Soil Drought Conditions Research Articles

Responses of Plant Community Composition and Eco-Physiological Characteristics of Dominant Species to Different Soil Hydrologic Regimes in Alpine Marsh Wetlands on Qinghai–Tibetan Plateau, China

Shifts in soil water content affect seasonal wetland plant communities worldwide, but little is known about the responses and influences of plant communities to soil water content on Qinghai–Tibetan Plateau. To determine the relationship between soil water content and plant community structure in seasonally wet alpine marshes, we investigated plant community structures at different soil water content amounts in the field station at sampling sites. We selected and determined eco-physiological characteristics of species belonging to three different functional types (sedges, grasses, forbs) exposed to the same soil water regimes as characteristics in field station and under experimental flooding conditions in controlled treatments on pots plants. The field investigation indicated that decreased soil moisture (from 57 to 43 % m3 m−3) during the growing season was significantly associated with reductions in aboveground biomass, average plant height, and species richness. A shift in dominant plant species within the communities from sedges and grasses to some forbs accompanied the decrease in soil moisture. The controlled treatments demonstrated that the sedge and grass species had higher net photosynthetic rates and higher instantaneous water-use efficiencies than the forb species in the studied communities. The results indicate that shifts in wetland plant community structure and function are the result of longer dry periods and more intense rainfall events. This positive feedback suggests that changes in plant community composition could intensify soil drought conditions in seasonally wet alpine marshes in the future.

Photosynthetic response of European beech to atmospheric and soil drought

Abstract Physiological response of European beech under soil and atmospheric drought conditions was investigated in this study. A group of six beech trees was irrigated during the growing season 2012, while the second group of non-irrigated (control) beech trees was treated under natural soil drought. During the experiment, we observed more than 45-day long period when no precipitation fell on the soil surface. The relationship of PN (CO2 assimilation rate) to gS (stomatal conductance) was very tight in both groups, which indicates that stomatal opening was the main factor limiting PN. The statistically significant differences in gS between the groups of trees were revealed only on the last measuring day. The significant differences in PN were confirmed on the days when the differences in soil water potential (ΨS) appeared. On these measurement days, the PN values of irrigated individuals were approximately 1.9 or 3.3 times greater than the values of non-irrigated individuals. At the level of primary photosynthetic processes (chlorophyll fluorescence parameters) we did not observe lower values of the control individuals in comparison with the irrigated trees in any of the evaluated parameters. Long-term soil water deficit caused strong decrease of leaf water potential (ΨL) in the control trees, but ΨL values of the irrigated trees were also rather low due to diurnal dynamics in higher parts of crown. Close relationship between ΨL and gS was confirmed for the control (non-irrigated) trees, but could not be confirmed for the irrigated trees. We revealed significant influence of VPD (vapour pressure deficit of the air) on gS only in the control group. On the days when ΨS decreased, the stomata of the non-irrigated trees were closed in spite of the low VPD values. Almost complete stomatal closure in both groups of trees was caused by the increase of VPD to 1.2 kPa.

Effect of seed priming with various concentrations of KNO3on sunflower seed germination parameters inin vitrodrought conditions

Seed priming is technique for controlled seed hydratation which allows activation of pre-germinative metabolic processes. It has been successfully used for improving germination parameters in different crops. The study was conducted in 2012, at the Institute of Field and Vegetable Crops, Novi Sad, Serbia, in order to assess the effect of seed priming on four germination parameters (germination, germination index, mean germination time and time to 50% germination) of sunflower in in vitro drought conditions. Drought conditions (−0.8 MPa) were simulated using PEG 6000. Seed priming was conducted with soaking seed in water and different KNO3 solutions (0.1, 0.5 and 1%) for 12 h. Seed priming treatment with 0.1% KNO3 showed the best results and improved germination parameters: germination index, mean germination time and time to 50% germination of untreated seed (control). Seed priming showed no improvement in germination percentage in comparison to the control. Control had high germination (98%) and there was no possibility for its significant improvement with seed priming. This study showed that seed priming could improve some germination parameters of sunflower seed in in vitro drought conditions. Seed priming could be used as a simple method for overcoming problems related to a poor germination and seedling establishment in soil drought conditions. Main advantages of seed priming method are its simplicity and no requirements for expensive equipment and chemicals.

