异株荨麻（Urtica dioica）生长及光合特性对不同土壤水分含量的响应

异株荨麻是我国一种重要中药植物,如何对其进行人工定向培植具有重要意义。以异株荨麻扦插苗为实验材料,采用盆栽水分梯度法,按土壤相对含水量93．58％、80．74％、67．90％、55．06％和42．22％分为对照组CK,实验处理组T1,T2,T3和T4五个处理组,通过观测不同水分条件下,异株荨麻生长量、生物量变化和光合特征变化,试图探讨异株荨麻的水分适应性,寻找出适合其生长的水分范围及对生物量分配的影响,为北方地区种植栽培此类植物提供科学依据。结果发现：随土壤水分含量的降低,异株荨麻生长量和生物量积累呈先上升后下降趋势,在土壤相对含水量80．74％生长达到高峰,土壤相对含水量67．90％～80．74％为异株荨麻生长的适宜范围。净光合速率、蒸腾速率、气孔导度、气孔限制值及叶绿体色素含量的变化趋势与之相近似。在上述水分范围以外其光合功能下降、同化作用减弱,使异株荨麻生长衰退和干物质积累的减少。此外,土壤水分直接调控光合产物在地上和地下的分配以及植株个体形状,生产实践中,通过控水可以达到控制株形和根冠比的作用,用以诱导植株朝种植的预期方向生长,以达到获取植株不同部位产量的实际应用需求。     

Quantification of the Daily Cytokinin Transport from the Root to the Shoot of Urtica dioica L.*

Cytokinins are predominantly root-born phytohormones which are distributed in the shoot via the xylem stream. In the hormone message concept they are considered as root signals mediating the transport of the photosynthates to the various sinks of a plant. In this paper the cytokinin relations of Urtica dioica L., the stinging nettle, are described, based on the daily flux from the roots to the shoot. Trans-zeatin-type cytokinins predominate in the various tissues of Urtica (Wagner and Beck, 1993), and accordingly trans-zeatin riboside and trans-zeatin are the forms transported by the xylem sap. The daily time-course of cytokinin concentration in root pressure exudates and in xylem sap collected from a petiole after pressurizing the root bed showed high concentrations in the morning, followed by a substantial drop to a level of 15–30% of the initial concentration which was then maintained during the afternoon. This time-course is interpreted as resulting from continuous synthesis and exudation of cytokinins into the xylem fluid of the roots whose cytokinin concentration is then modified by the dynamics of the transpiration stream. Loading of cytokinins into the xylem sap could be enhanced several times by increasing the flux rate of the xylem stream to the maximal transpiration rate when a maximum export rate was reached. The total daily cytokinin gain by the shoot depended on the nitrogen status of the plant. Roots of Urtica plants grown on a sufficient nitrogen supply had a significantly higher cytokinin content and exuded more cytokinins into the shoot than those of plants raised under nitrogen shortage. A positive correlation was found between the steady rates of cytokinin export measured during the afternoon and the shoot to root-ratios of biomass which, in turn, corresponded to the nitrogen status of the plants.     

The Effect of Nitrogen Supply to Urtica dioica L. Plants on the Distribution of Assimilate Between Shoot and Roots

In this study the influence of nitrogen nutrition on the patterns of carbon distribution was investigated with Urtica dioica. The nettles were grown in sand culture at 3 levels of NO^?₃, namely 3 (low), 15 (medium) and 22 (high) mM. These levels encompassed a range within which nitrogen did not affect total biomass production. The ratio of root: shoot biomass of the low nitrogen plants was, however, significantly higher than that of the nettles grown at medium and high N supply. Carbon allocation from one leaf of each pair of leaves was examined after a ¹⁴CO₂-pulse and a subsequent ¹⁴C distribution period of one night. Only the youngest two leaf pairs did not export assimilates. Carbon (¹⁴C) export to the shoot apex and to the roots, as measured at the individual nodes responded to the nitrogen status: At medium and high nitrogen supply the 3rd, 4th and 5th leaf pairs exported to the shoot apex, while lower leaves exported to the root. At low nitrogen supply only the 3rd leaf exported towards the shoot apex. The results illustrate the plastic response of carbon distribution patterns to the nitrogen supply, even when net photosynthesis, carbon export from the source leaves and biomass production were not affected by the nitrogen supply to the plant.     

