Evidence from Amazonian forests is consistent with isohydric control of leaf water potential

Climate modelling studies predict that the rain forests of the Eastern Amazon basin are likely to experience reductions in rainfall of up to 50% over the next 50-100 years. Efforts to predict the effects of changing climate, especially drought stress, on forest gas exchange are currently limited by uncertainty about the mechanism that controls stomatal closure in response to low soil moisture. At a through-fall exclusion experiment in Eastern Amazonia where water was experimentally excluded from the soil, we tested the hypothesis that plants are isohydric, that is, when water is scarce, the stomata act to prevent leaf water potential from dropping below a critical threshold level. We made diurnal measurements of leaf water potential (psi 1), stomatal conductance (g(s)), sap flow and stem water potential (psi stem) in the wet and dry seasons. We compared the data with the predictions of the soil-plant-atmosphere (SPA) model, which embeds the isohydric hypothesis within its stomatal conductance algorithm. The model inputs for meteorology, leaf area index (LAI), soil water potential and soil-to-leaf hydraulic resistance (R) were altered between seasons in accordance with measured values. No optimization parameters were used to adjust the model. This 'mechanistic' model of stomatal function was able to explain the individual tree-level seasonal changes in water relations (r2 = 0.85, 0.90 and 0.58 for psi 1, sap flow and g(s), respectively). The model indicated that the measured increase in R was the dominant cause of restricted water use during the dry season, resulting in a modelled restriction of sap flow four times greater than that caused by reduced soil water potential. Higher resistance during the dry season resulted from an increase in below-ground resistance (including root and soil-to-root resistance) to water flow.     

Nighttime sap flow of Acacia mangium and its implications for nighttime transpiration and stem water storage

Aims Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night. This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China. Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance.Methods Granier's thermal dissipation method was used to determine the nighttime sap flux of A. mangium. Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees.Important findings Nighttime sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow in A. mangium. Mean nighttime sap flow was higher in the less precipitation year of 2004 (1122.4 mm) than in the more precipitation year of 2005 (1342.5 mm) since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow. Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow, they could only explain a small fraction of the variance in nighttime sap flow. The total accumulated water loss (E L) by transpiration of canopy leaves was only ~2.6–8.5% of the total nighttime sap flow (E t) during the nights of July 17–18 and 18–19, 2006. Therefore, it is likely that the nighttime sap flow was mainly used for refilling water in the trunk. The stem diameter at breast height, basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features, such as tree height, stem length below the branch, and canopy size. The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3% depending on different DBH class and was considerably higher in the dry season compared to the wet season.     

Ecophysiological responses of two closely related Magnoliaceae genera to seasonal changes in subtropical China

Aims Plants use a variety of hydraulic strategies to adapt to seasonal drought that differ by species and environmental conditions. The early-diverging Magnoliaceae family includes two closely related genera with contrasting leaf habits, Yulania (deciduous) and Michelia (evergreen), which naturally inhabit temperate and tropical regions, respectively. Here, we evaluate the hydraulic strategy of species from both genera that have been ex situ conserved in a subtropical region to determine how they respond to the novel cool–dry season climatic pattern.Methods We measured ecophysiological traits in five Michelia and five Yulania species conserved in the South China Botanical Garden in both wet and dry season conditions and monitored the whole-year sap flow for four of these species.Important findings We found that Magnoliaceae species that have been ex situ conserved in a subtropical climate did not suffer from excessive water stress due to the mild drought conditions of the dry season and the ecophysiological adjustments the species made to avoid this stress, which differed by leaf habit. Specifically, deciduous species completely shed their leaves during the dry season, while evergreen species decreased their turgor loss points, dry mass based photosynthetic rates, stomatal conductance and specific leaf areas (SLAs) compared to wet season measurements. In comparing the two distinct leaf habits during the wet season, the leathery-leaved evergreen species had higher leaf hydraulic conductance and leaf to sapwood area ratios than the papery-leaved deciduous species, while the deciduous species had greater hydraulic conductivity calculated on both a stem and leaf area basis, dry mass based photosynthetic rates, leaf nutrients, SLAs and stomatal sizes than the evergreen species. Interestingly, species from both genera maintained similar sap flow in the wet season. Both photosynthetically active radiation and vapour pressure deficit affected the diurnal patterns of sap flow in the wet season, while only vapour pressure deficit played a dominant role in the dry season. This study reveals contrasting hydraulic strategies in Yulania and Michelia species under subtropical seasonal conditions, and suggests that these ecophysiological adjustments might be affected more by leaf habit than seasonality, thus reflecting the divergent evolution of the two closely related genera. Furthermore, we show that Magnoliaceae species that are ex situ conserved in a subtropical climate are hydraulically sound, a finding that will inform future conservation efforts of this ancient family under the threat of climatic change.     

