Optimal stomatal conductance in relation to photosynthesis in climatically contrasting Eucalyptus species under drought

Models of stomatal conductance (g_s) are based on coupling between g_s and CO₂ assimilation (A_net), and it is often assumed that the slope of this relationship (‘g₁’) is constant across species. However, if different plant species have adapted to different access costs of water, then there will be differences in g₁ among species. We hypothesized that g₁ should vary among species adapted to different climates, and tested the theory and its linkage to plant hydraulics using four Eucalyptus species from different climatic origins in a common garden. Optimal stomatal theory predicts that species from sub‐humid zones have a lower marginal water cost of C gain, hence lower g₁ than humid‐zone species. In agreement with the theory that g₁ is related to tissue carbon costs for water supply, we found a relationship between wood density and g₁ across Eucalyptus species of contrasting climatic origins. There were significant reductions in the parameter g₁ during drought in humid but not sub‐humid species, with the latter group maintaining g₁ in drought. There are strong differences in stomatal behaviour among related tree species in agreement with optimal stomatal theory, and these differences are consistent with the economics involved in water uptake and transport for carbon gain.     

Stomatal and nonstomatal limitations of photosynthesis in 19 temperate tree species on contrasting sites during wet and dry years

A unique approach was used to evaluate stomatal and nonstomatal constraints to photosynthesis in 19 naturally occurring, deciduous tree species on xeric, mesic and wetmesic sites in central Pennsylvania, USA, during relatively wet (1990) and dry (1991) growing seasons. All species exhibited significantly decreased stomatal conductance to CO₂ (g_c) in 1991 compared to 1990. The mesic species had drought related decreases in photosynthesis (A) attributed primarily to increased absolute stomatal limitation to A (L_g), whereas in the wet-mesic species, the absolute mesophyll limitation (Lm) was at least as important as L_g in limiting A during drought. The xeric species maintained relatively high A during drought despite decreased g_c. In the xeric and mesic species, L_m decreased and L_g increased during drought due to stomatal closure. From xeric to mesic to wet-mesic, the relative stomatal limitation (I_g) generally decreased faster, and relative mesophyll limitations to A increased faster, with increasing g_c suggesting greater photosynthetic capacity (i.e. greater potential maximum A) with increasing drought tolerance rank of species. Few species exhibited a significant drought-related decrease in photosynthetic capacity. The results of this landscape-based study indicate that the interaction of stomatal and nonstomatal limitations of A vary in a manner consistent with species' drought tolerance and site conditions, and that nonstomatal constraints to A in field plants during a moderate, season-long drought were generally not as severe as reported in controlled studies.     

Nutrient uptake and gas exchange of Great Basin plants provide insight into drought adaptations and coexistence

植物的养分吸收和气体交换为干旱适应和物种共存带来的启示尽管气候变化很可能会导致干旱生态系统的降水量变化幅度增大,但沙漠灌木群落对降水变化的响应模式尚不清楚。因为适应干旱的植物具有多种生存机制以利于其物种的共存,所以认识沙漠灌木群落对降水变化的响应具有重要的意义。我们在美国大盆地地区(美国加利福尼亚州欧文斯谷)开展了一项同质园实验,对8种干旱适应性植物(其中包括了禾本科植物、灌木和杂类草植物)在3种不同夏季降水 量(分别为每月1.3、2.6和3.9 cm)的响应进行了测定。检测了上述植物在矿物养分吸收(碳、氮、磷、钾、 钙、镁、锰、铜、硼、锌、铁和钠)和气体交换参数(光合速率和气孔导度)方面的变化。研究结果显示, 两种禾本科植物[鼠尾粟草(Sporobolus airoides)和匍匐野麦赖草(Leymus triticoides)]与一种耐盐灌木物种 [密叶滨藜(Atriplex confertifolia)]在水分供给增加时,光合速率和/或气孔导度增加。在这8个物种中,有 5个物种表现出了水分供给与营养物质吸收之间的显著相关性。光合速率更高的三齿蒿(Artemisia tridentata)含有更多的钾、铜和硼,而光合速率更高的北极灯心草(Juncus arcticus)具有更高的镁和铁含量 以及更低的钠含量。北极灯心草以及3种适盐性物种[密叶滨藜、盐草(Distichlis spicata)和鼠尾粟草]在气孔导度和营养素浓度方面表现出了相关性。这些结果表明,干旱适应性物种之间对水分增多情形的生理响应机制的差异以及相关的营养吸收策略可能有助于它们在夏季雨水增多时的相互共存。     

