Hydraulic and Chemical Responses of Citrus Seedlings to Drought and Osmotic Stress

In this work we investigated the function of abscisic acid (ABA) as a long-distance chemical signal communicating water shortage from the root to the shoot in citrus plants. Experiments indicated that stomatal conductance, transpiration rates, and leaf water potential decline progressively with drought. ABA content in roots, leaves, and xylem sap was also increased by the drought stress treatment three- to sevenfold. The addition of norflurazon, an inhibitor of ABA biosynthesis, significantly decreased the intensity of the responses and reduced ABA content in roots and xylem fluid, but not in leaves. Polyethylene glycol (PEG)-induced osmotic stress caused similar effects and, in general, was counteracted only by norflurazon at the lowest concentration (10%). Partial defoliation was able to diminish only leaf ABA content (22.5%) at the highest PEG concentration (30%), probably through a reduction of the active sites of biosynthesis. At least under moderate drought (3–6 days without irrigation), mechanisms other than leaf ABA concentration were required to explain stomatal closure in response to limited soil water supply. Measurements of xylem sap pH revealed a progressive alkalinization through the drought condition (6.4 vs. 7.1), that was not counteracted with the addition of norflurazon. Moreover, in vitro treatment of detached leaves with buffers iso-osmotically adjusted at pH 7.1 significantly decreased stomatal conductance (more than 30%) as much as 70% when supplemented with ABA. Taken together, our results suggest that increased pH generated in drought-stressed roots is transmitted by the xylem sap to the leaves, triggering reductions in shoot water loss. The parallel rise in ABA concentration may act synergistically with pH alkalinization in xylem sap, with an initial response generated from the roots and further promotion by the stressed leaves.     

抗旱性不同的冬小麦幼苗对渗透胁迫的生理效应

抗旱性不同的冬小麦幼苗在渗进胁迫下叶片相对透性、Pro含量和在总游离氨基酸中的Pro比例均增加,但品种间变化幅度有明显差别。在严重胁迫下,抗旱性弱的品种济南13,烟农15和鲁麦5号的叶片相对透性增加大于抗旱性强的品种昌乐5号,秦麦3号和山农587;而Pro含量和在总游离氨基酸中Pro比例的增加小于抗旱性强的品种、叶片Pro累积与RWC呈显著负相关。     

Plant Responses to Drought,Acclimation, and Stress Tolerance

At the whole plant level, the effect of stress is usually perceived as a decrease in photosynthesis and growth. That is why this review is focused mainly on the effect of drought on photosynthesis, its injury, and mechanisms of adaptation. The analysed literature shows that plants have evolved a number of adaptive mechanisms that allow the photochemical and biochemical systems to cope with negative changes in environment, including increased water deficit. In addition, the acquisition of tolerance to drought includes both phenotypic and genotypic changes. The approaches were made to identify those metabolic steps that are most sensitive to drought. Some studies also examined the mechanisms controlling gene expression and putative regulatory pathways.     

Responses of Tropical Understory Plants to a Severe Drought: Tolerance and Avoidance of Water Stress1

Shade-tolerant understory shrubs and subcanopy trees constitute most of the woody species in Neotropical moist forest, but studies demonstrating physiological differences among these species are few. Shade-tolerant species that coexist in the forest understory exhibit differences in leaf life span that have been associated with variation in physiological traits. We hypothesized that water relations of understory species with widely divergent leaf life spans differ in response to drought. Although severe drought is infrequent in Neotropical moist forest, we studied the water relations of shade-tolerant understory species with short or long leaf life spans during the severe 1991-1992 dry season on Barro Colorado Island, Panama. The predawn leaf water potential declined to -2.8 and -3.6 MPa during the dry season in Hybanthus prunifolius and Psychotria horizontalis, respectively, two species with short leaf life spans, but remained above -1.3 MPa in two species with long leaf life spans, Swartzia simplex and Ouratea lucens. The midday leaf water potential dropped as low as -3.4 and -4.5 MPa for H. prunifolius and P. horizontalis, respectively. The osmotic potential of H. prunifolius and P. horizontalis and another species with short leaf life span, Alms blackiana, decreased early in the dry season, a period during which all three had substantially negative predawn water potential. In contrast, the osmotic potential of S. Simplex, O. lucens, and Licania platypus, a third species with long leaf life span, declined late in the dry season, even though we observed little change in predawn water potential for S. simplex and O. lucens. We conclude that the variable and potentially severe dry season in Neotropical moist forest can be sufficiently intense to severely limit soil moisture availability for understory plants. H. prunifolius and P. horizontalis tolerated dehydration, whereas S. simplex and O. lucens postponed dehydration.     

