Regulation of transpirational water loss in Quercus suber trees in a Mediterranean-type ecosystem

Sap flux density in branches, leaf transpiration, stomatal conductance and leaf water potentials were measured in 16-year-old Quercus suber L. trees growing in a plantation in southern Portugal to understand how evergreen Mediterranean trees regulate water loss during summer drought. Leaf specific hydraulic conductance and leaf gas exchange were monitored during the progressive summer drought to establish how changes along the hydraulic pathway influence shoot responses. As soil water became limiting, leaf water potential, stomatal conductance and leaf transpiration declined significantly. Predawn leaf water potential reflected soil water potential measured at 1-m depth in the rhizospheres of most trees. The lowest predawn leaf water potential recorded during this period was -1.8 MPa. Mean maximum stomatal conductance declined from 300 to 50 mmol m(-2) s(-1), reducing transpiration from 6 to 2 mmol m(-2) s(-1). Changes in leaf gas exchange were attributed to reduced soil water availability, increased resistances along the hydraulic pathway and, hence, reduced leaf water supply. There was a strong coupling between changes in soil water content and stomatal conductance as well as between stomatal conductance and leaf specific hydraulic conductance. Despite significant seasonal differences among trees in predawn leaf water potential, stomatal conductance, leaf transpiration and leaf specific hydraulic conductance, there were no differences in midday leaf water potentials. The strong regulation of changes in leaf water potential in Q. suber both diurnally and seasonally is achieved through stomatal closure, which is sensitive to changes in both liquid and vapor phase conductance. This sensitivity allows for optimization of carbon and water resource use without compromising the root-shoot hydraulic link.     

Differential response to soil drought among co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand

? Introduction

In the context of global environmental changes, better understanding of tree response to soil drought in young mixed species stands is needed to anticipate forest adaptation and management practices for the future.

? Materials and methods

We investigated the functional response of five co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand in northeastern France during the 2006 summer drought. We measured functional traits related to water acquisition (phenology, rooting pattern and vulnerability of xylem to cavitation) and the ecophysiological response (sap flux density, leaf water potential) of these species to soil water shortage.

? Results

Our study highlights contrasted drought response strategies among these species and a trade-off between leaf phenology, resistance of xylem to cavitation and root system depth.

? Conclusion

At this site, a deep root system seemed to be a key functional trait for the species to cope with drought. Quercus robur and Salix capreae can be characterised as drought-avoidance species as they possess a deep root system and therefore did not strongly experience soil drought. Despite deep rooting capacity, Betula pendula did not really avoid soil drought and strongly regulated transpiration during dry periods. Nevertheless, the earliness of budburst of this species contributes to high annual growth rate. In contrast, Carpinus betulus and Fagus sylvatica both displayed typical characteristics of drought-sensitive species.     

Functional convergence in hydraulic architecture and water relations of tropical savanna trees: from leaf to whole plant

Functional convergence in hydraulic architecture and water relations, and potential trade-offs in resource allocation were investigated in six dominant neotropical savanna tree species from central Brazil during the peak of the dry season. Common relationships between wood density and several aspects of plant water relations and hydraulic architecture were observed. All species and individuals shared the same negative exponential relationship between sapwood saturated water content and wood density. Wood density was a good predictor of minimum (midday) leaf water potential and total daily transpiration, both of which decreased linearly with increasing wood density for all individuals and species. With respect to hydraulic architecture, specific and leaf-specific hydraulic conductivity decreased and the leaf:sapwood area ratio increased more than 5-fold as wood density increased from 0.37 to 0.71 g cm(-3) for all individuals and species. Wood density was also a good predictor of the temporal dynamics of water flow in stems, with the time of onset of sap flow in the morning and the maximum sap flow tending to occur progressively earlier in the day as wood density increased. Leaf properties associated with wood density included stomatal conductance, specific leaf area, and osmotic potential at the turgor loss point, which decreased linearly with increasing wood density. Wood density increased linearly with decreasing bulk soil water potential experienced by individual plants during the dry season, suggesting that wood density was greatest in individuals with mostly shallow roots, and therefore limited access to more abundant soil water at greater depths. Despite their taxonomic diversity and large intrapopulation differences in architectural traits, the six co-occurring species and their individuals shared similar functional relationships between all pairs of variables studied. Thus, rather than differing intrinsically in physiological responsiveness, the species and the individuals appeared to have distinct operating ranges along common physiological response curves dictated by plant architectural and structural features. The patterns of water uptake and access to soil water during the dry season appeared to be the main determinant of wood density, which constrained evolutionary options related to plant water economy and hydraulic architecture, leading to functional convergence in the neotropical savanna trees studied.     