Growth and water-use efficiency of 10 Triticum aestivum cultivars at different water availability in relation to allocation of biomass

In environments where the amount of water is limiting growth, water-use efficiency (biomass production per unit water use) is an important trait. We studied the relationships of plant growth and water use efficiency with the pattern of biomass allocation, using 10 wheat cultivars, grown at two soil moisture levels in a growth chamber. Allocation pattern and relative growth rate were not correlated, whereas allocation pattern and water use efficiency were. Variation in transpiration per plant resulted from variation in the rate of transpiration per unit leaf area or root weight, rather than from differences in leaf area or root weight per plant. Transpiration per unit leaf area or root weight was lower when the leaf area or root weight per unit plant weight was larger. Also, the efficiency of water use at the plant and leaf levels was higher for plants with a higher leaf area per unit plant weight, and it was not correlated with the plant's growth rate. Differences in water-use efficiency at the leaf level were related to variation in stomatal conductance, rather than in the rate of photosynthesis. A high photosynthetic water-use efficiency was associated with a low efficiency of nitrogen use for photosynthesis.     

Variability in mesophyll conductance between barley genotypes,and effects on transpiration efficiency and carbon isotope discrimination

Leaf internal, or mesophyll, conductance to CO₂ (g_m ) is a significant and variable limitation of photosynthesis that also affects leaf transpiration efficiency (TE). Genotypic variation in g_m and the effect of g_m on TE were assessed in six barley genotypes (four Hordeum vulgare and two H. bulbosum). Significant variation in g_m was found between genotypes, and was correlated with photosynthetic rate. The genotype with the highest g_m also had the highest TE and the lowest carbon isotope discrimination as recorded in leaf tissue (Δ_p). These results suggest g_m has unexplored potential to provide TE improvement within crop breeding programmes.     

The photosynthetic drought physiology of a diverse group of southern hemisphere conifer species is correlated with minimum seasonal rainfall

1. The instantaneous and integrated leaf gas exchange of 13 species of southern hemisphere conifers grown under identical glasshouse conditions were examined to determine whether there was any correlation between the characteristics of water use at the leaf level and environmental water availability.
2. In the conifer species examined, the minimum ratio of internal to ambient CO₂ measured in leaves during artificially imposed drought [( c _i/ c _a)_min] was strongly correlated with the minimum rainfall observed within the natural range of each species. This suggests that the distributions of these species are constrained by the drought tolerance of their photosynthetic apparatus.
3. A good correlation was found between the ratio of internal to ambient CO₂ measured in leaves under optimal conditions ( c _i/ c _a)_max and leaf δ¹³C (and hence inferred ∫[ c _i/ c _a]). Neither of these, however, correlated with the environmental parameters considered most likely to be limiting species distribution, i.e. precipitation and altitude.
4. These data suggest that decreasing water availability may have been the major factor responsible for the restriction and extinction of conifers in the southern hemisphere.     

Spatial distribution of leaf water-use efficiency and carbon isotope discrimination within an isolated tree crown

The spatial variations in the stable carbon isotope composition (δ¹³C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well‐watered, 20‐year‐old walnut tree growing in a low‐density orchard. The observed leaf carbon isotope discrimination (Δ) was compared with that computed by a three‐dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO₂ gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long‐term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in Δ. The main conclusions of this study are: (i) leaf Δ can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf Δ, and (iii) the spatial variations in Δ and water‐use efficiency resulted from the short‐term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long‐term acclimation of leaf characteristics to the local light regime.     

Photosynthesis and Water Use Efficiency in Twenty Tropical Tree Species of Differing Succession Status in a Brazilian Reforestation