陇东旱地果园覆沙对苹果树蒸腾耗水及果实品质的影响

针对甘肃陇东旱地苹果园季节性干旱问题,以15年生长富2号苹果树为试材,测定果园覆沙后土壤水分、温度,以及果树主干液流速率、叶片气孔导度、果实品质等指标.结果表明：果园覆盖5 cm厚河沙,2-4月地温升高幅度低于1 ℃,6、7月晴天地温升高2.44 ℃,阴天地温升高2.61 ℃;在果树生长季节土壤含水量始终保持在田间持水量的60%以上.土壤含水量较高的时期（H期）晴天,树干液流曲线呈“几”字形宽峰曲线,覆沙处理液流启动时间较对照提前0.6 h,峰值较对照高25.5%,阴天峰值较对照高165.6%,且液流活动时间延长;土壤含水量较低的时期（L期）晴天,覆沙处理液流呈单峰曲线,启动时间较对照提前0.5～1 h,峰值在794 g·h^-1左右,而阴天液流启动时间较对照提前近1 h,峰值较对照高311.0%.3-7月对照的棵间蒸发量（E_s）较覆沙处理高156.0%,过多的地面蒸发是造成果树缺水的主要原因.覆沙后果实单果质量显著提高,果实硬度略有降低,果实可溶性固形物、维生素C、总糖、有机酸含量均有所提高.     

基于树干液流和土壤-叶片水势梯度分析荷木干湿季整树水分利用特征

运用Granier热消散探针连续监测荷木的树干液流,于2009年的湿季(8月)和干季(11月)选择天气晴朗的3d测定叶片水势,同步连续监测林冠上方光合有效辐射、土壤含水量、气温和空气相对湿度.结果表明:干湿季下荷木树干液流存在显著差异,此外,土壤水势和液流有较好的相关性,且干季时的相关性更好;荷木的叶面积/边材面积比值平均为(0.416±0.033)m2·cm-2,并与树高呈指数函数下降关系;随着11月土壤水势下降,荷木的整树水力导度和午间叶片水势也有所下降,但不明显;对叶片水势和整树蒸腾进行回归分析,二者之间呈二次多项式关系(P＜0.01),叶片水势并非无限制下降;结果还表明,大气水汽压亏缺(D)和叶片水势呈负相关,这是否空气温度和相对湿度或共同作用影响叶片水势,需要进一步研究.     

广州地区荷木夜间树干液流补水的影响因子及其对蒸腾的贡献

明确树木夜间水分补充现象有助于提高总蒸腾量和冠层气孔导度估算的精确度,进一步认识冠层蒸腾与树干液流之间存在的时滞关系.本研究采用热消散探针法测定了广州地区的荷木树干液流密度,同步监测了主要的环境因子,从不同时间尺度分析了树干夜间液流的水分补充现象.结果表明:与白天相比,荷木夜间液流密度较小,旱季变化幅度比湿季大;夜间水分补充的时间段主要在前半夜(18:00-22:00);年内各季节夜间水分补充量之间没有显著差异,与环境因子之间的偏相关关系不显著,但与胸径、树高、冠幅、树干生物量、冠层生物量的回归曲线拟合很好,表明树形特征和生物量能更好地解释夜间补水的变化;各季节夜间水分补充量对总蒸腾量的贡献有显著差异,旱季明显高于湿季.     

