Xylem structure and water transport in a twiner,a scrambler,and a shrub of Lonicera (Caprifoliaceae)

Summary Wood structure and function was investigated in different growth forms of temperate honeysuckles (Lonicera spp.). All three species had many narrow vessels and relatively few wide ones, with the measured K _h (flow rate/pressure gradient) approximately 24–55% of the theoretical K _h predicted by Poiseuille's law. Only the twiner, Lonicera japonica, had some vessels greater than 50 m in diameter. The twiner also had the narrowest stem xylem diameters, suggesting the greater maximum vessel diameter hydraulically compensated for narrow stems. Conversely, the free-standing shrub, L. maackii, had the greatest annual increments of xylem but the least percent conductive xylem implying that a great portion of the wood was involved with mechanical support. The scrambler, L, sempervirens had low maximum vessel diameter, high Huber values (= xylem area/leaf area), and low specific conductivities (= measured K _h/xylem area), much like the shrub. The greatest vessel frequency occurred in the scrambler (901 vessels · mm^-2), the highest thus far recorded in vines. The lowest Huber value and highest specific conductivity occurred in the twiner, suggesting little self-support but relatively efficient water conduction. LSC (= measured K _h/leaf area) and maximum vessel diameter of Lonicera vines were near the low end of the range for vines in general.     

Aquaporin expression in response to different water stress intensities and recovery in Richter-110 (<Emphasis Type="Italic">Vitis</Emphasis> sp.): relationship with ecophysiological status

Aquaporins seem essential for the regulation of plant water status and expenses. Richter-110 is a Vitis hybrid (Vitis berlandieri × rupestris) reputed to be strongly drought-tolerant. Three irrigation treatments were established in Richter-110 plants growing outdoors defined by the resulting maximum stomatal conductance (g _s), and ensuring water stress situations not severe enough as to stop photosynthesis and growth: well-watered plants (g _s about 250 mmol H₂O m⁻² s⁻¹), moderate water stress (g _s about 150 mmol H₂O m⁻² s⁻¹) and severe water stress (g _s about 50 mmol H₂O m⁻² s⁻¹). Plants under water stress were kept at constant water availability for 7 days to check for possible acclimation. Finally, plants were re-watered, and allowed to recover, for 3 days. Stomatal conductance, leaf water potential, xylem abscisic acid (ABA) content and root and stem hydraulic conductivity were determined. The relative amounts of expression of mRNA encoding seven putative aquaporins were determined in roots and leaves by RT-PCR. The decrease in stomatal conductance with moderate and severe water stress was associated with increasing ABA contents, but not with the leaf water potential and hydraulic conductivities, which remained unchanged during the entire experiment. Aquaporin gene expression varied depending on which aquaporin, water stress level and the plant organ. We suggest that aquaporin expression was responsive to water stress as part of the homeostasis, which resulted in constant leaf water potential and hydraulic conductivity.     

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain

The C₄ grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO₂ uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO₂ uptake (A) for the emergent leaves were high with a mean of c. 30 mol m^-2 s^-1 at mid-day and occasional values of 40 mol m^-2 s^-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 mol m^-2 s^-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO₂ concentration (c _i/c _a). During the dry phase, a midday depression of rates of CO₂ assimilation was observed. The lowest leaf area index (F) was c. 2 in November–December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m^-2 mo^-1 throughout the period of flooding, with a peak of 37 mol C m^-2 mo^-1 in August, but declined to 13 mol C m^-2 mo^-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m^-2. This was insufficient to account for the 3.99 kg C m^-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO₂ from the CO₂-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Effects of plant size and water relations on gas exchange and growth of the desert shrub Larrea tridentata