Physiological variation among western redcedar (Thuja plicata Donn ex D. Don) populations in response to short-term drought

? Variation in the ability of western redcedar (Thuja plicata Donn ex D. Don) populations to withstand water stress may exist because this species is found in coastal and interior biogeoclimatic subzones representing the full range of precipitation regimes in British Columbia, Canada.

? Seven western redcedar populations from locations in British Columbia, representing a wide range of habitat types, were assessed for their gas exchange and water relations response to controlled drought.

? Before drought, population variation occurred in stomatal conductance, net CO₂ assimilation rate and intrinsic water use efficiency and the relative water content at turgor loss point. During drought, populations had different responses of net CO₂ assimilation to decreasing predawn shoot water potential. After drought, populations differed in stomatal conductance and intrinsic water use efficiency, plus osmotic potential at turgor loss point, osmotic potential at saturation and apparent cuticular transpiration. Western redcedar populations from drier-inland habitats had a lower osmotic potential at turgor loss point, lower relative water content at turgor loss point and lower apparent cuticular transpiration in response to drought than populations from coastal origin with temperate maritime habitat.

? Reduction of cuticular water loss and adjustments of cellular water relations in response to drought was found to occur among seven western redcedar populations originating along a precipitation gradient while; there were minimal population differences in the gas exchange response to drought.

     

Atmospheric and soil drought reduce nocturnal conductance in live oaks

Nocturnal and daytime whole-canopy transpiration rate (E) and conductance (g = E/VPD, where VPD is leaf to air vapor pressure difference) were assessed gravimetrically in drought-treated and well-watered 3-year-old saplings of live oak species (Quercus series Virentes Nixon) from the southeastern USA (Quercus virginiana Mill.) and Central America (Q. oleoides Cham. and Schlecter). Our objectives were to: (1) quantify nocturnal and daytime E and g in a controlled environment; (2) determine the impact of severe drought on nocturnal E and g; and (3) examine whether unavoidable water loss through the epidermis could account for nocturnal water loss. We calculated daytime E during peak daylight hours (between 0930 and 1330 h) and nocturnal E during complete darkness (between 2200 and 0500 h). In addition to reducing E and g during the daytime, drought-treated plants reduced nocturnal E and g on a whole-canopy basis by 62-64% and 59-61%, respectively, and on a leaf-level basis by 27-28% and 19-26%, respectively. In well-watered plants, nocturnal g declined with increasing VPD, providing evidence for stomatal regulation of nocturnal transpiration. In drought-treated plants, g was low and there was no relationship between nocturnal g and VPD, indicating that water loss could not be reduced further through stomatal regulation. Both daytime and nocturnal g declined curvilinearly with predawn water potential for all plants, but nocturnal g was unrelated to predawn water potentials below -1 MPa. The reductions in daytime and nocturnal E and g during drought were associated with decreases in whole-plant and leaf hydraulic conductances. Observed nocturnal g was within the same range as epidermal conductance for oak species determined in previous studies under a range of conditions. Nocturnal E rose from 6-8% of daytime E for well watered plants to 19-20% of daytime E for drought-treated plants. These results indicate that, during drought, saplings of live oak species reduce g to a minimum through stomatal closure, and experience unavoidable water loss through the epidermis.     

