Données préliminaires sur la transpiration du riz pluvial,variété IRAT 13, soumis àune carence hydrique

Summary The transpiration of a rice plant, variety IRAT 13, grown in a controlled environment, was investigated by means of a continuous record of the difference between the rate of water vapour in the air admitted in the growth cabinet and that in the air issued from it. The plant was submitted to water stress by stopping the irrigation to the soil in which the plant was grown.The plant reacted rapidly to the stress settlement, though the soil water potential was still high: on the one hand, the mean diurnal and nocturnal transpiration values, which increased linearly with time, became to decrease linearly. On the other hand, the transpiration cycles, over 24 h periods, became distorted, likely in relation with the rate of the nocturnal rehydration of the plant.     

氚水在模拟水稻－水－土壤生态系统中的行为

采用模拟污染物的同位素示踪技术研究HTO(流水)在水稻-水-土壤模拟生态系统中的迁移、消长行为,并应用三库室开系统模型和非线性回归方法确定了水稻、水和土壤分室的拟合方程．结果表明,田表水中的HTO不仅在系统各分室间转移和分配,而且迅速向系统外散逸;HTO中的流以自由水氚和结合态流形式存在于水稻中,以吸湿性水流和结晶水氚存在于土壤,其中自由水氚(或吸湿性水氚)的比活度大于结合态流(或结晶水氚);水稻植株和土壤中HTO比活度随时间增加至最大值后又趋于下降,而结合态氚则呈缓慢增加;水稻茎秆中的总氚比活度高于其它各部位,而后逐渐趋于动态平衡．对实验数据进行回归分析得：田表水、土壤和水稻植株中的总氚比活度分别为Cw(t)=32.19c^-0.0353t 99.94c^-0.330t、Cs(t)=20.42(e^-0.0353t-e^-0.330t)和Cr(t)=38.49c^-0.0353t-10.13e^-0.330t-28.36c^-2.5744t。方差分析结果表明,各回归方程较好地反映了HTO在水稻-水-土壤生态系统中的行为．     

Evaluation of the transpiration rate of lotus using the stem heat-balance method

To reveal the mechanism of transpiration by hydrophytes in the field, it is necessary to evaluate the transpiration rate without the effect of the evaporation from the water surface. In order to test the suitability for evaluating the transpiration rate of lotus (Nelumbo nucifera Gaertn.) leaves in the field, stem heat-balance method was applied and the obtained sap-flow rate was compared with the transpiration rate measured by weighing and with the overall canopy evapotranspiration rate by means of the eddy covariance technique. The transpiration rate estimated with the sap-flow measurements showed good agreement with that obtained from the weighing method. Lotus has many air canals in its petiole to carry oxygen-rich air to the rhizome and methane- and carbon dioxide-rich air back to the atmosphere, but there was little effect of the mass flow of air through these canals on the sap-flow rates. In the field observations, the canopy evapotranspiration rate (0.28 mm h⁻¹ at maximum) was nearly equal to the sum of the transpiration rate from all sunlit leaves (0.30 mm h⁻¹), and the contribution of the transpiration from shaded leaves and evaporation from the water surface was considered to be minor in the seasons when the leaves were fully developed. Evaluation of bulk leaf conductance revealed that the conductance in the leaf boundary layer of lotus could be low (ca. 0.23 mol m⁻² s⁻¹) because of its large leaf area. The low conductance in the leaf boundary layer would increase leaf temperature, which, in turn, would generate air circulation within the plant's ventilation system. Because there was a linear relationship between transpiration rate and the leaf-to-air vapor-pressure deficit, with no apparent maximum, high vapor-pressure deficits (3.4 kPa at maximum) did not appear to cause significant stomatal closure in lotus plants. The stomata of lotus leaves play a role as air inlets to carry oxygen-rich air to the rhizome, so their low sensitivity would help to increase air intake.     