Larrea tridentata is a xerophytic evergreen shrub, dominant in the arid regions of the southwestern United States. We examined relationships between gasexchange characteristics, plant and soil water relations, and growth responses of large versus small shrubs of L. tridentata over the course of a summer growing season in the Chihuahuan Desert of southern New Mexico, USA. The soil wetting front did not reach 0.6 m, and soils at depths of 0.6 and 0.9 m remained dry throughout the summer, suggesting that L. tridentata extracts water largely from soil near the surface. Surface soil layers (<0.3 m) were drier under large plants, but predawn xylem water potentials were similar for both plant sizes suggesting some access to deeper soil moisture reserves by large plants. Stem elongation rates were about 40% less in large, reproductively active shrubs than in small, reproductively inactive shrubs. Maximal net photosynthetic rates (P_max) occurred in early summer (21.3 mol m^-2 s^-1), when pre-dawn xylem water potential (XWP) reached ca. -1 MPa. Although both shrub sizes exhibited similar responses to environmental factors, small shrubs recovered faster from short-term drought, when pre-dawn XWP reached about -4.5 MPa and P_max decreased to only ca. 20% of unstressed levels. Gas exchange measurements yielded a strong relationship between stomatal conductance and photosynthesis, and the relationship between leaf-to-air vapor pressure deficit and stomatal conductance was found to be influenced by pre-dawn XWP. Our results indicate that stomatal responses to water stress and vapor pressure deficit are important in determining rates of carbon gain and water loss in L. tridentata.     

Embolism vulnerability of an evergreen tree

Leaf bearing stems ofCuratella americana L. were very susceptible to induced cavitation: embolisms began at a pressure of 0.5 MPa (15 %) and at 2.0 MPa most of the conductivity was lost (85 %). Nevertheless, in nature similar leaf specific conductivities, of about 90 x 10^-5 kg m^-2 s^-1 MPa^-1 during both wet and dry seasons indicated absence of drought induced embolisms. Leaf water potentials were neither very negative or considerably different between seasons but stomatal conductance decreased from 236 mmol m^-2 s^-1 measured during wet period to 100 mmol m^-2 s^-1 during drought season. Therefore, it was concluded thatCuratella had an accurate homeostatic balance of leaf water status to keep up xylem integrity. Acknowledgements: Financial support was provided by Decanato de Investigaciones - USB (S1-CB-811). Dr John Sperry (University of Utah) allowed me to learn how to use and to build the equipment used in this study. Mr. M. Edreida and Mr. T. Perez helped me in the field and in the laboratory, respectively. Dr D. HenrIquez corrected the English grammar.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Linkage between seasonal gas exchange and hydraulic acclimation in the top canopy leaves of Fagus trees in a mesic forest in Japan

We investigated how leaf gas exchange and hydraulic properties acclimate to increasing evaporative demand in mature beech trees, Fagus crenata Blume and Fagus japonica Maxim., growing in their natural habitat. The measurements in the top canopy leaves were conducted using a 16-m-high scaffolding tower over two growing seasons. The daily maxima of net photosynthetic rate for the early growing season were close to the annual maximum value (11.9 mol m^–2 s^–1 in F. crenata and 7.7 mol m^–2 s^–1 in F. japonica). The daily maxima of water vapor stomatal conductance were highest in the summer, approximately 0.3 mol m^–2 s^–1 in F. crenata and 0.15 mol m^–2 s^–1 in F. japonica. From the early growing season to the summer season, the leaf-to-air vapor pressure deficit increased and the daily minima of leaf water potentials decreased. However, there was no loss of leaf turgor in the summer as a result of effective osmotic adjustment. Both the soil-to-leaf hydraulic conductance per unit leaf area and the twig hydraulic conductivity simultaneously increased in the summer, probably as a result of production of new vessels in the xylem. These results suggest that both osmotic adjustment and increased hydraulic conductance resulted in the largest diurnal maximum of stomatal conductance in the summer, resulting in the lowest relative stomatal limitation on net photosynthetic rate, although the leaf-to-air vapor pressure deficit was highest. These results indicate that even in a mesic forest, in which excessive hydraulic stress does not occur, the seasonal acclimation of hydraulic properties at both the single leaf and whole plant levels are important for plant carbon gain.     

