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.     

Desiccation tolerance and drought acclimation in the Antarctic collembolan Cryptopygus antarcticus

The availability of water is recognized as the most important determinant of the distribution and activity of terrestrial organisms within the maritime Antarctic. Within this environment, arthropods may be challenged by drought stress during both the austral summer, due to increased temperature, wind, insolation, and extended periods of reduced precipitation, and the winter, as a result of vapor pressure gradients between the surrounding icy environment and the body fluids. The purpose of the present study was to assess the desiccation tolerance of the Antarctic springtail, Cryptopygus antarcticus, under ecologically-relevant conditions characteristic of both summer and winter along the Antarctic Peninsula. In addition, this study examined the physiological changes and effects of mild drought acclimation on the subsequent desiccation tolerance of C. antarcticus. The collembolans possessed little resistance to water loss under dry air, as the rate of water loss was >20% h(-1) at 0% relative humidity (RH) and 4 degrees C. Even under ecologically-relevant desiccating conditions, the springtails lost water at all relative humidities below saturation (100% RH). However, slow dehydration at high RH dramatically increased the desiccation tolerance of C. antarcticus, as the springtails tolerated a greater loss of body water. Relative to animals maintained at 100% RH, a mild drought acclimation at 98.2% RH significantly increased subsequent desiccation tolerance. Drought acclimation was accompanied by the synthesis and accumulation of several sugars and polyols that could function to stabilize membranes and proteins during dehydration. Drought acclimation may permit C. antarcticus to maintain activity and thereby allow sufficient time to utilize behavioral strategies to reduce water loss during periods of reduced moisture availability. The springtails were also susceptible to desiccation at subzero temperatures in equilibrium with the vapor pressure of ice; they lost approximately 40% of their total body water over 28 d when cooled to -3.0 degrees C. The concentration of solutes in the remaining body fluids as a result of dehydration, together with the synthesis of several osmolytes, dramatically increased the body fluid osmotic pressure. This increase corresponded to a depression of the melting point to approximately -2.2 degrees C, and may therefore allow C. antarcticus to survive much of the Antarctic winter in a cryoprotectively dehydrated state.     

Responses to acute and chronic desiccation stress in <Emphasis Type="Italic">Enchytraeus</Emphasis> (Oligochaeta: Enchytraeidae)

Enchytraeids are small soil living oligochaete worms with high sensitivity to low soil moisture. The effects of acute and chronic desiccation on survival and reproduction were determined in Enchytraeus albidus and Enchytraeus crypticus. Further, effects of acute drought stress on the water balance physiology and accumulation of osmolytes were investigated in E. albidus. Survival of E. crypticus and E. albidus was significantly influenced by exposure time. Reproduction was much more sensitive to desiccation than survival and was significantly reduced from −0.06 bar, which was surprising because no dehydration or change in the body fluid osmolality of E. albidus occurred until much harsher drought regimes occurred. The body fluid osmolality of E. albidus was relatively high, about 500 mOsm. Congruent with this no water loss or changes in osmotic pressure occurred until equivalent or higher water potential values of the environment were reached. Two osmolytes, glucose and alanine, were up-regulated in drought exposed E. albidus. Even though enchytraeids display moderate physiological protection to rapid changes in soil moisture (by having a high osmotic pressure) in the short term, populations subjected to long-term drought stress can be severely reduced even under moderate drought levels.     