Water relations of baobab trees (Adansonia spp. L.) during the rainy season: does stem water buffer daily water deficits?

Baobab trees are often cited in the literature as water-storing trees, yet few studies have examined this assumption. We assessed the role of stored water in buffering daily water deficits in two species of baobabs (Adansonia rubrostipa Jum. and H. Perrier and Adansonia za Baill.) in a tropical dry forest in Madagascar. We found no lag in the daily onset of sap flow between the base and the crown of the tree. Some night-time sap flow occurred, but this was more consistent with a pattern of seasonal stem water replenishment than with diurnal usage. Intrinsic capacitance of both leaf and stem tissue (0.07-0.08 and 1.1-1.43 MPa(-1), respectively) was high, yet the amount of water that could be withdrawn before turgor loss was small because midday leaf and stem water potentials (WPs) were near the turgor-loss points. Stomatal conductance was high in the daytime but then declined rapidly, suggesting an embolism-avoidance strategy. Although the xylem of distal branches was relatively vulnerable to cavitation (P50: 1.1-1.7 MPa), tight stomatal control and minimum WPs near--1.0 MPa maintained native embolism levels at 30-65%. Stem morphology and anatomy restrict water movement between storage tissues and the conductive pathway, making stored-water usage more appropriate to longer-term water deficits than as a buffer against daily water deficits.     

断根处理对银杏树体水力特征及生长的影响

以8年生银杏为试材,采用不同程度的断根处理,测定处理后树体超声（ultrasound acoustic emission,UAE）信号、枝条栓塞程度（percentage loss of hydraulic conductance,PLC）、树干液流通量、气孔导度、蒸腾速率以及叶片水势等生理指标,分析断根对银杏树体水力特征及生长状况的影响.结果表明:断根处理后,在短时间内有大量UAE信号产生,且随断根程度的加重而增大,在6 h出现最大值,随后逐渐降低;枝条的PLC在起初的12 h迅速增加,24 h后增速趋于停滞;银杏树干液流通量、叶片气孔导度、蒸腾速率和水势明显降低,各处理间差异显著.经统计分析,累积UAE（cumulative number of UAE, cUAE）信号量与PLC呈现显著的正相关关系,与液流通量和叶片水势呈负相关关系.次年的单叶面积、新生枝条长度显著下降,下降程度随断根处理的加重而加剧.断根处理在短时间内增加了银杏管胞气穴数量和木质部栓塞的程度,影响到水分输送和叶片蒸腾,最终对叶片和枝条的生长产生负面影响.     

Coordination of stomatal,hydraulic, and canopy boundary layer properties: Do stomata balance conductances by measuring transpiration?

A striking coordination is observed in sugarcane between prevailing levels of stomatal opening and the hydraulic capacity of the soil, roots and stem to supply the leaves with water. This coordination of vapor phase and liquid phase conductances is associated with decreases in stomatal conductance on a leaf area basis that compensate for increasing leaf area during canopy development, causing transpiration to approach a maximum value on a per plant or ground area basis rather than increase linearly with leaf area. The resulting balance between water loss and water transport capacity maintains leaf water status remarkably constant over a wide range of plant. sizes and growing conditions. These changes in stomatal conductance during development are determined by changes in the composition of the xylem sap rather than by changes in leaf properties. Changes in boundary layer conductance resulting from non-developmental changes in canopy structure such as loding cause additional changes in stomatal conductance mediated by altered humidity at the leaf surface. These maintain a constant level of total canopy vapor phase conductance (stomatal and boundary layer in series) and a constant level of canopy transpiration. These patterns indicate that stomata exert an active role in regulating transpiration even in dense canopies. This control function is consistent with stomatal metering of transpiration, mediated by fluxes of root-derived materials in the xylem sap.     