Response of antioxidant and osmoprotective systems of wheat seedlings to drought and rehydration

Complex study of the effect of soil drought (72 h) and subsequent rehydration for 24 and 48 h on the activities of antioxidant and osmoprotective systems in the leaves of young plants of winter wheat (Triticum aestivum L.) cvs. Ballada (high productivity) and Beltskaya (low productivity) was carried out. Under drought conditions, the content of water in the leaves of cv. Ballada reduced to a lesser degree than in the leaves of cv. Beltskaya. Drought did not affect the rate of leaf growth in cv. Ballada but retarded leaf growth in cv. Beltskaya. Under drought conditions, the content of ascorbate reduced in cv. Beltskaya but was not changed in cv. Ballada; the content of glutathione increased by 19% in cv. Ballada and by 30% in cv. Beltskaya. Under drought conditions, ascorbate peroxidase activity was not changed in cv. Ballada whereas in cv. Beltskaya there was a tendency to its decrease. Glutathione reductase activity in the leaves of cv. Beltskaya increased stronger than in cv. Ballada. Substantial differences between cultivars in the accumulation of reducing sugars and sucrose under water deficit were observed. In both cultivars, drought induced an active proline accumulation. Observed differences in the cultivar responses to water stress evidently indicate differences in the strategy of their adaptation to drought. Drought did not affect the contents of chlorophyll and MDA in both cultivars. The data obtained allow a suggestion that, under conditions of moderate soil drought, the coordinated system of antioxidant defense and osmotic control functioned sufficiently effective; as a result, oxidative stress was not developed in both cultivars. Young plants of both cultivars differing in their responses to water deficit retained the ability to recover after rehydration.

An increase in the content of cell wall-bound phenolics correlates with the productivity of triticale under soil drought

The objective of this study was to investigate whether the content of cell wall-bound phenolics can simultaneously influence both the productivity and the water status of triticale under soil drought conditions. Two parallel treatments were carried out. The T1 treatment involved plants being subjected to soil drought twice, during the tillering phase and then during the flowering phase. The T2 treatment included drought only during the flowering phase.After T1 treatment, the majority of cultivars exhibited better PSII functioning at the flowering phase in comparison to T2, which could be related to better adaptation of the photosynthetic apparatus to leaf dehydration. Simultaneously, the higher activity of the photosynthetic apparatus of flag leaves for T1 was significantly correlated with the higher content of cell wall-bound phenolics. The dry mass of plants was markedly lower in the T1 treatment and was correlated with a higher content of cell wall-bound phenolics. Moreover, cultivars subjected to the T1 treatment showed a significantly higher water content in comparison to the T2 treatment. The delay in the leaf rolling and the ageing of plants in the T1 treatment, which induced a higher level of cell wall-bound phenolics, was visual proof of the improvement in the water status of plants.Phenolic compounds that form cross-bridges with carbohydrates of the cell wall can be considered a more effective biochemical protective mechanism than free phenolics during the dehydration of leaves. This potentially higher level of effectiveness is likely the result of the double action of phenolic compounds, both as photoprotectors of the photosynthetic apparatus and hydrophobic stabilizers, preventing water loss from the apoplast.

Formation of rudimentary spike and productivity of spring wheat varieties under increasing soil drought conditions

The reaction of spring wheat varieties during advance of soil drought is studied. Disparity of the critical period of ontogeny is shown and it is established that depression of variety productivity depends on the degree of rudimentary spike development upon reaching the permanent wilting point of the plants.

Influence of Seasonal Variations in Soil Water Availability on Gas Exchange of Tropical Canopy Trees

AbstractSeasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree‐based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions.

