Plant Water Stress Indicators Research Articles

Potential application of spectral indices for olive water status assessment in (semi‐)arid regions: A case study in Khuzestan Province, Iran

AbstractSpectral indices can be used as fast and non‐destructive indicators of plant water status or stress. It is the objective of the present study to evaluate the feasibility of using several spectral indices including water index (WI) and normalized spectral water indices 1–5 (NWI 1–5) to estimate water status in olive trees in arid regions in Iran. The experimental treatments involved two olive cultivars (Koroneiki and T2) and four irrigation regimes (irrigated with 100%, 85%, 70%, and 55% estimated crop evapotranspiration [ETc]). The results obtained showed that olive trees subjected to the different irrigation regimes of 85%, 70%, and 55% ETc experienced soil water content (SWC) deficits by 4.5%, 12%, and 20.5% that of the control, respectively. Significant differences were observed among the treatments with respect to measured relative water content (RWC), SWC, and the spectral indices of WI and NWI 1–5. The normalized spectral indices combining NIR and NIR wavelengths were found more effective in tracking changes in RWC and SWC than those that combine NIR and VIS or VIS and VIS wavelengths, respectively. Spectral indices were closely and significantly associated with RWC () and SWC (). Among all the spectral indices investigated, NWI‐2 showed the least consistent associations with RWC (ranging from 4–15% lower than the other indices examined) and SWC (ranging from 1–23% lower than the others). Based on the pooled data on spectral indices, RWC, and SWC collected during the study period, WI, NWI‐1, NWI‐4, and NWI‐5 showed stronger correlations with RWC and SWC than did NWI‐3 and NWI‐2. In conclusion, the spectral indices of WI and NWI 1–5 measured at the leaf level are found useful as fast and non‐destructive estimators of plant water stress in arid regions.

Determination of Water Stress in Wheat Crops by Thermal Images Under Mediterranean Conditions

ABSTRACT Canopy temperature is used as an indicator of plant water stress. In recent years, technological advances have allowed improved applications in agriculture from temperature sensors clamped on leaves to short-range remote-sensing, such as infrared thermometry and thermal imaging. There are many scientific studies on the use of thermal imaging to determine crop water stress; however, no study on wheat crops has been found in the literature. Therefore, this study aimed to determine the detectability of water stress in wheat by using thermal images. In the experiment, drainage-type, weighable lysimeters were used to make precise irrigations and obtain components of the water budget for calculating the plant water consumption. Wheat crops were grown under four different irrigation levels including no water stress as control (I100), mild water stress (I75), moderate water stress (I50), and severe water stress (I25). When the thermal images were examined, it can be seen that pre-irrigation images reflect water stress conditions better than post-irrigation. The warm color distribution was more evident under severe water stress, whereas cold colors were dominant under no water stress. During the experimental period, the plant leaf temperatures varied from 17.5 to 34.2°C, while the spike temperatures ranged from 26.6°C to 36.3°C. The lowest plant leaf and spike temperature values were observed under control, whereas the highest values were obtained under severe water stress. It can be said that thermal cameras that provide fast and contactless data acquisition have the potential to be used in detecting water stress in wheat crops.

Study of Microclimate and Sapling Citrus Plant Transpiration in Tunnel Greenhouse Under Mediterranean Conditions

Abstract In Mediterranean basin, citrus culture is expanding rapidly with a growing demand for high-quality saplings. Protected cultivation with fine microclimate conditions control is the best process of citrus saplings growing with the required quality. In this context, the developed microclimate of a tunnel citrus saplings greenhouse and its effects on plant activity parameters, such as leaf temperature and transpiration, were investigated. The findings show that the temperatures of air and plant leaves inside the greenhouse are highly variable between 6 °C and 33 °C and from 8 °C to 30 °C, respectively. Results also show that greenhouse air humidity is very high – more than 80%. The analysis of plant transpiration based on greenhouse microclimate conditions and leaf temperature reveals that (i) plant transpiration varies throughout the day (between 1.9 and 65.7 mg m−2·s−1); (ii) leaf temperature simulation can be used as an indirect indicator of plant water stress; and (iii) the simulated temperature difference between the leaves and the internal air of the greenhouse can be used to define the minimum threshold air temperature for proper plant development under the greenhouse conditions.