Reversal of the Direction of Photosynthate Allocation in Urtica dioica L. Plants by Increasing Cytokinin Import into the Shoot

The natural cytokinin import from the root into the shoot of Urtica dioica plants was enhanced by supplying zeatin riboside (ZR) solutions of various concentrations to a portion less than 10 % of the root system after removal of their tips. After 6 h ZR pretreatment of the plants, ¹⁴CO₂ was supplied for 3 h to a mature (source) leaf or to an expanding leaf and the ¹⁴C-distribution in the whole plant was determined after a subsequent dark period of 14 h. ZR substantially increased ¹⁴C fixation by the expanding leaves and also enhanced export of carbon and transport to the shoot apex. The effect of the hormone treatment was, however, more pronounced when the ¹⁴CO₂ was supplied to a mature leaf. In the control plants these leaves exported carbon only to the roots: When the amount of the natural daily ZR input from the roots to the shoot was enhanced by 20%, the bulk of the ¹⁴C exported from a mature leaf moved to the shoot apex and only a minor portion of ¹⁴C was still detected in the root fraction. A several-fold increase of the natural daily ZR input into the shoot resulted in a flow of ¹⁴C only to the growing parts of the shoot. The results suggest control of the sink strength of the shoot apex by ZR in Urtica diocia.     

Population dynamics of Phalaris arundinacea L. and Urtica dioica L. in a floodplain during a dry period

The distribution of young and old Phalaris arundinacea and Urtica dioica plants across the Lunice River floodplain (Tebo Biosphere Reserve, South Bohemia, Czech Republic) was studied over a three-year dry period. Plants of both species that germinated in the current year were found on sites disturbed by floods and on emerged pool bottoms. Older P. arundinacea plants were only occasionally found on unshaded sites (pool bottoms, sandy drifts in river bed). U. dioica established itself more often on sites shaded by herbaceous plants or by neighboring shrubs. The long-term survival of U. dioica on wet sites seems uncertain because this species is less tolerant of flooding than P. arundinacea. Thus episodic expansion of U. dioica into unmown parts of the floodplain is expected.This paper was presented at the INTECOL IV International Wetlands Conference in Columbus, Ohio, 1992, as part of a session organized by Prof. S.E. Jørgensen and sponsored by the International Lake Environment Committee.Corresponding Editor: P. Denny     

Purification and characterization of polyphenol oxidase from nettle (Urtica dioica L.) and inhibitory effects of some chemicals on enzyme activity

Polyphenol oxidase (PPO) of nettle (Urtica dioica L.) was extracted and purified through (NH₄)₂SO₄ precipitation, dialysis, and CM-Sephadex ion-exchange chromatography and was used for its characterization. The PPO showed activity to catechol, 4-methylcatechol, L-3,4-dihydroxyphenylalanine (L-DOPA), L-tyrosine, p-cresol, pyrogallol, catechin and trans-cinnamic acid. For each of these eight substrates, optimum conditions such as pH and temperature were determined and L-tyrosine was found to be one of the most suitable substrates. Optimum pH and temperature were found at pH 4.5 and 30°C respectively and K_m and V_max values were 7.90?×?10^?4?M, and 11290?EU/mL for with L-tyrosine as substrate. The inhibitory effect of several inhibitors, L-cysteine chloride, sodium azide, sodium cyanide, benzoic acid, salicylic acid, L-ascorbic acid, glutathione, thiourea, sodium diethyl dithiocarbamate, β-mercaptoethanol and sodium metabisulfite were tested. The most effective was found to be sodium diethyl dithiocarbamate which acted as a competitive inhibitor with a K_i value of 1.79?×?10^?9?M. In addition one isoenzyme of PPO was detected by native polacrylamide slab gel electrophoresis.     

