Proline accumulation: A parameter for evaluation of sensitivity of tomato varieties to drought stress?

The pattern of proline accumulation and the growth response were followed in several tomato ( Lycopersicon esculentum Mill.) varieties which were exposed to 7 days of drought stress followed by a 15-day period of rewatering. During dehydration, water potential and leaf elongation rates decreased more in var. 'Hosen' and 'S-5' than in 'LX-11', '1970', 'Pakmor', 'Faculty-16', 'Alcobaca' and '475'. Proline accumulation during stress was greatest in the first two varieties. In 'Hosen' and 'S-5' rewatering resulted in a decrease of proline to control levels, whereas in the other varieties accumulation of proline continued long after turgor had been regained. The extent of this continued accumulation was not correlated with the degree to which each variety was dehydrated. Upon rewatering of the plants the rate of leaf elongation was increased, but the final leaf size as well as whole shoot and root fresh weight of the recovered plants were not colated with the degree of "suffering" that each variety experienced during the drought period. Incubation of detached young tomato leaves in polyethylene glycol solution for 48 h resulted in a substantial accumulation of proline. The varietal differences observed under these conditions were reminiscent of the differential responses in proline accumulation obtained in the intact plants. It is concluded that proline accumulation at the time of dehydration signals drought stress in tomato plants but does not correlate with the overall varietal sensitivity to transient dehydration in recovered plants.     

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.     

不同植物在水分胁迫条件下脯氨酸的累积与抗旱性的关系

Ca植物春小麦幼苗叶片累积脯氨酸的数量随水分胁迫时间和强度递增,累积的数量与品种的抗旱性没有相关性。C_4植物玉米和高粱累积脯氨酸的数量低于小麦,但高粱多于玉米。CAM植物落地生根脯氨酸含量略有变化。旱生植物细枝岩黄芪的同化枝累积脯氨酸的数量高于中生植物钻天杨和槐的叶片,而梭梭的同化校则低于槐而高于钻天杨。表明这些植物累积脯氨酸的数量与它们的抗旱性无夫。因此似不宜用脯氨酸数量的多少作为植物抗旱性的生理指标。     

Free proline accumulation in drought-stressed plants under laboratory conditions

Summary Free proline accumulation was measured in leaves of intact sorghum (Sorghum bicolor L. cv. Pioneer 846) and soybean (Glycine max. L. cv. Calland) grown in growth chambers and subjected to ‘normal’ drought stress. Stomatal diffusive resistance and leaf water potential were used to determine the degree of stress at the time of proline analysis. Free proline did not accumulate markedly in either species until each was severely stressed, indicating that proline is not a sensitive indicator of drought stress. Free proline accumulated under less stress in soybean than in sorghum. Since soybean is less drought resistant than sorghum, proline accumulation may be an indicator of drought resistance or susceptibility. re]19731003     

Comparison of response of two C3 species to leaf water relation,proline synthesis,gas exchange and water use under periodic water stress

Water relations, proline content and gas exchange of leaf were investigated under periodic water stress for two C3 plants (eggplant and tomato) in a greenhouse to study comparative adaptive responses. Although both species showed reduced water content of leaf and increased osmolality and proline content under low soil water potential, the recovery capacity after the stress was better in eggplant than tomato. Both species over-accumulated proline under low soil water potential and returned to its initial concentration during the recovery, indicating that proline may act as an osmoprotectant during drought. Proline was directly corresponding with osmolality during stress, and dehydration stress reduced the gas exchange parameters such as transpiration rate (ET), stomatal conductance (GS), and photosynthesis rate (Pn). At the final stage of the experiment both species showed 2.6 and 3.3 times lower Pn and 27 and 19 times lower GS for eggplant and tomato, respectively, as compared to control. But after stress was relieved by rewatering, both plants increased GS for 2 to 3 times and Pn for 4.5 times. Eggplant showed better water use efficiency (WUE) in relation to fruit production under the stress than tomato. Higher biomass allocation at root and fruit parts in eggplant indicated more efficient recovery than that of tomato. These findings inferred that both C3 plants developed internal complementary drought survival mechanism by lowering relative water content, increasing proline, and decreasing stomatal conductance but eggplants withstood the periodic draughting better than tomato, mainly due to its ability to recover from a water stress condition.     