How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponically were exposed to 5 degrees C for up to 4 d; stem girdling was used to inhibit the phloem transport from the leaves to the roots. After initial water stress, stomatal closure and an amelioration of root water transport properties allowed the plants to rehydrate and regain turgor. Chilling augmented the concentration of abscisic acid (ABA) content in leaves, roots and xylem sap. In intact plants stomatal closure and leaf ABA accumulation were preceded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdling did not affect the influence of chilling on plant water relations and leaf ABA content, but it reduced slightly the alkalinization of xylem sap and, principally, prevented the massive ABA accumulation in root tissues and the associated transport in the xylem that was observed in non-girdled plants. When the plants were defoliated just prior to chilling or after 10 h at 5 degrees C, root and xylem sap ABA concentration remained unchanged throughout the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humidity), there were no significant variations in endogenous ABA levels. The increase in root hydraulic conductance in chilled plants was a response neither to root ABA accretion, nor to some leaf-borne chemical signal transported downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with plants chilled at 100% relative humidity. It was concluded that the root system contributed substantially to the bean's ability to cope with chilling-induced water stress, but not in an ABA-dependent manner.     

Whole-tree level water balance and its implications on stomatal oscillations in orange trees [Citrus sinensis (L.) Osbeck] under natural climatic conditions

Sustained cyclic oscillations in stomatal conductance, leaf water potential, and sap flow were observed in young orange trees growing under natural conditions. The oscillations had an average period of approximately 70 min. Water uptake by the roots and loss by the leaves was characterized by large time lags which led to imbalances between water supply and demand in the leaves. The bulk of the lag in response between stomatal movements and the upstream water balance resided downstream of the branch, with branch level sap flow lagging behind the stomatal conductance by approximately 20 min while the stem sap flow had a much shorter time lag of only 5 min behind the branch sap flow. This imbalance between water uptake and loss caused transient changes in internal water deficits which were closely correlated to the dynamics of the leaf water potential. The hydraulic resistance of the whole tree fluctuated throughout the day, suggesting transient changes in the efficiency of water supply to the leaves. A simple whole-tree water balance model was applied to describe the dynamics of water transport in the young orange trees, and typical values of the hydraulic parameters of the transpiration stream were estimated. In addition to the hydro-passive stomatal movements, whole-tree water balance appears to be an important factor in the generation of stomatal oscillations.     

Ecological physiology of Pereskia guamacho, a cactus with leaves

The specialized physiology of leafless, stem-succulent cacti is relatively well understood. This is not true, however, for Pereskia (Cactaceae), the 17 species of leafy trees and shrubs that represent the earliest diverging lineages of the cacti. Here we report on the water relations and photosynthesis of Pereskia guamacho, a small tree of the semiarid scrubland of Venezuela's Caribbean coast. Sapwood-specific xylem conductivity (Ksp) is low when compared to other vessel-bearing trees of tropical dry systems, but leaf-specific xylem conductivity is relatively high due to the high Huber value afforded by P. guamacho's short shoot architecture. P. guamacho xylem is not particularly vulnerable to drought-induced cavitation, especially considering the high leaf water potentials maintained year round. This is confirmed by the lack of significant variation exhibited in Ksp between wet and dry seasons. In the rainy season, P. guamacho exhibited C3-like patterns of stomatal conductance, but during a prolonged drought we documented nocturnal stomatal opening with a concomitant accumulation of titratable acid in leaves. This suggests that P. guamacho can perform drought-induced crassulacean acid metabolism (CAM photosynthesis), although delta 13C values imply that most carbon is assimilated via the C3 pathway. P. guamacho leaves display very low stomatal densities, and maximum stomatal conductance is low whether stomata open during the day or night. We conclude that leaf performance is not limited by stem hydraulic capacity in this species, and that water use is conservative and tightly regulated at the leaf level.     

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short‐rotation bio‐energy culture

The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E_c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E_c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.     

Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.     