The fate of apoplastic sucrose in sink and source leaves of Urtica dioica

Experiments were performed with developing and mature leaves of Urtica dioica L. to trace differences which could be interpreted in terms of cell wall-bound acid invertase (EC 3.2.1.26) participating in phloem unloading in a sink leaf. The pH of apoplastic fluid that was collected by gentle centrifugation of entire leaves was identical (7.1) in the two types of leaves; also, fluorometric determination with esculetin showed a neutral apoplastic pH between 7.0 in the source and 7.2 in the sink leaf. To detect whether differences in apoplastic pH occur within limited leaf areas, such as of the tissue surrounding the veins, the metabolic fate of [¹⁴C]–(fructosyl)-sucrose that was administered via the xylem was investigated. In source leaves, there was a large transitory decrease in [¹⁴C]-sucrose followed by a substantial resynthesis of this compound. In sink leaves, resynthesis was less significant and carbon was incorporated mainly in starch, charged soluble compounds and cell walls. However, after correction for resynthesis, the two types of leaves showed an identical capacity for sucrose cleavage. Finally, activation of the apoplastic invertase by administering labelled sucrose in buffered solution of pH 5.0 did not result in an enhanced degradation. By contrast, apoplastic fluid collected from leaves which had been infiltrated with buffer solutions of pH 5.5 and 8.0, respectively, showed a rapid adjustment of the pH close to the natural neutral value by the mesophyll tissue. The results are incompatible with the idea of an active invertase in the sink (and the source) leaves apoplast, and hence do not lend support to the theory of apoplastic cleavage of sucrose being required for phloem unloading in this kind of a utilization sink.     

土壤水分对异株荨麻（Urtica dioica）保护酶和渗透调节物质的影响及其与叶片光合和生物量的相关性

通过对不同土壤水分处理下异株荨麻保护酶系(SOD、POD)活性、渗透调节物质(可溶性蛋白、可溶性糖和游离脯氨酸)和细胞膜脂过氧化产物(MDA)含量以及最大光合速率、生物量的测定,各项生理指标间线性相关性的分析,探讨异株荨麻对水分变化的响应特性和耐干旱适应能力.实验采用盆栽水分梯度法将异株荨麻扦插苗按土壤相对含水量93.58%、80.74%、67.90%、55.06%和42.22%分组,以充足水分93.58%为CK对照组,实验组水分由高到低为T1、T2、T3和T4共5组.结果表明,(1)异株荨麻体内SOD活性随水分含量下降而呈先上升后下降的变化趋势,POD活性与之成相似互补的变化趋势;可溶性蛋白、可溶性糖、游离脯氨酸和MDA含量随水分含量的下降而升高; (2)各保护酶系和渗透调节物质与土壤水分呈显著线性负相关,保护酶系、渗透调节物质与过氧化产物MDA线性正相关;(3)比较光合速率、生物量和各种酶系、渗透调节物质的关系发现,光合速率与各生理指标无明显相关性,渗透调节物质对生物量积累有显著负相关性.     

Growth characteristics ofNelumbo nucifera Gaertn. in response to water depth and flooding

Three experiments on the effects of water depth and flooding onNelumbo nucifera Gaertn. were made in the artificial environment of concrete ponds. First, plants were harvested in autumn after growing under seven different water levels ranging from 0.2–3 m The number of floating leaves, the total number of leaves and the leaf area index of emergent leaves were greatest in the tanks at 0.5 m depth. The petiole dry weight per unit length of emergent leaves and the ratio of aboveground to belowground biomass rose with increasing water depth up to 2 m. In contrast, that of floating leaves was constant at about 10 mg dry weight cm⁻¹. The proportion of biomass in tubers fell from 20% at 0.2 m to 6% at 2 m. Second, petiole elongation responses to the amplitude of flooding were investigated in early summer. The maximum rate of petiole elongation was 25 cm per day at 2.4 m water depth. This was the maximum depth at whichN. nucifera could grow. No petioles could elongate from 3 m to 5 m depth. Finally, the effects of timing of flooding on growth were investigated. At the end of growing season, the belowground biomass of plants in the flooding treatment in late summer was smallest among the flooding treatment plants (P<0.05), and was most severe when flooding occurred in this season. Based on the results of these experiments, the growth characteristics ofN. nucifera in relation to petiole elongation, biomass allocation, and flooding tolerance were discussed.     