Daily variations in tomato stem diameter as a criterion for irrigation management

The process of daily variation in tomato stem diameter is examined in order to justify the use of this parameter to control drip irrigation. Changes in the size of individual plant parts depend on the provision of water, light, heat and nutrients to the production process. Therefore, such plant parameters as leaf temperature, xylem flow rate, fruit and stem diameter can be indicators of availability of necessary resources. The research was carried out in Novosibirsk region in June - September 2020. The value of the range of daily variations in stem diameter, which has a close relationship to relative soil moisture, was used as an indicator of plant water stress. The source of the information is the results of measurements of soil moisture and stem diameter growth of tomato. Experiments to assess the effect of water deficit on stem parameters were carried out on a plant set out in the open ground separately from the rest. Artificial water stress conditions were created by watering once a week. Data were collected using a PM-11z phytomonitor, soil moisture and stem diameter growth sensors. The results of measurements were processed in Microsoft Office Excel program. It was found that the range of daily fluctuations of stem diameter growth depends on moisture availability. When soil moisture is below 30%, the plant experiences water stress and the range of stem diameter fluctuations increases. The maximum growth in stem diameter was observed at 7-10 a.m. and the minimum at 13-15 p.m. local time. The difference between the maximum and minimum of the daily stem diameter increase characterizes the range of the daily stem diameter difference, which correlates closely with soil moisture. The correlation coefficient between them is 0.72. The limit for the daily stem diameter difference is 0.025 mm at 30% soil moisture. If the actual value of this parameter exceeds the limit value, the irrigation system can be activated. The implementation of this approach makes it possible to automate the irrigation process and to take into account the indicator that signals water stress of the plant.

Long-Term Assessment of Reference Baselines for the Determination of the Crop Water Stress Index in Maize under Mediterranean Conditions

Canopy temperature has been proposed as a relevant variable for crop water stress monitoring. Since crop temperature is highly influenced by the prevailing climatic conditions, it is usually normalized with indices such as the crop water stress index (CWSI). The index requires the use of two baselines that relate canopy temperature under maximum stress and non-water stress conditions with vapor pressure deficit (VPD). These reference baselines are specific to each crop and climatic region. In maize, they have been extensively studied for certain climatic regions but very little is known on their suitability to be used under Mediterranean-type conditions nor their temporal stability, both diurnally and between seasons. Thus, the objective of this work was to determine the reference baselines for maize grown under Mediterranean conditions, as well as its diurnal and long-term stability. An experiment was conducted for 3 years in a maize breeding field, under well-watered and water-stressed irrigation treatments. The determined reference baselines for computing CWSI in maize have shown to be stable in the long term but markedly influenced by the meteorological variations between 10–17 h UTC (Coordinated Universal Time). These results indicate that several reference baselines should be used for CWSI computing throughout the abovementioned time interval. The CWSI values calculated for well-watered and water-stressed maize breeding plots using the reference baselines derived in this study were successfully correlated with other physiological indicators of plant water stress.

English

Regulated deficit irrigation (RDI) is considered among the best water-saving techniques for supplementing Regulated water to fully achieve the water needs of the plant while maximizing water productivity with little or no substantial decrease in final produce compared to the conventional forms of irrigating crops. The aim of this paper is to review existing RDI approaches used in citrus production as well as plant-water stress indicators. Most of the approaches employed in citrus RDI scheduling require weather data for evapotranspiration calculations which is very technical, laborious and time consuming. Nonetheless, the time domain reflectometer (TDR) offers a simple way of scheduling RDI based on the soil-water status at any given time. This approach will help address the challenges in setting up on-farm synoptic stations to measure weather data to compute evapotranspiration or from using data from weather stations which might be different from the farm conditions. The pros and cons of all the approaches have been discussed and recommended that the TDR can be adopted as an alternative to schedule irrigation in citrus orchards to ensure that plants are supplied with adequate volume of water for maximum water use efficiency.  Key words: Partial root-zone drying, plant-water requirement, plant-water stress, regulated deficit irrigation, remote sensing, subsurface irrigation. 