Interactions of SO2 with other environmental stresses in influencing leaf gas exchange

Summary Leaves of two field growing co-occuring perennial shrubs (drought-deciduous Diplacus aurantiacus and the evergreen Heteromeles arbutifolia) from the Californian chaparral were exposed to small doses of SO₂. During this exposure the leaf environment was manipulated to determine how the presence of SO₂ alters the response of gas exchange to other environmental stresses. The data show that no direct changes in stomatal conductance (g) or net assimilation rate (A) could be attributed to short-term (7 h) SO₂ (4.2 mol m^-3, 0.1 l l^-1) exposure. D. aurantiacus leaves possessed features which demonstrate that they were sensitive to changes in environment e.g. light flux and atmospheric relative humidity. The interspecific differences in stomatal sensitivity to water vapour were extremely important, as relative humidity is a major factor influencing carbon fixation and the rate of pollutant absorption. Conditions of high relative humidity and high xylem water potentials are suggested to pre-dispose leaves of D. aurantiacus to greater pollutant doses than the more stomatally-conservative evergreen, H. arbutifolia. In the presence of SO₂ there was some indication of increased g for both D. aurantiacus and H. arbutifolia as W became smaller. This SO₂-effect was only obvious as increasing atmospheric humidity induced further stomatal opening. The important consequences of an SO₂ enhanced g, were a reduction in WUE, which may cause earlier leaf abscission and a concomitant decline in productivity.Abbreviations A net photosynthesis - A _max maximum rate light saturated photosynthesis - E transpiration; g stomatal conductance to water vapour - QY apparent incident quantum yield - W water vapour mole fraction difference between the leaf and the air - SO₂ Sulphur dioxide - WUE water use efficiency (mol CO₂ fixed per mol H₂O^-1 transpired)     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.     

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) and sessile oak [<Emphasis Type="Italic">Quercus petraea</Emphasis> (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψ_pd) and midday water potential were measured in transpiring (Ψ_leaf) and non-transpiring leaves (Ψ_xyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (g_s) and sap flow (F_d) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (K_h) and whole tree hydraulic conductance (K_hsf) were calculated by using the simple analogy of the Ohm’s law. K_h was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψ^l_xyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψ^u_xyl) at the uppermost canopy. Predawn water potential (Ψ_pd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (K_h) decreased progressively as water stress increases. Minimum values of K_h and K_hsf were recorded when Ψ_pd was lower. However, Q. petraea showed higher K_h than F. sylvatica for the same Ψ_pd. The decrease in K_h with water stress was mainly linked to its fall from the soil to the lowermost canopy (K_srs). Nevertheless, a significant resistance in the petiole–leaf lamina (K_pl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in K_h was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (g_s) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.     

Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.): evidence that low light uncouples water transport capacity from leaf area

