Sap Flow System Research Articles

An Experimental Investigation of the Precipitation Utilization of Plants in Arid Regions.

What represents a water source for the ecological restoration of a plant in an arid region is still up to debate. To address this issue, we conducted an in situ experiment in the Ulan Buh Desert of China, to study desert plants absorbing atmospheric water vapor. We selected Tamarisk, a common drought-salt-tolerant species in the desert, for ecological restoration as our research subject, used a newly designed lysimeter to monitor precipitation infiltration, and a sap flow system to track reverse sap flow that occurred in the shoot, branch, and stem during the precipitation event, and observed the precipitation redistribution process of the Tamarisk plot. The results showed that Tamarisk indeed directly absorbs precipitation water: when precipitation occurs, the main stem, lateral branch, and shoot all show the signs of reversed sap flow, and the reversed sap flow accounted for 21.5% of the annual sap flow in the shoot and branch, and 13.6% in the stem. The precipitation event in the desert was dominated by light precipitation events, which accounted for 81% of the annual precipitation events. It was found that light precipitation can be directly absorbed by the Tamarisk leaves, especially during nighttime or cloudy days. Even when the precipitation is absent, it was found that desert plants can still absorb water from the unsaturated atmospheric vapor; even the absorbed atmospheric water vapor was transported from the leaves to the stem, forming a reversed sap flow, as a reversed sap flow was observed when the atmospheric relative humidity reached 75%. This study indicated that the effect of light precipitation on desert plants was significant and should not be overlooked in terms of managing the ecological and hydrological systems in arid regions.

Different Responses of Evapotranspiration Rates of Urban Lawn and Tree to Meteorological Factors and Soil Water in Hot Summer in a Subtropical Megacity

Urban evapotranspiration (ET) is an effective way to mitigate ecological challenges resulting from rapid urbanization. However, the characteristics of urban vegetation ET, especially how they respond to meteorological factors and soil water, remains unclear, which is crucial for urban ET regulation. Therefore, this study measured the actual ET rate of an urban lawn (ETlawn) using the Bowen ratio system and an urban tree (Ttree) by a sap flow system in the hot summer of a subtropical megacity, Shenzhen. The results showed that the daily ETlawn was more restricted by energy (Rs) and diffusion conditions (vapor pressure deficit, VPD), while the daily Ttree was more restricted by VPD and relative extractable water (REW) in the urban area. The daily Ttree decreased when the REW was lower than 0.18, while the daily ETlawn started to decrease when it was lower than 0.14. When REW was lower than 0.11, the Ttree stayed at a relatively low level. The impacts of VPD was more evident on the diurnal Ttree than on the diurnal ETlawn. Wind speed had a scarce impact as it was relatively low in urban areas. This study clarifies the different responses of ETlawn and Ttree to meteorological factors and soil water based on actual ET. The results are of great significance for the knowledge of urban forestry and urban hydrology.

Evapotranspiration and crop coefficient of tomato grown in a solar greenhouse under full and deficit irrigation

Information of crop evapotranspiration (ETc) and crop coefficient (Kc) is essential for improving water use and optimizing irrigation scheduling. Here, a three-year (2015, 2016 and 2019) experiment was conducted for tomato grown in a solar greenhouse under full and deficit drip irrigation to investigate the variation of ETc measured by sap flow system plus micro-lysimeter in 2015 and by weighing lysimeter in 2016 and 2019. The controlling meteorological factors on ETc in two irrigation treatments were analyzed by using path analysis method. A single crop coefficient model considering leaf senescence, soil water stress and fraction of canopy cover was proposed under two irrigation levels. The results showed that total seasonal ETc over the whole growth stage under full irrigation was 310−350 mm, which was 16–23 % higher than that under deficit irrigation. The maximum hourly ETc rate in each month during the three-years experiment varied from 0.15 to 0.89, and from 0.15 to 1.88 mm h−1, respectively, under deficit and full irrigation. Path analysis showed that the net radiation was the dominant meteorological factor aff ;ecting ETc through the direct eff ;ect, followed by the vapor pressure deficit, mainly through an indirect action on ETc. The Kc values at different growth stages estimated by the proposed single crop coefficient model under full and deficit irrigation agreed well with the measured ones, and the water stress coefficient, Ks, under deficit irrigation varied from 0.5 to 1.0. The proposed single crop coefficient model also estimate daily ETc of drip irrigated tomato reasonably in the solar greenhouse with regression coefficient of 0.93−0.99, determination of coefficient of 0.78−0.95 and root mean square root of 0.35−0.52 mm d−1.