Some chemical and physical factors affecting the rate and dunamics of nitrification in urine-affected soil

The effects of urinary chloride and nitrogen concentration and osmotic pressure on the nitrification of ammonium in a calcareous soil treated with cow urine were examined. Urinary chloride concentrations of up to 7.4 g L^–1 had no effect on the rate of nitrification, as determined by the accumulation of soil nitrate. Osmotic stress, generated using a mixed salt solution, had an inhibitory effect on nitrification at soil osmotic pressures lower than or equal to –1.0 PMa. Nitrification was completely inhibited at a soil osmotic pressure of –2.6 MPa. Accumulation of nitrate after a lag phase of 18 days was noted in the –2.0 MPa soil osmotic pressure treatment, indicating some degree of adaptation or osmo-regulation within the nitrifying population at this stress level. High urine-N concentrations resulted in considerable nitrite accumulations and reduced nitrification activity through the effect of free ammonia. It is concluded that in most temperate grassland soils at near-neutral pH, urinary chloride and nitrogen are unlikely to reduce nitrification rates, except where urine-N concentrations exceed 16 g N L^–1. Inhibition due to osmotic stress will be directly related to soil moisture status and may be particularly severe in dry, light-textured soils.     

Detrimental effect of rust infection on the water relations of bean

Bean plants (Phaseolus vulgaris L.) infected with the rust Uromyces phaseoli became unusually susceptible to drought as sporulation occurred. Under the conditions used (1,300 ft-c, 27 C, and 55% relative humidity) such plants wilted at soil water potentials greater than −1 bar, whereas healthy plants did not wilt until the soil water potential fell below −3.4 bars. Determinations of leaf water and osmotic potentials showed that an alteration in leaf osmotic potential was not responsible for the wilting of diseased plants. When diffusive resistance was measured as a function of decreasing leaf water content, the resistance of healthy leaves increased to 50 sec cm⁻¹ by the time relative water content decreased to 70%, whereas the resistance of diseased leaves remained less than 8 sec cm⁻¹ down to 50% relative water content. Apparently, water vapor loss through cuticle damaged by the sporulation process, together with the reduction in root to shoot ratio which occurs in diseased plants, upset the water economy of the diseased plant under mild drought conditions.     

模拟喀斯特生境条件下干旱胁迫对青冈栎苗木的影响

为了研究喀斯特"双层"地质结构对植物的影响,以及在干旱环境下,喀斯特地区植物的生理变化与适应策略,建立"土壤层-岩石(石灰岩)层-岩溶水层"水分供应分层模拟柱,对‘土壤层’设置不同水分梯度,种植青冈栎(Cyclobalanopsis glauca)苗木进行干旱胁迫试验,测定青冈栎的叶面积、比叶面积、生物量、渗透调节物质含量、相对叶绿素含量。结果表明:在模拟柱‘岩溶水层’加水条件下,植物根系能下扎至‘岩溶水层’,‘土壤层’干旱胁迫对青冈栎的生理变化没有产生影响;在‘岩溶水层’无水条件下,青冈栎生理变化受‘土壤层’干旱胁迫影响显著,‘土壤层’水分含量越低,青冈栎的叶面积、生物量、叶绿素含量、叶片相对含水量越小,青冈栎干鲜比、根冠比、可溶性糖、丙二醛和脯氨酸含量越高。青冈栎幼苗利用岩溶水层水分是适应喀斯特干旱环境的重要策略。     

Response of microbial activity and community structure to decreasing soil osmotic and matric potential

Low soil water content (low matric potential) and salinity (low osmotic potential) occur frequently in soils, particularly in arid and semi-arid regions. Although the effect of low matric or low osmotic potential on soil microorganisms have been studied before, this is the first report which compares the effect of the two stresses on microbial activity and community structure. A sand and a sandy loam, differing in pore size distribution, nutrient content and microbial biomass and community structure, were used. For the osmotic stress experiment, salt (NaCl) was added to achieve osmotic potentials from ?0.99 to ?13.13 MPa (sand) and from ?0.21 to 3.41 MPa (sandy loam) after which the soils were pre-incubated at optimal water content for 10d. For the matric stress experiment, soils were also pre-incubated at optimal water content for 10d, after which the water content was adjusted to give matric potentials from ?0.03 and ?1.68 MPa (sand) and from ?0.10 to 1.46 MPa (sandy loam). After amendment with 2% (w/w) pea straw (C/N 26), soil respiration was measured over 14d. Osmotic potential decreased with decreasing soil water content, particularly in the sand. Soil respiration decreased with decreasing water potential (osmotic?+?matric). At a given water potential, respiration decreased to a greater extent in the matric stress experiment than in the osmotic stress experiment. Decreasing osmotic and matric potential reduced microbial biomass (sum of phospholipid fatty acids measured after 14 days) and changed microbial community structure: fungi were less tolerant to decreasing osmotic potential than bacteria, but more tolerant to decreasing water content. It is concluded that low matric potential may be more detrimental than a corresponding low osmotic potential at optimal soil water content. This is likely to be a consequence of the restricted diffusion of substrates and thus a reduced ability of the microbes to synthesise osmolytes to help maintain cell water content. The study also highlighted that it needs to be considered that decreasing soil water content concentrates the salts, hence microorganisms in dry soils are exposed to two stressors.     