Effects of Arbuscular-Mycorrhizal Glomus Species on Drought Tolerance: Physiological and Nutritional Plant Responses

The tolerance of lettuce plants (Lactuca sativa L. cv. Romana) to drought stress differed with the arbuscular-mycorrhizal fungal isolate with which the plants were associated. Seven fungal species belonging to the genus Glomus were studied for their ability to enhance the drought tolerance of lettuce plants. These fungi had different traits that affected the drought resistance of host plants. The ranking of arbuscular-mycorrhizal fungal effects on drought tolerance, based on the relative decreases in shoot dry weight, was as follows: Glomus deserticola > Glomus fasciculatum > Glomus mosseae > Glomus etunicatum > Glomus intraradices > Glomus caledonium > Glomus occultum. In this comparative study specific mycorrhizal fungi had consistent effects on plant growth, mineral uptake, the CO(inf2) exchange rate, water use efficiency, transpiration, stomatal conductance, photosynthetic phosphorus use efficiency, and proline accumulation under either well-watered or drought-stressed conditions. The ability of the isolates to maintain plant growth effectively under water stress conditions was related to higher transpiration rates, levels of leaf conductance, and proline, N, and P contents. Differences in proline accumulation in leaves among the fungal symbioses suggested that the fungi were able to induce different degrees of osmotic adjustment. The detrimental effects of drought were not related to decreases in photosynthesis or water use efficiency. Neither of these parameters was related to P nutrition. The differences in P and K acquisition, transpiration, and stomatal conductance were related to the mycorrhizal efficiencies of the different fungi. Our observations revealed the propensities of different Glomus species to assert their protective effects during plant water stress. The greater effectiveness of G. deserticola in improving water deficit tolerance was associated with the lowest level of growth reduction (9%) under stress conditions. The growth of plants colonized by G. occultum was reduced by 70% after a progressive drought stress period. In general, the different protective effects of the mycorrhizal isolates were not associated with colonizing ability. Nevertheless, G. deserticola was the most efficient fungus and exhibited the highest levels of mycorrhizal colonization, as well as the greatest stimulation of physiological parameters.     

Effects of Drought on CAM and Water Relations in Plants of Peperomia carnevalii

The mechanisms underlying the drought tolerance of Peperomiacarnevalii Steyermark (Piperaceae), a succulent herb growingin the understorey of seasonally dry forests, were examined.Crassulacean acid metabolism (CAM) was studied in the fieldand laboratory, and measurements of water status were made inplants subjected to drought in the greenhouse. Nocturnal acidaccumulation and day and night-time CO₂assimilation rates weregreatest in watered plants and decreased in drought. The proportionof CO₂recycled through CAM in droughted plants, with nocturnalCO₂uptake close to zero, was higher than in watered plants.Maximum quantum yield of chlorophyll fluorescence remained unchangedduring drought, but the PSII quantum yield at the photosyntheticphoton flux density at which the plants were grown was significantlydecreased. Leaf anatomy consists of a chlorophyll-less hydrenchymalocated beneath the upper epidermis, and a two-layered mesophyll.Leaves nearer to the apex are thinner than those nearer to thebase of the shoot. Drought caused a reduction in leaf thicknessdue to shrinkage of the hydrenchyma, but not of the mesophyll.This was associated with the occurrence of a gradient of osmoticpotential between these tissues. Comparison of water loss fromthin leaves of watered and droughted plants, either partly defoliatedat the lower nodes or intact, suggested that water moved fromthe thick to the thin leaves. This process was related to theoccurrence of a gradient of water potential between the thickand the thin leaves. Drought tolerance in P. carnevalii is achievedby the operation of CAM and the occurrence of water movementwithin and between leaves. Copyright 2000 Annals of Botany Company Crassulacean acid metabolism, fluorescence, hydrenchyma, mesophyll, Peperomia carnevalii, water relations     

