moreover, the decrease in photosynthesis could result from a decrease in stomatal conductance, a protective mechanism against water loss to improve water use efficiency. These findings indicate that Atriplex halimus tolerates NaCl and drought through decreasing growth, reducing gas exchange parameters to improve water use efficiency, uptake Na⁺ and saving, if any, the photosynthetic enzyme particularly PEPC.     

Stimulation of intermolecular ligation with E. coli DNA ligase by high concentrations of monovalent cations in polyethylene glycol solutions.

In the presence of high concentrations of the nonspecific polymer polyethylene glycol (PEG), intermolecular cohesive-end ligation with the DNA ligase from Escherichia coli was stimulated by high salt concentrations: 200 mM NaCl or 300 mM KCl in 10% (w/v) PEG 6000 solutions, and 100-200 mM NaCl or 150-300 mM KCl in 15% PEG 6000 solutions. Intermolecular blunt-end ligation with this ligase was also stimulated at 100-150 mM NaCl or 150-250 mM KCl in 15% PEG 6000 solutions. The extent of such intermolecular ligation increased and the salt concentrations at which ligation was stimulated extended to lower concentrations when we raised the temperature from 10 to 37 degrees C.     

Role of Ca2+ in Drought Stress Signaling in Wheat Seedlings

Plants use complex signal transduction pathways to perceive and react to various biotic and/or abiotic stresses. As a consequence of this signaling, plants can modify their metabolism to adapt themselves to new conditions. One such change is the accumulation of proline in response to drought and salinity stresses. We have studied drought and salinity induced proline accumulation and the roles of Ca²⁺ (10 mM) and indoleacetic acid (IAA, 0.3 mM) in this response. Subjecting seedlings to both drought (6% polyethylene glycol, PEG) and salinity (150 mM NaCl) stress resulted in a dramatic increase in proline accumulation (7-fold higher than control level). However, the application of Ca²⁺ along with these stress factors had different effects. Unlike the salinity stress, Ca²⁺ prevented the drought induced proline accumulation indicating that these stress factors use distinct signaling pathways to induce similar responses. Experiments with IAA and EGTA (10 mM) supported this interpretation and suggested that Ca²⁺ and auxin participate in signaling mechanisms of drought-induced proline accumulation. Drought and salt stress-induced proline accumulation was compared on salt resistant (cv. Gerek 79) and salt sensitive (cv. Bezostaya) wheat varieties. Although proline level of the first was twofold lower than that of the second in control, relative proline accumulation was dramatically higher in the case of the salt resistant wheat variety under stress conditions.     

Comparison of ultrastructural and physiological changes of potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.) plantlets subjected to salt and modeling drought stresses

Salinity and drought are two severe abiotic stresses that affect plant growth and decrease food production worldwide. Compared to the field plants, test-tube plantlets could be more direct and fast to investigate the mechanism of stress adaptation. In the present study, the ultrastructural and physiological differences of potato (Solanum tuberosum L. c.v. “Longshu No. 3”) plantlets in response to the gradient saline (0, 25, 50, 100, and 200 mM NaCl) and the modeling drought stresses with polyethylene glycol (PEG) at the concentrations of 0, 2, 4, 6, and 8 % were analyzed. The results show that the severe salt (200 mM NaCl) and the modeling drought stresses (8 % PEG) inhibited the plantlet growth. There are considerable differences in their ultrastructural alteration under salt and modeling drought adaptation: PEG caused the increase in the number of stacked chloroplast, plastoglobuli, and starch; NaCl induced the decrease in the number of chloroplast and plastoglobuli. Moreover, plantlet has higher free proline content, less malondialdehyde (MDA) content, and higher activities of catalase (CAT) and superoxide dismutase (SOD) under the gradient NaCl treatments than the gradient PEG treatments. The results of this study will provide theoretical and practical insights into characterizing the ultrastructural and physiological differences of plants adapting to various stressful environments.     