强旱生小灌木绵刺劈裂生长过程中的水分特征

绵刺(Potaninia mongolica)是西鄂尔多斯-东阿拉善地区特有的单种属残遗植物。选取内蒙古磴口县境内具有绵刺群落的草原化荒漠区为研究样区,于2002~2003年每年8月1~5日采集未劈裂、正在劈裂和已劈裂植株,运用PV技术对不同劈裂生长状态绵刺的多种水分关系参数(${φ_{s}}^{sat}$、${φ_{s}}^{tlp}$、ROWC^tlp、RWC^tlp、Δφ、ε^max等)进行了测定,从绵刺保持膨压的能力和途径两方面进行了深入探讨;同时结合同一项目研究中绵刺劈裂生长过程中抗氧化酶系统和内源激素方面的研究成果,综合分析并探讨了绵刺劈裂生长的发生机理及其环境适应性。结果表明:1)未劈裂绵刺主要通过增加细胞内溶质(如脯氨酸),减少细胞内的水分丧失来进行渗透调节,从而在干旱胁迫下能够维持正常的膨压。2)已劈裂绵刺通过渗透调节和高的组织弹性两条途径来共同保持膨压,以抵抗不良的生存环境;同时对环境水分胁迫具有较高的敏感性。3)3种状态绵刺保持膨压的能力由强到弱依次为:未劈裂绵刺、正在劈裂绵刺、已劈裂绵刺。4)劈裂的发生导致绵刺保持膨压能力的降低,同时耐旱方式和途径发生了变化。     

Drought tolerance and germination response to light and temperature for seeds of saline and non-saline habitats of the habitat-indifferent desert halophyte <Emphasis Type="Italic">Suaeda vermiculata</Emphasis>

Few plants are habitat-indifferent halophytes (i.e., grow well in both saline and non-saline soils). These plants offer a good opportunity to study drought and salinity tolerances during germination for seeds developed and matured in soils differ in salinity. Here, we assessed drought tolerance during germination, as simulated with PEG, and response of germination to light and temperature for Suaeda vermiculata, a habitat-indifferent shrub. Seeds matured in saline and non-saline soils were germinated in six PEG concentrations (0 to ? 1.0 MPa) and put in three incubators adjusted at different temperatures in both light and dark regimes. Drought tolerance was greater for seeds of the non-saline than those of saline soils, especially at higher temperatures. Seeds of the saline soils germinated in the lowest osmotic potentials (? 0.8 and ??1.0 MPa) only at lower temperatures, but seeds of the non-saline soils germinated to higher levels at all temperatures. Tolerances to drought and high temperatures were greater in light for seeds of saline soils, but in darkness for seeds of non-saline soils. Germination rate index did not differ significantly between seeds of the two soil types in higher osmotic potentials, but was significantly greater in seeds of non-saline at lower osmotic potentials. Most seeds that failed to germinate in the PEG concentrations recovered their germination when transferred to distilled water. Germination recovery levels and speeds increased with the decrease in osmotic potentials. Seeds of the saline soil postpone their germination until arrival of suitable temperatures and effective rainfalls that ensure seedling survival in salty habitats of the arid unpredictable deserts.     

Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

Daily and seasonal patterns in water flow and water potentialwere investigated for the Crassulacean acid metabolism succulentAgave deserti during an extended summer drought and for a periodfollowing rainfall. Field measurements of transpiration andof osmotic pressure changes over selected 24 h periods wereused as input variables for a computer model of water flow thatwas based on an electrical circuit analog of the whole plant.Parameters such as root resistance and tissue capacitance werealso varied to reflect the effects of changing plant or soilwater status. The model predicted internal water flow and waterpotential during the drought cycle and was used to assess therole of tissue osmotic properties in water uptake from the soiland in internal water redistribution. For plants under wet soil conditions, 55% of the night-timetranspiration was derived from water storage, this storage beingrecharged during the day. As drought progressed, transpirationand the nocturnal increase in osmotic pressure declined, althoughthe osmotic pressure itself increased. The difference in osmoticpressure between the water storage tissue and the chlorenchymacaused a net flow of water into the chlorenchyma after 3 weeksof drought, thereby increasing chlorenchyma turgor pressure.Simulations also indicated that a large increase in root resistancemust occur to prevent substantial water loss from the plantto the dry soil. After rainfall, recharge of plant water storagewas complete within one week, although full recovery in theamplitude of daily osmotic pressure variations took longer. Key words: Agave deserti, transpiration, water potential, water storage     

The distribution of tree and grass roots in savannas in relation to soil nitrogen and water

Here we describe the fine root distribution of trees and grasses relative to soil nitrogen and water profiles. The primary objective is to improve our understanding of edaphic processes influencing the relative abundance of trees and grasses in savanna systems. We do this at both a mesic (737 mm MAP) site on sandy-loam soils and at an arid (547 mm MAP) site on clay rich soils in the Kruger National Park in South Africa. The proportion of tree and grass fine roots at each soil depth were estimated using the δ¹³C values of fine roots and the δ¹³C end members of the fine roots of the dominant trees and grasses at our study sites. Changes in soil nitrogen concentrations with depth were indexed using total soil nitrogen concentrations and soil δ¹⁵N values. Soil water content was measured at different depths using capacitance probes. We show that most tree and grass roots are located in the upper layers of the soil and that both tree and grass roots are present at the bottom of the profile. We demonstrate that root density is positively related to the distribution of soil nitrogen and negatively related to soil moisture. We attribute the negative correlation with soil moisture to evaporation from the soil surface and uptake by roots. Our data is a snapshot of a dynamic process, here the picture it provides is potentially misleading. To understand whether roots in this system are primarily foraging for water or for nitrogen future studies need to include a dynamic component.     

Facing drought in a Mediterranean post-fire community: tissue water relations in species with different life traits

Bulk shoot water potential, the osmotic component and the bulk modulus of elasticity were measured throughout one growing season in four species co-occurring in a post-fire Mediterranean community in southern Italy: Pinus halepensis, Phillyrea latifolia, Cistus salvifolius and Rosmarinus officinalis. A severe drought occurred throughout the measurement period. Large seasonal fluctuations have been observed for both predawn and afternoon water potential in all species. Although minimum values down to –4 MPa have been measured, plant water potential always recovered to less negative values after drought. Daily amplitude of water potential decreased with increasing plant water stress in all species. In Cistus and Rosmarinus less ability for short-term control of plant water status has been assessed. Osmotic potential at full turgor did not display clear seasonal patterns, with no consistent ranking of species by their osmotic values. In most cases, no osmotic adjustment (lowering of osmotic potentials) and no change in tissue elastic properties were observed in response to increasing summer drought and intensity of water stress.     