Water Relations and Photosynthesis of a Desert CAM Plant, Agave deserti

The water relations and photosynthesis of Agave deserti Engelm., a plant exhibiting Crassulacean acid metabolism, were measured in the Colorado desert. Although no natural stomatal opening of A. deserti occurred in the summer of 1975, it could be induced by watering. The resistance for water vapor diffusion from a leaf (R_WV) became less than 20 sec cm⁻¹ when the soil water potential at 10 cm became greater than −3 bars, as would occur after a 7-mm rainfall. As a consequence of its shallow root system (mean depth of 8 cm), A. deserti responded rapidly to the infrequent rains, and the succulent nature of its leaves allowed stomatal opening to continue for up to 8 days after the soil became drier than the plant. When the leaf temperature at night was increased from 5 to 20 C, R_WV increased 5-fold, emphasizing the importance of cool nighttime temperatures for gas exchange by this plant. Although most CO₂ uptake occurred at night, a secondary light-dependent rise in CO₂ influx generally occurred after dawn. The transpiration ratio (mass of water transpired/mass of CO₂ fixed) had extremely low values of 18 for a winter day, and approximately 25 for an entire year.     

Comparison of Osmotic Adjustment Responses to Water and Temperature Stresses in Spring Wheat and Sudangrass

This study explores the mechanisms of osmotic adjustment bycomparing the growth of spring wheat and sudangrass, which exhibitdifferent degrees of osmotic adjustment, under soil water andtemperature stresses. Leaf water potential ( ₁), osmotic potential(), and rate of leaf area growth of spring wheat and sudangrassseedlings were measured at combinations of five soil water potentials,from -0·03 to -0·25 MPa, and six root temperatures,from 14 to 36°C. Spring wheat exhibit little osmotic adjustment.The leaf osmotic potential was not affected by either soil wateror root temperature stress. Osmotic potential of sudangrassdecreased in parallel with the decreasing leaf water potentialas a result of osmotic adjustment. As soil water potential decreasedfrom -0·03 to -0·25 MPa, the rates of growth andphotosynthesis of spring wheat both decreased by about 30%.For sudangrass with the same range of soil water potential,the photosynthesis rate decreased by only 10% while the leafarea growth rate decreased by 49%. We introduce a dimensionlessindex (R) to quantify the degree to which environmental stressesalter the balance between production of photosynthates and theiruse for growth. The index, R, is equal to 1 when stress reducesgrowth and photosynthesis by the same degree, i.e. the balancebetween production and consumption of photosynthate is not disturbed.R is smaller than 1 when growth is reduced more than photosynthesis.R was equal to 1 for spring wheat where there was no osmoticadjustment. For sudangrass, R decreased from 1 to 0·25as osmotic potential decreased from -1·10 to -1·63MPa. These findings lead to the hypothesis that osmotic adjustmentcould result from an imbalance between production, consumptionand translocation of photosynthates under stressed conditions.Copyright1993, 1999 Academic Press Osmotic adjustment, water stress, root temperature     

Water and Osmotic Potentials of Ethephon-Treated Maize Genotypes Varying in Drought Resistance

Leaves of drought-resistant and drought-sensitive genotypes of maize (Zea mays L.) were sprayed with ethephon to determine its effect on water and osmotic potentials. With both sufficient and limited water supplies, ethephon decreased the water potential, but not the osmotic potential, of the drought-sensitive maize. It had no effect on the water and osmotic potentials of the drought-resistant maize under either water regime. The results showed that the response of a genotype of maize to ethephon depends upon its susceptibility to drought.     

Effects of Pressure Increase and Release on Temperature within a Pressure Chamber Used to Estimate Plant Water Potential

During a water-potential measurement sequence, temperature changewithin the pressure chamber exhibits certain distinct phases.At the introduction of gas into the chamber there is a rapidincrease in temperature, normally in excess of 8 ?C above ambient,which is directly related to the rate of pressure increase.After reaching a maximum, temperature begins to decline graduallyduring continued pressure increase, and subsequently falls toambient after gas entry coases. When pressure is rapidly releasedfrom the chamber at the end of a measurement, temperature instantlyfalls to subzero values. The causes of these temperature changes are explained in termsof simple thermodynamics, and ways of reducing them are described.     

