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PubMed2004

A combination of the Arabidopsis DREB1A gene and stress-inducible rd29A promoter improved drought- and low-temperature stress tolerance in tobacco by gene transfer.

The transcription factor DREB1A/CBF3 specifically interacts with the dehydration responsive element (DRE/CRT) and induces expression of genes involved in environmental stress tolerance in Arabidopsis: Overexpression of DREB1A improved drought- and low-temperature stress tolerance in tobacco. The stress-inducible rd29A promoter minimized the negative effects on the plant growth in tobacco. Furthermore, we detected overexpression of stress-inducible target genes of DREB1A in tobacco. These results indicate that a combination of the rd29A promoter and DREB1A is useful for improvement of various kinds of transgenic plants that are tolerant to environmental stress.

Acclimatization↗
PubMed1994

Influence of water relations on Quercus cerris-Hypoxylon mediterraneum interaction: a model of drought-induced susceptibility to a weakness parasite.

The association between water stress and susceptibility of Quercus cerris to the fungal parasite Hypoxylon mediterraneum was studied in field-grown trees and greenhouse-grown seedlings. Susceptibility of Q. cerris to the fungus, expressed as extension of wood discolorations, increased in field-grown trees during drought and increased in greenhouse-grown seedlings in response to limited water supply. In both seedlings and trees, spread of the fungus in the vascular system was higher when leaf water potentials were low than when leaf water potentials were high, and was significantly correlated to loss of hydraulic conductivity of xylem. The presence of the fungus in the xylem suggests that H. mediterraneum could use embolized vessels to spread in the host.

Journal Article↗
PubMed1994

Electric resistance as a measure of tree water status during seasonal drought in a tropical dry forest in Costa Rica.

Variation in electric resistance of stem tissues was used to measure differences and changes in water status among trees in a tropical dry forest in Costa Rica during the dry season. For more than 30 tree species, stem water content (SWC), measured as electric resistance between nails driven 20 mm deep into tree trunks, correlated well with wood density, saturation water content, dehydration, measured with the pressure chamber, and tree development during drought. At dry sites, SWC was lowest in hardwood trees (characterized by high wood density) and highest in stem-succulent lightwood trees (characterized by low wood density). Among hardwood trees, SWC varied with soil water availability. During the dry season, SWC declined before leaf shedding and increased during rehydration preceding bud break. The time course of seasonal changes in SWC apparently constitutes an indirect measure of variation in the relative water content of outer stem tissues, which determines development of dry-forest trees during the dry season.

Journal Article↗
PubMed1996

Carbon dioxide enrichment improves growth, water relations and survival of droughted honey mesquite (Prosopis glandulosa) seedlings.

Low water availability reduces the establishment of the invasive shrub Prosopis on some grasslands. Water deficit survival and traits that may contribute to the postponement or tolerance of plant dehydration were measured on seedlings of P. glandulosa Torr. var. glandulosa (honey mesquite) grown at CO(2) concentrations of 370 (ambient), 710, and 1050 micro mol mol(-1). Because elevated CO(2) decreases stomatal conductance, the number of seedlings per container in the elevated CO(2) treatments was increased to ensure that soil water content was depleted at similar rates in all treatments. Seedlings grown at elevated CO(2) had a greater root biomass and a higher ratio of lateral root to total root biomass than those grown at ambient CO(2) concentration; however, these seedlings also shed more leaves and retained smaller leaves. These changes, together with a reduced transpiration/leaf area ratio at elevated CO(2), may have contributed to a slight increase in xylem pressure potentials of seedlings in the 1050 micro mol mol(-1) CO(2) treatment during the first 37 days of growth (0.26 to 0.40 MPa). Osmotic potential was not affected by CO(2) treatment. Increasing the CO(2) concentration to 710 and 1050 micro mol mol(-1) more than doubled the percentage survival of seedlings from which water was withheld for 65 days. Carbon dioxide enrichment significantly increased survival from 0% to about 40% among seedlings that experienced the lowest soil water content. By increasing seedling survival of drought, rising atmospheric CO(2) concentration may increase abundance of P. glandulosa on grasslands where low water availability limits its establishment.