马占相思夜间树干液流的分配及其对整树蒸腾估算的影响

采用Granier热消散探针测定了马占相思(Acacia mangium)的树干液流,结合Li-6400光合测定系统测定的夜间叶片气孔导度和蒸腾,将夜间液流区分为夜间树干水分补充和叶片气孔蒸腾。叶片的蒸腾作用微弱,因此,夜间液流主要用于补充贮水部位的水分亏缺。马占相思夜间水分补充量年内和年际的变化不明显,树形特征的差异是解释夜间水分补充量变化的重要因子,夜间水分补充量对于整树蒸腾量的贡献因季节和树木径级的不同而有明显变化,但对整树总蒸腾量计算造成的误差可以忽略。     

Transpiration from a mature Eucalyptus globulus plantation in Portugal during a spring-summer period of progressively higher water deficit

The rates of transpiration from a mature Eucalyptus globulus Labill. stand in Portugal were evaluated during a drying period of the spring-summer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental period daily transpiration varied between 3.64 and 0.50 mm day⁻¹. For high-transpiration days, a good agreement was observed between Penman-Monteith estimates and sap flow measurements, both on a daily and on an hourly basis. However, for low-transpiration days, the Penman-Monteith model overestimated transpiration in comparison with the sap flow method. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods through a progressive stomatal closure. As soil moisture deficit increased, the daily maximum stomatal conductance decreased from 0.46 to 0.14 cm s⁻¹. The results also show that, on a seasonal basis, stomatal conductance and daily transpiration were mainly related to predawn leaf water potential and, thus, to soil moisture content. Received: 26 January 1996 / Accepted: 20 October 1996     

Response of citrus trees to modified radiation regime in semi-arid conditions

Citrus trees are characterized by a large canopy and low hydraulicconductivity. In Israel's semi-arid summer climate this couldcause transpiration to exceed water uptake and cause temporaryexcessive water deficits. It was hypothesized that reductionof radiative load would reduce transpiration and thus reducedeficits. Net radiation of lemon trees in the hottest season was reducedby shading hedgerows with reflective nets for approximatelyone month in both 1994 and 1995. Stem sap flow and climate variableswere measured continuously. Daily courses of leaf conductanceand leaf water potentials were measured on selected days. Midday net radiation below the dense and sparse shade net treatmentswas 47% and 73% of that above the control trees. Midday ‘sunlit’leaf temperatures below the nets were reduced by 2.7 and 1.6C,respectively. The reduction in net radiation caused large changes in leafconductance. Average midday sunlit leaf conductance measuredin 1995 under the dense and sparse treatments and control were4.1, 2.9 and 1.8mm s^–1, respectively (significantly differentat P <0.01). Similar differences in sunlit leaf conductancewere found in 1994. Shade leaf conductance was not affectedby the treatments. Daily total and midday sap flow under the dense net were reducedby 6–7% and 10–11%, respectively. Sap flow underthe sparse net did not change significantly in 1994, but in1995 daily and midday sap flows were reduced by 6% and 7%, respectively.Midday leaf water potentials increased by 0.2 and 0.1 MPa underdense shade in 1994 and 1995, respectively. Under sparse shademidday leaf water potentials increased by 0.1 MPa in 1994, butdid not change significantly in 1995. A modified Penman-Monteith model evaluated transpiration ifleaf conductance were constant in the different radiation environments.At leaf conductance levels found in the unshaded trees, denseshade was estimated to cause a 25% reduction in transpiration,while leaf conductance values found in trees under the denseshade would lead to an increase in transpiration of more than35% in unshaded trees. The ability of the tree to maintain almost constant transpirationin different radiation environments and thus avoid water deficitby adjusting the conductance of sunlit leaves is discussed interms of environmental influences and significance to the plant'swater balance. Key words: Tree transpiration, stomatal closure, climate modification, citrus     

Experimental Studies of the Factors Controlling Transpiration: III. THE INTERRELATIONS BETWEEN TRANSPIRATION RATE, STOMATAL MOVEMENT, AND LEAF-WATER CONTENT

