Osmoregulation in two aquatic oligochaetes from habitats with different salinity and comparison to other annelids

The osmoregulatory capacity of two oligochaete species, Enchytraeus albidus Henle, 1837, and Heterochaeta costata (Claparède, 1863), was investigated by direct measurements of the osmolality of the coelomic fluid. Terrestrial and marine (28 S) populations of Enchytraeus albidus and a brackish water population (14 S) of H. costata were used in the study. The range of salinity acclimation investigated was 0–40. The response to osmotic stress was measured (a) after a long-term maintenance (>14 days) in various salinities (E. albidus only), and (b) after a hyperosmotic shock as a short-term time-course sequence. The rate of water loss following a hyperosmotic shock was measured for E. albidus. Long-term acclimation. E. albidus maintained a hyperosmotic coelomic fluid over all salinities tested. In low salinities the osmolality of the coelomic fluid of the marine population was significantly higher than that of the terrestrial population. Possible genetic discrepancies or long-term acclimation may account for this difference. The coelomic fluid of H. costata was hyperosmotic at 15%. S and isoosmotic at 30 S. Short-term acclimation (hyperosmotic shock). Both species investigated, kept at 15 S and then exposed to a salinity of 30, showed fast responses: within the first two hours the internal concentrations were adjusted to the new external condition with only small subsequent changes. Regulation of the body-water content after an exposure to a hyperosmotic shock was much slower: individuals of terrestrial E. albidus, acclimated for two weeks to either 0 or 15 S, had the same water content; hence, they showed a 100% regulation. However, after exposure to 30 S, a 100% regulation was still not attained 4 days after the hyperosmotic shock. Enchytraeus albidus is capable of actively reducing water loss following the hyperosmotic shock: the observed loss of water was only 40% of that expected for a passive osmotic flow. The observed reactions are compared with those in other annelids. It seems that an active transport of ions combined with a changeable permeability of the body wall play a major role in the regulation of body fluids.     

A role for nickel in osmotic adjustment in drought-stressed plants of the nickel hyperaccumulator <Emphasis Type="Italic">Stackhousia tryonii</Emphasis> Bailey

The hypothesis that hyperaccumulation of certain metals in plants may play a role in osmotic adjustment under water stress (drought) was tested in the context of nickel hyperaccumulator Stackhousia tryonii. Field-collected mature plants of S. tryonii, grown in native ultramafic soil, were pruned to soil level and the re-growth exposed to five levels of water stress (20, 40, 60, 80 and 100% field capacity; FC) for 20 weeks. Water stress had significant (P<0.05) influence on growth (biomass), water potential and shoot Ni concentrations, with progressively more impact as water stress was increased from 80 to 40% FC. Shoot Ni concentration increased significantly from 3,400 μg g⁻¹ dry weight (at 100% FC) to 9,400 μg g⁻¹ dry weight (at 20% FC). Assuming that Ni is uniformly distributed through the shoot tissue, the Ni concentration could account for 100% at the 80 and 60% FC conditions, and 50% at the 40 and 20% FC conditions of plant osmotic regulation. The results are consistent with a role of Ni in osmotic adjustment and protection of S. tryonii plants against drought.     

Effects of long-term experimental night-time warming and drought on photosynthesis, <Emphasis Type="Italic">F</Emphasis>v/<Emphasis Type="Italic">F</Emphasis>m and stomatal conductance in the dominant species of a Mediterranean shrubland

We conducted a night-time warming and drought field experiment for 7 years (1999–2005) in a Mediterranean shrubland. We focused on the two dominant shrub species, Erica multiflora L. and Globularia alypum L. and the tree Pinus halepensis L. and the final years to study the effects of the experimental night-time warming and drought on Fv/Fm, photosynthesis, and stomatal conductance. Warming treatment increased mean air temperature and mean soil temperature through the years by an average of 0.7 and 0.9°C respectively, and drought treatment reduced soil moisture through the years by an average of 19%. Warming tended to increase photosynthetic rates in E. multiflora, G. alypum and P. halepensis mostly in the cold seasons, when plants were more limited by temperature, as shown by the lowest values of Fv/Fm being detected in winter in the three studied species. A negative effect of warming was only detected for E. multiflora in summer 2003. Drought treatment generated different responses of net photosynthetic rates depending on the species, season and year. Stomatal conductance showed the same pattern as photosynthesis for the three studied species, displaying seasonal and inter-annual variability, although with an overall negative effect of drought for P. halepensis. Photosynthetic rates decreased significantly in the dry winter 2005 and spring 2005 in comparison to the same seasons of 2003 and 2004. There were positive correlations between the photosynthetic rates in different seasons for E. multiflora, G. alypum and P. halepensis and the soil moisture of the week prior to measurements. The great variation in the photosynthetic rates was thus explained in a significant part by soil moisture levels. The lowest Fv/Fm values usually corresponded with lowest stomatal conductances suggesting that drought stress could be associated to stress by low temperatures in winter.     