Journal Article↗
PubMed1996

Leaf water relations of Eucalyptus globulus ssp. globulus and E. nitens: seasonal, drought and species effects.

In August 1990, a 2-ha plantation was established in an area where rainfall (about 515 mm year(-1)) was insufficient to meet evaporative demand. On nine occasions between September 1991 and April 1993, pressure-volume curves were constructed for irrigated and rainfed Eucalyptus globulus ssp. globulus Labill. and E. nitens (Deane and Maiden) Maiden trees. During the experiment, rainfed trees experienced six periods when predawn water potential was significantly lower than that of irrigated trees. In early spring of 1991 and 1992, osmotic potentials at full turgor and turgor loss point in the irrigated E. nitens were significantly lower than at other times of the year, probably because of winter hardening. Water stress reduced osmotic potential and increased bulk elastic modulus in E. nitens, whereas the reverse occurred in E. globulus. However, treatment differences with respect to changes in osmotic and elastic properties were commonly overshadowed by interspecific differences. These were most apparent at the end of the sixth period of water stress when osmotic potentials at full and zero turgor were significantly higher and bulk elastic modulus and relative water content at turgor loss point were significantly lower in E. globulus than in E. nitens. We conclude that the drought-tolerance responses of E. globulus make it a more suitable species than E. nitens for establishment on sites where moderate water stress is experienced.

Journal Article↗
PubMed1996

Root growth potential as an indicator of drought stress history.

Container-grown quiescent Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) seedlings were air dried to plant water potentials of -0.2, -2.2 or -3.8 MPa (unstressed, moderate, and severe stress treatments, respectively). Trees from each treatment were either placed in root mist chambers held at 10, 20, or 28 degrees C for 28 days and root growth potential (RGP) and plant water potential (PWP) measured weekly, or potted in a 1/1 mix of peat and vermiculite, watered only once, and height growth and survival recorded after 10 weeks in an unheated greenhouse. Root growth potential of unstressed trees was greater than that of moderately stressed trees at all temperatures. Root growth potential of severely stressed trees was zero. Predawn plant water potentials of unstressed and moderately stressed trees were initially high, fell to -0.5 to -0.8 MPa, and then increased. Predawn plant water potential of severely stressed trees declined continuously over the 28-day experiment. Survival and height growth of the severely stressed trees were reduced compared to the unstressed and moderately stressed trees. Among the root growth potential measurements, RGP measured after 7 days at 10 degrees C was most sensitive to drought stress history and revealed differences in vigor that were not apparent from the survival and height growth data.

Journal Article↗
PubMedDate not supplied

Fine roots dynamics in a Mediterranean forest: effects of drought and stem density.

Summer drought is a characteristic trait of Mediterranean climates and can limit primary production. The holm oak (Quercus ilex L.) forest at the Prades Experimental Complex of Catchments (PECC, NE Spain) was managed by clearcutting until the 1950s. As a result of this practice, the forest has a high density (13000 stems ha(-1)) that is causing growth stagnation and intense competition for water. A thinning experiment was conducted to release resources, reduce stem competition and improve forest productivity. Minirhizotrons were installed in two plots in each of the thinned and control stands and fine root dynamics quantified to a soil depth of 60 cm over a 2-year period (June 1994-May 1996). Soil water content was higher and soil water penetrated deeper in the soil (30-50 cm) in the thinned plots than in the control plots. In the control plots, soil temperature did not vary substantially with depth (mean = 11.65 degrees C), whereas soil temperature was higher in shallow horizons compared with deep horizons in the thinned plots. Mean soil temperature was one degree higher in the thinned plots than in the control plots (12.9 versus 11.9 degrees C). There were always more roots in the top 20 cm of soil in thinned plots than in control plots. In the thinned plots, more than 50% of root density (root no. cm(-2)) was located in the top 20 cm of soil, whereas root density never reached 50% in the 0-20 cm soil layer of the control plots. Roots longevity varied from 35 to 471 days, and there were significant differences between soil depths. Root longevity was greater in the control plots than in the thinned plots.