Physiological and antioxidant responses of Erica multiflora to drought and warming through different seasons

Climate change projections forecast a warming and an associated change in the distribution and intensity of rainfalls. In the case of the Mediterranean area, this will be reflected in more frequent and severe drought periods in the future. Within a long-term (9 years) manipulation experiment, we aimed to study the effect of the soil drought (15–20% decreased soil moisture) and warming conditions (+1°C warming) projected for the next decades onto photosynthetic rates and water relations, and onto the antioxidant and anti-stress defense capacity of Erica multiflora, a common species of the dry Mediterranean coastal scrublands, in two different seasons, spring and summer. Results indicated that none of the applied treatments was severe enough to induce a pronounced negative response of photosynthesis in this species well adapted to harsh Mediterranean conditions, but also highlighted important seasonal differences. Photosynthesis was limited by photoinhibition in spring and by stomatal closure in summer. Isoprenoid emission and the level of enzymatic and non-enzymatic antioxidants were lower in summer than in spring, whereas pigment and total phenolic contents were generally higher in summer. Volatile isoprenoid emissions were largely inhibited by drought and were not stimulated by warming. Drought and warming increased the oxidation state of ascorbate and reduced total glutathione levels in spring, but not in summer. It is concluded that E. multiflora plants can adapt to prolonged drought and warming, at least at the level simulated by our manipulative experiment, through changes in the seasonal physiological and antioxidant response of leaves.

Drought effects on water relations in beech: The contribution of exchangeable water reservoirs

The exceptional soil drought and heat wave that occurred in Europe in summer 2003 provided a good opportunity to analyze the response of forest ecosystems to extreme climatic conditions. This work aimed at studying the functioning of exchangeable water reservoirs of beech trees under climatic and edaphic constraints, in a 37-year-old beech stand located in north-eastern France. We characterised the impact of drought on seasonal variations in water fluxes at the tree (sap flow measurements) and the forest (eddy covariance measurements) scales and estimated (i) the daily water storage capacity of beech trees as the difference between stand-scaled sap flow and water vapour flux over the stand and (ii) the contribution of exchangeable water in tree reservoirs to the total tree transpiration, under non-limiting soil water (summer 2002) or severe soil drought conditions (summer 2003). In parallel, daily variations in trunk circumference were analyzed to estimate the contribution of elastic tissues as a compartment for water storage to tree transpiration. Stand transpiration was strongly reduced by soil water shortage (up to 80% at the peak of drought in August 2003). From the beginning of August 2003, we observed daytime contraction of stem circumference, with only partial, or even no night recovery, suggesting that trees were less and less able to refill the elastic and strongly depleted reservoirs. Even if those elastic reservoirs were active throughout the season, the corresponding volume of water withdrawn for tree transpiration remained very low (maximum 1% of the daily transpiration). Thus, elastic tissue reservoirs play a minor role in the total water budget of beech trees. The amount of water depleted daily from the whole reservoirs was much higher than the water extracted from elastic tissues. Furthermore, the contribution of total exchangeable water within trees to the transpiration stream increased during the dry period (from a few percent to 67% at the peak of the drought) and was positively correlated with soil water shortage. Our results thus clearly demonstrated the strong sensitivity of beech to both climate and drought and the major role of whole tree water reservoirs to maintain leaf transpiration under severe drought.

Change of Zinc Forms in Rhizosphere and Nonrhizosphere Soils of Maize (Zea mays L.) Plants as Influenced by Soil Drought Condition