NaCl alleviates polyethylene glycol-induced water stress in the halophyte species Atriplex halimus L

Atriplex halimus L. is a C4 xero-halophyte species well adapted to salt and drought conditions. To collect information on the physiological impact of low salt levels on their water-stress resistance, seedlings were exposed for 6 d to nutrient solution containing either 0% or 15% polyethylene glycol 10,000 (PEG), in the presence or in the absence of 50 mM NaCl. Similar experiments were performed with one PEG-resistant and one PEG-sensitive selected cell line exposed for 50 d to 0% or 15% PEG on standard Linsmaier and Skoog (LS) medium, on LS medium supplemented with 50 mM NaCl, or on Na+-free medium. NaCl mitigated the deleterious impact of PEG on growth of both whole plants and PEG-sensitive cell lines and improved the ability of stressed tissues to perform osmotic adjustment (OA). Water stress reduced CO2 net assimilation rates quantified in the presence of high CO2 and low O2 levels (A), stomatal conductance and transpiration, but NaCl improved water use efficiency of PEG-treated plants through its positive effect on A values, especially in young leaves. PEG increased the internal Na+ concentration. The resistant cell line accumulated higher concentration of Na+ than the PEG-sensitive one. The complete absence of Na+ in the medium endangered the survival of both cell lines exposed to PEG. Although Na+ by itself contributed only for a small part to OA, NaCl induced an increase in proline concentration and stimulated the synthesis of glycinebetaine in response to PEG in photosynthetic tissues. Soluble sugars were the main contributors to OA and increased when tissues were simultaneously exposed to PEG and NaCl compared with PEG alone, suggesting that Na+ may influence sugar synthesis and/or translocation.     

Influence of NaCl and mannitol on peroxidase activity and lipid peroxidation in Centaurea ragusina L. roots and shoots

Centaurea ragusina L. is a Croatian endemic plant species growing on cliffs above the Adriatic Sea, but there is no information about its physiological behavior or stress tolerance. To investigate the response of C. ragusina plants to salinity and drought, we have analysed soluble peroxidase (POD; EC 1.11.1.7) activity, anionic isoperoxidase pattern, levels of malondialdehyde (MDA) and hydrogen peroxide in C. ragusina plants exposed to these stresses. Rooted plantlets grown on MS 1/2 nutrient medium supplemented with mannitol (300 mM) and different concentrations of NaCl (150, 300, 450 or 600 mM) were harvested after 5, 10 and 15 days. Both osmotic treatments significantly increased MDA and hydrogen peroxide contents in C. ragusina shoots after 10 days of stress, while in roots these parameters showed no significant difference compared to control in overall. POD activity of salt-stressed plants changed with respect to different saline treatments and plant organs - in shoots enzymatic activity markedly increased in response to lower saline treatments, especially 300 mM NaCl; otherwise it was similar as in control plants while in roots of plants grown under 450 and 600 mM NaCl it significantly decreased. Drought increased POD activity of both shoots and roots especially after 10 days of experiment. Generally, change in the POD isoenzyme pattern of treated plants was in accordance with the activity change in time. Several POD isoforms (P3, P4 and P9) were specifically induced by salinity and drought.     

Relationships between polyamines, ethylene, osmoprotectants and antioxidant enzymes activities in wheat seedlings after short-term PEG- and NaCl-induced stresses

Contents of ethylene, osmoprotectants, levels and forms of polyamines (PAs) and activities of antioxidant enzymes in the leaves and roots were investigated for five wheat cultivar seedlings (differing in drought tolerance) exposed to osmotic stress (?1.5 MPa). Stress was induced by 2-day-long treatment of plants with polyethylene glycol 6000 (PEG) or NaCl added to hydroponic cultures. Nawra, Parabola and Manu cv. (drought tolerant) showed a marked increase in osmoprotectors (proline and soluble carbohydrates, mainly glucose, saccharose and maltose), free PAs (putrescine Put, spermidine Spd and spermine Spm) and Spd-conjugated levels, in both leaves and roots, after PEG-treatments. Radunia and Raweta (drought sensitive) exhibited smaller changes in the content of these substances. The analysis of enzymes involved in proline metabolism revealed the glutamate as a precursor of proline synthesis in PEG-induced stress conditions. The increase in the activity of antioxidative enzymes, especially catalase and peroxidases, was characteristic for tolerant wheat plants, but for sensitive ones, a decrease in superoxide dismutase and an increase in mainly glutathione reductase activities were observed. After NaCl-treatment smaller changes of all biochemical parameters were registered in comparison with PEG-induced stress. Exceptions were the higher values of ethylene content and a significant increase in saccharose, raffinose and maltose levels (only in stress sensitive plants). The proline synthesis pathway was stimulated from both glutamate and ornithine precursors. These results suggest that the accumulation of inorganic ions in NaCl-stressed plants may be involved in protective mechanisms as an additional osmoregultor. Thus, a weaker stressogenic effect as determined as water deficit by leaf relative water content and relative dry weight increase rate and differences in metabolite synthesis in comparison with PEG stress was observed. Proline seems to be the most important osmo-protector in osmotic stress initiated by both PEG and NaCl. The synthesis of sugars and PAs may be stimulated in a stronger stress conditions (PEG).     