Journal Article↗
PubMed1999

Predisposition of trees to drought stress by ozone.

This paper reviews research on the effects of ozone on stomatal behavior, and draws attention to the direct link between stomatal function and air pollution. Contradictory results concerning the effects of air pollutants on stomatal activity are attributed to the use of univariate statistical treatments of data. Alternative experimental methods that subject stomata to a functional test are described. These methods verify the hypothesis that ozone predisposes trees to drought stress. It is proposed that ozone directly attacks the walls of guard and subsidiary cells, leading to impairment of the sensory mechanism of the stomata.

Journal Article↗
PubMed2001

Effects of thinning on wood production, leaf area index, transpiration and canopy interception of a plantation subject to drought.

We conducted thinning trials in a 5-year-old Eucalyptus globulus ssp. globulus Labill plantation near Warrenbayne, northeastern Victoria, Australia, where soil salinization and waterlogging are common, and assessed treatment effects on tree growth, water use and survival. Half-hectare plots were thinned from the original density of 1100 stems ha(-1) to densities of 800, 600 and 400 stems ha(-1), and stem diameter increment, leaf area index, transpiration, canopy interception and depth of tree water source monitored for 21 months. Two drought periods occurred during the study, rainfall was 30% below the long-term average and there was severe mortality in all three plots. Analysis of deuterium abundance in soil and xylem water indicated that the trees accessed water only from the top meter of the soil profile. Transpiration rates were higher in the most heavily thinned plot than in the least thinned plot, which underwent a reduction in basal area during the study. The most heavily thinned plot increased in basal area by 10% during the study. Edge trees had significantly greater diameters than trees from the middle of the plots.

Disasters↗
PubMed2003

Seasonal trends in photosynthetic parameters and stomatal conductance of blue oak (Quercus douglasii) under prolonged summer drought and high temperature.

Understanding seasonal changes in photosynthetic parameters and stomatal conductance is crucial for modeling long-term carbon uptake and energy fluxes of ecosystems. Gas exchange measurements of CO2 and light response curves on blue oak leaves (Quercus douglasii H. & A.) were conducted weekly throughout the growing season to study the seasonality of photosynthetic capacity (Vcmax) and Ball-Berry slope (m) under prolonged summer drought and high temperature. A leaf photosynthetic model was used to determine Vcmax. There was a pronounced seasonal pattern in Vcmax. The maximum value of Vcmax, 127 micromol m(-2) s(-1), was reached shortly after leaf expansion in early summer, when air temperature was moderate and soil water availability was high. Thereafter, Vcmax declined as the soil water profile became depleted and the trees experienced extreme air temperatures, exceeding 40 degrees C. The decline in Vcmax was gradual in midsummer, however, despite extremely low predawn leaf water potentials (Psipd, approximately -4.0 MPa). Overall, temporal changes in Vcmax were well correlated with changes in leaf nitrogen content. During spring leaf development, high rates of leaf dark respiration (Rd, 5-6 micromol m(-2) s(-1)) were observed. Once a leaf reached maturity, Rd remained low, around 0.5 micromol m(-2) s(-1). In contrast to the strong seasonality of Vcmax, m and marginal water cost per unit carbon gain (partial partial differential E/ partial partial differential A) were relatively constant over the season, even when leaf Psipd dropped to -6.8 MPa. The constancy of partial partial differential E/ partial partial differential A suggests that stomata behaved optimally under severe water-stress conditions. We discuss the implications of our findings in the context of modeling carbon and water vapor exchange between ecosystems and the atmosphere.

California↗
PubMed2004

Ozone uptake, water loss and carbon exchange dynamics in annually drought-stressed Pinus ponderosa forests: measured trends and parameters for uptake modeling.