Leaf gas exchange characteristics were measured in twenty woody species that differ in succession status ranging from pioneer species (PS) to late succession species (LS) in a Brazilian rain-reforestation ecosystem. Photon-saturated photosynthetic rate, calculated per either a leaf area (P _NA) or a dry mass (P _NM) basis, differed among species. P _NA and P _NM were highest in PS and lowest in LS. Variation among species was 3-fold (from 7 to 23 mol m^–2 s^–1) for P _NA, and 5-fold (from 50 to 275 mol kg^–2 s^–1) for P _NM. The highest P _NA (23 mol m^–2 s^–1) and P _NM (275 mol kg^–2 s^–1) values were recorded in PS Croton urucurana, while the lowest P _NA (7 mol m^–2 s^–1) and P _NM (50 mol kg^–2 s^–1) values were recorded in LS Aspidosperma cylindrocarpon. A considerable overlap was recorded between PS and LS in values of stomatal conductance (g _s), transpiration rate (E), and leaf mass to area ratio (ALM). However, C. urucurana also showed highest g _s and E. P _NM was highly correlated with ALM in both PS and LS (r=–0.75 and –0.90, respectively). The high values of instantaneous transpiration efficiency (ITE) and intrinsic water use efficiency (WUE_i) were also observed in the PS when compared with the LS.     

CO2浓度和土壤含水量对植物个体尺度短期水分利用效率的影响

分析植物个体短期水分利用效率(WUE_p)对CO₂浓度(C_a)和土壤含水量(SWC)的响应,可提高对气候变化下个体生存策略的认识。本研究以侧柏幼树为对象,在模拟气候箱中进行培养试验,设400(C₄₀₀)、600(C₆₀₀)和800 μmol·mol^-1CO₂(C₈₀₀)浓度处理和35%～45%田间持水量(FC)、50%～60%FC、60%～70%FC、70%～80%FC、95%～100%FC土壤含水量处理,共15个处理。WUE_p对C_a和SWC的响应用包裹式茎流计、称重法结合静态同化箱测定。结果表明: 个体日间(0.12～1.87 mol·h^-1)和夜间蒸腾速率(0.01～0.16 mol·h^-1)均在C₄₀₀×70%～80%FC时达到最大值,个体日间净光合速率(2.12～22.10 mmol·h^-1)在C₈₀₀×70%～80%FC时达到最大值,而个体夜间呼吸速率(0.84～4.41 mmol·h^-1)随SWC的增加而增加,随C_a的增加而减小,在C₄₀₀×95%～100%FC时达到最大值。WUE_p(5.37～24.35 mmol·mol^-1)在C₈₀₀×50%～60%FC时达到最大值,表明高C_a和干旱条件下,植物个体可通过生理可塑性调整,利用较少的水分固定更多的碳;此外,当个体间形态特征差异较小时,叶片瞬时水分利用效率可以较好地指示WUE_P的变化。     

Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO₂, and in full sun and 80% shade for 90 days. The shaded seedlings received 30-min sun patches twice during the course of the day. Gas exchange and tissue–water relations were measured at midday in the sun plants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO₂ on constraints to sun-patch utilization in these species.
2. Elevated CO₂ had the largest stimulation of photosynthesis in B. papyrifera sun plants and A. rubrum shade plants.
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO₂ responses that would facilitate tolerance to leaf water deficits in large sun patches, including osmotic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increases in photosynthetic nitrogen-use efficiency.
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO₂, in ways that are consistent with the habitats in which they typically occur.     

Leaf hydraulics and drought stress: response, recovery and survivorship in four woody temperate plant species

Efficient conduction of water inside leaves is essential for leaf function, yet the hydraulic-mediated impact of drought on gas exchange remains poorly understood. Here we examine the decline and subsequent recovery of leaf water potential ( Ψ _leaf), leaf hydraulic conductance ( K _leaf), and midday transpiration ( E ) in four temperate woody species exposed to controlled drought conditions ranging from mild to lethal. During drought the vulnerability of K _leaf to declining Ψ _leaf varied greatly among the species sampled. Following drought, plants were rewatered and the rate of E and K _leaf recovery was found to be strongly dependent on the severity of the drought imposed. Gas exchange recovery was strongly correlated with the relatively slow recovery of K _leaf for three of the four species, indicating conformity to a hydraulic-stomatal limitation model of plant recovery. However, there was also a shift in the sensitivity of stomata to Ψ _leaf suggesting that the plant hormone abscisic acid may be involved in limiting the rate of stomatal reopening. The level of drought tolerance varied among the four species and was correlated with leaf hydraulic vulnerability. These results suggest that species-specific variation in hydraulic properties plays a fundamental role in steering the dynamic response of plants during recovery.     