Environmental controls on sap flow in a northern hardwood forest

Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (D) and soil water interact to control transpiration in the dominant canopy species of a mixed hardwood forest in northern Lower Michigan. An improved understanding of how these environmental factors affect whole-tree water use in unmanaged ecosystems is necessary in assessing the consequences of climate change on the terrestrial water cycle. We used continuously heated sap flow sensors to measure transpiration in mature trees of four species during two successive drought events. The measurements were scaled to the stand level for comparison with eddy covariance estimates of ecosystem water flux (Fw). Photosynthetically active radiation and D together explained 82% of the daytime hourly variation in plot-level transpiration, and low soil water content generally resulted in increased stomatal sensitivity to increasing D. There were also species-specific responses to drought. Quercus rubra L. showed low water use during both dry and wet conditions, and during periods of high D. Among the study species, Acer rubrum L. showed the greatest degree of stomatal closure in response to low soil water availability. Moderate increases in stomatal sensitivity to D during dry periods were observed in Populus grandidentata Michx. and Betula papyrifera Marsh. Sap flow scaled to the plot level and Fw demonstrated similar temporal patterns of water loss suggesting that the mechanisms controlling sap flow of an individual tree also control ecosystem evapotranspiration. However, the absolute magnitude of scaled sap flow estimates was consistently lower than Fw. We conclude that species-specific responses to PAR, D and soil water content are key elements to understanding current and future water fluxes in this ecosystem.     

Adaptations to soil drying in woody seedlings of African locust bean, (Parkia biglobosa (Jacq.) Benth.)

Stomatal conductance, transpiration and xylem pressure potential of African locust bean (Parkia biglobosa (Jacq.) Benth.) seedlings subjected from the sixth week after emergence to four weeks of continuous soil drought did not differ from those of well-watered, control plants until two-thirds of the available soil water had been used. In both well-watered and drought-treated plants, stomatal conductance was highest early in the day when vapor pressure deficits were low, but decreased sharply by midday when evaporative demand reached its highest value. There was no increase in stomatal conductance later in the day as vapor pressure deficit declined. The relationship between transpiration rate and xylem pressure potential showed non-linearity and hysteresis in both control and drought-treated plants, which seems to indicate that the plants had a substantial capacity to store water. The rate of leaf extension in African locust bean seedlings subjected to six consecutive 2-week cycles of soil drought declined relative to that of well-watered, control plants, whereas relative root extension increased. It appears that African locust bean seedlings minimized the impact of drought by: (1) restricting transpiration to the early part of the day when a high ratio of carbon gain to water loss can be achieved; (2) utilizing internally stored water during periods of rapid transpiration; (3) reducing the rate of leaf expansion and final leaf size in response to soil drought without reducing the rate of root extension, thereby reducing the ratio of transpiring leaf surface area to absorbing root surface area.     

Species selection in hardwoods research: variations in baseline physiological responses of select temperate hardwood tree species

Drought periods are becoming more extreme worldwide and the ability of plants to contribute towards atmospheric flux is being compromised. Properly functioning stomata provide an exit for water that has been absorbed by the roots, funneled into various cell parts, and eventually released into the atmosphere via transpiration. By observing the effects that weather conditions such as climate change may have on stomatal density, distribution, and functioning, it may be possible to elucidate a portion of the mechanisms trees use to survive longer periods of water stress. This study analyzed stomatal density (SD), stomatal conductance (gs ), CO2 assimilation (A), instantaneous water-use efficiency (WUEi ), and transpiration (E) rates in six native tree species in the Midwestern USA and showed that trees within the same ecotype followed similar trends, but that trees within the same family did not when exposed to identical greenhouse conditions. Naturally drought tolerant tree species demonstrated lower g s and higher WUEi , while intolerant species had higher SD. This study showed negative or no correlation between SD and g s , A, E, and WUEi and positive correlations between E and A and gs and E.     

Disturbances in European beech water relation during an extreme drought

Context

In the context of a probable increase in intensity and frequency of extreme summer drought events, a better understanding of the key processes involved in water relations is needed to improve the theoretical foundations of predictive process-based models.

Aims

This paper aims to analyse how temperate deciduous trees cope with water shortage.

Methods

The exceptional summer drought of 2003 in Europe provided an opportunity to monitor stomatal conductance and twig water potential in European beech (Fagus sylvatica L.) at predawn and midday and to analyse variations with respect to leaf height within the canopy. By comparing our field measurements of twig water potential to values found in the literature, we confirmed the strong impact of soil water shortage on crown water relations.

Results

This paper shows that (1) the vertical gradient of stomatal conductance within the crown disappeared under extreme soil water depletion; (2) at maximum drought intensity, predawn twig water potential (ψ _pd) reached ?2.3 MPa at a height of 14 m in the crown and ?2.0 MPa at a height of 10 m. The significant differences in ψ _pd between the two measurement heights in the canopy may be due to night transpiration; (3) there was a close relationship between predawn twig water potential and relative extractable soil water; (4) as drought conditions intensified, there was a close relationship between canopy radiation interception and predawn water potential, as estimated daily from relative extractable soil water.     

