Low Sap Flow Research Articles

Xylem Hydraulic Conductance Role in Kiwifruit Decline Syndrome Occurrence

Kiwifruit decline syndrome (KiDS) has affected kiwifruit orchards for more than ten years in the Mediterranean area, severely compromising productivity and causing extensive uprooting. The affected plants go through an irreversible and fast wilting process. The problem has not been solved yet, and a single cause has not been identified. In this work, we carried out a survey on ten five-year-old healthy kiwifruit cv. Hayward plants cultivated in an area strongly affected by KiDS and characterised by a rising temperature and vapor pressure deficit (VPD). Five plants were located in a KiDS-affected orchard. Our goal was to assess the hydraulic conductance of asymptomatic plants in a KiDS-affected area where rising climate change stress is underway. Our hypothesis was that a rising temperature and VPD could impair xylem functionality, leading the plants to develop strategies of tolerance, such as vessel narrowing, or stress symptoms, such as cavitation or implosion, inducing a higher risk of KiDS onset. Hydraulic conductance was investigated using a physiological and morphological approach to detect trunk sap flow, trunk growth and daily diameter variations, leaf gas exchanges and temperature, stem water potential, and the root xylem vessel diameter and vulnerability to cavitation. A strong xylem vessel narrowing was observed in all plants, with the highest frequency in the 30–45 µm diameter class, which is an indicator of long-term adaptation to a rising VPD. In some plants, cavitation and implosion were also observed, which are indicative of a short-term stress response; this behaviour was detected in the plants in the KiDS-affected orchard, where a high leaf temperature (>39 °C), low stomatal conductance (<0.20 mol H2O m−2 s−1) and transpiration (<3 mmol H2O m−2 s−1), low stem water potential (<−1 MPa), high vulnerability to cavitation (3.7 μm mm−2), low trunk sap flow and high daily stem diameter variation confirmed the water stress status. The concurrence of climate stress and agronomic management in predisposing conditions favourable to KiDS onset are discussed, evidencing the role of soil preparation, propagation material and previous crop.

Partitioning of respired CO2 in newly sprouted Moso bamboo culms.

Stem respiration (R s) plays a vital role in ecosystem carbon cycling. However, the measured efflux on the stem surface (E s) is not always in situ R s but only part of it. A previously proposed mass balance framework (MBF) attempted to explore the multiple partitioning pathways of R s, including sap-flow-transported and internal storage of R s, in addition to E s. This study proposed stem photosynthesis as an additional partitioning pathway to the MBF. Correspondingly, a double-chamber apparatus was designed and applied on newly sprouted Moso bamboo (Phyllostachys edulis) in leafless and leaved stages. R s of newly sprouted bamboo were twice as high in the leafless stage (7.41 ± 2.66 μmol m-2 s-1) than in the leaved stage (3.47 ± 2.43 μmol m-2 s-1). E s accounted for ~80% of R s, while sap flow may take away ~2% of R s in both leafless and leaved stages. Culm photosynthesis accounted for ~9% and 13% of R s, respectively. Carbon sequestration from culm photosynthesis accounted for approximately 2% of the aboveground bamboo biomass in the leafless stage. High culm photosynthesis but low sap flow during the leafless stage and vice versa during the leaved stage make bamboo an outstanding choice for exploring the MBF.

Temporal and spatial diurnal dynamics of the hysteresis of weighed transpiration and water transport in tomatoes