采煤塌陷影响下土壤含水量变化对柠条气孔导度、蒸腾与光合作用速率的影响

采煤塌陷引起的土壤环境因子的变化对矿区植物生长的影响越来越受到人们的关注,气孔导度、蒸腾与光合作用作为环境变化响应的敏感因子,研究植物气孔导度、蒸腾与光合作用的变化是揭示荒漠矿区自然环境变化及其规律的重要手段之一。研究采煤塌陷条件下植物光合生理的变化是探究煤炭开采对植物叶片水分蒸腾散失和CO_2同化速率影响的关键环节,是探讨采煤塌陷影响下植物能量与水分交换动态的基础,而采煤矿区植物叶片气孔导度、蒸腾与光合作用速率对采煤塌陷影响下土壤含水量变化的响应如何尚不清楚。选取神东煤田大柳塔矿区52302工作面为实验场地,以生态修复物种柠条为研究对象,对采煤塌陷区和对照区柠条叶片气孔导度、蒸腾和光合作用速率以及土壤体积含水量进行监测,分析了采煤塌陷条件下土壤含水量的变化以及其对柠条叶片气孔导度、蒸腾与光合作用速率的影响。结果显示:(1)煤炭井工开采在地表形成大量裂缝,破坏了土体结构,潜水位埋深降低,土壤含水量均低于沉陷初期,相对于对照区,硬梁和风沙塌陷区土壤含水量分别降低了18.61%、21.12%;(2)柠条叶片气孔导度、蒸腾和光合作用速率均与土壤含水量呈正相关关系;煤炭开采沉陷增加了地表水分散失,加剧了土壤水分胁迫程度,为了减少蒸腾导致的水分散失,柠条叶片气孔阻力增加,从而气孔导度降低,阻碍了光合作用CO_2的供应,从而导致柠条叶片光合作用速率的降低,蒸腾速率也显著降低。     

Etude de la reponse à un stress hydrique de quelques varietés de riz pluvial et de riz irrigué

Résumé La transpiration de plants de riz cultivés sur sol en chambre de culture, est suivie en continu pendant une séquence déssèchement-irrigation. Les cycles jouraliers de transpiration sont présentés avant et pendant le stress hydrique. Quatre variétés montrent une augmentation de la transpiration nocturne qui disparait lors du stress hydrique. Il apparait que la régulation stomatique est indépendante du potentiel hydrique du sol jusqu'à une valeur seuil au-delà de laquelle la baisse de transpiration est proportionnelle à la baisse du potentiel hydrique. Cette valuer seuil est plus basse pour les variétés de type irrigué.

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Water stress response for some cultivars of upland- and lowland riceI. Effects on transpiration

Summary The transpiration of rice seedlings grown on soil in a growth chamber was observed during a soil drying/rewetting sequence. The diurnal cycles of transpiration are shown before and during the water stress period. Four cultivars showed an increase of transpiration during the night. This increase disappeared during the water stress period. The stomatal regulation appeared not to depend on the soil water potential until a certain value. Below this soil-water threshold, the decrease of transpiration was proportion to the decrease of water potential. This threshold value is lower for the rain-fed cultivars than for the irrigated cultivars.

     

Transpiration and Calcium Deposition by Unifoliate Leaves of Phaseolus vulgaris Differing in Maturity

Unifoliate leaves were individually enclosed in clear, plastic chambers for the 24 hour treatment periods and then sacrificed for Ca analysis. Two transpiration rates were obtained by passing dry air through the chambers tising flow rates of 160 and 260 cm³/min. A third rate was obtained by a combination of shade and the lower air flow rate. Neither the transpiration rate nor solution-Ca concentration (0.5mM and 2.5 mM of 0.1, and 0.5 strength Hoagland solution) altered the amount of Ca deposited in the unifoliate leaves of 22 day old bean plants (Phaseolus vulgaris). The transpiration rate per unit area of leaf remained constant for all ages studied (1l–20 days) and was 1.8, 2.7, 3.6 g H₂O per dm² day for the three different imposed conditions. A definite pattern of Ca deposition occurred. With all the transpiration rates there was a maximum rate of calcium deposition at 13 days of growth and a gradual decrease thereafter. When the Ca concentration of the nutrient solution was 20 μg/ml the daily Ca deposition in terms of water transpired by the unifoliate leaves exceeded this amount, except for the oldest leaf tested, and, the maximum Ca to water ratios were 250, 320, and 430 (μg Ca/g) in order of decreasing transpiration rates. The uptake of Ca against a concentration gradient and approximately the same total uptake regardless of transpiration rates and solution concentrations used, firmly suggest that Ca secretion into root-xylem elements from a surrounding low level Ca solution requires energy expenditure by the plant. A possible explanation was proposed for the decreased rate of Ca deposition by the unifoliate leaves subsequent to the 13th day.     