Evaluation of Several Reference Evapotranspiration Models and Determination of Crop Water Requirement for Tomato in a Solar Greenhouse

Studies on dual crop coefficient method in a greenhouse require accurate values of reference evapotranspiration (ETo). This study was conducted in a solar greenhouse at the experimental station of the Chinese Academy of Agricultural Sciences during 2015 and 2016. An automatic weather station was installed in the center of the same greenhouse to record weather parameters at 30-minute intervals. Five ETo models (Penman-Monteith, Penman, radiation, pan evaporation, and Priestley-Taylor) were employed, and their performance was evaluated using the dual crop coefficient method. The basal crop coefficient Kcb and soil evaporation coefficient Ke were adjusted according to the surrounding climate inside the greenhouse. Crop evapotranspiration (ETc) was continuously measured using sap flow system combined with microlysimeter in 2015 and weighing lysimeters in 2016. Daily ETo was simulated from the five models and compared with the measurements. Results show that the adjusted Kcb values were 0.15, 0.94, and 0.65 in 2015 and 0.15, 1.02, and 0.70 in 2016 at initial, midseason, and late-season, respectively. The Ke varies between 0.10 and 0.45 during the whole growth period. The ETc was ≈345 mm for drip-irrigated tomato in solar greenhouse at the whole growth stage. The radiation and pan evaporation models tend to overestimate ETo values. Results of the Penman-Monteith, Penman, and Priestley-Taylor models show comparatively good performance in estimating ETo. Considering the robustness and simplicity, the Priestley-Taylor was recommended as the first choice to estimate ETo of tomato grown in a solar greenhouse. This work can help farmers to optimize the irrigation scheduling based on an ETo model for solar greenhouse vegetables in northern China.

Surface soil water content dominates the difference between ecosystem and canopy water use efficiency in a sparse vineyard

Water use efficiency (WUE) is an important eco-physiological index that reflects the relationship between ecosystem water and carbon cycles. Investigating the ecosystem WUE contributes to a better understanding of the water-carbon coupling mechanism in ecosystems. We explored the responses of the daily ecosystem WUE (WUEe) and canopy WUE (WUEc) to several influential factors, the dominant biophysical factor and the dominant water/carbon flux component of the difference between WUEe and WUEc based on continuous measurements collected via an eddy covariance system, sap flow system, and microlysimeters over two growing seasons from 2017 to 2018 in a sparse vineyard in northwestern China. The results showed that the seasonal averages for WUEe and WUEc were 0.52 g C kg H2O−1 and 1.96 g C kg H2O−1, respectively. WUEe and WUEc responded weakly to the air temperature (Ta) and showed a negative linear correlation to the photosynthetic active radiation (PAR) and vapor pressure deficit (VPD). WUEe decreased significantly with the rising soil water content (SWC) while WUEc showed a weak response. Both WUEe and WUEc exhibited positive linear relationships with the leaf area index (LAI). The results from a structural equation model (SEM) indicated that among the biophysical controllers, SWC and LAI were the top two controlling factors on the difference between WUEe and WUEc, and SWC played a dominant role. The difference between WUEe and WUEc increased with the rising SWC and decreased with the rising LAI. Among the flux components, including soil evaporation (E), transpiration (T), net ecosystem productivity (NEP), and ecosystem respiration (Reco), E dominated the discrepancy between WUEe and WUEc due to the strong control of SWC on E. This study highlights the strong control by the SWC in regulating the ecosystem WUE under a sparse canopy in an arid area.