In studies of terrestrial water cycles and farmland irrigation management, plant transpiration is an important reference data source. There is a hysteresis effect between plant transpiration and meteorological factors, which is important for predicting plant transpiration. However, the hysteresis effect in diurnal dynamics (time) and between the processes of water from soil to air in the soilplantatmosphere continuum (SPAC) system (space) remains unclear. In this study, the intensity of light radiation (high radiation, HR; low radiation, LR) and the amount of water used for irrigation (high irrigation, HW; low irrigation, LW) were used to form four treatments (HRHW, HRLW, LRHW, and LRLW) on tomato plants in a greenhouse. The diurnal dynamics of the transpiration of the whole plant by weighting (weighted transpiration, WT), sap flow (SF), and lag time in the process of water transport in tomato plants were analyzed by the data of every 10 min. The results showed that the WT exceeded the SF in the HRHW during 10:00–18:00 and in the HRLW during 10:00–16:00. In the HRHW, the proportion of SF in the upper part to the lower part of the plants was significantly lower than that in the other treatments during the daytime. The hysteresis effect was prevalent in the process of water transport from soil through plants to the atmosphere during the daytime. The lag time of WT to net radiation (Rn) and SF to WT changed more dramatically during the daytime under HR- conditions, and the maximum lag time was 60 min. Compared with the hysteresis of middle SF to low SF of plants, the lag time of top SF to middle SF had a greater fluctuation, with a maximum of 50 min (LRLW) and 60 min (HRHW and HRLW), respectively. The longest times at which of stem diameter (SD), fruit diameter (FD) and substrate water content (SWC) lagged behind SF were 60 min (LRHW), 70 min (HRHW and LRHW) and 50 min (HRHW and HRLW), respectively. After temporal and spatial dynamic hysteresis were added to the structural equation modeling (SEM), the path coefficients between most variables were improved, and the overall effect of the model was enhanced in all treatments. This provides a valuable direction for the future improvements in transpiration models, irrigation efficiency, and crop yield.

Low sap flow of Picea crassifolia and its influencing factors in Qilian Mountains, China

As an important part of ecological hydrology, transpiration is the basis for analyzing forest water cycle and healthy growth, and important for forest protection and scientific management. We used thermal diffusion probes (TDP) to continuously monitor sap flow of Picea crassifolia in the Qilian Mountains from 2017 to 2018 to explore water consumption of P. crassifolia, and analyze the main controlling factors for the growth and transpiration of spruce. The results showed that the instantaneous change of P. crassifolia sap flow showed a single-peak curve in sunny days, a multi-peak or double-peak curve in cloudy days, and basically no obvious regularity in rainy days. The sap flow density of Qinghai spruce was consistent with the dynamics of solar radiation. The sap flow started earlier and ended later on sunny days, and lasted for 12 to 14 hours. Due to the high altitude (2700 m), low air temperature, and low vapor pressure difference (VPD) in this area, the overall sap flow density was low, with an average of (0.86±0.49) kg·d-1. On the hourly scale, the instantaneous rate of sap flow was significantly affected by solar radiation and VPD. On the daily scale, soil temperature and soil moisture content of the 0-40 cm layer were significantly related to sap flow density. The spruce sap flow density decreased with the decreases of solar radiation, air temperature, and VPD. In the high-altitude forest area of Qilian Mountains, lower soil and air temperature as well as lower VPD and solar radiation were the causes of low sap flow in Picea crassifolia in this area.

Optimisation of CAG technique: An algorithm to estimate low and reverse sap flow in roots

The present study aims at developing a specific algorithm to automatize the calibrated average-gradient (CAG) heat pulse method and estimating low and reverse sap flow in roots, termed as CAGALG. The innovative algorithm is based on an automatic data processing to make easier the complexity in interpreting data from sap flow sensors, without losing the reliability on low and even reverse sap flows quantification. The CAGALG algorithm resorts to two mathematical tools to indicate reverse sap flow occurrence and clustering the roots per reverse sap flow patterns. Results showed a good correlation between the average temperature gradient (ΔTavg) and the corrected heat pulse velocity (VC) that was confirmed by the simulation study. The simulation provided a model aiming to estimate the thermal diffusivity, which has been shown to be below the default values usually attributed. Quantitative and temporal reverse sap flow occurrences were identified along the two years in which presented the results from the CAGALG in accordance with the traditional CAG.

An improved single probe method for sap flow measurements using finite heating duration

Because of its low cost and simple fabrication, it is easy to advocate for the single probe method as a method of choice in sap flow studies. An improved single probe method with finite heating duration (F-SPHP) is verified both in cut stem segments and in the field using mature beech (Fagus sylvatica L.) trees. The F-SPHP method is based on an analytical solution of the partial differential equation for combined heat conduction and convection, which shows large relative sensitivity to thermal conductivity (K). The F-SPHP method is able to measure sap flux densities (SFD) between 2 and 36 cm3 cm−2 h−1 (heat pulse velocity (Vh): 3–60 cm h−1) in the cut stem segment experiment. This is an improvement compared to the instantaneous single probe heat pulse (I-SPHP) method which cannot accurately measure low (Vh < 20 cm h−1) sap flow. Sapflow+ (a four-needle heat pulse method) is used for field validation, and when compared with gravimetric measurements, slopes of 0.853 and 0.730 are obtained for F-SPHP and Sapflow+, respectively. Patterns measured with F-SPHP and Sapflow+ in mature beech trees in the forest are similar at all measurement positions and temperature correction is needed for both methods when natural temperature gradients are steep. Compared with multi-probe methods, the single probe method with finite heating duration has the advantage of causing less damage to conductive tissue.