A rhizobox experiment was conducted to study the changes of various zinc (Zn) forms in rhizosphere and nonrhizosphere soils of maize (Zea mays L.) plants grown under well-watered and drought conditions. The tested soil was earth-cumulic orthic anthrosol sampled from the Shaanxi Province of China. The experiment was set at two levels of Zn, 0 and 5.0 mg Zn kg−1 soil, and at two treatments of soil water content, 45%–50% (drought) and 70%–75% (well watered) of soil water-holding capacity. A completely randomized factorial design (2 Zn treatments × 2 water levels × 3 replicates) was set up. Adequate soil water supply enhanced growth and Zn accumulation of maize plants. Applying Zn increased plant biomass and Zn content more notably under well-watered conditions rather than drought conditions. Soil Zn was defined as water-soluble plus exchangeable (WSEXC) Zn, carbonate-bound Zn (CA), iron–manganese oxide–bound Zn (FeMnOX), organic matter–bound Zn (OM), and residual Zn (RES) forms using the sequential extraction procedure. Most of Zn was predominantly in the RES fraction. Zinc application increased the contents of WSEXC Zn, CA Zn, and FeMnOX Zn in soil. When Zn was added to the soil, the concentrations of CA Zn within 0–2 mm and 0–4 mm apart from the central root compartment (CC) were greater than other zones under the conditions of adequate and limited soil water supplies, respectively. Zinc application also resulted in an accumulation of FeMnOX fractions at a distance of 2 mm from CC. The FeMnOX Zn content in this compartment increased with soil drought. Under well-watered conditions, dry-matter weight and Zn concentration of shoots presented better correlations with CA Zn and FeMnOX Zn fractions in and near the rhizosphere as compared with drought conditions. It is suggested that in an earth-cumulic orthic anthrosol, soil moisture conditions affect the transformation of the added Zn into the CA and FeMnOX fractions near the rhizosphere and their bioavailability to maize plants.

Physiological acclimation of two psammophytes to repeated soil drought and rewatering

To understand physiological acclimation of psammophyte to repeated soil drought and rewatering, two psammophytes (Setaria viridis and Digitaria ciliaris) were subjected to three cycles of soil drought and rewatering. The response process of leaf relative water content (RWC), membrane permeability, lipid peroxidation, gas exchange characteristics, antioxidant enzymes, soluble protein, and free proline was examined. Leaf RWC, the net photosynthesis rate, stomatal conductance, and water use efficiency decreased, while membrane permeability, lipid peroxidation, intercellular CO2 concentration, soluble protein, and free proline increased during three soil drought periods for both psammophytes. These physiological characteristics were recovered to the control levels following rewatering for 4 days. However, activities of SOD, CAT, and POD were induced continuously under soil drought conditions, and remained higher than those in the control throughout the whole experiment period, which agrees with our hypothesis that drought hardening activates defensive systems of both psammophytes continuously. Decreasing level of leaf RWC and increasing levels of leaf membrane permeability and lipid peroxidation were suppressed with increasing the number of drought cycles, suggesting that drought hardening alleviates damages of both psammophytes and improves their drought tolerance and acclimation to soil drought conditions in the future. Additionally, the photosynthesis decreased more slowly in the subsequent drought cycles than in the first cycle, allowing both psammophytes to maximize assimilation in response to repeated soil drought conditions. Thus, both psammophytes acclimatize themselves to repeated soil drought.

Poplar vulnerability to xylem cavitation acclimates to drier soil conditions

Xylem vulnerability to cavitation differs between tree species according to their drought resistance, more xerophilous species being more resistant to xylem cavitation. Variability in xylem vulnerability to cavitation is also found within species, especially between in situ populations. The origin of this variability has not been clearly identified. Here we analyzed the response of xylem hydraulic traits of Populus tremula x Populus alba trees to three different soil water regimes. Stem xylem vulnerability was scored as the xylem water potential causing 12, 50 and 88% loss of conductivity (P(12), P(50) and P(88)). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P(50) values of -1.82, -2.03 and -2.45 MPa in well-watered, moderately water-stressed and severely water-stressed poplars, respectively. The value of P(12), the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (psi m) experienced by each plant, the difference between the two parameters being approximately 0.5 MPa, consistent with the absence of any difference in embolism level between the different water treatments. These results support the hypothesis that vulnerability to cavitation is a critical trait for resistance to drought. The decrease in vulnerability to cavitation under growing conditions of soil drought was correlated with decreased vessel diameter, increased vessel wall thickness and a stronger bordered pit field (t/b)(2). The links between these parameters are discussed.