珍稀濒危植物堇叶紫金牛对持续干旱的生理响应

采用盆栽控水法,研究了珍稀濒危植物堇叶紫金牛(Ardisia violacea)在持续干旱条件下的生理响应。随着持续干旱时间的延长,堇叶紫金牛应对持续干旱的阶段可分为适应期、轻度干旱期、中度干旱期和重度干旱期。在适应期和轻度干旱期,堇叶紫金牛叶片游离脯氨酸和可溶性糖含量稳定在一个较低水平,可溶性蛋白质含量先下降后快速上升,细胞膜系统和抗氧化酶系统能主动进行生理调节;中度干旱期,丙二醛(MDA)含量和质膜相对透性迅速升高,细胞膜系统受损加剧,游离脯氨酸、可溶性糖含量均急剧增加,对抵御干旱起到重要的渗透调节作用。在轻度干旱期和中度干旱期,光合色素中叶绿素a和叶绿素b含量显著提高,以抵抗干旱胁迫。重度干旱期,细胞膜系统、抗氧化酶SOD、游离脯氨酸和可溶性糖含量上升,但MDA略微下降,这时可能达到植物耐受干旱的极限,不再发生膜脂过氧化作用。综上表明,堇叶紫金牛具有较强的耐旱性,RWC为49.94%是细胞膜系统、抗氧化酶系统和渗透调节物质含量变化的拐点,渗透调节和抗氧化酶系统的主动适应是其耐旱的主要机制。     

砂藓RcPLD基因的抗旱功能分析

砂藓(Racomitrium canescens)是一种具有极强耐脱水性的苔藓植物,编码磷脂酶D的基因RcPLD能够在砂藓的脱水和复水过程中产生显著的表达响应,它可能参与了砂藓的强耐脱水性功能。该研究使用已克隆的RcPLD编码序列构建拟南芥(Arabidopsis thaliana)过量表达转基因株系rcpld-oe,初步考察过表达株系的干旱胁迫耐受能力及其相关的生理生化指标,分析RcPLD增强拟南芥抗旱性的机制。结果表明:(1)利用已克隆的RcPLD编码序列构建了植物中的过表达载体,成功构建了RcPLD的过表达转基因拟南芥株系rcpld-oe,并获得了多个T_3代rcpld-oe纯合体株系。(2)在正常生长条件下,rcpld-oe株系T_3代纯合体植株比野生型拟南芥植株体积小,但营养生长期较长,抽薹较晚,莲座叶衰老速率较慢;在干旱处理条件下,rcpld-oe株系表现出比野生型拟南芥更强的干旱耐受能力。(3)在干旱胁迫处理过程中,rcpld-oe株系莲座叶的水分散失速率降低,可能在一定程度上降低了干旱对膜完整性的损伤和光合作用的抑制,但其渗透调节物质含量的变化相对较小。研究发现,在干旱胁迫条件下,rcpld-oe植株莲座叶的水分散失速率和光合作用抑制程度显著降低,从而表现出明显强于野生型的干旱耐受能力,这为后续RcPLD功能的深入研究和更多砂藓抗旱功能基因的挖掘奠定了基础。     