Morphology, water relations, and xylem anatomy of high-light (sun)- and low-light (shade)-grown Vitis vinifera L. shoots were studied to determine the effects of shading on the hydraulic conductance of the pathway for water flow from the roots to the leaves. Shade shoots developed leaf area ratios (leaf area: plant dry weight) that were nearly threefold greater than sun shoots. Water-potential gradients (·m^–1) in the shoot xylem accounted for most of the ·m^–1 between soil and shoot apex at low and high transpiration rates in both sun and shade shoots, but the gradients were two- to fourfold greater in shade-grown plants. Low light reduced xylem conduit number in petioles, but had an additional slight effect on conduit diameter in internodes. The hydraulic conductance per unit length (Kh) and the specific hydraulic conductivity (k_s, i.e. Kh per xylem cross-sectional area) of internodes, leaf petioles, and leaf laminae at different developmental stages leaf plastochron index was calculated from measurements of water potential and water flow in intact plants, from flow through excised organs, and from vessel and tracheid lumen diameters according to Hagen-Poiseuille's equation. For all methods and conductance parameters, the propensity to transport water to sink leaves was severalfold greater in internodes than in petioles. The Kh and k_s increased logarithmically until growth ceased, independent of treatment and measurement method, and increased further in pressurized-flow experiments and Hagen-Poiseuille predictions. However, the increase was less in shade internodes than in sun internodes. Mature internodes of shade-grown plants had a two- to fourfold reduced Kh and significantly lower k_s than sun internodes. Except very early in development, leaf lamina conductance and k_s from shade-grown plants was also reduced. The strong reduction in Kh with only a slight reduction in leaf area (17% of sun shoots) in the shade shoots indicated a decoupling of water-transport capacity from the transpirational surface supplied by that capacity. This decoupling resulted in strongly reduced leaf specific conductivities and Huber values for both internodes and petioles, which may increase the likelihood of cavitation under conditions of high evaporative demand or soil drought.Abbreviations A_c total cross-sectional area (internodes, petioles, leaf laminae) - A_x xylem cross-sectional area - HV Huber value - Kh hydraulic conductance per unit length - k_s specific hydraulic conductivity - LPI leaf plastochron index - LSC leaf specific conductivity - water potential - water-potential gradient - q volume flow of water per unit time Hans R. Schultz was supported in part by the Deutsche Forschungsgemeinschaft (grant Ki-114/8-1). We wish to thank Dr. Thomas Geier, Institut für Biologie, Forschungsanstalt D-6222 Geisenheim, Germany for his advice on sample preparation and microscopy, and two anonomous reviewers for their helpful comments.     

云南文山石漠化区车桑子叶脉密度与叶氮含量关系对生境的响应

植物叶脉特征和叶氮含量的变化影响着叶片经济谱的形成,为验证叶片结构中叶脉网络构建提供了理论依据。该文以单位质量叶氮含量(N_(mass))和单位面积叶氮含量(N_(area))分别表示叶氮含量,采取主成分分析、线性回归分析的方法,研究了云南文山石漠化区旷地(Ⅰ)、林缘(Ⅱ)和林下(Ⅲ)3种自然生境下车桑子的叶脉密度(Vein density,VD)与N_(mass)和N_(area)的异速关系。结果表明:从乔灌群落的旷地到林下,车桑子的比叶面积、叶绿素总含量、光能利用率和N_(mass)逐渐增大,光饱和点、光补偿点、水分利用效率、VD、N_(area)逐渐减小,净光合速率、蒸腾速率、气孔导度呈先增大后减小的趋势。VD与叶氮含量呈不同程度的相关性,在生境I和III,VD与N_(mass)和N_(area)分别具有显著的负相关(P0.05)和正相关(P0.05);在生境Ⅱ,VD与N_(mass)和N_(area)分别呈不显著负相关(P0.05)和正相关(P0.05)。车桑子在旷地强光生境,高VD的叶片含有低N_(mass)高N_(area),而林下荫蔽生境偏向于相反的配置模式,反映了石漠化区植物较强的叶脉可塑性及其与氮利用性状的权衡机制。     

Effects of flooding and drought on stomatal activity, transpiration, photosynthesis, water potential and water channel activity in strawberry stolons and leaves

Transpiration, xylem water potential and water channel activity were studied in developing stolons and leaves of strawberry (Fragaria × ananassa Duch.) subjected to drought or flooding, together with morphological studies of their stomata and other surface structures. Stolons had 0.12 stomata mm^–2 and a transpiration rate of 0.6 mmol H₂O m^–2 s^–1, while the leaves had 300 stomata mm^–2 and a transpiration rate of 5.6 mmol H₂O m^–2 s^–1. Midday water potentials of stolons were always less negative than in leaves enabling nutrient ion and water transport via or to the strawberry stolons. Drought stress, but not flooding, decreased stolon and leaf water potential from –0.7 to –1 MPa and from –1 to –2 MPa, respectively, with a concomitant reduction in stomatal conductance from 75 to 30 mmol H₂O m^–2 s^–1. However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from stolons of flooded strawberry plants showed no change in water channel activity. In these stolons, turgor may be preserved by maintaining root pressure, an electrochemical and ion gradient and xylem differentiation, assuming water channels remain open. By contrast, water channel activity was reduced in stolons of drought stressed strawberry plants. In every case, the effect of flooding on water relations of strawberry stolons and leaves was less pronounced than that of drought which cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. This stomatal closure under flooding may be due to release of stress ethylene. In the relative absence of stomata from the stolons, cellular (apoplastic) water transport in strawberry stolons was primarily driven by water channel activity with a gradient from the tip of the stolon to the base, concomitant with xylem differentiation and decreased water transport potential from the stolon tip to its base. Reduced water potential in the stolons under drought are discussed with respect to reduced putative water channel activity.     