Regulation of gene expression governing proline metabolism in <Emphasis Type="Italic">Thellungiella salsuginea</Emphasis> by NaCl and paraquat

Differential expression of the proline metabolism genes in Thellungiella salsuginea (Pall) E. Schulz was investigated under salinity (100 and 300 mM NaCl), upon the effect of paraquat (0.1 μM), and at their joint action. It was shown that, depending on the intensity of stress factor, expression of the P5CS1 gene was induced in the leaves (at 100 mM NaCl) or roots (at 300 mM NaCl). When the plants on control medium were treated with paraquat, the proline content changed only in the leaves. Time course of proline content in the leaves complied with the dynamic of P5CS1 gene expression, while expression of PDH gene essentially did not change. When the plants, which experienced salt stress, were treated with paraquat, the content of proline and the P5CS1 mRNA level increased even more. The obtained results suggest a complicated nature of signaling between the organs of the halophyte Th. salsuginea causing expression of the proline biosynthesis genes in the leaves and roots under the effect of salinity, paraquat, or upon their joint action. The proline catabolism in these plants was maintained essentially unchanged, which is probably related to the participation of proline and/or the products of its degradation in the pathways of other metabolite biosynthesis. We suggested that proline took part in ROS scavenging process and proline level was under strong control in Th. salsuginea.     

干旱胁迫对远志种子萌发及幼苗生长和生理特性的影响

以远志(Polygala tenuifolia Willd.)为研究对象,采用不同浓度(2.5%~25%)聚乙二醇(PEG-6000)模拟不同程度的干旱胁迫,探讨干旱胁迫对远志种子萌发及幼苗生理生化特性的影响。结果表明:(1)随着干旱胁迫强度的增加,远志种子的发芽启动时间推迟,发芽率、发芽势、发芽指数和活力指数降低,但种子发芽率在2.5%~15%PEG胁迫下与对照无显著性差异,而在20%PEG胁迫下均显著低于对照,在25%PEG胁迫下种子不能萌发;在干旱胁迫条件下,远志幼苗生物量降低,胚芽生长受到显著抑制,胚根长度则先伸长后缩短。(2)远志幼苗叶绿素含量在2.5%~10%PEG范围内随胁迫强度的增加和时间的延长而持续上升,在15%和20%PEG胁迫下则表现为先上升后下降,在10%PEG胁迫处理第15天时含量最高,为对照的1.34倍。(3)幼苗叶片的游离脯氨酸、可溶性糖和可溶性蛋白含量随PEG胁迫强度的增加和时间的延长而增加,各指标均在20%PEG胁迫处理第15天时含量最高,分别为对照的1.99倍、1.53倍和1.50倍。(4)随着PEG胁迫时间的延长,远志幼苗叶片超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性先上升后下降,并在10%PEG胁迫处理第10天时活性最强;过氧化物酶(POD)活性随着胁迫时间的延长表现出先上升后下降又上升的特性,并在20%PEG胁迫处理第5天时活性最强;叶片丙二醛(MDA)含量在15%和20%PEG胁迫处理下持续上升,在2.5%~10%PEG胁迫范围内先上升后又有所下降。研究发现,远志种子在轻、中度干旱胁迫下仍可正常萌发,而且幼苗能通过调节自身生长、渗透调节物质含量和抗氧化酶活性主动适应干旱环境,对干旱环境表现出较好的适应能力。     