岩生植物雅安紫云菜对干旱及复水的生理响应

选择自然状态下生长在岩石边坡上的岩生植物雅安紫云菜为研究材料,通过人工控制土壤含水量的盆栽试验模拟自然界中的干湿交替现象,对雅安紫云菜中与其抗旱相关的叶绿素含量、膜质过氧化、渗透调节物质及抗氧化酶活性等11个生理生化指标进行了测定和分析,探讨雅安紫云菜对干旱胁迫的适应性及其生理机制。结果显示:(1)在干旱胁迫条件下,雅安紫云菜叶片具有较强的水分保持能力,叶绿素含量减少幅度较小,且复水后叶片相对含水量和叶绿素含量均能迅速恢复。(2)干旱胁迫9d后,叶片中超氧阴离子含量、过氧化氢含量以及丙二醛含量持续增加,同时抗氧化酶SOD和POD活性显著升高,而其叶片电解质外渗率、渗透调节物质可溶性糖含量和脯氨酸含量在干旱胁迫12d后开始大幅增加,复水后这些指标都恢复到接近对照水平。(3)在持续21d的干旱胁迫后,土壤含水量降低到4.26%,雅安紫云菜植株叶片呈现严重萎蔫,植株倒伏,但是复水后植株仍能较快恢复正常生长。研究表明,雅安紫云菜具有较强的渗透调节能力、酶促抗氧化活力以及叶片保水能力,从而表现出较强抗干旱能力,并且在恢复水分供应后能迅速恢复植物生长势,可以作为西南地区岩石边坡绿化的潜力景观植物。     

不同品种油橄榄离体叶片对渗透胁迫的生理响应及其抗旱机制

以油橄榄3个主栽品种(‘戈达尔’、‘城固32’、‘弗奥’)离体叶片为研究对象,在实验室条件下考察了30%PEG溶液模拟渗透胁迫对叶片相关生理生化指标的影响,探讨不同品种间的响应差异及其生理机制。结果显示:(1)随着渗透胁迫时间的延长,各油橄榄品种离体叶片含水量和叶绿素含量呈下降趋势,相对电导率和MDA含量大幅度升高,即渗透胁迫造成了油橄榄离体叶片水分状况恶化、叶绿素分解、脂质过氧化和细胞膜损伤程度加重;同时,3个品种油橄榄叶片的束缚水与自由水相对含量、超氧阴离子产生速率、SOD活性及渗透调节物质含量表现出升高的趋势。(2)品种间相比较,随着胁迫时间的延长,‘城固32’叶片脂质过氧化产物MDA含量相对较低、细胞膜损伤较小,‘戈达尔’的表现则相反,‘弗奥’介于二者之间;同期各品种间叶片束缚水与自由水相对含量、超氧阴离子产生速率、SOD活性及渗透调节物质含量的变化表现相似。研究表明,3个油橄榄主栽品种中‘城固32’对30%PEG溶液渗透胁迫的抗性最强,‘弗奥’次之,‘戈达尔’最差;在30%PEG溶液渗透胁迫下,‘城固32’叶片具有较高的束缚水与自由水含量比值、较低的超氧阴离子产生速率、较高的SOD活性及较高渗透调节物质含量,这在一定程度上保证其叶片具有较强的抗脱水能力、抗氧化能力和渗透调节能力,从而在整体上表现出对渗透胁迫的较强抗性。     

Plant Morphological and Biochemical Responses to Field Water Deficits : II. Responses of Leaf Glycerolipid Composition in Cotton