Comparison of Shuttleworth-Wallace model and dual crop coefficient method for estimating evapotranspiration of tomato cultivated in a solar greenhouse

The accurate determination of crop evapotranspiration (ETc) is important for improving irrigation water use efficiency and optimizing fruit quality in solar greenhouses. Taking drip-irrigated tomato in a solar greenhouse as an example, this study simulated the variations of ETc by the Shuttleworth–Wallace (S–W) and Dual crop coefficient (Dual-Kc) models at initial, development, mid and late stages in 2015 and 2016 respectively. Continuous measurements of crop ETc with weighing lysimeter, and sap flow system combined with micro-lysimeter were used to validate the performance of two approaches. Results indicated that the total soil evaporation accounted for 25% of ETc over the whole growth period, and decreased gradually with the increasing of canopy coverage ratio. The locally developed basal crop coefficients values during the initial, mid, and late stages were 0.15, 0.94 and 0.65 in 2015, and 0.15, 1.02 and 0.70 in 2016, respectively. The variations of the estimated ETc from S–W and Dual-Kc models were similar to the measured ETc. The S–W model performed well in estimating ETc when LAI was between 0.5 and 2.7, but slightly overestimated and underestimated at initial and mid stages, respectively. Additionally, the performance of the S–W model was also better under different weather conditions as well as after irrigation. However, the Dual-Kc method was superior to the S–W model in estimating daily soil evaporation and daily plant transpiration. The good agreements were found between the predicted ETc using the Dual-Kc method and the measurements for the greenhouse tomato, with the coefficients of regression of 1.03 (R2=0.93) in 2015, and 0.96 (R2=0.93) in 2016. Therefore, the Dual-Kc method was finally recommended for estimating evapotranspiration of tomato cultivated in the solar greenhouse.

Comparison of maize water consumption at different scales between mulched and non-mulched croplands

Plastic mulch can significantly improve soil water and temperature conditions in croplands, altering ecohydrological processes in agroecosystems. Therefore, analysis of maize water consumption in response to plastic mulch is of importance to agricultural water management. A field experiment was conducted in non-mulched (CK) and mulched (PM) maize fields of eastern Loess Plateau to monitor maize evapotranspiration at different scales. Portable photosynthesis system, sap flow system and eddy covariance systems were utilized to measure transpiration rate at leaf scale, sap flow at plant scale and evapotranspiration (ET) at field scale, respectively. The results indicated PM provided a wetter condition (+2.8% and +4.2% in soil water content for two growing seasons respectively) for maize, leading to higher leaf area index, plant height and aboveground biomass under PM treatment. PM increased leaf transpiration by 9.8%˜33.3% and sap flow by 1.4%˜34.5% under different weather conditions. However, seasonal ET and the ratio of soil evaporation to ET decreased from 326.3˜341.1 mm and 39.9%˜42.8% under CK to 305.0˜313.9 mm and 23.8%˜34.9% under PM respectively, indicating PM declined seasonal ET and altered ET components, as evaporation decreased while crop transpiration increased. Thus, more water was used for maize growth, finally resulted in significantly higher yield and water use efficiency (p < 0.05). Our results shed light on how plastic mulch saves water and improves water use efficiency, which can provide important information for crop production modeling, water balance evaluation and sustainable management of dryland farming in this area.

Improved application of the Penman–Monteith model using an enhanced Jarvis model that considers the effects of nitrogen fertilization on canopy resistance

Nitrogen partially regulates stomatal conductance and crop transpiration. We conducted greenhouse experiments into the effects of different nitrogen applications on the transpiration (T) of greenhouse pot-grown tomato plants with plastic film mulch in 2016–2017. We modeled the relationship between soil mineral nitrogen content and canopy resistance (rc) and incorporated our model into the Jarvis canopy resistance model to create the nitrogen modified Jarvis (N-Jarvis) model. We calculated transpiration under three nitrogen application levels, each with four irrigation programs using the Penman–Monteith (P–M) model with the Jarvis model and with the N-Jarvis model. Canopy resistance estimated by the Jarvis model differed greatly from observed canopy resistance, which was calculated by reformulating the P–M model and using the measured transpiration by sap flow system (TSF). The N-Jarvis model is more accurate than the Jarvis model in predicting rc. Similarly, the transpiration estimated by the P–M model with N-Jarvis model is more accurate than with the Jarvis model when compared to TSF, with higher values for the coefficient of determination and the modified efficiency coefficient, and lower values for the root mean square error and the Akaike information criterion. Including predictions from the N-Jarvis model improves the accuracy of the P–M model in estimating tomato transpiration under different nitrogen application levels.