This paper describes 3 years of physiological measurements on ponderosa pine (Pinus ponderosa Dougl. ex Laws.) growing along an ozone concentration gradient in the Sierra Nevada, California, including variables necessary to parameterize, validate and modify photosynthesis and stomatal conductance algorithms used to estimate ozone uptake. At all sites, gas exchange was under tight stomatal control during the growing season. Stomatal conductance was strongly correlated with leaf water potential (R2=0.82), which decreased over the growing season with decreasing soil water content (R2=0.60). Ozone uptake, carbon uptake, and transpirational water loss closely followed the dynamics of stomatal conductance. Peak ozone and CO2 uptake occurred in early summer and declined progressively thereafter. As a result, periods of maximum ozone uptake did not correspond to periods of peak ozone concentration, underscoring the inappropriateness of using current metrics based on concentration (e.g., SUM0, W126 and AOT40) for assessing ozone exposure risk to plants in this climate region. Both Jmax (maximum CO2-saturated photosynthetic rate, limited by electron transport) and Vcmax (maximum rate of Rubisco-limited carboxylation) increased toward the middle of the growing season, then decreased in September. Intrinsic water-use efficiency rose with increasing drought stress, as expected. The ratio of Jmax to Vcmax was similar to literature values of 2.0. Nighttime respiration followed a Q10 of 2.0, but was significantly higher at the high-ozone site. Respiration rates decreased by the end of the summer as a result of decreased metabolic activity and carbon stores.

California↗
PubMed2004

A comparison of three approaches to modeling leaf gas exchange in annually drought-stressed ponderosa pine forests.

We tested, compared and modified three models of stomatal conductance at the leaf level in a forest ecosystem where drought stress is a major factor controlling forest productivity. The models were tested against 2 years (1999 and 2000) of leaf-level measurements on ponderosa pine (Pinus ponderosa Dougl. ex Laws.) growing in the Mediterranean climate of California, USA. The Ball, Woodrow and Berry (1987) (BWB) model was modified to account for soil water stress. Among the models, results of the modified BWB model were in the closest agreement with observations (r2 = 0.71). The Jarvis (1976) model showed systematic simulation errors related to vapor pressure deficit (r2 = 0.65). Results of the Williams, Rastetter, Fernandes et al. (1996) (SPA) model showed the poorest correlation with empirical data, but this model has only one calibration parameter (r2 = 0.60). Sensitivity analyses showed that, in all three models, predictions of stomatal conductance were most responsive to photosynthetically active radiation and soil water content. Stomatal conductance showed little sensitivity to vapor pressure deficit in the Jarvis model, whereas in both the BWB and SPA models, vapor pressure deficit (or relative humidity) was the third most important variable. Parameterization of the SPA model was in accordance with the parameterization of the modified BWB model, although the two models differ greatly. Measured and modeled results indicate that stomatal behavior is not water conservative during spring; however, during summer, when soil water content is low and vapor pressure deficit is high, stomatal conductance decreases and, according to the models, intrinsic water- use efficiency increases.

Climate↗
PubMed2006

Modeling photosynthesis in olive leaves under drought conditions.

We quantified parameters for a model of leaf-level photosynthesis for olive, and tested the model against an independent dataset. Specific temperature-dependence parameters of the model for olive leaves were measured, as well as the relationship of the model parameters with area-based leaf nitrogen (N) content. The effect of soil water deficit on leaf photosynthesis was examined by applying two irrigation treatments to 29-year-old trees growing in a plantation: drip irrigation sufficient to meet the crop water requirements (I) and dry-farming (D). In both treatments, leaves had a higher photosynthetic capacity in April than in August. In August, photosynthetic capacity was lower in D trees than in I trees. Leaf photosynthetic capacity was linearly and positively related to leaf N content on an area basis (N(a)) and to leaf mass per unit area (LMA), and the regression slope varied with irrigation treatment. The seasonal reduction in N(a) was used in the model to predict photosynthesis under drought conditions. Olive leaves showed a clear limitation of photosynthesis by triose phosphate utilization (TPU) even at 40 degrees C, and the data suggest that olive invests fewer resources in TPU than other species. The seasonal decrease in photosynthetic capacity moderated the stomatal limitation to carbon dioxide (CO(2)) fixation as soil water deficit increased. Further, it enabled leaves to operate close to the transition point between photosynthetic limitation due to RuBP carboxylation capacity and that due to RuBP regeneration capacity, and resulted in a near constant value of internal CO(2) concentration from April to August. Under well watered conditions, N-use efficiency of the olive leaves was enhanced at the expense of reduced water-use efficiency.