The effects of water deficits on leaf glycerolipid composition were analyzed in two photoperiodic strains of field grown cotton (Gossypium kirsutum L.) that differ in sensitivity to drought. Leaves from plants grown under dryland conditions exhibited increased dry weight and specific leaf weight. The average midday leaf water potential in the dryland treatment decreased to −1.9 and −2.4 megapascals, respectively, for the T25 and T185 genotypes. Total leaf lipid content of plants exposed to dryland conditions was 5.9 and 7.5% of leaf dry weight for strain T25 and T185, respectively. The difference in leaf lipid content between these genotypes was caused by water deficits and was attributed to loss of both phospholipids and glycolipids in strain T25. There was no apparent loss of phospholipids due to water deficits in the T185 genotype; however, a significant loss of glycolipids was partially compensated by a 2-fold increase in triacylglycerol. No change in triacylglycerol was found between treatments in T25 leaves. Water deficit caused a significant decline in the relative degree of acylunsaturation in phospholipids and glycolipids from both genotypes; however, the double bond index for triacylglycerol increased in both genotypes. It is believed that the observed responses of leaf lipid composition to dryland conditions may be an additional criterion for characterization and selection of new drought-tolerant cotton genotypes.     

The Arrhenius Equation as a Model for Explaining Plant Responses to Temperature and Water Stresses

The Arrhenius equation describes the response of biologicalprocesses to temperature. This study was conducted to examinethe applicability of the Arrhenius equation to whole plant processesand to explore the application of the Arrhenius equation asa basis for characterizing plant responses to water stress.Rates of growth of leaf area and shoot dry mass of spring wheatseedlings were measured at combinations of five soil water potentials(–0.03, –0.06, –0.10, –0.17 and –0.25MPa) and seven root temperatures (12, 14, 17, 22, 27, 29 and32 C). A non-linear least square procedure was used to fitthe modified Arrhenius equation to experimental observations.Adequate distribution of experimental observations with respectto temperature reduces the uncertainties in parameter evaluations.The standard error of the estimate of optimum temperature forleaf area growth increased from 1.4 C to 6.3 C when one ofthe data points was omitted. The optimum temperature and theenthalpy of denaturalization of enzyme systems were independentof soil water potential. A linear relation was found betweenthe rate constant and the activation energy: The Arrhenius equation was modified using this linear relation,leaving the activation energy as the only parameter affectedby water stress. The activation energy increased linearly assoil water potential decreased, with slopes of –27.18 10₃ and –28.09 10₂ K MPa^–1 for the rates ofgrowth of leaf area and shoot dry mass, respectively. Theseslopes could be used as indicators of the sensitivity of plantprocesses to water stress. Temperature, water, plants, Arrhenius equation     

Differential Responses of Nitrogen-Fixing Cyanobacteria to Salinity and Osmotic Stresses

Two nitrogen-fixing Anabaena strains were found to be differentially tolerant to salinity and osmotic stresses. Anabaena torulosa, a brackish-water, salt-tolerant strain, was relatively osmosensitive. Anabaena sp. strain L-31, a freshwater, salt-sensitive strain, on the other hand, displayed significant osmotolerance. Salinity and osmotic stresses affected nitrogenase activity differently. Nitrogen fixation in both of the strains was severely inhibited by the ionic, but not by the osmotic, component of salinity stress. Such differential sensitivity of diazotrophy to salinity-osmotic stresses was observed irrespective of the inherent tolerance of the two strains to salt-osmotic stress. Exogenously added ammonium conferred significant protection against salinity stress but was ineffective against osmotic stress. Salinity and osmotic stresses also affected stress-induced gene expression differently. Synthesis of several proteins was repressed by salinity stress but not by equivalent or higher osmotic stress. Salinity and osmotic stresses induced many common proteins. In addition, unique salt stress- or osmotic stress-specific proteins were also induced in both strains, indicating differential regulation of protein synthesis by the two stresses. These data show that cyanobacterial sensitivity and responses to salinity and osmotic stresses are distinct, independent phenomena.     