Seed Germination and Seedling Emergence of Scotch Broom (Cytisus scoparius)

Scotch broom is a large, leguminous shrub that has invaded 27 U.S. states. The species produces seeds with a hard coat that remain viable in the soil for years. Growth-chamber studies were conducted to determine effects of temperature regime and cold-stratification period on seed germination. Seedling emergence, mortality, and biomass also were studied in response to sulfometuron and metsulfuron herbicides and variation in soil texture and watering regime. Germination was greatest for a dark/light temperature regime of 15/20 C. Initial rates of germination increased as stratification period was varied from 0 to 60 d, but final germination after 90 d did not differ significantly among periods. Applied alone or in combination, sulfometuron and metsulfuron decreased biomass and increased mortality of seedlings. Mortality from simulated soil drought was greater in the presence versus absence of sulfometuron (20 and 6% mortality, respectively) probably because the herbicide reduced root biomass by 58 to 95%. Invasiveness of Scotch broom is facilitated by a prolonged period of germination across a broad temperature range. Increased control of Scotch broom seedlings with sulfometuron is likely if application is timed to expose recently emerged seedlings to developing conditions of soil drought.

Net ecosystem exchange and whole plant isoprenoid emissions by a mediterranean shrubland exposed to experimental climate change

We tested the effect of forecasted soil drought and warming climate conditions for the next decades on emission rates of isoprenoids by mediterranean shrublands. We measured isoprenoid emissions by whole dominant mediterranean woody plants (Erica multiflora L. and Globularia alypum L.) inhabiting the studied shrublands. Monoterpene emissions were detected in both species, but isoprene was emitted only by E. multiflora. Maximum emission rates were found during the hottest periods (except for G. alypum, in which they occurred in autumn), and minimum emission rates in winter in E. multiflora. Terpene emission rates ranged from 0.08 μg/(g dry wt h) in winter in E. multiflora to 8.8 μg/(g dry wt h) in G. alypum in autumn. In E. multiflora, the terpene emission rates decreased in response to soil drought only in summer, but increased in response to warming in spring and autumn. Isoprene emissions ranged from 0.1 μg/(g dry wt h) in spring to 4.4 μg/(g dry wt h) in summer. The effect of the treatments was only detected in autumn when soil drought and warming had a negative effect on isoprene emission rates. These data might improve our knowledge of isoprenoid emissions at the canopy level and in response to climate change, soil drought, or warming.

Starch-to-sugar conversion in wood parenchyma of field-growing Laurus nobilis plants: a component of the signal pathway for embolism repair?

The ability of stems of Laurus nobilis (L.) to refill embolised xylem conduits was studied in plants both at optimal water supply (W) and under conditions of soil drought inducing xylem pressures (Px) of -1.54 (S1) and -2.35 MPa (S2). Starch depolymerisation in wood parenchyma was measured as percentage of cells 'with high starch content' (HSC-cells) counted under a microscope. HSC-cells decreased during embolism and increased again in refilled stems. A direct relationship was found between percentage of HSC-cells and Px, with HSC-cells between 65 and 75% of the total at Px ≥ -0.6 MPa, at which recovery from PLC was recorded. At low transpiration, starch re-appeared in wood parenchyma cells but only in plants that showed diurnal stomatal opening (W- and S1-plants). In S2-plants showing diurnal stomatal closure and nocturnal opening with Px between -1.2 to -2.4 MPa, HSC-cells were only 25% and plants did not recover from PLC. This finding suggests that (i) the Px threshold for embolism repair was ≥ -0.6 MPa, and (ii) impeded phloem loading limits starch content in wood parenchyma and embolism repair. We conclude that starch depolymerisation acts as a signal to phloem unloading sugars to embolised conduits thus generating the necessary osmotic gradients driving refilling.