Effects of water table depth and calcium perioxide application on cowpea (Vigna unguiculata) and soybean (Glycine max)

Summary The effects of three water table (WT) depths (0, 15 and 40 cm) and calcium peroxide (Calper) on the growth and yield of cowpea (Vigna unguiculata, L.) and soybean (Glycine max) were investigated in field lysimeters for a sandy loam soil. Cowpea growth was the best at 40 cm WT depth. Leaf area, plant height, dry matter production, number of leaves and pods, grain yield and consumptive water use of cowpea increases with deeper (lower) WT depth. Application of calcium peroxide improved per cent emergence, leaf area, dry matter, number of leaves and pods, weight of 100 seeds, grain yield and water use in cowpea. The optimum WT depth for vegetative growth of soybean was 15 cm, although the highest grain yield was obtained at 40 cm WT depth. Number of pods, grain yield and water use efficiency of soybean increased with deeper water table depth. Application of calcium peroxide to soybean increased number of leaves and pods per plant, and grain yield for the 15 cm WT depth treatment.     

模拟喀斯特不同土壤生境下黑麦草对水分胁迫的生长和光合生理响应

以多年生草本黑麦草为对象,模拟喀斯特生境土壤特征设置浅而宽(Shallow and wide, SW:30×30×5 cm³)和深而窄(Deep and narrow, DN:10×10×45 cm³)两种土壤容器,以正常降水量为对照供水(W100%),设减水50%(W50%)和减水70%(W30%)共3种水分处理进行盆栽实验,探究了两种不同土壤生境中土壤水分变化对黑麦草生长及光合生理的影响,以进一步理解喀斯特地区植物的适应对策。结果显示:(1)SW生境对水分变化敏感,随供水减少土壤含水量显著下降。轻度减水下植物叶面积增大,光合速率提高,地上部分生长得到促进,但水分严重减少对其生长和光合生理有抑制作用,但地上质量分数和水分利用效率却显著升高;(2)DN生境保水能力较好,随供水减少土壤水分含量下降较为平缓。叶片相对含水量、气孔导度和比叶面积在各水分处理之间差异不显著,但严重减水条件下总生物量、地上质量分数和水分利用效率均有回升。研究表明:浅而宽生境中植物倾向于通过提高地上部分的生长,保持较高的光合速率,并向地上部分分配较多生物量来应对水分胁迫;...     

Differential effects of water depth and sediment type on clonal growth of the submersed macrophyte <Emphasis Type="Italic">Vallisneria natans</Emphasis>

The response of clonal growth and ramet morphology to water depth (from 60 to 260 cm) and sediment type (sand versus organic clay) was investigated for the stoloniferous submersed macrophyte Vallisneria natans in an outdoor pond experiment. Results showed that water depth significantly affected clonal growth of V. natans in terms of clone weight, number of ramets, number of generations, clonal radius and stolon length. V. natans showed an optimal clonal growth at water depths of 110–160 cm, but at greater depths clonal growth was severely retarded. A high allometric effect was exhibited in ramet morphology. Along the sequentially produced ramet generations, ramet weight and plant height decreased while stolon length and the root:leaf weight ratio increased. When using ramet generations as covariate, sediment type rather than water depth more strongly affected the ramet characteristics. For plants grown in clay, ramet weight, ramet height and stolon length were greater, and plants exhibited lower root:leaf weight ratio. These data suggest that water depth and sediment type have differential effects on clonal growth of V. natans: Water depth appears primarily to affect numerical increase in ramets and spatial spread, whereas sediment type mainly affects biomass accumulation and biomass allocation. Handling editor: S. M. Thomaz     