Water use patterns of four co-occurring chaparral shrubs

Summary Mixed stands of chaparral in California usually contain several species of shrubs growing close to each other so that aerial branches and subterranean roots overlap. There is some evidence that roots are stratified relative to depth. It may be that root stratification promotes sharing of soil moisture resources. We examined this possibility by comparing seasonal water use patterns in a mixed stand of chaparral dominated by four species of shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a neutron probe and soil phychrometers to follow seasonal depletion and recharging of soil moisture and compared these patterns to seasonal patterns of predawn water potentials, diurnal leaf conductances, and diurnal leaf water potentials. Our results indicated that 1) Quercus was deeply rooted, having high water potentials and high leaf conductances throughout the summer drought period, 2) Heteromeles/Adenostoma were intermediate in rooting depth, water potentials, and leaf conductances, and 3) Rhamnus was shallow rooted, having the lowest water potentials and leaf conductances. During the peak of the drought, predawn water potentials for Quercus corresponded to soil water potentials at or below a depth of 2 m, predawn water potentials of Heteromeles/ Adenostoma corresponded to a depth of 0.75 m, and predawn water potentials of Rhamnus corresponded to a depth of 0.5 m. This study supports the concept that co-occurring shrubs of chaparral in California utilize a different base of soil moisture resources.     

Responses of nonstructural carbohydrates to shoot removal and soil moisture treatments in <Emphasis Type="Italic">Salix nigra</Emphasis>

Aboveground disturbances are common in dynamic riparian environments, and Salix nigra is well adapted with a vigorous resprouting response. Soil moisture stresses are also common, and S. nigra is flood tolerant and drought sensitive. The objective of this study was to quantify nonstructural carbohydrate (NSC) reserves in S. nigra following shoot removal and soil moisture treatments. NSC reserves provide energy for regeneration of shoot tissue until new functional leaves are developed. Three soil moisture treatments: well-watered (W), periodic flooding (F) and drought (D); and three shoot removal treatments: no shoots removed (R0), partial shoot removal (R1), and complete shoot removal (R2) were applied. Plants were harvested when new shoot development was observed (day 13). Statistical significance in the 3 × 3-factorial design was determined in two-factor ANOVA at P < 0.05. Both roots and cuttings were important reservoirs for NSC during resprouting response, with decreases in root (31%) and cutting (14%) biomass in R2 compared to R0. Rapid recovery of photosynthetic surface area (from 15 to 37% of R0) was found in R1. A clear pattern of starch mobilization was found in roots in R0, R1 and R2, with lowest root starch concentration in W, F higher than W, and D higher than F. Shoot starch concentration was lower in F and D compared to W in R0, however, in R1 shoot starch was reduced in W compared to F and D, possibly indicating reduced rates of translocation during soil moisture stress. Evidence of osmotic adjustment was found in roots and shoots with higher total ethanol-soluble carbohydrates (TESC) during soil moisture stress in F and D treatments. Total plant NSC pool was greater in F and D treatments compared to W, and progressively reduced from R0 to R1 to R2. Results indicated negative effects of drought, and to a lesser extent periodic flooding on resprouting response in S. nigra, with implications for reduced survival when exposed to combined stresses of aboveground disturbance and soil moisture.     

Accumulation of free amino acids during exposure to drought in three springtail species

Springtails are closely related to insects, but they differ from these with respect to water balance, in particular because springtails are small and have high integumental permeability to water. Here we report a series of experiments addressing the dynamics of osmoregulation, water content and accumulation of free amino acids (FAAs) in three springtail species during exposure to a gradually increasing environmental desiccation simulating conditions in drought exposed soil. Folsomia candida and Protaphorura fimata (both living in the deeper soil layers; euedaphic species) were active throughout the 3 week exposure, with the developing drought regime ending at −3.56 MPa (the soil water activity at the permanent wilting point of plants is −1.5 MPa) and remained hyperosmotic (having an body fluid osmolality higher than the corresponding environment) to their surrounding air. Sinella curviseta (living in upper soil/litter layers; hemiedaphic species) also survived this exposure, but remained hypoosmotic throughout (i.e. with lower osmolality than the environment). The body content of most FAAs increased in response to drought in all three species. Alanine, proline and arginine were the most significantly upregulated FAAs. By combining our results with data in the literature, we could account for 82% of the observed osmolality at −3.56 MPa in F. candida and 92% in P. fimata. The osmolality of S. curviseta was only slightly increased under drought, but here FAAs were considerably more important as osmolytes than in the two other species. We propose that FAAs probably have general importance in drought tolerance of springtails.     