Contributions of inorganic ions,soluble carbohydrates,and multiatomic alcohols to water homeostasis in <Emphasis Type="Italic">Artemisia lerchiana</Emphasis> and <Emphasis Type="Italic">A. pauciflora</Emphasis>

Two morphological forms of wormwood Artemisia lerchiana (f. erecta and f. nutans) and A. pauciflora Web. (morphological form erecta) were grown on sand culture at a range of NaCl concentrations in the nutrient medium and then assayed for Na⁺, K⁺, and Cl^? content in various organs. In addition, the content of mono-, di-, and trisaccharides and multiatomic alcohols (mannitol and glycerol); water content; and organ biomass were determined. All plants examined showed high NaCl tolerance, comparable to that of halophytes. They were able to maintain high tissue hydration under conditions of salinity-induced growth suppression. The intracellular osmotic pressure in wormwood organs was mainly determined by the presence of Na⁺, K⁺, and Cl^?, as well as by mono-, di-, and trisaccharides, mannitol, and glycerol. The high content of Na⁺ and Cl^? in wormwood organs was also observed in the absence of salinity, which implies the ability of these organs to absorb ions from diluted NaCl solutions and accumulate ions in cells of their tissues. With the increase in salinity, the content of Na⁺ and Cl^? in roots and leaves increased to even higher levels. It is concluded that the ability of wormwood plants to absorb and accumulate inorganic ions provides for sustainable high intracellular osmotic pressure and, accordingly, low water potential under drought and salinity conditions. Growing plants under high salinity lowered the content of monosaccharides in parallel with accumulation of the trisaccharide raffinose. It is supposed that soluble carbohydrates and multiatomic alcohols are not only significant for osmoregulation but also perform a protective function in wormwood plants. The lower osmotic pressure in root cells compared to that in leaf cells of all plants examined was mainly due to the gradient distribution of K⁺ and Cl^? between roots and leaves. The two Artemisia species and two morphological forms of A. lerchiana did not differ appreciably in the ways of water balance regulation. It is found that different morphologies of two A. lerchiana forms are unrelated to variations in intracellular osmotic and turgor pressures.     

荒漠植物蒙古扁桃水分生理特征

蒙古扁桃(Prunus mongolica)是荒漠区和荒漠草原的水土保持植物和景观植物,是蒙古高原古老残遗植物,对其深入研究对于了解蒙古高原植被演替以及对当地生态环境的稳定和恢复有着重要意义。该实验采用PV技术和自然脱水法探讨了蒙古扁桃的水分生理特性。结果表明:在自然状态下,蒙古扁桃幼苗叶片的相对含水量为69%,饱和含水量为117%,临界饱和亏为48%,水势为-0.85 MPa。经 5% PEG-Hoagland (-0.46 MPa)干旱胁迫处理3 d后,其相对含水量、临界含水量和水势分别下降到48%、39%和 -1.97 MPa,而饱和含水量和束缚水与自由水比值分别增加到187%和11.94。对失水率分析的结果表明:在正常水分状态下,蒙古扁桃幼苗经102 h自然脱水后失水达到平衡,而经过干旱胁迫处理3 d后,其失水率曲线斜率变小,失水过程明显减缓,失水最终达到平衡的时间延长到152 h,其保水能力显著提高。将旱生植物蒙古扁桃的失水率曲线与中旱生植物长柄扁桃(P. pedunculata)的失水率曲线相比较发现,蒙古扁桃的耐脱水能力明显强于中旱生植物长柄扁桃。PV曲线(Pressure-volume curve)分析结果表明: 蒙古扁桃饱和含水量渗透势(Ψπ¹⁰⁰)和零膨压渗透势 (Ψπ⁰)很低,分别为-2.49 MPa和-3.11 MPa,而Ψπ¹⁰⁰和Ψπ⁰差值较大(0.62 MPa),表明其维持膨压的能力很强。其细胞壁弹性模量值低(4.18 MPa)进一步表明,蒙古扁桃具有很强的膨压调节能力。蒙古扁桃幼苗失去膨压时的渗透含水量(ROWC^tlp)为80%,这是其细胞壁特性所决定的渗透调节能力的基础。蒙古扁桃质外体含水量(AWC, %)较高(79%),因而具有较高的束缚水与自由水比值(7.76),这是其耐脱水性的生理基础。总之,蒙古扁桃叶水势、渗透势低有利于其根部对深层土壤水分的吸收,而较高的束缚水与自由水比值及较低的细胞壁弹性模量是其耐脱水的生理基础。     