等渗盐分、干旱胁迫下冬小麦叶片部分渗透调节物质的动态变化

分别以PEG-6000、NaCl模拟干旱胁迫及盐胁迫,采用水培方法研究了抗旱耐盐冬小麦沧-6001在干旱胁迫、盐胁迫条件下叶片可溶性糖、脯氨酸和可溶性蛋白质含量的动态变化以及Na⁺、K⁺在地上部和根系的分布。结果表明可溶性糖和可溶性蛋白变化趋势相似,其含量随干旱胁迫或盐胁迫时间延长而增加,但在胁迫处理后期下降,并且随胁迫强度增加,二者出现下降时间提前;脯氨酸在干旱胁迫条件下快速积累达到峰值后下降但在胁迫处理后期再次增加,在盐胁迫条件下,随胁迫强度的增加和胁迫时间的延长而增加;Na⁺在干旱胁迫下随胁迫程度增加而下降,盐胁迫条件下随胁迫程度的增加而增加,K⁺在干旱胁迫或盐胁迫下均随胁迫程度的增加而下降,且在根系中下降的速度大于地上部。     

Comparative evaluation of hydro-, chemo- , and hormonal-priming methods for imparting salt and PEG stress tolerance in Indian mustard (<Emphasis Type="Italic">Brassica</Emphasis><Emphasis Type="Italic">juncea</Emphasis> L.)

The present study was aimed to evaluate the effect of different seed priming methods to enhance the sodium chloride (NaCl) and polyethylene glycol-8000 (PEG-8000) stress tolerance in Indian mustard (Brassica juncea L.). Seeds subjected to different priming treatments such as water (hydro-priming), calcium chloride (CaCl₂) (chemo-priming), and abscisic acid (ABA) (hormonal-priming) showed increased rate of germination as compared to non-primed seeds. The primed and non-primed seeds were grown for 15 days and then the seedlings were independently subjected to iso-osmotic salt (150 mM NaCl) or PEG-8000 (20%) stress. The different biochemical responses were studied 10 days after treatment. Under NaCl and PEG stress, the dry weight and total chlorophyll content were higher in primed sets as compared to non-primed treatment which was also evident by the phenotype of the seedlings. In general, the higher activities of superoxide dismutase and glutathione reductase resulted in lower oxidative damage, in terms of malondialdehyde content, under NaCl and PEG stress in hydro-primed set as compared to non-primed, ABA-, and CaCl₂-primed treatments. Besides, the level of total phenolics and accumulation of osmolytes such as free proline, glycine betaine, and total soluble sugars was also lower in hydro-primed set as compared to other primed and non-primed treatments. The study thus suggests the use of hydro-priming as a simple and cost-effective strategy to alleviate the NaCl and PEG induced stress in B. juncea.     

Growth inhibition by exogenous proline and its metabolism in saltgrass (Distichlis spicata) suspension cultures

The growth of Distichlis spicata suspension cultures in LS medium without NaCl was inhibited 54% by 2 mM proline. In medium containing 260 mM NaCl, 10 mM proline inhibited growth by only 22%. The uptake and metabolism of 10 mM L-[1-¹³C] proline was followed by ¹³C NMR and ninhydrin analyses of suspensions cultured in the presence of 0 or 260 mM NaCl. Uptake of 85 to 92% of the exogenous proline occurred within 72 h in all media. In 10 mM proline and no NaCl, cellular proline reached a maximm of 51.5 moles/g FW compared to 1.9 moles/g FW in suspensions not grown on proline. In medium containing 260 mM NaCl and proline, cellular proline reached 59–65 moles/g FW compared to 30–40 moles/g FW in controls grown without proline. The ¹³C-label in the proline-C₁ was either retained in proline or disappeared, presumably released as carbon dioxide, by catabolism through the TCA cycle. Since no metabolite of ¹³C-proline was detected by NMR, proline was considered to be the molecule which inhibited the suspension culture growth.Abbreviations LS Linsmaier and Skoog medium - FW fresh weight - DW dry weight - P5C 1-pyrroline-5-carboxylate - TCA tricarboxylic acid cycle - FID free-induction-decay - NMR nuclear magnetic resonance spectroscopy - T₁ spin-lattice relaxation time - NOE Nuclear Overhauser Effect.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Adaptability to drought in sugar beet cultivars