Predicting the decline in daily maximum transpiration rate of two pine stands during drought based on constant minimum leaf water potential and plant hydraulic conductance

The effect of drought on forest water use is often estimated with models, but comprehensive models require many parameters, and simple models may not be sufficiently flexible. Many tree species, Pinus species in particular, have been shown to maintain a constant minimum leaf water potential above the critical threshold for xylem embolism during drought. In such cases, prediction of the relative decline in daily maximum transpiration rate with decreasing soil water content is relatively straightforward. We constructed a soil-plant water flow model assuming constant plant conductance and daily minimum leaf water potential, but variable conductance from soil to root. We tested this model against independent data from two sites: automatic shoot chamber data and sap flow measurements from a boreal Scots pine (Pinus sylvestris L.) stand; and sap flow measurements from a maritime pine (Pinus pinaster Ait.) stand. To focus on soil limitations to water uptake, we expressed daily maximum transpiration rate relative to the rate that would be obtained in wet soil with similar environmental variables. The comparison was successful, although the maritime pine stand showed carry-over effects of the drought that we could not explain. For the boreal Scots pine stand, daily maximum transpiration was best predicted by water content of soil deeper than 5 cm. A sensitivity analysis revealed that model predictions were relatively insensitive to the minimum leaf water potential, which can be accounted for by the importance of soil resistance of drying soil. We conclude that a model with constant plant conductance and minimum leaf water potential can accurately predict the decline in daily maximum transpiration rate during drought for these two pine stands, and that including further detail about plant compartments would add little predictive power, except in predicting recovery from severe drought.     

Responses of trembling aspen and hazelnut to vapor pressure deficit in a boreal deciduous forest

The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornuta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). During the same period, daytime measurements of stomatal conductance (g(s)) and leaf water potential (Psi(leaf)) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between daytime g(s) and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Psi(leaf) was below -2 MPa in aspen and near -1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Psi(leaf) above a minimum threshold value. Reductions in g(s) on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than D. These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal forest. However, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.     

Water relations characteristics of competing singleleaf pinyon seedlings and sagebrush nurse plants

Encroachment of singleleaf pinyon (Pinus monophylla) into adjacent low sagebrush (Artemisia arbuscula) and basin big sagebrush (Artemisia tridentata ssp. tridentata) communities may be enhanced by the efficient use of limited water resources by tree seedlings. Seedlings and sagebrush nurse plants were monitored over two growing seasons to determine water-use patterns. Predawn xylem water potential of low sagebrush declined rapidly, reaching −3.5 to −5.5 MPa by late summer. Big sagebrush values dropped to −2.0 to −3.0 MPa during summer drought. The drop in sagebrush xylem water potential was related to the decline in soil water potential (r=0.68 and 0.82). The change in pinyon predawn xylem water potential was moderate, declining to values of −1.5 to −2.5 MPa. An apparent diurnal threshold xylem water potential (−2.3 to −3.0 MPa) that results in stomatal closure enables pinyon seedlings to maintain a seasonally stable xylem water potential. Water use by pinyon seedlings declined by 50% from May to August in association with a reduction in stomatal conductance. Despite reduced stomatal conductance, sagebrush water use continued to increase during summer and reached levels up to five times greater (per unit leaf area) than associated pinyon. Pinyon seedlings appear to have greater drought avoidance than sagebrush nurse plants.     

Some ecophysiological characteristics of artà (Calligonum comosum Ľ Hérit) in response to drought stress

Water deficit is an important environmental factor restricting plant growth and photosynthesis. The effect of water deprivation on leaf water status, photosynthetic gas exchange, chlorophyll content and fluorescence parameters of artà (Calligonum comosum) was studied. Five-month-old artà seedlings, grown in pots in the open air, were subjected to one of four drought treatments (i.e., mild, moderate, severe and extreme drought stress) and compared to control seedlings (normal watering regime). Results show that leaf water potential, net photosynthesis, stomatal conductance, transpiration, photosynthetic pigment content (chlorophyll a and b) decreased with increasing levels of drought stress. Inactivation of the photosynthetic apparatus was accompanied by changes in the fluorescence characteristics, providing evidence that reduction of photosynthetic rate could be attributed to non-stomatal limitations. Alterations imply changes in photochemical conversion efficiency of photosystem II by which plants could reduce water transpiration or protect their photosynthetic apparatus from destruction. These adaptations are discussed in relation to the strategies developed to grow drought-resistant artà seedlings in desert environments.     