美国海滨桤木和薄叶桤木水分生理特性的比较

采取盆栽、人工控水的方式, 研究并比较了美国本土海滨桤木(Alnus maritima)和薄叶桤木(A. incana)的气孔导度(G_s)、叶片水势(ψ_leaf)以及渗透调节能力对土壤水分条件的响应, 以探讨引起两种桤木生态分布差异巨大的生理生态原因。结果表明: 1)正常水分条件下, 海滨桤木的G_s低于薄叶桤木, 其与大气温度、相对湿度和水蒸气亏缺等气象因子的相关性低于薄叶桤木; 干旱胁迫下, 海滨桤木的G_s对其自身ψ_leaf下降信号的敏感度低于薄叶桤木; 复水后, 其G_s恢复更为缓慢。2)正常水分条件下, 海滨桤木的ψ_leaf高于薄叶桤木, 且引起气孔关闭的ψ_leaf临界值较高; 干旱胁迫下, 海滨桤木的ψ_leaf下降幅度高于薄叶桤木。3)正常水分条件下, 海滨桤木和薄叶桤木的渗透调节能力无显著差异; 干旱胁迫下, 尽管两种桤木均表现出饱和状态渗透势(ψ_s^sat)下降、膨压与水势关系的最大变化率降低、初始失膨点渗透势(ψ_s^tlp)增加、细胞渗透调节能力范围(ψ_s^sat-ψ_s^tlp, Dψ_s)减小的趋势, 但与薄叶桤木相比, 海滨桤木的ψ_s^tlp较高, Dψ_s较小。从以上生理生态指标可以看出, 较高的叶片水势、较低的气孔调节能力、干旱下较低的渗透调节能力是造成海滨桤木分布范围狭小的重要原因。     

Evaluation of wild Solanum species for drought resistance: 1. Solanum gandarillasii Cardenas

Since drought is a major factor limiting global potato production, identification of Solanum germplasm with drought resistance features is essential. The current study compared responses of Solanum tuberosum L. ‘Kennebec’ to those of the wild tuber bearing species, Solanum gandarillasii Cardenas, with respect to drought and heat stress. The cultivar Kennebec exhibited more leaf water loss as well as increased osmotic adjustment compared to S. gandarillasii during the imposition of progressively severe drought. In Kennebec, this stress led to severe leaf wilting and eventual canopy loss. However, S. gandarillasii was less sensitive to prolonged drought in terms of reduced loss of above ground biomass. The conservative “water saving” responses of S. gandarillasii included drought sensitive stomata resulting in low transpiration rates. Coupled with this apparent loss of an effective cooling mechanism, S. gandarillasii demonstrated superior thermal tolerance. Decreased intrinsic water use efficiency (WUE) at the leaf level was evident in Kennebec compared to S. gandarillasii when exposed to increasing levels of soil moisture stress and regardless of radiation level. This difference in WUE could be attributed to differences in transpiration rate and not to photosynthetic rate. S. gandarillasii may be appropriate for growth areas exhibiting drought conditions where reduced desiccation and thermal damage to leaf tissues are assets. Kennebec, however, was a ‘water spender’ that would be more appropriately grown under temperate growing conditions with an adequate water supply.     