Contrasting Species‐Specific, Compound‐Specific, Seasonal, and Interannual Responses of Foliar Isoprenoid Emissions to Experimental Drought in a Mediterranean Shrubland

We aimed to test the effect of soil drought conditions projected by general circulation models and ecophysiological models for the next few decades on emission rates of isoprenoids by Mediterranean shrublands. We conducted a field experiment in which we generated soil drought (ca. 12%–20% decreased soil moisture) using automatically sliding curtains, and we measured foliar isoprenoid emissions in the three dominant species of the studied shrubland (Erica multiflora L., Globularia alypum L., and Pinus halepensis L.) in two different annual periods. Monoterpene emissions were detected in the three studied species, but isoprene emissions were significantly detected only in E. multiflora. Main volatile terpenes emitted by the three species were α‐pinene, β‐myrcene, limonene, and Δ3‐carene. In general, maximum isoprenoid emission rates were found in the hottest periods, and minimum emission rates were found in winter. Isoprene emissions in E. multiflora ranged between practically 0 μg g−1 (dry matter) h−1 in winter 2005 and 57 μg g−1 (dry matter) h−1 in summer 2003. Isoprene emissions were 75% lower during the dry second annual period, 2004–2005, than during the first year, 2003–2004. In E. multiflora, there was an overall decrease of 19% in isoprene emissions in response to soil drought. Conversely, monoterpene emissions increased by 26.4% in drought treatment. In G. alypum, there was an overall increase of 75% in terpene emissions in response to soil drought. In P. halepensis, drought treatment increased terpene emission rates by 156%. Drought treatment affected the emissions mainly in the hottest seasons, spring and summer. There were strong compound‐specific, species‐specific, interannual, and seasonal changes in the emission rates and in their response to the treatments. These data might help to improve prediction algorithms, inventories, and modeling of isoprenoid emissions and of their response to climate change (decreased isoprene emissions and increased monoterpene emissions under moderate or short‐term drought and decreased emissions under severe or long‐term drought), but the great variability highlights the difficulty of the task.

COMPARATIVE STUDY OF DROUGHT TOLERANCE AND TOLERANCE MECHANISMS IN WILD AND CULTIVATED DICHONDRA REPENS

Aims Dichondra repens is a warm-season lawn substitute that remains green for long periods and reproduces strongly. Dichondra repens forms short, dense cover and is important in conservation of water and soil. It retains its green color during winter to -8 ℃ with only slight leaf browning and is resistant to diseases and heavy metals contamination. Little is known about the drought tolerance of wild and cultivated D. repens. Our objectives were to determine the drought-tolerant characteristics of wild and cultivated D. repens and explore the physiological foundations for drought tolerance in both. Methods We watered wild and cultivated D. repens every 3, 7 and 15 days from March 1 to May 1, 2004 and made growth and development measurements after water treatment. As soil drought conditions developed, we measured changes in superoxide dismutase activity (SOD), peroxidase activity (POD), nitrate reductase activity, soluble sugar content, free proline content, soluble protein, nitrate and nitrite (NO2-/NO3-) content and DNA fragments of leaves. Important findings Under drought stress, the content of free proline, soluble sugar and NO2-/NO3- in wild and cultivated D. repens leaves was enhanced. Activities of SOD and POD were regulated, protecting leaf cells of wild and cultivated D. repens from oxidative damage caused by drought stress. There were marked differences in drought resistance between the wild and cultivated D. repens. Antioxidant enzyme activities and osmotic adjustment substance content in wild D. repens leaves were higher than in cultivated D. repens leaves, while damage to DNA of wild D. repens was less serious than in cultivated D. repens. NO2-/NO3- content and nitrate reductase activity in wild D. repens leaves were significantly higher than in cultivated D. repens. Maximum NO2-/NO3- content and nitrate reductase activity in wild D. repens leaves were 10 and 2.2 times larger, respectively, than that in cultivated D. repens leaves. Wild D. repens had more antioxidant response to drought stress than cultivated D. repens. Under drought stress, change of NO2-/NO3- content may be associated with endogenous NO concentrations, which probably cause the drought tolerance of wild D. repens to be greater than that of cultivated D. repens.