Competition for soil water between perennial bunch-grass (Elymus glaucus B.B.) and blue oak seedlings (Quercus douglasii H. & A.)

The competition effects of the perennial bunch-grass (Elymus glaucus B.B.) on the growth and survival of the oak seedlings (Quercus douglasii H. & A.) were investigated. There were four levels of Elymus competition, replicated three times. The three densities ofElymus employed were zero (control), 50 (Low — ‘L’ -), 116 (Medium — ‘M’ -) and 199 (High — ‘H’ -) plants m^?2. Rates of soil water depletion, stomatal conductance, transpiration, shoot elongation and leaf expansion rates were measured between 23 March and 26 May 1988. Rates of soil water depletion, stomatal conductance and transpiration differed amongst the treatments and were higher in the control for the duration of the experiment. Shoot elongation rate (SER) and leaf expansion rate (LER) of blue oak seedling were directly related to soil water potentials. Zero values of LER rates for all treatments were observed at soil water potentials lower than?1.91 MPa, and concurrent reductions of stomatal conductance indicated stomatal closure due to the soil water deficit. In the control treatment, transpiration alone was not high enough to deplete soil moisture and to reduce LER of the oak seedlings. Leaf dessication occurred first in the H and M treatments (53% of seedlings dessicated) and two weeks later in the L treatment (37% dessicated) when the soil water potential was approximately ?4.0 MPa. The number of reproductive tillers and seed dry weight indicated thatElymus plants were under water stress from April 25 and concluded on May 25 with an early summer dormancy in all treatments. Data indicated that light intensity of 50% of ambient did not limit the development of oak seedlings. The results suggested that density of the perennial bunch-grassElymus glaucus lower than 50 plants m^?2 could allow survival and successful establishment of blue oak in understories.     

The ecological and functional correlates of nocturnal transpiration

Contrary to the conventional theory of optimal stomatal control, there is substantial transpiration at night in many tree species, but the functional significance of this phenomenon remains uncertain. To investigate the possible roles of nocturnal transpiration, we compared and contrasted the correlations of both nocturnal and diurnal sap flow with a range of traits in 21 temperate deciduous tree species. These traits included soil water affinity, shade tolerance, cold hardiness, nitrogen concentration of tissues, minimum transpiration rate of excised leaves, growth rate, photosynthetic capacity, stomatal length and density, and the water potential and relative water content of leaves at the wilting point. Nocturnal sap flow was higher in species with higher leaf nitrogen concentrations, higher rates of extension growth and lower shade tolerances. Diurnal sap flow was higher in species with higher leaf nitrogen concentrations and photosynthetic capacities on a leaf area basis. Because leaf metabolism and dark respiration, in particular, are strongly related to leaf nitrogen concentration, our findings suggest that nocturnal transpiration functions to sustain carbohydrate export and other processes driven by dark respiration, and that this function is most important in fast- growing shade-intolerant tree species.     

Physiological response of irrigated and non-irrigated Norway spruce trees as a consequence of drought in field conditions

Physiological reactions of 25-year-old Norway spruce (Picea abies (L.) Karst.) trees to drought were examined during 2009 vegetation period. During the second half of summer, the decrease in soil water content was observed and irrigation was applied to a group of spruce trees, while the second group was treated under natural soil drought. The response to water deficit was recorded at the level of leaf water potential (Ψ_L). However, it appears that Ψ_L plays minor role in early stomata regulation of Norway spruce as CO₂ assimilation rate (P _N) and stomatal conductance (g _S) were reduced already before water potential decrease. Leaf water potential decreased significantly only in case when soil water content was low in the long run and when transpiration losses were simultaneously relatively high. Almost complete stomatal closure even of the irrigated trees was caused by the increase in the vapour pressure deficit of the air (D) above the value of approximately 1.5?kPa. Low values of D were accompanied by partial stomata opening of drought-treated trees. In non-irrigated spruce trees, the values of P _N decreased by 35–55% in comparison with irrigated trees. No drought-induced significant changes were found either in chlorophyll and carotenoid concentration (chl a?+?b, car) or in maximal photochemical efficiency of photosystem II (F _v/F _m). High rates of sap flow (F) did not always lead to stomatal closure during midday. It?appears that high transpiration rates do not control stomatal response to D.     