Transpiration rates of single leaves of Pelargonium and wheatwere measured under constant conditions of light, temperature,and air flow. Concurrently, stomatal movement was followed withthe resistance porometer during cycles of changing water contentof the leaf and changes induced by light and darkness. Stomatalmovement was found to exert a large controlling influence onthe transpiration rate, whereas water content had an extremelysmall or negligible effect. An approximately inverse linearrelation between transpiration rate and logarithm of resistanceto viscous flow through the leaf is believed to be the resultantof an inverse curvilinear relationship between the diffusiveconductance of the stomata and log. leaf resistance and thedecreasing difference of vapour pressure arising from the highertranspiration rates with increasing stomatal conductances. Nevertheless,the relation demonstrates that the transpiration rate is influencedby the degree of stomatal opening throughout its entire range. There was some evidence of lower transpiration rates duringand after recovery from wilting than before wilting. This isattributed to a decrease in a cell-wall conductance, the evaporatingsurface being located within the cell wall. During wilting partiallyirreversible contraction of the cell wall occurs. There wasalso evidence of slow changes in cell volume at full turgidityattributable to plastic flow. These occurred when the leaf wastransferred from environments of a high to low potential forevaporation. Extensive movement of the stomata followed changes in leaf water,passive opening resulting from decrease and closure from increaseof leaf water. It is suggested that the direction and extentof stomatal changes induced by water deficits is a consequenceof the rate of change of leaf water content and not of the absolutevalues. The stomata also showed an enhanced tendency to closein dry moving air following a period of wilting even after theleaf had regained turgidity.     

Stem flow and porometer measurements of transpiration from honey mesquite (Prosopis glandulosa)

The objective of this study was to compare stem flow and porometermethods of measuring transpiration of honey mesquite (Prosopisglandulosa) trees on a semiarid site. Stem flow was measuredusing heat balance stem flow gauges. Porometer measurementsof leaf stomatal conductance (g_s) were made within foliage layersof each stem and scaled to transpiration values for the entirestem (E_stem) using stem leaf area. Simultaneous measurementsusing both methods were made diurnally and under artificiallyimposed stem shading or defoliation in June and October 1990.Stem flow and E_stem had similar diumal patterns except on 2d in June when E_stem increased during the afternoon while stemflow declined relative to midday values. During October, E_stemwas greater than stem flow throughout the day. This was attributedto sampling error in which only undamaged leaves were used forporometer measurements yet, by this time in the growing season,many leaves on each stem were damaged from insects or wind andlikely had lower transpiration rates. A regression coefficientbetween E_stem and stem flow of 0.79 in June and 0.91 in Octobersuggested the two methods were comparable, but there was considerablevariation between methods during peak transpiration rates. Bothtechniques detected that artificial shading or defoliation causedsimilar relative declines in transpiration. Results imply thatestimates of stem transpiration can be obtained by scaling porometermeasurements of leaves but accuracy declines at higher transpirationrates. Key words: Sap flow, evapotranspiration, stomatal conductance, scaling, water relations     

Changes with seasonal flooding in sap flow of the tropical flood-tolerant tree species, <Emphasis Type="Italic">Campsiandra laurifolia</Emphasis>

In order to determine how flooding affects sap flow and hydraulic conductivity of the tolerant species, Campsiandra laurifolia, trees growing in a tropical seasonally flooded forest in Venezuela were studied. We hypothesized that trees respond to rising-waters with a decrease in root-water absorption, caused by hypoxia, and stomatal conductance, and that this is reverted later on through a process of acclimation that involves improvement in water absorption. We followed the seasonal changes, of trees with the whole or part of the canopy exposed to air, in sap flow density, leaf stomatal conductance, leaf transpiration rate and xylem water potential. The highest daytime sap flow density occurred at noon and its proportion relative to the yearly maximum (drainage at falling-waters) was 41 (dry season), 15 (flooding by rising-waters for 2 weeks), 54 (2 months of flooding) and 41% (6 months of flooding). Since at rising-waters dawn xylem water potential remained high, it became apparent that the initial stages of flooding imposed a restriction to sap flow unrelated to water deficit. The decrease at rising-waters in highest daytime sap flow density was due to reduced leaf-specific hydraulic conductivity, whereas the recovery observed 1.5 months later was correlated to an increase in hydraulic conductivity, and attributed to acclimation. Sap flow density was highly and positively correlated with radiation at all seasons but rising-waters; also, the relationship with air water vapor saturation deficit was high and significant on dates other than at rising-waters. Results suggest that early flooding inhibited water absorption by roots and that this inhibition was overcome later on at a higher water column through an acclimation process involving the improvement of internal aeration by adventitious roots.