Recovery responses of photosynthesis,transpiration, and stomatal conductance in kidney bean following drought stress

Photosynthesis, transpiration, stomatal conductance and chlorophyll fluorescence characteristics were examined in kidney bean plants, with developing gradually water stress for several days after watering and then permitted to recover by re-watering. The photosynthetic rate, transpiration rate, and stomatal conductance decreased rapidly by withholding water for 2 days. The F_v/F_m of chlorophyll fluorescence characteristics slightly decreased when the water was withheld for 7 days. After re-watering the rate of recovery of photosynthesis, transpiration, and stomatal conductance decreased gradually as the days without watering became longer. The differences existed in rates of recovery of photosynthesis, transpiration, and stomatal conductance following drought stress. Among the fractional recoveries the highest was photosynthesis, and the lowest was stomatal conductance. Photosynthesis rate following drought stress was rapidly recovered until 2 days after re-watering, then recovered slowly. The critical time for the recovery of photosynthesis was recognized. The results show clearly a close correlation between the leaf water potential and the recovery level and speed of photosynthesis, transpiration, and stomatal conductance.     

Genotypic variation within sorghum for transpiration response to drying soil

Aims

Sorghum is commonly grown under dryland conditions, where yields are limited by soil water deficits. Yield increase may be possible by selecting genotypes that express traits that are desirable for water-limited conditions. Water conservation is one possible trait for increasing yield since this trait could increase water availability during critical stages of crop development. Water conservation could be achieved by slower transpiration rate with soil drying initiated at a high fraction of transpirable soil water (FTSW) so that the use of soil water is extended over a longer period of time. This water conservation strategy may allow the crop to have water available during the critical phase of grain filling. Therefore, the objective of this study was to compare genotypes of sorghum for possible differences in the threshold for the decline in transpiration.

Methods

Sixteen sorghum genotypes were selected for this study based on differences in their sensitivity to elevated vapor pressure deficit (VPD). These genotypes were subjected to dry-down experiments in pots in a greenhouse to determine the threshold FTSW for the decrease in transpiration rate as the soil dried.

Results

Differences in the FTSW threshold for transpiration decline were observed to range among genotypes from 0.32 to 0.48. The genotypes differed between low and high FTSW thresholds in the same manner as they did for a breakpoint in the VPD or lack of breakpoint, respectively. Those genotypes with high FTSW thresholds exhibited the hypothesized desired trait. However, they did not exhibit the water conserving trait of limited transpiration rate at high VPD. On the other hand, those genotypes with a low FTSW threshold were those selected for limited transpiration rate at high VPD. These genotypes also differed based on their transpiration rate under well-watered conditions with the genotypes with a low FTSW threshold exhibiting a low transpiration rate.

Conclusions

These results demonstrated that among the sorghum genotypes there are several alternative traits for enhancing soil water conservation for growth under dry land conditions.     

永定河沿河沙地杨树人工林蒸腾耗水特征及其环境响应

杨树是我国北方最常见的人工造林树种之一。一直以来在干旱、半干旱地区,速生杨树用材林和生态防护林的耗水问题备受关注。研究不同生长发育阶段杨树人工林蒸腾耗水及其对各环境因子的响应对于实现杨树人工林可持续经营具有重要价值。采用树干液流法结合微气象观测系统和土壤水分观测,在2010—2011年对位于北京南郊大兴林场、林龄为13a的杨树人工林林分蒸腾耗水和环境因子进行了同步观测,以期能够探究该林分的蒸腾耗水及其对环境因子的响应。结果表明,树干液流密度(Js)日变化呈明显的单峰曲线,单株样木耗水量随着胸径的增加而增大。在半小时尺度上,单株树木Js与浄辐射(Rn)、饱和水气压差(VPD)存在时滞,这种时滞现象随土壤水分条件不同而变化。林分蒸腾耗水总量在2010和2011年生长季内分别为113.7 mm和174.8 mm,占同期降雨的30.2%和36.9%,与该杨树人工林前期研究相比,随着林龄的增长2010—2011年的蒸腾量呈减小趋势。日尺度上,该人工林蒸腾耗水与净辐射(Rn)、饱和水汽压差(VPD)和土壤体积含水率(SWC)显著相关,在不同土壤水分条件下Rn与林分蒸腾的相关关系发生变化,而VPD过高会对林分蒸腾产生抑制。林分月蒸腾和年总蒸腾主要取决于同期降雨量,因此,降雨年际差异较大时,蒸腾的年际变化也相应较大。     