Gas exchange and photosynthesis of Eucalyptus camaldulensis seedlings inoculated with different ectomycorrhizal symbionts

Eucalyptus camaldulensis Dehnh. seedlings inoculated with Pisolithus tinctorius (Pers.) Coker & Couch and Thelephora terrestris Ehrl. per Fr. were grown in well watered soil (_s –0.03 MPa) or subjected to a long-term soil water stress of up to –1.0 MPa over 13-week period in a glasshouse. After 13 weeks, all seedling containers were watered to field capacity and then water was withheld from the E. camaldulensis seedlings to induce a short-term drought. Diurnal measurements of seedling photosynthesis rate (A), leaf stomatal conductance (g) and leaf water potential (_p) were completed before, during, and after the short term drought. Although they were growing in an equal soil volume, photosynthesis rate (A), leaf stomatal conductance and leaf water potential (_p) of larger seedlings with P. tinctorius ectomycorrhizae were similar to those of smaller seedlings colonized with T. terrestris during the short-term drought period. Seedlings inoculated with Pisolithus tinctorius maintained higher photosynthesis rates over the course of the short-term drought. Thus, P. tinctorius ectomycorrhizae appear to be more efficient than those of T. terrestris in assisting seedlings to maintain gas exchange and photosynthesis under limited soil moisture conditions.     

盐分和干旱对沙枣幼苗生理特性的影响

以沙枣(Elaeagnus angustifolia L.)幼苗为实验材料,分别对其进行轻度干旱(土壤含水量7%—9%)、重度干旱(土壤含水量3%—5%)、100 mmol/L NaCl以及100 mmol/L NaCl处理下不同程度的盐旱共胁迫处理,处理2周后测其生理指标,包括生长指标、光合指标、渗透调节指标以及复水后生长指标,研究盐旱共胁迫对沙枣幼苗生理特性的影响。结果表明:和对照相比,轻度干旱对沙枣幼苗的生物量没有显著影响,重度干旱处理明显降低了沙枣的生物量,无论是轻度干旱还是重度干旱,都显著降低了沙枣幼苗的净光合速率、K~+含量,显著增加了Na~+含量、脯氨酸含量、可溶性糖含量、有机酸含量、总酚和类黄酮含量;和对照相比,100 mmol/L NaCl处理显著降低了沙枣幼苗的生物量、净光合速率和K~+含量,显著增加了Na~+含量、脯氨酸含量、可溶性糖含量、有机酸含量、总酚和类黄酮含量;和盐处理相比,轻度干旱和盐分共胁迫对沙枣幼苗的各项指标没有显著差异,而重度干旱和盐分共胁迫明显降低了沙枣幼苗的生物量、净光合速率;复水一周后,只有轻度干旱可以回复到对照水平。以上结果表明,盐分和干旱处理明显抑制了沙枣幼苗的生长,轻度干旱和盐分共胁迫条件下,沙枣幼苗表现出一定的交叉适应现象,而重度干旱却加重了盐害。     

Characterisation of the FAM69 family of cysteine-rich endoplasmic reticulum proteins

Myo-inositol is one of the major organic osmolytes in the brain and the kidney. The accumulation of intracellular organic osmolytes allows cells to regulate intracellular osmolality without altering cytoplasmic ionic strength and to adapt to hyperosmotic conditions. Two types of myo-inositol transporters, sodium/myo-inositol transporter and H⁺/myo-inositol transporter (HMIT), have been identified. Sodium/myo-inositol transporters are induced by osmotic stress and might be involved in the intracellular accumulation of myo-inositol in mammals. The role of HMIT, however, remains unknown. In the present study, we characterized three Caenorhabditis elegansHMIT genes, hmit-1.1, hmit-1.2, and hmit-1.3. hmit-1.1 was expressed in the intestine, and hmit-1.2 was expressed in the glia and the excretory canal, which is an osmotic regulatory organ that is functionally analogous to the kidney. hmit-1.3 was expressed in the intestine and the glia. The expression of hmit-1.1 and hmit-1.2 but not hmit-1.3, was markedly induced under hyperosmotic conditions. Animals with mutant hmit-1.1 and hmit-1.2 were hypersensitive to osmotic stress. The defects of hmit-1.1 and hmit-1.2 mutants were rescued by hmit-1.1 and hmit-1.2 transgenes, respectively, and by modified human HMIT. In human cell lines, HMIT expression was induced in hyperosmotic conditions. These findings indicate that the C. elegans HMIT family has a crucial role in the osmoprotective response.     