Physiological advantages of C4 grasses in the field: a comparative experiment demonstrating the importance of drought

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C₄ grasses inhabit drier habitats than C₃ relatives, but that both C₃ and C₄ photosynthesis are susceptible to drought. However, C₄ plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C₄ and C₃ sister lineages. Photosynthesis was always higher in the C₄ than C₃ grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C₃ than C₄ grasses, with the peak monthly average for C₃ species being similar to that of C₄ leaves. In water‐limited, rain‐fed plots, the photosynthesis of C₄ leaves was between 2.0 and 7.4 μmol m^?2 s^?1 higher, stomatal conductance almost double, and transpiration 60% higher than for C₃ plants. Although C₄ average instantaneous water‐use efficiencies were higher (2.4–8.1 mmol mol^?1) than C₃ averages (0.7–6.8 mmol mol^?1), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C₃ than C₄ species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C₄ plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C₃ species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C₄ and C₃ species. We suggest that these drought responses are important for understanding the advantages of C₄ photosynthesis under field conditions.     

Net photosynthetic and transpiration rates in a chlorophyll-deficient isoline of soybean under well-watered and drought conditions

The gas exchange traits of wild type soybeans (cv. Clark) and a near-isogenic, chlorophyll-deficient line homozygous for the recessive allele y9 (y9y9) were compared under either well-watered or water-stress conditions. Mature leaves of y9 had a 65% lower chlorophyll content than wild type. However, the net photosynthetic rate (PN) of y9 leaves was only 20% lower than in the wild type, irrespective of water availability. Transpiration rates (E) were significantly higher in leaves of y9, compared to the wild type, either under well-watered or stress conditions. The higher E of y9 correlated with increased stomatal conductance, particularly in the abaxial epidermis, where more than 70% of the stomata were located. The combination of lower PN and increased E resulted in a significant decrease of water use efficiency in y9, at both water availability levels. The relative water content decreased in stressed leaves, much more in y9 than in wild type leaves, probably because of the higher E of the mutant line. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanistic basis of differences in water-use efficiency between a CAM and a C3 species of Peperomia (Piperaceae)

Under ecologically realistic environmental conditions, the water-use efficiency (WUE) of Peperomia scandens , a CAM plant, was higher than that of the C₃ congener P. obtusifolia . This difference has been attributed to differences in stomatal activity between C₃ and CAM plants, coupled with differences in the evaporative demand of the atmosphere during which the stomata are open. This explanation has apparently not, however, been experimentally tested. Thus, WUEs were compared in these species in two experiments in which the atmospheric evaporative demand was identical (or nearly so) during the period of stomatal opening (i.e. during the night for the CAM plant and during the day for the C₃ species). In both experiments, the WUE of the CAM species was higher than that of the C₃ species. These results suggest that factors other than differences in atmospheric environmental conditions must also be responsible for the observed differences in WUE. Because CO₂ uptake rates of the CAM species were substantially lower than those of the C₃ species, the lower WUE in the CAM species resulted primarily from lower transpiration rates. Lower rates of water loss in P. scandens , relative to rates in P. obtusifolia , were ascribed, in part, to lower stomatal densities. Thus, leaf morphological differences, in addition to differences in atmospheric evaporative demand, help to explain the high WUE typically measured in CAM plants.     

The response of stomatal conductance to seasonal drought in tropical forests

Stomata regulate CO₂ uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (g_s) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water‐use‐efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψ_leaf), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf‐mass‐per‐area (LMA), leaf carboxylation capacity (V_c,max), leaf water content, the degree of isohydry, and predawn Ψ_leaf. We first investigated how the choice of four commonly used leaf‐level g_s models with and without the inclusion of Ψ_leaf as an additional predictor variable influence the ability to predict g_s, and then explored the abiotic (i.e. month, site‐month interaction) and biotic (i.e. tree‐species‐specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψ_leaf did not improve model performance and that the models that represent the response of g_s to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each g_s model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R² = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of g_s dynamics over seasonal drought, and identifies a practical, trait‐based approach to improve modeling of carbon and water exchange in tropical forests.     

On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments

Background and Aims

Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g_s) at the expense of lowering CO₂ uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g_s declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g_s, an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO₂ concentration (c_a).

Methods

Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c_a and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂g_s)/(∂E/∂g_s) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO₂ concentration (c_i) were examined using A/c_i curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO₂ Enrichment experiment (North Carolina, USA).

Key Results

Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated c_a increased η and λ, and at given c_a and needle age-class, the two quantities varied among needles in an approximately complementary manner.