水分胁迫对沙棘(Hippophae rhamnoides)和中间锦鸡儿(Caragana intermedia)蒸腾作用影响的比较

多年生灌木沙棘和中间锦鸡儿是黄土高原生态重建的重要物种,设计人工模拟水分胁迫实验,测量沙棘和中间锦鸡儿蒸腾作用的各种指标,研究其蒸腾特性对水分胁迫的适应方式。结果表明,同等水分处理条件下,中间锦鸡儿单叶水平上的蒸腾速率高于沙棘。沙棘和中间锦鸡儿的蒸腾速率日进程在晴天、阴雨天和生长发育的不同阶段明显不同。夜间蒸腾占全天蒸腾的比例相当大,夜间蒸腾在不同物种之间、不同供水量之间存在明显差异,而且越干旱的环境比例越大。两种植物气孔阻力的季节变化格局在不同水分处理间大体相似。沙棘的昼夜蒸腾节律在各种水分处理条件下都表现出明显的气孔振荡现象,而中间锦鸡儿没有。叶片温度、光合有效辐射和气孔阻力是各种水分条件下沙棘和中间锦鸡儿蒸腾作用的共同的限制因子,相对于沙棘,中间锦鸡儿还更多地受到空气相对湿度的影响。     

A reinterpretation of stomatal responses to humidity

The stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ?g/?E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance g_m obtained by extrapolation to zero transpiration, and a maximum rate of transpiration E_m obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO₂ concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.     

CO2浓度倍增对8种作物叶片光合作用、蒸腾作用和水分利用效率的影响

揭示作物光合作用、蒸腾作用和水分利用效率(WUE)对大气CO₂浓度变化的响应, 对预测未来大气CO₂浓度升高条件下作物生产力与需水规律的变化具有重要意义。在自然CO₂浓度、CO₂倍增和倍增后恢复到自然CO₂浓度3种情况下, 对大豆(Glycine max)、甘薯(Ipomoea batatas)、花生(Arachis hypogaea)、水稻(Oryza sativa)、棉花(Gossypium hirsutum)、玉米(Zea mays)、高粱(Sorghum vulgare)和谷子(Setaria italica) 8种作物的气体交换参数进行了研究。结果表明: CO₂浓度倍增可以提高光合速率, 降低蒸腾速率, 从而提高WUE, 其中光合速率提高的贡献更大; C₃比C₄作物的光合速率、WUE增幅大, C₃作物光合速率提高对WUE的贡献大于C₄作物; 通过对比倍增后恢复到自然CO₂浓度时气体交换参数随环境条件变化的响应确定了其内在调控机制; 倍增后恢复到自然CO₂浓度时作物光合速率低于自然CO₂浓度下的光合速率, 而蒸腾速率无明显差异。由此判断: CO₂浓度倍增下存在光合下调现象, 这可能是由于Rubisco酶蛋白含量、活化水平和比活性降低等“非气孔因素”造成的, 并非由气孔导度的降低引起的。     

Quantitative analysis of transpiration stream dynamics in an intact cucumber stem by a heat flux control method

Water flux of transpiration stream in an intact stem of the 10 leaf stage cucumber plant (Cucumis sativus L. cv. Chojitsu-Ochiai) was measured by a novel system of heat flux control method with a resolution of 1 × 10⁻³ grams per second and a time constant of 1 minute; two heat flux control sensors were attached to the seventh internode and the stem base. The transpiration stream responded clearly to leaf transpiration and root water absorption when the plant was exposed to light, and the water flux at the stem base corresponded to the transpiration rate per plant in steady state. Root water absorption lagged about 10 minutes behind leaf transpiration. Dynamics of water fluxes were affected by the lag of water absorption in roots, and temporary water loss caused by rapid increase in leaf transpiration was buffered by about 5% of the water content in the stem.     