Comparative ecophysiology ofLarix kaempferi (Lamb.) Carr. andAbies veitchii Lindl. II. Mechanisms of higher drought resistance of seedlings ofL. kaempferi as compared withA. veitchii

The underlying mechanisms of drought resistance in seedlings ofLarix kaempferi as compared with those inAbies veitchii were investigated. First, seedlings of the two species were desiccated in air of differing water potential, and their survival rates were compared to detect any difference in desiccation tolerance between them. Contrary to differences in their drought resistance,A. veitchii showed higher survival. Next, the shoot heights and root depths of the two species were compared to detect any difference in desiccation avoidance. Although the shoot height differed little between the two species, the roots ofL. kaempferi seedlings were much deeper than those ofA. veitchii seedlings, and the death rate under progressing drought was well correlated with the root extensibility of each species. It was thus concluded thatL. kaempferi seedlings are able to avoid desiccation by extending their roots in deep soil faster thanA. veitchii seedlings, thereby resisting to drought. This paper is dedicated to Prof. Toshiro Saeki to mark his fruitful career in plant ecology.     

Response of liquid flow resistance to soil drying in seedlings of four deciduous angiosperms

Summary Soil-leaf resistance to liquid water flow (R) in moist and drying soil was compared in three-month-old seedlings of two drought tolerant (white [Quercus alba L.], post oak [Q. stellata Wangenh.]) and two drought sensitive forest species (sugar maple [Acer saccharum Marsh.], black walnut [Juglans nigra L.]). At high soil moisture (_s–0.3 MPa), R was higher in J. nigra than in the other species, and as soil water was depleted R increased most in this species. In contrast, the lowest resistance at all levels of soil moisture was observed in Q. stellata. At _s of –1.5 MPa, R of drought-sensitive J. nigra and A. saccharum was about twice as high as that of the two drought-tolerant Quercus species. The difference in R between the two Quercus species was much smaller than that between this pair and the other two species. These differences among species in flow resistance may be attributable to: 1) variation in the balance between root surface area and leaf area, 2) variation in the inherent absorption capacity of the root systems and in xylem water conducting systems or 3) differences in root permeability, shrinkage and mortality in severely stressed seedlings. As the soil dried, seedlings of all species exhibited pronounced reductions in transpiration rate, which prevented development of large water potential gradients between leaves and the soil. Reduction in transpiration in J. nigra was especially pronounced, resulting in a decrease in the soil-to-leaf water potential gradient in dry soil despite high flow resistance. The observed differences among species in flow resistance are correlated with natural distribution patterns.     

Influence of long-term drought stress on osmolyte accumulation in sugar beet (Beta vulgaris L.) plants

Accumulation of various osmolytes was examined in plants of sugar beet cv. Janus grown under two soil water treatments: control (60% of the field water capacity; FWC) and drought (30–35% FWC). The water shortage started on the 61st day after emergence (DAE), at the stage of the beginning of tap-roots development and was imposed for 35 days. Osmotic potential of sugar beet plant organs, particularly tap-roots, was decreased significantly as a consequence of a long-term drought. Water shortage reduced univalent (K⁺, Na⁺) cations concentrations in the petioles and divalent (Ca²⁺, Mg²⁺) ions level in the mature and old leaves. Cation concentrations in the tap-roots were not affected by water shortage. The ratio of univalent to divalent cations was significantly increased in young leaves and petioles as a consequence of drought. Long-term water deficit caused a significant reduction of inorganic phosphorus (P_i) concentration in young and old leaves. Under the water stress condition, the concentration of proline was increased in all individual plant organs, except proline concentration in the youngest leaves. Drought treatment caused a significant increase of glycine betaine content in shoot without any change in tap-roots. Glucose concentrations were significantly increased only in tap-roots as the effect of drought. In response to water shortage the accumulation of sucrose was observed in all the examined leaves and tap-roots. Overall, a long-term drought activated an effective mechanism for osmotic adjustment both in the shoot and in the root tissues which may be critical to survival rather than to maintain plant growth but sugar beet organs accumulate different solutes as a response to water cessation.     