Convergence of carbon sink magnitude and water table depth in global wetlands

Wetlands are strategic areas for carbon uptake, but accurate assessments of their sequestration ability are limited by the uncertainty and variability in their carbon balances. Based on 2385 observations of annual net ecosystem production from global wetlands, we show that the mean net carbon sinks of inland wetlands, peatlands and coastal wetlands are 0.57, 0.29 and 1.88 tons of carbon per hectare per year, respectively, with a mean value of 0.57 tons of carbon per hectare per year weighted by the distribution area of different wetland types. Carbon sinks are mainly in Asia and North America. Within and across wetland types, we find that water table depth (WTD) exerts greater control than climate- and ecosystem-related variables, and an increase in WTD results in a stronger carbon sink. Our results highlight an urgent need to sustain wetland hydrology under global change; otherwise, wetlands are at high risk of becoming carbon sources to the atmosphere.     

Modulation of carbon and nitrogen allocation in Urtica dioica and Plantago major by elevated CO2: Impact of accumulation of nonstructural carbohydrates and ontogenetic drift

Doubling the atmospheric CO² concentration from 350 to 700 μ1 1^?1 increased the relative growth rate (RGR) of hydroponically grown Urtica dioica L. and Plantago major ssp. pleiosperma Pilger only for the first 10–14 days. Previous experiments with P. major led to the conclusion that RGR did not respond in proportion to the rate of photosynthesis. The present paper is focussed on the analysis of the impact of changes in leaf morphology, dry matter partitioning, dry matter chemical composition and ontogenetic drift on this discrepancy. Soon after the start of the treatment, carbohydrate concentrations were higher at elevated CO²: a reaction that was largely due to starch accumulation. An increase in the percentage of leaf dry matter and decreases in the specific leaf area (SLA) and the shoot nitrogen concentration were correlated with an increase in the total nonstructural carbohydrate concentration (TNC). A combination of accumulation of soluble sugars and starch and ontogenetic drift explains the decrease in SLA at the elevated CO² level. A similar ontogenetic effect of elevated CO² was observed on the specific root length (SRL). Other variables such as shoot nitrogen concentration and percentage leaf dry matter were not affected by correction of data for TNC levels. The net diurnal fluctuation of the carbohydrate pool in P. major was equal for both CO² concentrations, indicating that the growth response to elevated CO² may be ruled by variables other than photosynthesis, as for instance sink strength. Elevated CO² did not greatly influence the partitioning of nitrogen between soluble and insoluble, reduced N and nitrate, nor the allocation of dry matter between leaf. stem and root. The finding that the root to shoot ratio (R/S) was not affected by elevated CO² implies that, in order to maintain a balanced activity between roots and shoot, no shift in partitioning of dry matter upon doubling of the atmospheric CO² concentration is required. Our data on R/S are in good agreement with the response of R/S to high CO² predicted by models based on such a theorem.     

Effects of Nigella sativa L. and Urtica dioica L. on selected mineral status and hematological values in CCl4-treated rats