Evaluation of canopy transpiration rate by applying a plant hormone “abscisic acid”

A method for evaluation of temporal changes in canopy transpiration rate and stomatal conductance in crop fields by using a plant hormone abscisic acid (ABA) has recently been developed. The method was applied to a corn canopy at different growth stages in the upper Yellow River basin, China. Diurnal changes in the canopy transpiration rate and stomatal conductance were evaluated at the initial stage with a leaf area index (LAI) of 0.37 on June 7 and the crop development stage with an LAI of 4.39 on July 15, 2005. The proportions of the accumulated transpiration rate during daytime to the accumulated evapotranspiration were 24% and 74% at the initial and crop development stages, respectively. Stomatal conductance varied in parallel with transpiration rate in the initial stage of the crop. However, in the crop development stage with low soil water content, stomatal conductance reached the maximum value at 10:00 a.m. and thereafter decreased rapidly at around noon with high evaporative demand to corn canopy. This shows the midday stomatal closure was caused by excessive water stress to corn canopy in the crop development stage. Thus, the proposed method with ABA application is useful for evaluation of temporal changes in transpiration rate and stomatal conductance, and hence, can detect the plant water stress.     

Movement of Water and Solutes in Sieve Tubes of Willow in Response to Puncture by Aphid Stylets. Evidence against a mass flow of solution

Experiments are described in which bark strips of willow were sealed to polythene tubes having two compartments. This allowed investigations to be made of the transport along the sieve tubes of tritiated water, ¹⁴C-labelled sugars, and ³²P-phosphates from one compartment, towards a stylet situated in the bark over the other compartment. Although activity from both ¹⁴C and ³²p was detected in the stylet exudate usually within 1 hour from isotope application, tritium activity was never detected even after a period of 8 hours in most experiments, though in certain cases, very low activities were detected after 4 hours. Subsequent experiments in which stylets were sited over both compartments showed that tritium activity moved laterally into the punctured sieve element more rapidly than either ¹⁴C or ³²P. Experiments using both live and dead bark in which stylets were not employed, showed that within 4 hours tritium activity had moved by diffusion along the whole length of a bark strip, therefore after this time tritium activity could have moved into the stylet exudate by a diffusional process. The lack of rapid longitudinal movement of tritiated water along the sieve tubes, indicates that the transport process is unlikely to be a mass flow of solution.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The water relations of Picea abies in a healthy stand with green trees only and a declining stand with trees showing different stages of needle yellowing were investigated in northern Bavaria. The present study is based on observations of trees differing in their nutritional status but apparently green on both sites in order to identify changes in the response pattern which might be caused by atmospheric concentrations of air pollutants and could lead to the phenomenon of decline. Transpiration was measured as water flow through the hydroactive xylem using an equilibrium mass-flow measurement system. Total tree transpiration was monitored diurnally, from July 1985 until October 1985 at both sites. The relationship between transpiration and meteorological measurements indicated that transpiration was a linear function of the vapor pressure deficit. No differences in transpiration of green trees were observed between the two sites. Canopy transpiration was 57%–68% of total throughfall and 41%–54% of total rainfall. Due to this positive water balance, soil water potential at 10 and 20 cm depths remained close to-0.02 MPa (max.-0.09 MPa) for most of the summer. Soil water potential was correlated with the difference between the weekly precipitation and transpiration. No differences in the water relations of apparently healthy trees in the two P. abies stands were observed. It is concluded that differences between green trees at the two sites in terms of nutrient relations or growth rate cannot be explained by changes in whole-tree transpiration or soil water status.     

Effect of Abscisic Acid on the Transpiration of Zea mays

Intact plants of Zea mays L. were treated with foliar sprays of cis-trans-abscisic acid (ABA) at concentrations from 10⁻⁹ to 10⁻⁴M. Even the lowest concentration caused a reduction of the transpiration rate as measured between 1 and 33 h after spraying. With increasing ABA concentrations, there was a nearly linear relationship between the logarithm of the ABA concentration and the (decreasing) transpiration rate within that period. Subsequently a partial recovery of the transpiration rate set in, beginning progressively later as the ABA concentration was increased. After 5 1/2 days the transpiration rate of plants treated with 10⁻⁹ and 10⁻⁸M was nearly back to normal, whereas plants treated with 10⁻⁴M transpiration at only about 2/3 their normal rate. In experiments with detached maize leaves supplied with water or ABA solutions (10⁻⁸ to 10⁻⁵M) through their cut bases, the transpiration of control leaves decreased gradually to a low level in 24 h. ABA caused a marked and rapid reduction of the transpiration rate compared to that of the controls. After a few hours, the transpiration of the treated leaves decreased at a slower rate than that of the controls, thus approaching the control values. After 35 h, the transpiration of leaves treated with 10⁻⁵M ABA was nearly the same as in untreated leaves. Exchanging the ABA solution for distilled water after 24 h had little effect on the subsequent course of the transpiration rate.     