Water relations of native and introduced C4 grasses in a neotropical savanna

Introduced African grasses are invading Neotropical savannas and displacing the native herbaceous community. This work, which is part of a program to understand the success of the African grasses, specifically investigates whether introduced and native grasses differ in their water relations. The water relations of the native Trachypogon plumosus and the successful invader Hyparrhenia rufa were studied in the field during two consecutive years in the seasonal savannas of Venezuela. The two C₄ grasses differed clearly in their responses to water stress. H. rufa consistently had higher stomatal conductance, transpiration rate, leaf water and osmotic potential and osmotic adjustment than the native T. plumosus. Also, leaf senescence occurred much earlier during the dry season in H. rufa. Both grasses showed a combination of water stress evasion and tolerance mechanisms such as stomatal sensitivity to atmospheric or soil water stress, decreased transpiring area and osmotic adjustment. Evasion mechanisms are more conspicuous in H. rufa whereas T. plumosus is more drought tolerant and uses water more conservatively. The evasion mechanisms and oportunistic use of water by H. rufa, characteristic of invading species, contribute to, but only partially explain, the success of this grass in the Neotropical savannas where it displaces native plants from sites with better water and nutrient status. Conversely, the higher water stress tolerance of t. plumosus is consistent with its capacity to resist invasion by alien grasses on shallow soils and sites with poorer nutrient and water status.     

温带草原7种针茅植物根系特征及其对环境因子变化的适应

为比较针茅(Stipa)植物适应策略,以大尺度梯度下(>1600 km)温带草原主要针茅植物为对象,系统研究了8个样点7种针茅根系生物量、根冠比、解剖结构和生理调节物质的差异及其对环境因子的适应。在由东北至西南的区域上,随降水量下降针茅植物根系的抗旱特征增强或适应策略趋于复杂,不同针茅植物根系对水分变化(或旱季和雨季)有着不同的适应策略。综合分析表明贝加尔针茅(S.baicalensis)、大针茅(S.grandis)及克氏针茅(S.krylovii)(多伦样点)的生长受干旱制约,对降水高度敏感,雨季降水促进其生物量快速积累。沙生针茅(S.glareosa)、短花针茅(S.breviflora)、戈壁针茅(S.gobica)、本氏针茅(S.bungeana)等通过增大根冠比和渗透调节物质累积等途径提高根系吸水和保水能力,抵御干旱胁迫。偏相关分析显示实验区域针茅植物根系性状与降水量和海拔高度存在显著的相关性。     

Seawater drinking restores water balance in dehydrated harp seals

The purpose of this study was to answer the question of whether dehydrated harp seals (Phoca groenlandica) are able to obtain a net gain of water from the intake of seawater. Following 24 h of fasting, three subadult female harp seals were dehydrated by intravenous administration of the osmotic diuretic, mannitol. After another 24 h of fasting, the seals were given 1,000 ml seawater via a stomach tube. Urine and blood were collected for measurement of osmolality and osmolytes, while total body water (TBW) was determined by injections of tritiated water. In all seals, the maximum urinary concentrations of Na⁺ and Cl⁻ were higher than in seawater, reaching 540 and 620 mM, respectively, compared to 444 and 535 mM in seawater. In another experiment, the seals were given ad lib access to seawater for 48 h after mannitol-induced hyper-osmotic dehydration. In animals without access to seawater, the mean blood osmolality increased from 331 to 363 mOsm kg⁻¹ during dehydration. In contrast, the blood osmolality, hematocrit and TBW returned to normal when the seals were permitted ad lib access to seawater after dehydration. In conclusion, this study shows that harp seals have the capacity to gain net water from mariposa (voluntarily drinking seawater) and are able to restore water balance after profound dehydration by drinking seawater.     

Stomatal responses and water relations of Eucalyptus pauciflora in summer along an elevational gradient