Climate↗
PubMed1988

Drought tolerance, growth partitioning and vigor in eucalypt seedlings and rooted cuttings.

To clarify the physiological basis of productivity differences among rooted cuttings and seedlings of eucalypt species, relationships between morphology and water relations were examined in 4-month-old seedlings of Eucalyptus grandis W. Hill ex Maiden, E. urophylla S.T. Blake and E. cloeziana F. Muell. and in 4-month-old rooted cuttings of three E. grandis cultivars. Four-month-old seedlings had greater dry weights, lower leaf area/root dry weight (LA/RDW) ratios and lower shoot/root dry weight (S/R) ratios than 4-month-old rooted cuttings. For all cultivars of E. grandis, tall rooted cuttings, as defined by height at age 4 weeks, had greater dry weights by age 4 months and lower LA/RDW and S/R ratios than short rooted cuttings. There were differences in height growth, dry matter productivity and relative shoot and root development among cuttings of different E. grandis cultivars, but these differences were not as great as the differences between short and tall grades of the same cultivar and between seedlings and cuttings. Consistent with the differences in LA/RDW and S/R ratios, seedlings had higher daytime water potentials (Psi(x)) than cuttings, and tall cuttings had higher daytime values of Psi(x) than short cuttings. Differences in Psi(x) were also related to stomatal conductance (g(wv)), which was up to 300% greater in short cuttings than in tall cuttings. Among seedlings, those of E. cloeziana, which had the smallest dry weight at age 4 months, had the highest g(wv), whereas those of E. grandis, which had the greatest dry weight at age 4 months, had the lowest g(wv). Unlike seedlings and the tall cuttings, short cuttings lost turgor when subjected to drought. The differences observed in susceptibility to water stress may account in part for the associated differences in dry matter production. Xylem pressure potential and relative water deficit at zero turgor did not differ significantly among the types of plants studied, which suggests that differences in growth rates were not the result of differences in dehydration tolerance.

Journal Article↗
PubMed1990

Gas exchange in Quercus rubra (northern red oak) during a drought: analysis of relations among photosynthesis, transpiration, and leaf conductance.

Development of water stress in leaves of mature Quercus rubra L. caused a marked midday depression in photosynthesis (A) and transpiration (E). At external CO(2) partial pressures of 100-110 Pa, a constant temperature of 30 degrees C and a constant photosynthetic photon flux density of about 1000 micromol m(-2) s(-1), A was 8 micromol m(-2) at low leaf water potentials (-1.5 to -2.0 MPa), whereas it was 20 micromol m(-2) s(-1) in non-stressed leaves (-1.0 MPa). At lower external CO(2) partial pressures, the effect of low leaf water potential on A was less. The midday depression in gas exchange was relieved by an overnight rain of 2.5 cm. No difference in carboxylation efficiency or CO(2) compensation point was found between leaves before and after rain, The relationship between A and E was linear for a given external CO(2) partial pressure, but the slope varied with CO(2) concentration. Modification of the model of stomatal response proposed by Ball et al. (1987) produced a linear relationship between leaf conductance and a factor incorporating A, relative humidity, and CO(2). The data indicate that gas exchange in leaves of mature northern red oak respond rapidly to relief of drought with no indication of long-term photoinhibition.

Journal Article↗
PubMed2004

The effects of a severe drought on mouflon lamb survival.