A Comparison of Sap Flow Measurements and Potometry in Two Tropical Lowland Tree Species with Contrasting Wood Properties

We evaluated the performance of the Heat Dissipation Technique (HDT) to measure sap flow in whole trees by comparison with potometric water uptake. Two tropical lowland species, Ochroma lagopus (balsa), a pioneer species with light wood and Hyeronima alchorneoides (pilón), a late-successional species with hard wood were examined. Diurnal courses of sap flow measured with the HDT showed good agreement with potometry. At the low sap flow rates (below 1 Kg h(-1)) occurring during nocturnal recharge HDT consistently underestimated sap flow rates. This resulted in the failure of the current version of the HDT to measure nocturnal water uptake, an important component of the water budget of at least one of the two species examined.

Quantifying water use in a plant–fungal interaction

Fungal rust galls are parasitic fungi on Acacia host plants. The amount of water required by fungal parasites has yet to be widely measured. Sap flow instrumentation was installed on branches of Acacia implexa to measure water use of a fungal rust gall. Host water use was also measured. Partial regression analysis, using gall sap flow against environmental variables, was performed to determine a possible driving mechanism of gall sap flow. Following the removal of galls from branches, sap flow immediately returned to, and stayed at, zero flow. Gall sap flow was 4.63 % of total host tree sap flow on a daily and cumulative basis. Vapour pressure deficit explained the greatest amount of variation in gall sap flow, suggesting gradients of water potential was the driving mechanism of water use. This study demonstrated that instruments which can measure low sap flow can be used to quantify fungal water use.

Environmental drivers of leaf phenology in trees of the tropical species Ficus obtusifolia

Vegetative as well as reproductive phenology of tropical trees frequently occur synchronously at the end of the dry season. It has been repeatedly observed on two individuals of the tropical species Ficus obtusifolia growing in Caracas that they lose their entire foliage in a period of 48 h during the dry season and renew it within the next 72 h. In order to gain knowledge on the mechanisms that govern leaf exchange in F. obtusifolia, we followed in these two individuals the seasonal changes in leaf and fruit phenology, water relations and environmental variables, and in another 25, phenology only. Although foliar exchange was always observed before the rainy season, it also occurred near the end of the rainy season. An increase in leaf turgor potential of mature leaves and a low sap flow velocity coincided with leaf fall. Foliar exchange in all 27 individuals, growing in locations with different elevation and, possibly, access to underground water, was bi-modal and occurred in a period of approximately one month around both equinoxes. The presence of fruits was also bi-modal, occurring after leaf renewal. The sequence of phenological events repeated itself in all the individuals studied, although with inter-annual and individual variations in date of foliar exchange. Leaf fall was independent of leaf water status and significantly correlated with, in decreasing order, day length, minimum air temperature, rainfall, maximum air temperature and total radiation. We conclude that in F. obtusifolia high nocturnal temperature, increasing daytime temperature and radiation, and low cloud cover apparently drove foliar exchange. Day length showed a strong correlation with leaf renewal possibly to due to the significant relationships between day length and environmental variables. Direct effects of day length alone were not evidenced, as foliar exchange took place during a long time-span around the equinoxes.

Determination of the water use and water use response of canola to solar radiation and temperature by using heat balance stem flow gauges