Transpiration of female and male parents of seed maize in northwest China

Drip irrigation under mulch (DM) has been widely promoted and applied in arid regions. The process of crop growth and surface water and energy transfer will change significantly under the dual control of mulch and drip irrigation compared with those under the traditional border irrigation under mulch (BM) method, which will further affect the regional eco-hydrology process. The Hexi Corridor has become the largest seed maize production base in China due to its abundant solar radiation, and heat resources and large diurnal temperature difference. Unlike grain maize, seed maize can be divided into female and male parent plants. The growth conditions of the parent plants differ; male plants are planted 7–14 d later than female plants to produce preferable fertilization conditions, which distinctly influences the energy transfer process between the different parent plants and the soil and ultimately leads to significantly different plant water consumption in the maize field.To determine the difference in the transpiration rate between a BM field and a DM field and to reveal the difference in water consumption characteristics between female and male plants, eddy covariance system (EC) measurements, a thermal balance sap flow system and micro-lysimeters were applied to conduct continuous, fixed-location comparison measurements of the total evapotranspiration from 2014 to 2016. The total evapotranspiration (ET), transpiration rates of the female (Tf) and male plants (Tm) and the soil evaporation (Es) in seed maize fields were separately analysed in both the BM field and DM fields. Results indicated that Tf/ET, Tm/ET and Es/ET dynamically varied with the crop growth during the whole growth period. Transpiration rates under the DM treatment were 2–8% higher than those under the BM treatment under the local agricultural management conditions. Besides, the female plant transpiration rate exceeded the male plant transpiration rate by 9–20% in the BM treatment and by 14–32% in the DM treatment field during the whole growth stages. However, male plant transpiration produced after the pollination of female plants is useless to final seed production. It is more practical to decrease the planting ratio of male to female plants to the appropriate level and remove the male plants earlier to increase the water use efficiency in seed maize fields. Moreover, the seed maize field in this study had a smaller proportion of soil evaporation than those in other grain maize field studies conducted around the world, which was probably due to the mulching effect and higher plant density.

A proposed surface resistance model for the Penman-Monteith formula to estimate evapotranspiration in a solar greenhouse

Greenhousing is a technique to bridge season gap in vegetable production and has been widely used worldwide. Calculation of water requirement of crops grown in greenhouse and determination of their irrigation schedules in arid and semi-arid regions are essential for greenhouse maintenance and have thus attracted increased attention over the past decades. The most common method used in the literature to estimate crop evapotranspiration (ET) is the Penman-Monteith (PM) formula. When applied to greenhouse, however, it often uses canopy resistance instead of surface resistance. It is understood that the surface resistance in greenhouse is the result of a combined effect of canopy restriction and soil-surface restriction to water vapor flow, and the relative dominance of one restriction over another depends on crop canopy. In this paper, we developed a surface resistance model in a way similar to two parallel resistances in an electrical circuit to account for both restrictions. Also, considering that wind speed in greenhouse is normally rather small, we compared three methods available in the literature to calculate the aerodynamic resistance, which are the r a 1 method proposed by Perrier (1975a, b), the r a 2 method proposed by Thom and Oliver (1977), and the r a 3 method proposed by Zhang and Lemeu (1992). We validated the model against ET of tomatoes in a greenhouse measured from sap flow system combined with micro-lysimeter in 2015 and with weighing lysimeter in 2016. The results showed that the proposed surface resistance model improved the accuracy of the PM model, especially when the leaf area index was low and the greenhouse was being irrigated. We also found that the aerodynamic resistance calculated from the r a 1 and r a 3 methods is applicable to the greenhouse although the latter is slightly more accurate than the former. The proposed surface resistance model, together with the r a 3 method for aerodynamic resistance, offers an improved approach to estimate ET in greenhouse using the PM formula.