The mouflon population of Caroux-Espinouse, southern France, inhabits a highly seasonal area with dry summers. We monitored summer lamb survival during a severe drought in 2003, from early June to late August. The survival of 35 radio-tagged lambs over nine two-week periods was strongly affected by the timing of rainfall. Survival depended on the amount of rainfall recorded at a given 14 day period and in the previous 14-21 day period. Survival was not influenced by the exceptionally high mean daily temperature recorded during some periods. Male lamb survival (0.68) tended to be less than female survival (0.81), although not significantly, possibly because of a low sample size. The high lamb mortality (25.7%) recorded during a four-month period is much higher than previous estimates of first-year mortality (less than 10%). We recommend accounting for climatic variation in summer when studying the population dynamics of ungulates.

Analysis of Variance↗
PubMed2002

Overexpression of a 9-cis-epoxycarotenoid dioxygenase gene in Nicotiana plumbaginifolia increases abscisic acid and phaseic acid levels and enhances drought tolerance.

The plant hormone abscisic acid (ABA) plays important roles in seed maturation and dormancy and in adaptation to a variety of environmental stresses. An effort to engineer plants with elevated ABA levels and subsequent stress tolerance is focused on the genetic manipulation of the cleavage reaction. It has been shown in bean (Phaseolus vulgaris) that the gene encoding the cleavage enzyme (PvNCED1) is up-regulated by water stress, preceding accumulation of ABA. Transgenic wild tobacco (Nicotiana plumbaginifolia Viv.) plants were produced that overexpress the PvNCED1 gene either constitutively or in an inducible manner. The constitutive expression of PvNCED1 resulted in an increase in ABA and its catabolite, phaseic acid (PA). When the PvNCED1 gene was driven by the dexamethasone (DEX)-inducible promoter, a transient induction of PvNCED1 message and accumulation of ABA and PA were observed in different lines after application of DEX. Accumulation of ABA started to level off after 6 h, whereas the PA level continued to increase. In the presence of DEX, seeds from homozygous transgenic line TN1 showed a 4-d delay in germination. After spraying with DEX, the detached leaves from line TN1 had a drastic decrease in their water loss relative to control leaves. These plants also showed a marked increase in their tolerance to drought stress. These results indicate that it is possible to manipulate ABA levels in plants by overexpressing the key regulatory gene in ABA biosynthesis and that stress tolerance can be improved by increasing ABA levels.

Abscisic Acid↗
PubMed2003

Drought-induced changes in the redox state of alpha-tocopherol, ascorbate, and the diterpene carnosic acid in chloroplasts of Labiatae species differing in carnosic acid contents.

To assess antioxidative protection by carnosic acid (CA) in combination with that of other low-molecular weight (M(r)) antioxidants (alpha-tocopherol [alpha-T] and ascorbate [Asc]) in chloroplasts, we measured endogenous concentrations of these antioxidants, their redox states, and other indicators of oxidative stress in chloroplasts of three Labiatae species, differing in their CA contents, exposed to drought stress in the field. Damage to the photosynthetic apparatus was observed neither in CA-containing species (rosemary [Rosmarinus officinalis]) and sage [Salvia officinalis]) nor in CA-free species (lemon balm [Melissa officinalis]) at relative leaf water contents between 86% and 58%, as indicated by constant maximum efficiency of photosystem II photochemistry ratios and malondialdehyde levels in chloroplasts. The three species showed significant increases in alpha-T, a shift of the redox state of alpha-T toward its reduced state, and increased Asc levels in chloroplasts under stress. Lemon balm showed the highest increases in alpha-T and Asc in chloroplasts under stress, which might compensate for the lack of CA. Besides, whereas in rosemary and sage, the redox state of CA was shifted toward its oxidized state and the redox state of Asc was kept constant, lemon balm displayed a shift of the redox state of Asc toward its oxidized state under stress. In vitro experiments showed that both CA and Asc protect alpha-T and photosynthetic membranes against oxidative damage. These results are consistent with the contention that CA, in combination with other low-M(r) antioxidants, helps to prevent oxidative damage in chloroplasts of water-stressed plants, and they show functional interdependence among different low-M(r) antioxidants in chloroplasts.

Abietanes↗