Summary The spatial and temporal variation of lead conductance (g) in Eucalyptus pauciflora was analysed with respect to photon flux area density (I), temperature (T), water vapour concentration deficit (w), and leaf water potential () at four different sites between 940 m and 2,040 m altitude in the Snowy Mountains of south-eastern Australia. Along this altitudinal gradient the precipitation/evaporation ratio increases from 1 to 4. The results show that gas diffusion in this tree species is primarily controlled by I and w at all sites, independently of the specific soil moisture regime. Even under dry midsummer conditions with predawn leaf water potentials of-1 MPa at the lowest altitude, had no striking effect on g.The humidity threshold for the onset of stomatal closure does not vary greatly between the study sites (12.2±1.3 Pa kPa^-1). The highest and lowest values observed for , the osmotic potential at water saturation (from pressure/volume curves), the mean and maximum g and stomatal dentity, all increase with elevation. The highest (least negative) osmotic potentials were obtained at all sites in midsummer. It therefore appears that there is no osmotic adjustment to drought in the seasonal course. The maximum difference between osmotic potentials obtained at the lowest and highest sites is 0.46 MPa. In general osmotic potential varies less than has been reported for other plant species exposed to varying water regimes. This may be the consequence of the pronounced feed-forward response of the stomata to evaporative demand, which led to only moderate tissue desiccation, never exceeding the turgor loss point. E. pauciflora is a tree species with a very conservative utilisation of soil water, which adjusts to drought via stomatal control of water loss, rather than via osmotic properties.These results explain previous reports of the comparatively high susceptibility of E. pauciflora to severe drought and its positive influence on the hydrological balance of mountain ecosystems in the Australian Alps.     

Contribution of Pre-Anthesis Assimilates and Current Photosynthesis to Grain Yield,and their Relationships to Drought Resistance in Wheat Cultivars Grown Under Different Soil Moisture

We investigated the relative importance of pre-anthesis assimilates stored in plant parts, mainly in the stem, and post-anthesis photosynthesis to drought resistance in wheat (Triticum aestivum L.) cultivars Hongwangmai (drought resistant) and Haruhikari (drought sensitive) subjected to two soil moisture regimes: irrigated and non-irrigated. In the irrigated treatment, soil moisture was maintained near field capacity throughout the growing season, while in the non-irrigated treatment water was withheld from 81 d after sowing until maturity. Drought stress reduced grain yield of Hongwangmai and Haruhikari by 41 and 60 %, respectively. Remobilization of pre-anthesis assimilates to the grain (remobilization) was reduced by drought in Hongwangmai but increased in Haruhikari. The contribution of pre-anthesis assimilates to the grain decreased under non-irrigated treatment in Hongwangmai. However, under water stress, Hongwangmai maintained a higher net photosynthetic rate in the flag leaf than Haruhikari. These results indicated that maintenance of post-anthesis photosynthetic rate was related to drought resistance in Hongwangmai rather than to remobilization under drought stress.     

土壤水分及其垂直分布对内蒙古草原冰草和冷蒿竞争的影响

内蒙古退化冷蒿草原围封恢复演替3-4年,植物群落更替出现半灌木冷蒿（Artemisia frigida）向禾草冰草（Agropyron cristatum）群落突然转变的现象,这对内蒙古草原植被恢复具有积极的作用,但目前关于冰草-冷蒿演替和竞争机制的研究鲜见报道。水分作为草原植被生长的主要限制因子,是影响群落演替的重要因素。气候干旱和过度放牧导致草原土壤水分垂直分布发生明显变化。由于冰草和冷蒿的根系分布深浅不同,土壤水分垂直变化可能会影响冰草-冷蒿的竞争。为探讨土壤水分及其垂直分布对冰草和冷蒿竞争的影响,本盆栽试验设置冰草、冷蒿单种以及混种处理,并进行不同土壤深度（0-30cm,30-60cm）的水分处理（上干下湿、上湿下干、上干下干）,结果表明：（1）与冷蒿相比,冰草受土壤水分变化影响显著,相对于上干下干处理,冰草在湿润（上干下湿或者上湿下干）处理的地上、地下生物量均显著增加,而冷蒿没有显著变化;（2）上干下湿和上湿下干处理间,冰草、冷蒿的地上、地下总生物量和根长差异均不显著;（3）不同水分处理,冰草和冷蒿的根系均可分布在30-60cm土壤中,且土壤30-60cm层冰草单种的根生物量和根长显著高于冷蒿单种;（4）相对竞争强度和竞争攻击力系数表明：湿润处理冰草的竞争力大于冷蒿;上干下干处理冷蒿的竞争力大于冰草。土壤水分变化引起冰草、冷蒿的明显竞争,上湿下干与上干下湿处理间冰草与冷蒿竞争力差异不显著。由于冰草和冷蒿均为多年生物种,冰草-冷蒿的竞争实验仍需在今后的研究中反复地验证。