Effects of nutrition and soil water availability on water use in a Norway spruce stand

We investigated effects of nutrition and soil water availability on sap flux density, transpiration per unit leaf area (EL), and canopy stomatal conductance (GS) of Norway spruce (Picea abies L. (Karst.)) in northern Sweden during the 1996 growing season. Our objectives were to determine (1) if artificially imposed drought (65% rain diversion) reduces soil water sufficiently to cause physiological limitations to whole-tree and plot-scale water transport, and (2) whether increased capacity for water transport resulting from fertilization-induced increases in leaf (> 3-fold) and sapwood areas (> 2.3-fold) deplete soil water sufficiently to cause a negative feedback on GS and EL. We monitored soil water content (theta) and soil water potential (PsiS) in control (C), drought (D), fertilized (F) and irrigated + fertilized (IL) treatment plots, along with site meteorological conditions. Ten trees per plot were monitored for sap flow. Although there were significant treatment differences in mean daily EL (C > D > F; P < 0.01) and GS (C > D > F; P < 0.05), variation in absolute magnitudes was small. Therefore, transpiration differences on a unit ground area basis (EC) were nearly proportional to leaf area differences. Precipitation was well distributed throughout the study period and so PsiS remained high, except during short dry periods in Plot F when it declined rapidly. Thus, although soil water was not limiting to GS, EL or EC when precipitation was uniformly distributed throughout the growing season, we cannot conclude that water availability would not limit GS in fertilized stands if the seasonal distribution of precipitation were altered.     

观赏海棠品种对土壤干旱胁迫的响应差异

探讨观赏海棠适应土壤干旱胁迫及复水的生长生理机制,筛选观赏海棠抗旱性鉴定的生长生理指标。研究盆栽人工模拟干旱条件及复水条件下的13个观赏海棠品种的叶片相对含水量(RWC)、叶片失水量(WWL)、新梢增长率(SGR)、比叶重(SDW)、净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Cs)、水分利用效率(WUE)、相对电导率(REC)、脯氨酸含量(PRO)、可溶性蛋白含量(SPC)和旱害指数(DI)的变化,并采用聚类分析法对品种的抗旱性进行综合评价。土壤干旱胁迫明显降低观赏海棠的RWC,WWL,SGR,Pn,Tr和Cs;提高DI,WUE和REC;对SDW,PRO和SPC的影响不一。供试观赏海棠品种抗旱性可分4类,比利时直立和王族为特抗型;比利时垂枝、凯尔斯和钻石为高抗型;当娜、高原之火、红花、绚丽和雪球为中抗型;红丽、红玉和印第安魔力为低抗型。不同观赏海棠品种对土壤干旱胁迫和复水的响应不同,其抗旱性存在差异。     

Variations in transpiration rate and leaf cell turgor maintenance in saplings of deciduous broad-leaved tree species common in cool temperate forests in Japan

To clarify mechanisms underlying variation in transpiration rate among deciduous broad-leaved tree species, we measured diurnal changes in stomatal conductance (gs) and leaf water potential, and calculated the maximum transpiration rate (Emax), leaf-specific hydraulic conductance (K(s-l)) and difference between the soil water potential and the daily minimum leaf water potential (Psis - Psi(l,min)). Pressure-volume (P-V) measurements were made on leaves. Saplings of eight broad-leaved tree species that are common in Japanese cool temperate forests were studied. Maximum transpiration rate varied significantly among species. There was a statistically significant difference in Psis - Psi(l,min), but not in K(s-l). Species with large Emax also had large Psis - Psi(l,min) and gs. The results of the P-V analyses showed that species with a large Psis - Psi(l,min) maintained turgor even at low leaf water potentials. The similar daily minimum leaf pressure potentials (Psip) across all eight species indicate that Psip values below this minimum are critical. Based on these results, we suggest that the leaf cell capacity for turgor maintenance strongly affects Psis - Psi(l,min) and consequently Emax via stomatal regulation.     

Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought

In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.