Characteristics of water relations in seedling of Machilus yunnanensis and Cinnamomum camphora under soil drought condition

The soil drought stress experiment in different durations (no watering within 3d, 6d, 9d, 11d individually) was conducted to study the drought-resistant capacity of one-year-old seedlings for the native tree species (Machilus yunnanensis) in Yunnan Province and the introduced tree species (Cinnamomum camphora). The leaf water potential, chlorophyll content, proline content and plasma membrane permeability for two species seedlings were measured in different soil drought conditions. The results showed that, on the 9th day of drought stress, the leaf water potential of two species decreased obviously, whereas the free proline content and plasma membrane permeability increased sharply. On the 11th day, the leaf water potential of C. camphora seedlings was lower than that of M. yunnanensis seedlings; the plasma membrane permeability in C. camphora seedling leaves increased much more than that in M. yunnanensis seedling leaves, which showed that the injury to the former by soil drought stress was more severe than that to the latter. The free proline content in M. yunnanensis seedling leaves continued to increase on the 11th day, but that in the C. camphora seedling leaves started to drop obviously, indicating that the reduction of osmotic regulation substance in C. camphora seedling leaves after the 11th day was unable to maintain the osmotic balance between the plasma system and its surroundings and the water loss occurred inevitably. Comprehensively, M. yunnanensis seedlings enhanced the drought-resistance in the course of soil drought stress by maintaining higher leaf water potential and by increasing osmotic regulation substance to promote cell plasma concentration and maintain membrane structure integrity so as to reduce water loss. The subordination function index evaluated with fuzzy mathematic theory also showed that the drought-resistant capacity of M. yunnanensis seedlings was stronger than that of C. camphora seedlings.

Investigation on water consumption characteristics and water use efficiency of poplar under soil water deficits on the Loess Plateau

The water consumption characteristics and water use efficiency (WUE) of poplar ( Populus simonii) were investigated under the condition of three different soil water regimes, which were 70%, 55% and 40% of field capacity (FC), respectively. The results showed that water potential, water content, photosynthetic rate, WUE of leaves, and plant growth rate decreased significantly with reduction in soil water content. At the level of adequate and medium soil water content, rapid growth and biomass accumulation in poplar occurred during May and June, whereas this phenomenon occurred only during May under severe soil drought condition. Total water consumption and biomass growth were the greatest under adequate soil water condition, decreased under medium soil drought condition, and lowest under severe soil drought condition. Total WUE was the highest under medium soil water condition and lowest under severe soil condition. Changing patterns of water consumption for daily rate, every 10-day rate, and month rate were quite different under these three soil water regimes. For all these three treatments, the highest monthly water consumption rate occurred during July and June. The highest water consumption over a 10-day period was during in the second 10 days of July, the first 10 days of July, and the last 10 days of June for these three treatments, respectively. The day for the highest water consumption in the medium and severe drought treatments occurred 1 or 2 months earlier than the adequate soil water treatment. The daily time for the greatest water consumption was different throughout the life span of poplar under these soil water levels. According to these results, we concluded that poplar did not have the characteristics of drought-resistance plants, and we do not recommend that this tree species be planted over a wider range of the Loess Plateau in China.

Effects of Soil Moisture on Gas Exchange, Partitioning of Fed 14CO2 and Stable Carbon Isotope Composition (δ13C) of Leymus chinensis Under Two Different Diurnal Temperature Variations

AbstractIn a pot experiment under controlled condition, Leymus chinensis was grown at five soil water contents and two diurnal temperature variation levels. The leaf relative water content of L. chinensis decreased under soil drought conditions, and increased at high diurnal temperature variation (30/20 °C). Severe and very severe soil drought remarkably reduced photosynthetic rate, stomatal conductance, transpiration rate and water use efficiency (WUE), especially at the low temperature variation. Severe drought mainly increased the specific radioactivity of 14C of sheaths, roots and rhizomes, but significantly decreased that of fed leaves and the not fully expanded leaves. Root percentage of total radioactivity remaining in the whole plant increased from 15 % at 30/25 °C to 28 % at 30/20 °C. Leaf carbon stable isotope composition (δ13C) increased with soil drought, ranged from −26 ‰ of the well‐watered to −24 ‰ of severe drought treatments. High diurnal temperature variation improved leaf water status, and increased partitioning of carbon to root and δ13C values, especially under severe soil drought conditions. In conclusion, higher diurnal temperature variation enhanced the resistance of the plant to drought.