Performance of AquaCrop and SIMDualKc models in evapotranspiration partitioning on full and deficit irrigated maize for seed production under plastic film-mulch in an arid region of China

It is difficult – though important – to assess the necessary allocation of increasingly scarce water resources and yield estimation, by determining the exact amount of evapotranspiration (ET) and the partitioning of ET into transpiration (T) through the stomata of plants and evaporation (E) from the soil. An accurate and effective method for ET partitioning and estimation is, therefore, desirable. This paper aims to evaluate the performance of two main models for ET partitioning and estimation in terms of feasibility and accuracy: the AquaCrop (Version 4.0) model, and the SIMDualKc model. Field experiments were carried out between 2011 and 2015 in an arid region of Northwest China on two sites using full and deficit irrigation under plastic film-mulch, with the ET of maize for seed production and its partitioning components (T and E) being strictly measured by using the eddy covariance (EC) system or the sap flow system and micro-lysimeter cylinders. Subsequently, part of the measured data was used to calibrate the two models, so that the calibrated models could then be used to assess whether the agreement between simulation and measurement had proved successful, thus validating the models, or not. The results showed that the two models performed well with regard to their simulation of ET and T under full irrigation conditions. Under deficit irrigation conditions, the ET and T values simulated by the AquaCrop model were much closer to the actual measurement when compared with the results simulated by the SIMDualKc model. This was particularly the case when the soil was re-watered after a period of long-term water stress. For the simulated E, however, both models generated data that were distant from the actual measurements taken under full or deficit irrigation conditions using plastic film-mulch, although when the SIMDualKc model simulated E data, it came closer to the measurement than did the AquaCrop model. The comparison of two models in terms of their accuracy and feasibility based on the data analysis is discussed.

Comparison of dual crop coefficient method and Shuttleworth–Wallace model in evapotranspiration partitioning in a vineyard of northwest China

The objective of this study was to evaluate the potential use of dual crop coefficient method in FAO-56 (FAO dual-Kc) and Shuttleworth–Wallace (S–W) model in estimating evapotranspiration (ET) and its components (plant transpiration and soil evaporation) of a vineyard in arid region of northwest China. Continuous measurements of ET with eddy covariance, plant transpiration (T) with sap flow system and soil evaporation (E) with micro-lysimeter in 2013 and 2014, were used to validate the performance of two approaches. Results indicate that sap flow system and micro-lysimeter can provide accurate measurements of T and E at hourly and daily scales if compared to eddy covariance, respectively. The FAO dual-Kc method in partitioning ET was acceptable when using the site-specific basal crop coefficient obtained from sap flow, with the slope and intercept of linear regression of 0.96 and −0.13mmd−1 (R2=0.81) for ET, 0.92 and −0.07mmd−1 (R2=0.76) for E, 0.93 and 0.16mm d−1 (R2=0.80) for T, respectively. The S–W model can better estimate ET, but overestimated T and underestimated E when using site-specific soil surface resistance, with the slope and intercept of linear regression of 0.98 and 0.28mmd−1 (R2=0.79) for ET, 0.49 and 0.42mmd−1 (R2=0.46) for E, 1.10 and 0.38mmd−1 (R2=0.81) for T, respectively. Both approaches had obvious discrepancies of E after rainfall and irrigation, especially the S–W model, and overestimated T after a snowfall. Sensitivity analysis indicates that estimated ET and its components were sensitive to soil field capacity and wilting point in both approaches, and in the S–W model, predicted T was also sensitive to leaf area index (LAI) and minimum stomatal resistance and predicted E sensitive to soil surface resistance and LAI. Thus two approaches can estimate ET with good accuracy, but the FAO dual-Kc method had higher accuracy in estimating E and T.

Canopy transpiration obtained from leaf transpiration, sap flow and FAO‐56 dual crop coefficient method

AbstractWith a maize seed planting area of about 67 000 hm2, Zhangye city supplies the seeds for more than 40% of the maize planting area in China. Irrigation water is often overused to ensure the quality of the maize seeds, leading to serious water shortage problems in recent years. An accurate and convenient estimate of canopy transpiration is of particular importance to ease the problem. In this paper, leaf transpiration and sap flow in a maize field were measured in 2012 using a portable photosynthesis system and a heat balance sap flow system. Based on a large amount of meteorological data and relevant maize plant‐growing parameters, canopy transpiration was up‐scaled from both leaf transpiration (Tl) and sap flow (Tf), and also calculated by the FAO‐56 dual crop coefficient method (T). Comparing these three types of transpiration, Tf was proved to be more reliable than Tl. Taking Tf as a benchmark, the basal crop coefficient (Kcb, the key parameter of FAO‐56 dual crop coefficient method) was further adjusted and verified for the maize plants in this region. In addition, the errors when using up‐scaling methods and FAO‐56 dual crop coefficient method are summarized. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