Photosynthetic acclimation to elevated CO2 is dependent on N partitioning and transpiration in soybean

Physiological processes that modulate photosynthetic acclimation to rising atmospheric CO₂ concentration are subjects of intense discussion recently. Apparently, the down-regulation of photosynthesis under elevated CO₂ is not understood clearly. In the present study, the response of soybean (Glycine max L.) to CO₂ enrichment was examined in terms of nitrogen partitioning and water relation. The plants grown under potted conditions without combined N application were exposed to either ambient air (38 Pa CO₂) or CO₂ enrichment (100 Pa CO₂) for short (6 days) and long (27 days). Plant biomass, apparent photosynthetic rate, transpiration rate and ¹⁵N uptake and partitioning were measured consecutively after elevated CO₂ treatment. Long-term exposure reduced photosynthetic rate, stomatal conductance and transpiration rate. In contrast, short-term exposure increased biomass production of soybean due to increase in dry weight of leaves. Leaf N concentration tended to decrease with CO₂ enrichment, however such difference was not true for stem and roots.A close correlation was observed between transpiration rate and ¹⁵N partitioned into leaves, suggesting that transpiration plays an important role on nitrogen partitioning to leaves. In conclusion existence of a feed back mechanism for photosynthetic acclimation has been proposed. Down-regulation of photosynthetic activity under CO₂ enrichment is caused by decreasing leaf N concentration, and reduced rate of transpiration owing to decreased stomatal conductance is partially responsible for poor N translocation.     

Effects of Kinetin on Transpiration Rate and Abscisic Acid Content of Water Stressed Young Wheat Plants

Effects of kinetin on transpiration rate and abscisic acid content were determined. Leaves from 9-day-old wheat plants (Triticum aestivum L. cv. Weibull's Starke II) were used. —Transpiration rate decreased in excised leaves put in water, but it was maintained at a higher rate when kinetin was supplied. When excised leaves were water stressed by air-drying for 1 h, addition of kinetin resulted in a considerable stimulation of transpiration rate. The effect reached its maximum after 15 h and this level remained relatively unchanged for at least 10 h. Intact seedlings which were stressed before leaf excision, showed only a slight stimulation of kinetin on transpiration rate. — Abscisic acid content slowly increased up to three-fold in 2 days in excised leaves put in water. In excised and water-stressed leaves the abscisic acid content was reduced during the first 24 h and then increased. As the leaves were fully turgid, the increase could not have been caused by water stress. However, both in stressed and unstressed leaves kinetin addition reduced the increase in abscisic acid content. — It is suggested that the stimulation by kinetin on transpiration rate in excised and water stressed leaves was mainly due to the combined effect of (1) a reduction in the activity of endogenous cytokinins, (2) kinetin acting as a ‘substitute’ for the inactivated cytokinins but exerting a stronger effect on transpiration than the endogenous cytokinins, and (3) the ‘extra’ reduction in abscisic acid content caused by the kinetin treatment. Furthermore, the results indicate that changes in cytokinins might be partly responsible for the aftereffect on transpiration.     

Hydraulic architecture and water use of selected species from a lower montane forest in Panama

 Plant water relations of nine woody species were studied in a lower montane rain forest in Panama. These data provide a partial test of the hypothesis that hydraulic architecture of lower montane species might limit transpiration and thus leaf size or nutrient transport (as suggested by J. Cavelier and E. G. Leigh, respectively). Diurnal variation in leaf transpiration was closely correlated with changes in net radiation. Peak transpiration rates (7 × 10^–5 kg s^–1 m^–2) were as high as peak transpiration rates from tropical lowland forests but mean daily water use [0.39 ± 0.08 (SEM) kg m^–2 day^–1] were mostly lower than comparable data from tropical lowland forests. Thus transpiration rates are sufficiently high for sufficiently long periods to make it unlikely that nutrient transport is limited by transpiration. Another objective of this study was a comparison of two different methods to measure hydraulic conductance (K_h = flow rate per unit pressure gradient) and leaf specific conductance of stem segments (K_L = K_h/leaf area distal to the segment). The results obtained with the traditional conductivity apparatus and the high pressure flow meter method, yielded similar results in six out of seven cases. Received: 20 March / Accepted: 21 October 1997