Water stress induced changes in concentrations of proline and total soluble sugars in nodulated alfalfa (Medicago sativd) plants

Susceptibility of alfalfa ( Medicago saliva L. cv. Aragón) nodules and leaves to water stress has been investigated. Nodule acetylene reduction activity (ARA), leaf CO₂ exchange rate (CER) as well as soluble protein, proline and total soluble sugar (TSS) contents were determined during drought. Water status was estimated as water potential (Ψ_w) and Relative water content (RWC) of the respective tissues. Maximum rates of ARA required higher Ψ_w than CER. Nodules had lower RWC for a given Ψ_w than leaves. Water stress reduced soluble protein content in both tissues; however, the decline in soluble protein content was detected at greater Ψ_w in nodules than in leaves. Proline and TSS increased in leaves and nodules, and again the threshold Ψ_w triggering such accumulation was higher in nodule tissues. Oior results suggest that alfalfa nodules are more susceptible to water shortage than leaves. Effects of accumulated TSS and proline upon leaf and nodule physiology are discussed in relation to protein stability (proline), pH control (proline) and osmotic adjustment (proiine and TSS). The TSS accumulation induced by water stress suggests that substrate shortage would not be the primary effect of drought on nodule activity.     

Accumulation of Free Proline at Low Temperatures

The accumulation of free proline in the first leaves of barley, Hordeum distichum L., and wheat, Triticum aestivum L., in response to a range of low temperatures was examined with 10-day-old plants. In barley (cv. Prior) no proline accumulated at 8°C or above, but in wheat (cv. Gabo) proline accumulated at 12°C and lower temperatures. In barley, the first leaf survived for 29 days following transfer to 5°C and continued to accumulate proline throughout this period. In contrast, the first leaves of plants maintained at 20°C survived for 13 days only and accumulated no proline. Proline accumulation at low temperature was shown to be light-dependent, both in intact plants and excised leaf sections, and the light requirement could not be replaced by supplying leaf segments with precursors of proline. Proline accumulation in response to water stress was not light-dependent at 20°C but was at 5°C. Inter-specific and intra-specific variation in the extent of accumulation in response to low temperature was also examined. Considerable variation was encountered but there was no clear relationship with geographical distribution or chilling sensitivity for the species and no correlation with accumulation in response to water stress in the cultivars of barley examined.     

Proline Metablism During Water Stress in Mulberry

The influence of water stress on proline metabolism was studiedin 3-month-old mulberry plants at four levels of water stress.Leaf water potential was drastically decreased in all treatments.Though leaf area and relative water content were decreased,drastic decrease was observed only in very severe stress treatments.Proline accumulation was observed both in roots and leaves instress treatments; but accumulation was greater in roots thanin leaves. The enzymes, proline dehydrogenase and proline oxidase,were inhibited under stress conditions. Proline oxidase wasmore inhibited in roots than in leaves. The significance ofthe relative activities of these two enzymes is discussed. Key words: Water stress, proline dehydrogenase, proline oxidase     

Effects of Water Stress on the Ultrastructure of Leaf Cells of Sorghum bicolor

The subcellular changes which occurred in sorghum leaves during increasing water stress and subsequent rewatering are described. Stomata were closed, abscisic acid levels were elevated, and the amounts of starch in the bundle sheath chloroplasts were much reduced by - 14 bars leaf water potential. Swelling of the outer chloroplast membrane, and reorganization of the tonoplast to form small vesicles from the large central vacuole, occurred by a leaf water potential of - 37 bars. Complete structural disruption of the tonoplast, as previously described for maize was not found. On rewatering, large amounts of starch reappeared within three hours. These findings strengthen the hypothesis that maintenance of tonoplast integrity is an important factor in the ability of plants to withstand drought.     