Xylem water flow in a crack willow tree (Salix fragilis L.) in relation to diurnal changes of environment

Summary The diurnal course of the xylem water flow in a solitary Salix fragilis L. tree in a wet grassland was measured using the tissue heat-balance method. There was considerable variation due to meteorological factors. Maximum flow rate was 0.4 kg h^-1 m^-2 of crown projection area, or 5.9 kg h^-1 kg^-1 leaf dry weight. The daily total was 2.4 kg m^-2 day^-1 or 36 kg kg^-1 day^-1. Water flow decreased immediately at the tree base and at the branches after start of rain, and in a branch, after cutting it off: the time constant of the system was 600–700 s in both cases. The part of the crown oriented to the sun transpired up to ten times as much as the shaded part. Over 70% of the total cross-sectional area of the conductive xylem vessels of the trunk was used by the transpiration flow. The water content of the trunk tracked the diurnal changes of the xylem water flow rate with a short time-lag. During the day, 1% of the trunk volume was temporarily exploited as water reserve, an amount equalling 3% of daily water loss. The stereometric configuration of the crown significantly influenced its water loss. During the summer period, about 33 mature (polycormic) trees per ha may drain 100% of water consumed by the present-day sedge-grass marsh.     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

正常和环割条件下不同形态氮素添加对红椎幼苗光合特性的影响

采用盆栽试验和韧皮部环割方法研究了碳水化合物供应、氮素形态及其交互作用对红椎幼苗叶片光合特性的影响。无机氮源采用硝酸铵(AN)、铵态氮(NH_4~+-N)、硝态氮(NO_3~--N),有机氮源采用尿素(Urea)、精氨酸(Arg)和甘氨酸(Gly),氮素施用量均为10g N/m~2,处理时间为10 d。研究结果表明,环割、氮素形态及其交互作用均显著影响了红椎幼苗叶片的净光合速率(Pn);单一环割处理显著降低了红椎幼苗叶片Pn、气孔导度(Gs)、胞间CO_2浓度(Ci)、蒸腾速率(Tr)、相对叶绿素含量和CO2利用效率(CUE),但显著提高了叶片水分利用效率(WUE)和气孔限制值(Ls),其叶片Pn降低主要是由气孔限制和叶绿素含量降低所导致的。正常条件下,所有氮素形态处理均显著降低了红椎幼苗叶片的Pn、Gs、Ci和Tr,但显著升高了叶片Ls和WUE,其叶片Pn降低的主要原因是供氮过高引起的叶片气孔限制。正常条件下,除Arg处理外,其他氮素形态均显著升高了红椎幼苗叶片的CUE,其中以Gly处理的促进作用最大。环割条件下,3种有机态氮素的添加均显著缓解了单一环割处理对红椎幼苗叶片Pn的抑制作用,尤其以精氨酸最为明显,供应精氨酸的叶片Pn值从单一环割处理的强烈抑制中恢复到了对照水平,而无机态氮的供应均未显著改变这种抑制。结果表明短期碳水化合物供应的阻断会显著抑制红椎幼苗叶片的光合能力,适量供应精氨酸、甘氨酸和尿素等有机态氮会有效缓解这种抑制作用,其中以精氨酸的作用最为明显。     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.