This study was designed to investigate the effects of Nigella sativa L. (NS), known as black seed, or/and Urtica dioica L. (UD), known as stinging nettle root, treatments on serum Na, K, Cl, and Ca levels and some hematological values of CCl₄-treated rats. Sixty healthy male Sprague-Dawley rats, weighing 250–300 g, were randomly allotted into 1 of 4 experimental groups: A (CCl₄-only treated), B (CCl₄+UD treated), C (CCl₄+NS treated), and D (CCl₄+UD+NS treated), each containing 15 animals. All groups received CCl₄ (0.8 mL/kg of body weight, subcutaneously, twice a week for 90 d starting d 1). In addition, B, C, and D groups also received the daily ip injection of 0.2 mL/kg NS and/or 2 mL/kg UD oils for 45 d starting d 46. Group A, on the other hand, received only 2 mL/kg normal saline solution for 45 d starting d 46. Blood samples for the biochemical analysis were taken by cardiac puncture from five randomly chosen rats in each treatment group at the beginning, d 45, and d 90 of the experiment. The CCl₄ treatment for 45 d significantly (p<0.05) increased the serum K and Ca and decreased (p<0.05) the red blood cell count (RBC), white blood cell count (WBC), packed cell volume (PCV), and Hb levels without changing (p>0.05) the serum Na and Cl levels. NS or UD treatments (alone or combination) for 45 d starting d 46 significantly (p<0.05) decreased the elevated serum K and Ca levels and also increased (p<0.05) the reduced RBC, WBC, PCV, and Hb levels. It is concluded that NS and/or UD treatments might ameliorate the CCl₄-induced disturbances of anemia, some minerals, and body’s defense mechanism in CCl₄-treated rats.     

Water relations and growth responses of Uniola paniculata (sea oats) to soil moisture and water-table depth

Summary This study examined the water relations and growth responses of Uniola paniculata (sea oats) to (1) three watering regimes and (2) four controlled water-table depths. Uniola paniculata is frequently the dominant foredune grass along much of the southeastern Atlantic and Gulf coasts of the United States, but its distribution is limited in Louisiana. Throughout most of its range, U. paniculata tends to dominate and be well adapted to the most exposed areas of the dune where soil moisture is low. Dune elevations in Louisiana, however, rarely exceed 2 m, and as a result the depth to the water table is generally shallow. We hypothesized that if U. paniculata grows very near the water-table, as it may in Louisiana, it will display signs of water-logging stress. This study demonstrated that excessive soil moisture resulting from inundation or shallow water-table depth has a greater negative effect on plant growth than do low soil moisture conditions. Uniola paniculata's initial response to either drought or inundation was a reduction of leaf (stomatal) conductance and a concomitant decrease in leaf elongation. However, plants could recover from drought-induced leaf xylem pressures of less than-3.3 MPa, but prolonged inundation killed the plants. Waterlogging stress (manifested in significantly reduced leaf stomatal conductances and reduced biomass production) was observed in plants grown at 0.3 m above the water table. This stress was relieved, however, at an elevation of 0.9 m above the water table. As the elevation was increased from 0.9 to 2.7 m, there were no signs of drought stress nor a stimulation in growth due to lower soil moisture. We concluded that although U. paniculata's moisture-conserving traits adapt it well to the dune environment, this species can grow very well at an elevation of only 0.9 m above the water table. Field measurements of water-table depth in three Louisiana populations averaged about 1.3 m. Therefore, the observed limited distribution of U. paniculata along the Louisiana coast apparently cannot be explained by water-logging stress induced by the low dune elevations and the corresponding shallow water-table depth.     

Short-term and long-term effects of plant water deficits on stomatal response to humidity in Corylus avellana L.

Short-term (hours) changes in plant water status were induced in hazel (Corylus avellana L.) by changing the evaporative demand on a major portion of the shoot while maintaining a branch in a constant environment. Stomatal conductance of leaves on the branch was influenced little by these short-term changes in water status even with changes in leaf water potential as great as 8 bars. Long-term (days) changes in plant water status were imposed by soil drying cycles. Stomatal conductance progessively decreased with increases in long-term water stress. Stomata still responded to humidity with long-term water stress but the range of the conductance response decreased. Threshold responses of stomata to leaf water potential were not observed with either short-term or long-term changes in plant water status even when leaves wilted. It is suggested that concurrent measurements of plant water status may not be sufficient for explaining stomatal and other plant responses to drought.     