Population-specific Traits and their Implication for the Evolution of a Drought-adapted Ecotype in Armeria maritima

Traits contributing to drought resistance of Armeria maritima were investigated by comparing six populations of this species from sandy grasslands, heavy metal mines and salt marsh sites. The sandy soil ecotype that is influenced by periodic drought was found to allocate constitutively a higher proportion of biomass to the root system, especially to the fine roots, than the other two ecotypes. The selective advantage of this lower shoot-root ratio is presumably the delayed onset of the critical water saturation deficit because of the ability to take up additional water from deeper soil layers. Under water stress, all populations of A. maritima showed a decrease in the shoot-root ratio. Additionally, under moderate long-term water stress a decrease in (= more negative) osmotic potential was found in leaves of plants from all populations. Lowest osmotic potentials were shown by the salt marsh ecotype and the highest by one of the heavy metal populations. Osmotic adjustment was achieved passively by a reduction in osmotic volume. Accumulation of osmotically active compounds was seen only under severe water stress, when the considerable betaine contents found in all populations of A. maritima even under control conditions were further increased by severe drought. The evolution of traits related to drought adaptation in the sandy soil ecotype of A. maritima is discussed.     

Effect of nitrogen form on maize response to drought stress

Two hybrids of maize (Zea mays L.) differing in resistance to drought, were grown in chernozem soil in a greenhouse and were fertilized with two different forms of nitrogen: Ca(NO₃)₂ and (NH₄)₂SO₄ in concentrations corresponding to 100 kg of N ha^-1. After emergence of the 4th leaf, plants were exposed to drought. During the drought period, the parameters of plant water status (water potential, osmotic potential, turgor pressure and relative water content) and chlorophyll a+b concentration were monitored every two days. N and K concentration and accumulation over the drought period were also monitored.Next to differences in adaptability of the two hybrids to drought, the results demonstrate different adaptability of NH₄ and NO₃-treated plants within each hybrid. NH₄-plants of each hybrid maintain higher turgor pressure during the drought by better osmotic adaptation. Especially significant differences appear between chlorophyll (a+b) values of NH₄ and NO₃-treated plants and as affected by drought. Chlorophyll concentrations of NH₄-plants are higher than those of NO₃-plants both in control and droughted plants. NH₄ plants show a characteristic initial chlorophyll increase at the beginning of the drought period while in NO₃ plants chlorophyll constantly decreases throughout the whole drought period. The influence of the nitrogen form on chlorophyll concentration changes during drought does not appear to be affected by regulation of the K concentration.     

Soil salinity as a selection pressure is a key determinant for the evolution of salt tolerance in Blue Panicgrass (Panicum antidotale Retz.)

To assess the role of selection pressure in plant adaptation to saline environment, a hydroponic experiment was conducted on six Panicum antidotale Retz. populations collected from a wide range of habitats with varying selection pressure in the form of soil salinity. The soil electrical conductivity of six different habitats ranged from 3.39 to 19.23 dS m⁻¹ and pH from 5.86 to 7.65. Plants of all populations collected from varying habitats were established in pots containing normal soil and allowed to grow for 6 months. Newly grown tillers from each plant were separated and 10 of them each formed a composite sample for a particular population. They were then transplanted in plastic containers each containing 10 l of half strength Hoagland's nutrient solution alone or with 150 mol m⁻³ NaCl. After 42 days growth in salt treatment, the populations collected form highly saline habitats proved to be more salt-tolerant compared with those from mild or non-saline habitats in terms of growth performance. The populations adapted to high salinity showed less decrease in leaf K⁺/Na⁺ and Ca²⁺/Na⁺ ratios under salinity stress. Moreover, under stress the salt-tolerant populations showed less reduction in photosynthetic capacity than the salt-sensitive populations. In addition, hyper-accumulation of organic solutes such as glycinebetaine and proline and thereby higher osmotic adjustment seemed to be associated with the higher degree of adaptability of the salt-tolerant populations to salt stress. From the data presented, it is plausible to conclude that selection pressure (soil salinity) must have been one of the important determinants bringing about the evolution of salt-tolerance trait in Blue Panic grass.