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     

Growth characteristics and proline content in relation to water status in twoZea mays L. Cultivars during rehydration

Growth behaviour, proline changes and water saturation deficit (WSD) changes were studied in two maize cultivars, Agati-76, a drought resistant oultivar, and cv. Vijay, a susceptible one after stress conditions were removed. Plants of both the cultivars showed a tendency to recover during rehydration. Phenotypically the plants appeared just normal on 7th day of rehydration, although recovery was never complete. Significant positive correlation existed between free proline content and concomitant change in water saturation deficit. Significantly higher proline content in cv. Agati-76 was reoorded in cv. Vijay, immediately after the stress was released, thereafter continuous decline was observed up to 7th day of rehydration in both the cultivars. Proline changes in relation to recovery of plants from stress conditions are discussed.     

Reduction of shoot fly damage in irrigated post-rainy season sorghum by manipulating irrigation

Soil moisture was manipulated in an attempt to control shoot fly (Atherigona soccata Rondani) incidence in irrigated post-rainy season sorghum grown under a rainout shelter (ROS) and in field conditions. After uniform irrigation at sowing, the plants were subjected to water stress at young seedling stage (7–28 days after emergence, DAE) for different lengths of time. Soil water had profound effects on the production of water droplets on the surface of the central whorl leaf of seedlings (leaf surface wetness, LSW) of sorghum genotypes. LSW, which facilitates movement of the larvae, was more drastically affected in susceptible (CSH 5) than in moderately resistant (IS 1054) sorghum genotypes. Shoot fly oviposition (infestation) and deadhearts (crop damage) were much higher in treatments with full irrigation (control) than in treatments to which less water was applied during the first 3 wk after seedling emergence. This resulted in higher plant biomass and overall grain yield in the latter treatments than in the control. Using insecticides to control shoot fly infestation, it was shown that a simple cultural practice of inducing plant stress by reduced soil moisture content during early plant growth gave the same or better control of shoot fly damage and the same or higher grain yield than insecticide-protected plots with full irrigation. Thus the costs associated with irrigation requirement and insecticide can be greatly reduced in the former management option compared with the latter. It is suggested that manipulation of soil water content during the vulnerable early stages of crop growth can reduce shoot fly damage in irrigated post-rainy season sorghum.     

Changes in abscisic acid and proline levels in maize varieties of different drought resistance

Four varieties of maize differing in drought resistance and geographical origin (Swabi White and Shaheen from Pakistan, Garbo and Goldprinz from Germany) were analyzed for their proline and abscisic acid (ABA) accumulation during a prolonged water stress period. Proline levels increased continuously during the stress period in all the four varieties, but to different amounts. The drought-susceptible varieties Shaheen and Goldprinz produced higher levels of proline than the drought-resistant varieties Swabi White and Garbo. A negative correlation was also found between maximal ABA contents and degree of drought resistance during prolonged stress of younger plants. ABA levels did not increase steadily, but reached a maximum long before the end of the stress phase, and then declined. The results are discussed in relation to the possibility of using proline and ABA levels as biochemical indicators of resistance against drought.     

Long term water stress inactivates Rubisco in subterranean clover

In long-term field experiments, during consecutive years, microswards of subterranean clover were irrigated to minimise water deficits or subjected to progressively increasing drought over 30 days. Both leaf water potential and relative water content steadily decreased during the experiments. Plants affected by drought grew more slowly and photosynthesis was decreased. Photosynthetic rate (A) and Rubisco were analysed in relation to midday water potentials and relative water contents. The difference in A between draughted and irrigated plants increased progressively, in part as a result of decreased stomatal conductance and CO₂ concentration within leaf (Ci). However, A-Ci curves suggest that the photosynthetic capacity in plants experiencing long-term stress was reduced by 50% when compared with irrigated plants. Drought decreased both the initial and the total Rubisco activity per unit area in a similar way but did not reduce the amount of Rubisco protein per unit leaf area. Thus, the specific activity of Rubisco, rather than its activation state, decreased suggesting that under water stress the active sites were blocked by inhibitors.     