抗旱性不同的小麦叶片的渗透调节与水分状况的关系

两年的试验结果表明：在土壤水分胁迫下抗旱性强的小麦品种叶片的相对含水量和水势均高于抗旱性弱的品种;渗透势与水势为线性关系,水势每变动一个单位,渗透势变动0.71- 0.93个单位;渗透势与相对含水量的对数化关系为两条直线组成的一条折线,第一条直线渗透势的下降完全由渗透调节引起;第二条直线渗透势下降主要是细胞失水浓缩的结果。渗透调节能力为：秦麦3号>昌乐5号>山农587>济南13>烟农15>鲁麦5号。     

玉米对渗透胁迫的反应和生理适应

利用不同渗透势的培养液模拟土壤干旱条件,研究了玉米杂交种“中单2号”在此条件下的生长和生理的变化。实验表明,各指标对于渗透胁迫的变化敏感性顺序为叶片延伸速率、叶水势、脯氨酸含量>净光合速率>相对透性。玉米幼苗在渗透胁迫下具有一定的生理适应能力,表现在一定的渗透胁迫范围内随胁迫时间的延长,生长、生理变化有趋于缓和或恢复的趋势。     

Focus on Water: Monitoring Plant Drought Stress Response Using Terahertz Time-Domain Spectroscopy

We present a novel measurement setup for monitoring changes in leaf water status using nondestructive terahertz time-domain spectroscopy (THz-TDS). Previous studies on a variety of plants showed the principal applicability of THz-TDS. In such setups, decreasing leaf water content directly correlates with increasing THz transmission. Our new system allows for continuous, nondestructive monitoring of the water status of multiple individual plants each at the same constant leaf position. It overcomes previous drawbacks, which were mainly due to the necessity of relocating the plants. Using needles of silver fir (Abies alba) seedlings as test subjects, we show that the transmission varies along the main axis of a single needle due to a variation in thickness. Therefore, the relocation of plants during the measuring period, which was necessary in the previous THz-TDS setups, should be avoided. Furthermore, we show a highly significant correlation between gravimetric water content and respective THz transmission. By monitoring the relative change in transmission, we were able to narrow down the permanent wilting point of the seedlings. Thus, we established groups of plants with well-defined levels of water stress that could not be detected visually. This opens up the possibility for a broad range of genetic and physiological experiments.Climate change simulations predict an increase in the occurrence of drought events in the Mediterranean area and in central Europe due to smaller amounts of precipitation, especially during summer periods (IPCC, 2007). With the exception of the boreal zone, this leads to an increase in drought risks for every region on the European continent (Iglesias et al., 2007). Water availability is very important for a variety of plant species. Trees and crops play major roles regarding ecosystem stability and food supply. Forest trees are keystone elements in shaping long-term, regional ecosystem composition and stability and are, like most forest species, highly vulnerable to increases in drought severity (Breshears et al., 2005; Choat et al., 2012). Drought-induced forest die-offs thereby directly reduce ecosystem services such as carbon sequestration and timber supply (Allen et al., 2010). Further research is clearly necessary to elucidate the physiological traits and responses of plants regarding their water status.European silver fir (Abies alba) is an important forest tree species of ecological and economic relevance. This study is embedded in the European project LinkTree, “linking genetic variability with ecological responses to environmental changes: forest trees as model systems.” Our group is concerned with the identification of genes involved in the water stress response of silver fir. This species is of special interest because of its lower water-use efficiency compared with other conifer species (Guehl and Aussenac, 1987; Guehl et al., 1991).For this purpose, monitoring plant water status without inducing other forms of stress is instrumental in order to apply well-defined levels of water stress. Obtaining information regarding the water status of a plant is highly problematic without using invasive and destructive methods that usually only allow a retrospective assessment. These include commonly established methods, such as the gravimetric water content and pressure chamber techniques, most notably Scholander’s pressure bomb (Scholander et al., 1965).Chlorophyll fluorescence, stomatal conductance, and visual assessment are examples of nondestructive and noninvasive measurement techniques. The former two only provide indirect information about the plant stress status and, therefore, the water content via photosynthetic activity (Lichtenthaler and Rinderle, 1988; Tardieu and Davies, 1993). The latter is difficult to standardize and highly dependent on the morphology of the studied plant species. Conifers especially are challenging subjects for visually assessing drought stress. Due to their needle morphology, it is nearly impossible to detect early signs of dehydration.Measurement techniques using electromagnetic radiation in the terahertz (THz) regime have shown promising results, due to the nondestructive nature and high sensitivity of THz waves to water. With THz waves, we refer to frequencies in the electromagnetic spectrum between 0.1 and 1 THz, corresponding to wavelengths between 3 and 0.3 mm, which are located between infrared light (thermal radiation) and microwave radiation (used in common wireless data communication systems). In the last decade, terahertz time-domain spectroscopy (THz-TDS) has proven to be a very strong and accurate tool for characterizing and imaging various materials (for review, see Jepsen et al., 2011). Crucial for our study is the remarkably high absorption coefficient of water in this part of the electromagnetic spectrum. Thus, it is a robust technique hardly affected by physiological concentrations of soluble substances. Using transmission geometry, the resulting absorption by plant tissues directly reflects the quantity of water molecules.Furthermore, THz-TDS does not suffer from the disadvantages of other radiation-based techniques. These are mainly focused on the infrared or microwave spectrum but either lack the sensitivity for small changes in leaf water status or are affected by the plant’s inorganic salt content, leading to significant disturbances (Ulaby and Jedlicka, 1984). Moreover, the applicability of emitting microwave radiation is limited to minimal wavelengths of approximately 2.5 mm. The Abbe diffraction limit, therefore, restricts the minimum diameter of a measurable object to approximately 1.25 mm. In order to measure small leaves, such as coniferous needles, electromagnetic radiation with shorter wavelengths is necessary.Although presenting a useful alternative, THz-TDS was not feasible until recently, due to the difficulty of generating and detecting electromagnetic radiation with wavelengths in the THz spectrum. Despite its promising applicability in plant sciences, until now this relatively novel method relied exclusively on measurement setups that allowed only a single measurement per alternating plant (Hadjiloucas et al., 1999; Jördens et al., 2009; Breitenstein et al., 2012; Castro-Camus et al., 2013; Gente et al., 2013). For the purpose of continuously monitoring multiple plants, these setups are only of limited use, since the plants must be relocated for every measurement. This results in two problems: (1) an increase in possible disturbances (e.g. mechanical), influencing the plant’s stress response, and (2) the necessity to precisely target the same measurement spot on every analyzed plant at every consecutive measurement. The latter is of crucial importance for the exact monitoring of any individual plant’s water status because, as we will show in this study, the transmission varies substantially across the area of plant leaf tissue.We present a novel measurement procedure that overcomes the drawbacks of previously proposed methods. Our approach enables us to precisely monitor changes in the water content of multiple plants simultaneously.In the course of this study, three different experiments were performed. The profile measurement and the rehydration experiment were preliminary investigations to examine the influences of needle and tissue thickness and to define a nonlethal stress level. The main experiment established groups of plants with comparable levels of water stress.     