Sap flow gauges using a heat balance have been reliable for measuring real-time transpiration in a number of crops. However, information on the accuracy of sap flow gauges in canola is lacking. Therefore, a study was conducted to validate the sap flow system in canola and to observe sap flow response to variations in temperature and solar radiation. There were strong relationships between sap flow measured with sap flow gauges and actual transpiration measured by the gravimetric method over short periods of 1 h (r2 = 0.93 and RMSE = 2.34 g h-1), and over longer periods of 1 d (r2 = 0.83 and RMSE = 48 g d-1), although sap flow slightly overestimated transpiration. In both cases the slope was not significantly different from 1. Water use in canola, estimated with sap flow gauges or from actual transpiration measurement, was dependent upon temperature (r2 = 0.94 to 0.96). Water use increased until daytime temperatures reached 36°C, after which water use decreased. Sap flow followed solar radiation trends in the field. Heat is lost or dissipated from the gauges convectively as the sap flows through the stem, conductively through the solid stem material, and radially into the surrounding air. As the convective proportion of the heat loss from the gauge increased, the accuracy of the water use estimation using the sap flow gauges increased. For sunny days, convective heat loss through sap flow accounted for a major portion of the total heat input to the gauges, while on cloudy days radial heat loss from the gauges accounted for a major portion of the heat input. Thus, at low sap flow rates during cloudy days, the possibility of error in the sap flow system was high. Overall, sap flow in canola was strongly related to daily solar radiation (r2 = 0.92). The sensitive response to weather variations and the possibility of improving the accuracy at high flow rates in the field makes the use of sap flow gauges a viable option for measuring real-time transpiration in canola. Key words: Brassica napus, canola, heat balance, sap flow, transpiration, temperature, solar radiation

Influence of natural temperature gradients on measurements of xylem sap flow with thermal dissipation probes. 1. Field observations and possible remedies.

The thermal dissipation method is simple and widely used for measuring sap flow in large stems. As with several other thermal methods, natural temperature gradients are assumed to be negligible in the sapwood being measured. We studied the magnitude and variability of natural temperature gradients in sapwood of Acacia trees growing in the Sahelian zone of Senegal, analyzed their effects on sap flow measurements, and investigated possible solutions. A new measurement approach employing cyclic heating (45 minutes of heating and 15 minutes of cooling; 45/15) was also tested. Three-day measurement sequences that included 1 day without heating, a second day with continuous heating and a third day with cyclic heating were recorded during a 6.5-month period using probes installed at three azimuths in a tree trunk. Natural temperature gradients between the two probes of the sensor unit, spaced 8 to 10 cm vertically, were rarely negligible (i.e., < 0.2 degrees C): they were positive during the night and negative during the day, with an amplitude ranging from 0.3 to 3.5 degrees C depending on trunk azimuth, day and season. These temperature gradients had a direct influence on the signal from the continuously heated sensors, inducing fluctuations in the nighttime reference signal. The resulting errors in sap flow estimates can be greater than 100%. Correction protocols have been proposed in previous studies, but they were unsuitable because of the high spatial and temporal variability of the natural temperature gradients. We found that a measurement signal derived from a noncontinuous heating system could be an attractive solution because it appears to be independent of natural temperature gradients. The magnitude and variability of temperature gradients that we observed were likely exacerbated by the combination of open stand, high solar radiation and low sap flow rate. However, for all applications of the thermal dissipation method, it is wise to check regularly for natural temperature gradients by switching off the heater.

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

The tissue heat-balance method (Cy and the heat-dissipation method (Granier) were compared in three Scots pines and two Norway spruces in a forest in central Sweden. The Granier system measured up to 50% lower sapflow than the Cy system at high flow rates. New coefficients for the Granier system were estimated, based on sapflow density from the Cy measurements. Wi- thout compensation, natural temperature gradients may cause large errors in measurements made by the Granier system. By using a hori- zontal reference sensor, no compensation was necessary. It was also shown that radial flow patterns must be considered when calculating total tree sapflow. Transpiration of two adjacent stands, one measured by the Granier method and the other by the Cy method, sho- wed good agreement internally and with total evaporation measured by the eddy-correlation method.

Radial variation in sap flow in five laurel forest tree species in Tenerife, Canary Islands.

Variations in radial patterns of xylem water content and sap flow rate were measured in five laurel forest tree species (Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng., Myrica faya Ait., Erica arborea L. and Ilex perado Ait. ssp. platyphylla (Webb & Berth.) Tutin) growing in an experimental plot at Agua García, Tenerife, Canary Islands. Measurements were performed around midday during warm and sunny days by the heat field deformation method. In all species, water content was almost constant (around 35% by volume) over the whole xylem cross-sectional area. There were no differences in wood color over the whole cross-sectional area of the stem in most species with the exception of E. arborea, whose wood became darker in the inner layers. Radial patterns of sap flow were highly variable and did not show clear relationships with tree diameter or species. Sap flow occurred over the whole xylem cross-sectional area in some species, whereas it was limited to the outer xylem layers in others. Sap flow rate was either similar along the xylem radius or exhibited a peak in the outer part of the xylem area. Low sap flow rates with little variation in radial pattern were typical for shaded suppressed trees, whereas dominant trees exhibited high sap flow rates with a peak in the radial pattern. Stem damage resulted in a significant decrease in sap flow rate in the outer xylem layers. The outer xylem is more important for whole tree water supply than the inner xylem because of its larger size. We conclude that measurement of radial flow pattern provides a reliable method of integrating sap flow from individual measuring points to the whole tree.