Evaluation of the stress-inducible production of proline in transgenic sugarcane (Saccharum spp.): osmotic adjustment, chlorophyll fluorescence and oxidative stress

Proline accumulates in a variety of plant species in response to stresses such as drought, salinity and extreme temperatures. Although its role in plant osmotolerance remains controversial, proline is thought to contribute to osmotic adjustment, detoxification of reactive oxygen species and protection of membrane integrity. In the present study, we evaluated the effects of stress-inducible proline production on osmotic adjustment, chlorophyll fluorescence and oxidative stress protection in transgenic sugarcane transformed with a heterologous P5CS gene. In well-watered conditions, free proline, malondialdehyde (MDA) levels, Fv/Fm ratios and chlorophyll contents (Chls) in transgenic sugarcane were not statistically different from non-transformed control plants. After 9 days without irrigation, proline content in transgenic events was on the average 2.5-fold higher than in controls. However, no osmotic adjustment was observed in plants overproducing proline during the water-deficit period. The photochemical efficiency of PSII observed was higher (65%) in the transgenic events at the end of the water-deficit experiment. The effects of proline on lipid peroxidation as MDA levels and on the decline of Chl in paraquat-treated leaf discs along the drought period suggest that proline protected the plants against the oxidative stress caused by the water deficit. The overall capacity of transgenic plants to tolerate water-deficit stress could be assessed by the significantly higher biomass yields 12 days after withholding water. These results suggest that stress-inducible proline accumulation in transgenic sugarcane plants under water-deficit stress acts as a component of antioxidative defense system rather than as an osmotic adjustment mediator.     

Effects of Russian wheat aphid infestation on barley plant response to drought stress

The influence of Russian wheat aphid ( Diuraphis noxia Mordvilko) infestation on the response of barley ( Hordeum vulgare L. ev Hazen) plants to drought stress was investigated. Fourteen-day-old plants were infested with eight apterous adult aphids, which were removed 7 days later with systemic insecticide. Leaves previously infested with aphids had lower relative water content, reduced stomatal conductance, more negative water potential, lower levels of chlorophyll and higher levels of amino-N, proline and glycinebetaine than corresponding leaves from uninfested plants. When water was withheld for a period of 7 days after aphids were removed, the relative water content of previously infested plants dropped steadily from 0.89 to 0.60, while the relative water content of uninfested plants remained at about 0.94 for the first 4 days of the drought stress period followed by a steady drop to about 0.77 by the end of the drought stress period. Leaf water potentials dropped steadily during the drought stress period in both previously infested (-1.14 to -1.91 MPa) and unin-fested (-0.54 to -1.52 MPa) plants. Analysis of glycinebetaine and proline levels at the end of the drought stress period indicated that leaves of previously infested plants accumulated lower levels of these solutes than leaves from uninfested plants. Upon alleviation of drought stress, plants previously infested with aphids showed little increase in dry weight while younger leaves and tillers from uninfested plants showed large increases. It is concluded that Russian wheat aphids cause drought-stress symptoms in leaves of infested plants even in the presence of ample root moisture. The observations of low levels of glycinebetaine and proline present in leaves after water was withheld from roots and lack of leaf growth upon alleviation of drought stress in previously-infested plants, suggest that aphid infestation limits the capacity of barley plants to adjust successfully to drought stress.     