► Sap flow and evaporation were measured separately by heat balance sap flow system and micro-lysimeters in our vineyard . Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. ► Sap flow rate fluctuated diurnally and seasonally along with solar radiation and vapor pressure deficit. Volumetric soil water content to a depth of 1.0 m (VWC) influenced sap flow rate only when it fell below 0.188 cm 3 cm −3 . However, soil evaporation increased exponentially with VWC. ► When scaling vineyard transpiration from sap flow, leaf area, rather than stem diameter or vine cross-sectional area, should be selected as an indicator of individual sap flow variability. ► The sum of transpiration and evaporation was satisfactorily consistent to evapotranspiration measured by the Bowen ratio energy balance system, demonstrating the applicability of sap flow for measuring grape water use in this region. ► Soil evaporation accounted for 47.3% of total evapotranspiration. Keeping the VWC in such vineyards slightly above ∼60% of the field capacity could reduce the ratio of soil evaporation to evapotranspiration, according to the different response to VWC for sap flow rate and soil evaporation. Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration ( ET ) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET , were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate ( SR l ) increased linearly with solar radiation ( R s ), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR l . The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm 3 cm −3 , which was ∼60% of the field capacity. Conversely, soil evaporation ( E s ) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration ( T s ) was scaled from sap flow by using leaf area ( A l ) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.

Change of trunk sap flow of Ginkgo biloba L. and its response to inhibiting transpiration treatment

For this paper, GREENSPAN sap flow system was used to monitor the dynamics of trunk sap flow of Gingkgo biloba. Results indicate that sap flow velocity is significantly different among different heights, depths, and directions of the trunk. Sap flow velocity at the upper position of the trunk is higher than that of the middle and lower position, but cumulative flux is not significantly different among the upper, middle and lower sections. Sap flow velocity at 10 mm reached the most and that at 20 mm the least. However, sap flow velocity at 5 mm and 15 mm was similar and was second among the four depths. Results also showed that sap flow velocity of the south was the highest, and that of the west was next. An Automatic Weather Station of HOBO was synchronously applied to measure these meteorological parameters, and the relationship between these parameters and the changes of trunk sap flow velocity were analyzed. We found that the change of sap flow velocity was a single-crest curve on clear days and multi-crest curve on cloudy and rainy days. In addition, it is also revealed that by stepwise regression analyses photosynthetical active radiation (PAR), temperature and wind speed are the main environmental factors affecting sap flow velocity. The efficient methods of reducing water transpiration of trees, including leaf pruning, overshadowing and antitranspirant spraying, were found by investigating the effects on inhibiting transpiration, which indicated that spraying of antitranspirants, leaf pruning and overshadowing could significantly reduce transpiration but the effects of leaf pruning and overshadowing were far better than that of antitranspirant spraying.

Measuring and predicting evapotranspiration in an insect-proof screenhouse

This study addresses the current lack of accurate measurements and predictions of evapotranspiration below modern screening materials. Energy balance components, microclimatic and physiological parameters were measured in a 50-mesh insect-proof screenhouse cultivated with sweet pepper. A lysimeter-calibrated sap flow system and an eddy covariance system installed inside the screenhouse measured canopy transpiration and evapotranspiration, respectively. Sap flow (SF) and eddy covariance (EC) data showed good agreement ( R 2 = 0.95) and although EC exceeded SF by 9%, this was less than the standard error of inter-specific sap flow, supporting previous findings that mid-day soil evaporation was negligible. Screenhouse bulk resistance was inversely correlated with external wind speed and simple mass balance estimations gave reasonable values of evapotranspiration on a daily basis. A one-dimensional screenhouse model was derived, based on a modified Penman–Monteith equation incorporating an additional boundary layer resistance. The model, along with a greenhouse model, was used to calculate evapotranspiration of the screenhouse crop. Half hourly predictions of both models yielded good agreement with measured SF ( R 2 = 0.94–0.95). Daily crop water use, predicted with the greenhouse model was in better agreement with measurements than that predicted by the screenhouse model. Measured and predicted transpiration rates inside the screenhouse were approximately 1.8–2.1 mm day −1 during the most active stage of growth, while those simulated for a similar crop grown outside would be 4.5–5.3 mm day −1 on average. Model sensitivity analysis showed that reduced radiation and wind speed and modified vapour pressure deficit were the main factors influencing transpiration. Calculations of the decoupling factor Ω suggest that the screenhouse evaporative climate is predominantly “decoupled”.