The effects of NaCl and polyethylene glycol (PEG) on superoxide dismutase (SOD) and peroxidase (P) activities, lipid peroxidation (LP) and proline content in seeds and leaves of drought tolerant (FC-506 and MS-100) and drought sensitive (MS-612 and MS-13) sugar beet cultivars were examined. After PEG and NaCl treatment in tolerant cultivars both in seeds and leaves SOD activity mainly increased, though P activity increased only in leaves of tolerant cultivars. In drought sensitive cultivars the decrease of SOD and P activity was mostly observed. LP increased in seeds and leaves of all examined cultivars. The proline content increased in the leaves of examined cultivars and was significantly higher in drought tolerant plants. On the other hand, in the seeds only slight increase in proline content was found. The results obtained indicated that drought tolerance could be correlated with high proline content and enzymatic defense against lipid peroxidation.     

Effects of salt and osmotic shocks on unadapted and adapted callus lines of tobacco

Growth, viability and proline content of adapted and unadapted calluses of Nicotiana tabacum L. var. Jayasri, affected due to osmotic stresses and particularly to stress-shocks treated with different osmotica like NaCl (ionic-penetrating), mannitol (non-ionic-penetrating) and polyethylene glycol, (PEG) (non-ionic-non penetrating) were studied to evaluate the physiological differences of stress effects. The tissues adapted to a low concentration of NaCl (85 mM) showed low growth with high proline content compared to the tissues adapted to a low concentration of mannitol (165 mM). Proline content was similar in tissues adapted to high concentrations of NaCl (171 mM) and mannitol (329 mM) but growth in the latter case was relatively low. Growth and viability were subsequently correlated with the pattern of retention in or diffusion of proline out of the tissues after shock-treatments. The loss of tissue viability of the adapted calluses was comparatively less than the unadapted callus even after shock-treatments with 1282 mM NaCl and 823 mM mannitol. The former calluses retained the capability of regrowth though at a slow rate. Such adapted tissues also retained more proline. The mannitol-adapted tissues, when shocked with PEG (200 g l-1), showed low viability with more diffusion and a very little retention of proline while, in the unadapted tissue, all the proline was leached out. The results indicated that the effects of different osmotica on plant tissue varied depending upon the physico-chemical nature of the compounds used as stress-inducing-agents, and retention and diffusion of proline was altered when the tissues were shocked with high concentrations of all these compounds. This revised version was published online in June 2006 with corrections to the Cover Date.     

An inland and a coastal population of the Mediterranean xero-halophyte species Atriplex halimus L. differ in their ability to accumulate proline and glycinebetaine in response to salinity and water stress

Soil salinity and drought compromise water uptake and lead toosmotic adjustment in xero-halophyte plant species. These importantenvironmental constraints may also have specific effects onplant physiology. Stress-induced accumulation of osmocompatiblesolutes was analysed in two Tunisian populations of the Mediteraneanshrub Atriplex halimus L.—plants originating from a salt-affectedcoastal site (Monastir) or from a non-saline semi-arid area(Sbikha)—were exposed to nutrient solution containingeither low (40 mM) or high (160 mM) doses of NaCl or 15% polyethyleneglycol. The low NaCl dose stimulated plant growth in both populations.Plants from Monastir were more resistant to high salinity andexhibited a greater ability to produce glycinebetaine in responseto salt stress. Conversely, plants from Sbikha were more resistantto water stress and displayed a higher rate of proline accumulation.Proline accumulated as early as 24 h after stress impositionand such accumulation was reversible. By contrast, glycinebetaineconcentration culminated after 10 d of stress and did not decreaseafter the stress relief. The highest salt resistance of Monastirplants was not due to a lower rate of Na⁺ absorption; plantsfrom this population exhibited a higher stomatal conductanceand a prodigal water-use strategy leading to lower water-useefficiency than plants from Sbikha. Exogenous application ofproline (1 mM) improved the level of drought resistance in Monastirplants through a decrease in oxidative stress quantified bythe malondialdehyde concentration, while the exogenous applicationof glycinebetaine improved the salinity resistance of Sbikhaplants through a positive effect on photosystem II efficiency. Key words: Atriplex halimus, glycinebetaine, halophyte, NaCl, osmotic adjustment, proline, salinity, water stress     

Inducible and constitutive mechanisms of salt stress resistance in <Emphasis Type="Italic">Geum urbanum</Emphasis> L.