Conclusions

The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing c_a influences ecosystem water use.     

Biomass production and water use efficiency in perennial grasses during and after drought stress

Drought is a great challenge to agricultural production, and cultivation of drought‐tolerant or water use‐efficient cultivars is important to ensure high biomass yields for bio‐refining and bioenergy. Here, we evaluated drought tolerance of four C₃ species, Dactylis glomerata cvs. Sevenop and Amba, Festuca arundinacea cvs. Jordane and Kora, Phalaris arundinacea cvs. Bamse and Chieftain and Festulolium pabulare cv. Hykor, and two C₄ species Miscanthus × giganteus and M. lutarioriparius. Control (irrigated) and drought‐treated plants were grown on coarse and loamy sand in 1 m² lysimeter plots where rain was excluded. Drought periods started after harvest and lasted until 80% of available soil water had been used. Drought caused a decrease in dry matter yield (DM; P < 0.001) for all species and cultivars during the drought period. Cultivars Sevenop, Kora and Jordane produced DM at equal levels and higher than the other C₃ cultivars in control and drought‐treated plots both during and after the drought period. Negative correlations were observed between stomatal conductance (g_s) and leaf water potential (P < 0.01) and positive correlations between g_s and DM (P < 0.05) indicating that g_s might be suitable for assessment of drought stress. There were indications of positive associations between plants carbon isotope composition and water use efficiency (WUE) as well as DM under well‐watered conditions. Compared to control, drought‐treated plots showed increased growth in the period after drought stress. Thus, the drought events did not affect total biomass production (DM_total) of the whole growing season. During drought stress and the whole growing season, WUE was higher in drought‐treated compared to control plots, so it seems possible to save water without loss of biomass. Across soil types, M. lutarioriparius had the highest DM_total (15.0 t ha^?1), WUE_total (3.6 g L^?1) and radiation use efficiency (2.3 g MJ^?1) of the evaluated grasses.     

The effects of soil and atmospheric drought on photosynthesis and stomatal control of gas exchange in three coniferous species

The responses of steady-state CO₂ assimilation rate (A), transpiration rate (E), and stomatal conductance (g_s) to changes in leaf-to-air vapour pressure difference (δW) on one hand and to increasing soil drought on the other hand were examined in 2-year-old seedlings of Pseudotsuga menziesii, Pseudotsuga macrocarpa and Cedrus atlantica. Analysing the data through A vs intercellular CO₂ molar fraction (c_i) graphs, we could determine stomatal and mesophyll contributions to changes in A as δW or soil drought were increased. Increasing soil drought affected g_s and mesophyll photosynthesis independently, since clearly distinct predawn leaf water potential (ψ_p) regions appeared in which either stomatal or mesophyll effects prevailed for explaining the changes in A. The two Pseudotsuga species exhibited a large ψ_P range (between ca -0.8 and -1.5 to -1.9 MPa) in which only stomata were responsible for the decrease in A. A dramatic decline in mesophyll photosynthesis was noticed starting from values as high as -1.2 MPa ( C. atlantica ), -1.5 MPa ( P. macrocarpa ) and -1.9 MPa ( P. menziesii ). Increasing ΔW at high soil water content led to a sharp decline in A primarily due to an alteration of mesophyll photosynthesis. Stomatal conductance for CO₂ diffusion was affected in a lesser extent and in close correlation with the changes in mesophyll photosynthesis, which could suggest the existence of a functional linkage between mesophyll photosynthesis and stomata. Surprisingly, the drought resistant P. macrocarpa exhibited the least conservative water use efficiency in response to the two types of drought. In this species drought adaptation seems to be mainly due to its high root growth and soil prospection ability.     

土壤干旱对沙棘苗木生长及水分利用的影响

研究了土壤干旱条件下盆栽沙棘苗木生长、光合和水分关系的变化。结果表明,同样环境条件下,蒸腾速率随干旱胁迫程度加重而降低,而蒸腾日进程差异不大,均为单峰曲线,峰值出现在11：00;轻、中度干旱胁迫下沙棘叶含水量、水势、渗透势、膨压下降幅度较小,光合速率、侧枝生长速率保持在较高水平,能维持基本的生长量,表现出耐旱植物的生理特征;重度干旱胁迫使上述指标下降较显著,且长时间胁迫后,33．3％沙棘幼苗死亡。