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

WHOLE-PLANT WATER USE OF SOME TROPICAL AND SUBTROPICAL TREE CROPS AND ITS APPLICATION IN IRRIGATION MANAGEMENT

Whole-plant water use provides a baseline for a plant's irrigation requirement. The measurement of whole-plant water use in tree crops is much more difficult than in field crops, owing mainly to their large size. The development of thermo-electrical methods for xylem sap flow measurement has provided tree physiologists with an accurate and convenient tool to study whole-tree water use and water relations. However, sap flow measurement in orchard grown trees is particularly challenging because of horticultural practices, such as grafting, training and structural pruning. Whole-tree water use on a range of tropical and subtropical tree crops, such as mango, cashew, mangosteen, banana and macadamia, was measured using a Granier sap flow measuring system. The Granier system was found suitable for tree crops with very diverse morphologies and xylem structures, e.g. mango, mangosteen and banana. For a given species, radial variation in water flux in the sapwood has a more or less regular pattern, but the circumferential (aspect) variation was substantial and random. To obtain a reasonable estimate of the whole-plant or orchard water use, a large number of sap flow sensors and several dataloggers are required. To assist in taking account of spatial variabilities, a modified Granier method was developed and proved to be effective. A summary of the quantitative estimates of the whole-plant water use of several tropical and subtropical crops was provided. Sap flow was found to be a very sensitive indicator of plant water status useful for irrigation scheduling for plants grown in containers or on soil with limited soil water holding capacity but was not so sensitive for plants with deep roots and/or on soil with high water holding capacity. In its current design the sap flow system should remain as a research tool for researchers rather than an irrigation scheduling tool for farmers.

An assessment of potato sap flow as affected by soil water status, solar radiation and vapour pressure deficit

Water-use of three field-grown potato cultivars (Atlantic, Monona and Norchip) was examined using a commercially available sap flow monitoring system over three consecutive growing seasons. The objectives of the investigation were to utilize the sap flow system to assess the water use of three field-grown potato cultivars. This included an assessment of the relationship between environmental conditions, water status and measured sap flow including the plant-to-plant variation in sap flow and an evaluation of relative transpiration in relation to the soil water status.Each cultivar maintained daily sap flow close to the atmospheric potential transpiration until approximately 70% of the available water was depleted. Under conditions where the soil was drier (&gt;70% depleted), Monona potato plants exhibited a more rapid decline in transpiration than Norchip and Atlantic.Hourly sap flow rates were closely related to solar irradiance, especially under well-watered conditions, with no apparent light saturation point. Vapour pressure deficit effects on sap flow were less pronounced, although maximum vapour pressure deficits encountered were only 2 kPa. Key words: Water use, sap flow, transpiration, potato

Humate-based Biostimulants Affect Early Post-transplant Root Growth and Sapflow of Balled and Burlapped Red Maple

Application of biostimulants, humate-based products marketed as aids to plant establishment, may increase early post-transplant root growth and water uptake of landscape trees. We tested three distinct types of biostimulants on root growth and sapflow of balled and burlapped red maple (Acer rubrum L. `Franksred') trees. Treatments included: humate, 1) as a wettable powder formulation, applied as a soil drench; 2) as a liquid formulation to which various purported root growth—promoting additives had been added, also applied as a soil drench; 3) as a dry granular formulation, applied as a topdress; and 4) a nontreated control. Root growth was monitored through single-tree rhizotrons, and sap flow was measured with a heat balance sapflow system. Roots were first observed in the rhizotron windows 38 days after planting. No biostimulant-treated trees had more root length than nontreated controls, and the two soil drench treatments had the lowest root length throughout the 20 weeks of post-transplant observation. All biostimulants increased sapflow.