The avens (Geum urbanum L.) seedlings were grown for 6 weeks until the expansion of five to six leaves and then exposed to salinity shock (300 mM NaCl in the nutrient medium) or to a gradual (within 4 days) increase in NaCl concentration from 100 to 400 mM. The dynamics of stress-dependent accumulation of Na⁺, Cl^?, proline, and polyamines in leaves and roots was measured, together with activities of antioxidant enzymes, namely, superoxide dismutase (SOD) and guaiacol-dependent peroxidase occurring in soluble, ionically bound, and covalently bound forms. It is shown that avens plants can adapt to gradual salinization by mobilizing stressinducible protective mechanisms (accumulation of proline and spermine) and by activating constitutive enzyme systems (SOD and peroxidase).     

Glutamine synthetase and glutamate dehydrogenase contribute differentially to proline accumulation in leaves of wheat (Triticum aestivum) seedlings exposed to different salinity

To investigate the roles of ammonium-assimilating enzymes in proline synthesis under salinity stress, the activities of glutamine synthetase (GS; EC 6.3.1.2) and NADH-dependent glutamate dehydrogenase (NADH-GDH; EC 1.4.1.2) were determined in leaves of wheat (Triticum aestivum) seedlings exposed to salt stress at 150 and 300 mM NaCl for 5d. At the lower salinity, only GS activity increased markedly. At 300 mM NaCl, however, NADH-GDH activity increased while GS activity decreased. A significant accumulation of proline was found only at high-salinity exposure while glutamate, a proline precursor, increased dramatically under both low and high salinity. These data suggests that GS-catalysis might be the main glutamate synthesis pathway under low salinity. At 300 mM NaCl, glutamate seems to be preferentially produced through the process catalyzed by NADH-GDH. The increase of ammonium in salinity-stressed wheat seedlings might have resulted from increased photorespiration, which is responsible for the higher NADH-GDH activity. The activity of Delta(1)-pyrroline-5-carboxylate reductase (P5CR; EC 1.5.1.2) was significantly enhanced at 300 mM NaCl but remained unchanged at 150 mM. Delta(1)-Pyrroline-5-carboxylate synthetase (P5CS) activity did not show a specific response, indicating that P5CR might be the limiting step in proline synthesis from glutamate at high salinity.     

Combined effects of CO2 enrichment,diurnal light levels and water stress on foliar metabolites of potato plants grown in naturally sunlit controlled environment chambers

Experiments were conducted in outdoor, naturally sunlit, soil–plant–atmosphere research (SPAR) chambers using plants grown in pots. Drought treatments were imposed on potato plants (Solanum tuberosum cv. Kennebec) beginning 10 days after tuber initiation. A total of 23 out of 37 foliar metabolites were affected by drought when measured 11 days after initiating water stress treatments. Compounds that accumulated in response to drought were hexoses, polyols, branched chain amino acids (BCAAs) and aromatic amino acids, such as proline. Conversely, leaf starch, alanine, aspartate and several organic acids involved in respiratory metabolism decreased with drought. Depending upon harvest date, a maximum of 12 and 17 foliar metabolites also responded to either CO₂ enrichment or diurnal treatments, respectively. In addition, about 20% of the measured metabolites in potato leaflets were simultaneously affected by drought, CO₂ enrichment and diurnal factors combined. This group contained BCAAs, hexoses, leaf starch and malate. Polyols and proline accumulated in response to water stress but did not vary diurnally. Water stress also amplified diurnal variations of hexoses and starch in comparison to control samples. Consequently, specific drought responsive metabolites in potato leaflets were dramatically affected by daily changes of photosynthetic carbon metabolism.