Responses of Camellia sinensis to Drought and Rehydration

The effects of drought and rehydration on tea seedlings were significant. After five days of drought imposition the contents of chlorophylls, carotenoids, ascorbate and glutathione, and activities of guaiacol peroxidase and glutathione reductase decreased. Simultaneously, contents of proline, H₂O₂ and superoxide anion, lipid peroxidation and activities of catalase and superoxide dismutase increased. These parameters recovered to different degrees during subsequent rehydration.     

植物导水系统对土壤质地与辐射强度的形态性适应研究

以棉花(Gossyp ium herbaceum L.)为实验材料,在全生长期为盆栽植株设置3个土壤质地梯度、3个蒸发力梯度.在播种后50 d和90 d,测定总根长与总叶面积;同时,在90 d时,使用热脉冲探头和HR-33T露点微伏计测定植株茎流和叶水势,并由此确定导水度,以期了解植物导水系统对土壤质地与大气蒸发力的长期适应.结果表明:在蒸发需求相同的条件下,生长在沙土中、单位根导水度低的植株,形成的吸收根要多于生长在粘土中、单位根导水度高的植株;单位叶片导水度在3种不同的土壤中没有显著差别.在土壤质地相同条件下,高蒸发力下生长的植株比低蒸发力下生长的植株产生更多的吸收根、形成更高的单位叶片导水度.最终证实:当土壤质地或大气蒸发力发生变化时,植物个体可以通过导水系统的形态性适应来调节根-叶比例,以达到协调其自身水分供需平衡.