水深梯度下湿地植被空间分布与生态适应

采用模糊数学排序方法对黄河三角洲国家级自然保护区不同水深梯度下芦苇湿地植被进行了．研究,揭示了水深对植被空间分布的影响。结果表明,不同水深梯度下植物生境和群落类型都表现出较大差异,水深-30～40cm,为水陆过渡地带,旱生、水生植物并存,物种最为丰富,该段水深上植被盖度最大;水深在-30-50cm,由于地下水深较低,该段水深是研究区盐碱化程度最大处;水深低于-50cm时,地表较为干旱,盐碱化程度有所降低,植被类型被耐干旱植被代替。不同水深梯度影响了土壤水分、空气和土壤的生物、物理、化学过程,引起植被生长环境中土壤水分、盐碱化程度的改变,进而对植被空间分布和植被生态特征产生影响。     

Growth and water relations of Tamarix ramosissima and Populus euphratica on Taklamakan desert dunes in relation to depth to a permanent water table

The hypothesis that water relations and growth of phreatophytic Tamarix ramosissima Ledeb. and Populus euphratica Oliv. on dunes of varying height in an extremely arid Chinese desert depend on vertical distance to a permanent water table was tested. Shoot diameter growth of P. euphratica was inversely correlated with groundwater depth (GD) of 7 to 23 m (adj. R² = 0.69, P = 0.025); growth of T. ramosissima varied independent of GD between 5 and 24 m (P = 0.385). Pre‐dawn (pd) and midday (md) water potentials were lower in T. ramosissima (minimum pd ?1.25 MPa, md ?3.6 MPa at 24 m GD) than in P. euphratica (minimum pd ?0.9 MPa, md ?3.05 MPa at 23 m GD) and did not indicate physiologically significant drought stress for either species. Midday water potentials of P. euphratica closely corresponded to GD throughout the growing season, but those of T. ramosissima did not. In both species, stomatal conductance was significantly correlated with leaf water potential (P. euphratica: adj. R² = 0.84, P < 0.0001; T. ramosissima: adj. R² = 0.64, P = 0.011) and with leaf‐specific hydraulic conductance (P. euphratica: adj. R² = 0.79, P = 0.001; T. ramosissima: adj. R² = 0.56, P = 0.019); the three variables decreased with increasing GD in P. euphratica. Stomatal conductance of P. euphratica was more strongly reduced (> 50% between ?2 and ?3 MPa) in response to decreasing leaf water potential than that of T. ramosissima (30% between ?2 and ?3 MPa). Tolerance of lower leaf water potentials due to higher concentrations of leaf osmotically active substances partially explains why leaf conductance, and probably leaf carbon gain and growth, of T. ramosissima was less severely affected by GD. Additionally, the complex below‐ground structure of large clonal T. ramosissima shrub systems probably introduces variability into the assumed relationship of xylem path length with GD.     

Effects of water depth on clonal characteristics and biomass allocation of Halophila ovalis (Hydrocharitaceae)

Aims Halophila ovalis is a dioecious seagrass with a wide geographical and water depth range. The objective of this study was to understand its plasticity in clonal characteristics and biomass and also its allocation between above- and belowground in seagrass beds at different water depths.Methods Four monospecific H. ovalis beds, Shabei, Xialongwei, Beimu and Yingluo, which have different water depths at maximum tide level (MTL) but otherwise similar environmental conditions, were studied. We measured main clonal characteristics, i.e. horizontal internodal length, branching angle, shoot height, leaf length and width, and rhizome diameter. Above- and belowground biomasses of H. ovalis were also estimated using a harvest method.Important findings We found no significant differences in coverage, leaf pair density or number of stem nodes per square meter between the four study sites. However, horizontal internodal length, leaf length, width, rhizome diameter and shoot height all increased significantly with the increases in water depth from 2- to 9-m MTL and decreased when the water depths were greater than 9-m MTL. No significant difference in above- or belowground biomass between the seagrass beds was found. However, the ratio of above- to belowground biomass was significantly higher in the shallowest site compared to the other three seagrass beds, indicating that more biomass was stored belowground in deeper water. The results demonstrated plastic responses in clonal characteristics and biomass allocation in H. ovalis across the water depth gradient.