Sap flow index as an indicator of plant water status.

Night and especially predawn tree water status is an important indicator of drought stress in trees. Leaf water potential (LWP) is frequently used as a measure of tree water status and hence drought stress; however, there are difficulties associated with sampling foliage from tall trees and determining LWP automatically. The current study was undertaken to determine whether sap flow index (SFI), which can be automatically and continuously recorded even during very low flows, can be used to estimate drought stress in trees caused by dry air under non-limiting soil water conditions. We made simultaneous measurements of LWP, heat pulse velocity (HPV) and SFI on apple trees (Malus domestica Borkh.) in the semiarid climate of southern Ukraine over several growing seasons. Predawn values of LWP were highly correlated with SFI. Over the range of low sap flow rates occurring at nighttime, where other methods of measuring sap flow are not sensitive, the SFI method was linear and very sensitive. Additional information about tree water status was obtained by comparing nighttime and daytime values of SFI. The ratio of predawn SFI to midday SFI and the period between the two daily SFI maxima (the first SFI peak occurred in the morning and the second peak occurred in the evening on cloudless days) can be used to characterize internal plant water balance. Although the daily course of SFI was variable, specific patterns were identified that reflected particular stages in the development of plant drought stress. An "air-drought-stress curve" was used to characterize the development of water stress in trees subjected to air drought during the growing season.

Limitations of a compensation heat pulse velocity system at low sap flow: implications for measurements at night and in shaded trees.

Unlike an ideal system, the return time to thermal balance (t(b)) between upstream and downstream thermistors, as measured by the (compensation) heat pulse velocity method, effectively depends on the heat input and the water content of the wood at zero and low sap flow. Even when these factors were held constant and ambient temperature was stabilized, a twofold variation in t(b) at zero flow was observed within and among Greenspan Technology sensors implanted in wooden posts, making it impossible to distinguish zero flow from low sap velocities (< 0.01-0.02 mm s(-1)). This limitation has serious consequences because the contribution of low flow rates to water movement is important during both daytime and nighttime in tropical understory and overstory trees. Measurements in an artificial flow system showed that this technical limitation is exacerbated by erratic variation in sensor response at both zero and low flow rates. The limited sensitivity of the tested sap flow sensors may be caused by their poor thermal contact with wood. Interim procedures are suggested for estimating minimum detectable sap flow and delimiting the hydroactive zone until the sensitivity and interchangeability of sap flow probes are improved.

Abscisic acid concentrations and fluxes in droughted conifer saplings

ABSTRACTWe present the first study of abscisic acid (ABA) concentrations and fluxes in the xylem sap of conifers during a drought cycle. In both Pinus sylvestris and Picea sitchensis the concentration of ABA in the sap rose 11‐fold as the drought progressed. There were clear diurnal trends in this concentration, which reached its maximum (6–8.ininol ABA m−3) near the middle of the day. The fluxes of ABA were calculated by multiplying the xylem ABA concentration by the sap flow rate. The ABA fluxes in the droughted plants in the middle of the day were usually no higher than those of the controls, as a result of the very low sap flow in the droughted plants at that time. However, the ABA flux in the droughted plants was higher than in the controls in the morning, and we postulate that the stomata are responding to these ‘morning doses’Stomatal conductance, gs, could not be related statistically to leaf turgor or to the ABA flux. However, £s did display a negative exponential relationship with ABA concentration in the xylem.Pinus displayed more acclimation to drought than Picea, Its ABA concentration rose and its stomatal conductance fell at day 6 of the drought, as opposed to day 17 for Picea, and its osmotic potential fell during the drought treatment.

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long‐Distance Water Transport

AbstractThe rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below‐atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg−1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (−3 to −6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage‐ and/or protein‐related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h−1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity‐independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity‐dependent) convectional flows. Density‐driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.