Physiological responses of somaclonal variants of triploid bermudagrass (<Emphasis Type="Italic">Cynodon transvaalensis</Emphasis> × <Emphasis Type="Italic">Cynodon dactylon</Emphasis>) to drought stress

Eight somaclonal variants with enhanced drought tolerance were isolated from regenerated plants of triploid bermudagrass (Cynodon dactylon × Cynodon transvaalensis cv., TifEagle). Three of them (10-17, 89-02, 117-08) with strong drought tolerance were selected for investigations of physiological responses to drought stress. Compared to the parent control, TifEagle, the somaclonal variants had higher relative water contents and relative growth, and lower ion leakages in the greenhouse tests, while no difference in evapotranspirational water losses and soil water contents was observed between the variants and TifEagle. The variants also had less leaf firing in the field tests under drought stress. Superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities decreased gradually in responses to drought stress in all plants and exhibited negative correlations with ion leakage, indicating that the declined activities of these antioxidant enzymes were associated with drought injury in the triploid bermudagrass. However, CAT activities were significantly higher in all three variants than in TifEagle during drought stress. Two variants, 10-17 and 89-02, also had significantly higher APX activities than TifEagle before and during the first 4 days of drought treatments. These two lines also showed higher SOD activities after prolonged drought stress. Proline, total soluble sugars and sucrose were accumulated under drought stress in all plants and exhibited positive correlations with ion leakage. More proline and sugars were accumulated in TifEagle than in the variants. The results indicated that higher activities of the antioxidant enzymes in the variants during drought stress are associated with their increased drought tolerance.     

Over-expression of Osmotin Induces Proline Accumulation and Confers Tolerance to Osmotic Stress in Transgenic Tobacco

Osmotin has been implicated in conferring tolerance to drought and salt stress in plants. We have over-expressed the osmotin gene under the control of constitutive CaMV 35S promoter in transgenic tobacco, and studied involvement of the protein in imparting tolerance to salinity and drought stress. The transgenic plants exhibited retarded leaf senescence and improved germination on a medium containing 200mM NaCl. Further, the transgenics maintained higher leaf relative water content (RWC), leaf photosynthesis and free proline content than the wild type plants during water stress and after recovery from stress. When subjected to salt stress (200mM NaCl), the transgenic plants accumulated significantly more proline than the wild type plants. These results suggest the involvement of the osmotin-induced increase in proline in imparting tolerance to salinity and drought stress in transgenic plants over-expressing the osmotin gene.     

Proteome response of Elymus elongatum to severe water stress and recovery

Tall wheatgrass (Elymus elongatum Host) is a drought-tolerant, cool-season forage grass native to Iran. A proteomic approach has been applied to identify mechanisms of drought responsiveness and tolerance in plants undergoing vegetative stage drought stress and then recovery after rewatering. Uniformed clones were reproduced from a parent plant collected from Brojen (central region of Iran). Clones were grown in pots and drought was initiated by withholding water for 16 d. The leaf samples were taken in triplicate from both stressed/rewatered plants and continuously watered controls at five times: (i) 75% FC, (ii) 50% FC, (iii) 25% FC, (iv) 3 d after rewatering, and (v) 14 d after rewatering. Changes in the proteome pattern of shoots were studied using two-dimensional gel electrophoresis. Following the 16 d water stress, both shoot dry weight and leaf width decreased up to 67% compared with the well-watered plants, whereas proline content increased up to 20-fold. Leaf relative water contents (RWC) also declined from 85% to 24%. Out of about 600 protein spots detected on any given two-dimensional gel, 58 protein spots were reproducibly and significantly changed during drought stress and recovery. Only one protein (abscisic acid- and stress-inducible protein) showed significant changes in expression and position in response to severe drought. The fifty-eight responsive proteins were categorized in six clusters including two groups of proteins specifically up- and down-regulated in response to severe drought stress. Eighteen proteins belonging to these two groups were analysed by liquid chromatography tandem mass spectrometry leading to the identification of 11 of them, including the oxygen-evolving enhancer protein 2, abscisic acid- and stress-inducible protein, several oxidative stress tolerance enzymes, two small heat shock proteins, and Rubisco breakdown. The results suggest that E. elongatum may tolerate severe drought stress by accumulating proline and several proteins related to drought-stress tolerance. Recovery after rewatering might be another mechanism by which plants tolerate erratic rainfall in semi-arid regions.