Dependence of CWSI-Based Plant Water Stress Estimation with Diurnal Acquisition Times in a Nectarine Orchard

Unmanned aerial vehicle (UAV) remote sensing has become a readily usable tool for agricultural water management with high temporal and spatial resolutions. UAV-borne thermography can monitor crop water status near real-time, which enables precise irrigation scheduling based on an accurate decision-making strategy. The crop water stress index (CWSI) is a widely adopted indicator of plant water stress for irrigation management practices; however, dependence of its efficacy on data acquisition time during the daytime is yet to be investigated rigorously. In this paper, plant water stress captured by a series of UAV remote sensing campaigns at different times of the day (9h, 12h and 15h) in a nectarine orchard were analyzed to examine the diurnal behavior of plant water stress represented by the CWSI against measured plant physiological parameters. CWSI values were derived using a probability modelling, named ‘Adaptive CWSI’, proposed by our earlier research. The plant physiological parameters, such as stem water potential (ψstem) and stomatal conductance (gs), were measured on plants for validation concurrently with the flights under different irrigation regimes (0, 20, 40 and 100 % of ETc). Estimated diurnal CWSIs were compared with plant-based parameters at different data acquisition times of the day. Results showed a strong relationship between ψstem measurements and the CWSIs at midday (12 h) with a high coefficient of determination (R2 = 0.83). Diurnal CWSIs showed a significant R2 to gs over different levels of irrigation at three different times of the day with R2 = 0.92 (9h), 0.77 (12h) and 0.86 (15h), respectively. The adaptive CWSI method used showed a robust capability to estimate plant water stress levels even with the small range of changes presented in the morning. Results of this work indicate that CWSI values collected by UAV-borne thermography between mid-morning and mid-afternoon can be used to map plant water stress with a consistent efficacy. This has important implications for extending the time-window of UAV-borne thermography (and subsequent areal coverage) for accurate plant water stress mapping beyond midday.

ASSESMENT OF WATER RESOURCES USING LANDSAT SATELLITE IMAGERY

Observation of surface water is a very important for studying hydrological processes and last advances in satellite-based optical remote sensors have promoted the field of sensing surface water of a certain area or from a catchment area. This paper has considered analysing data from satellite image and geographical information system (GIS) for a catchment area. Banat hydrographical area is situated in the western part and south-western part of Romania and Barzava River basin drains a relatively small area in Banat Hydrographical Area. GIS techniques were used for extract data NDWI, MNDWI, AWEI indexes from in Landsat-8 satellite in images to evaluate their performances for the extraction of surface water. This paper shows the monitoring of Barzava river basin based on the indices resulting from the processing of satellite images. The objective of this research was to extract the surface water bodies from the hydrographic basin of river Barzava. Then, these data are utilized for purpose finding correlation and regression equations between the reflectance of the satellite image and the water parameters. NDWI being an indicator of plant water stress, it is observed that for 2018 compared to 2019 the basin of the Barzava River was affected by the drought.

Sensitivity of spectral vegetation indices for monitoring water stress in tomato plants

Innovations in irrigation water management are required to optimize agricultural water use in water stressed regions of the world, and physiological response of plants to water stress is an important criterion. Remotely sensed plant stress indicators, based on the visible and near-infrared spectral regions, provide an alternative to traditional field measurements of plant stress parameters, as this provides information about the spatial and temporal variability of crops and soil. The present study is a proof of concept on the feasibility of using narrow-band hyperspectral derived indices for monitoring water stress in tomato plants (Solanum Lycopersicum L.). Spectral reflectance data were acquired from tomato plants, with five different irrigation regimes namely 100, 80, 60, 40, and 20% of plant available water, in a completely randomized design. Also, plant water stress indicators including canopy temperature (Tc) and relative leaf water content (RWC), as well as volumetric soil moisture content (SMC) were concurrently measured with spectral data acquisition. Normalized Difference Vegetation Index (NDVI), Renormalized Difference Vegetation Index (RDVI), Optimized Soil Adjusted Vegetation Index (OSAVI), Photochemical Reflectance Index centered at 570 nm (PRI570), normalized PRI (PRInorm), Water Index (WI), and Normalized Water Index (NWI) were computed from the spectral data. The relationships between canopy reflectance and water stress indicators were analyzed at different water stress levels. The result showed that the PRI centered at 550 nm wavelength (PRI550), WI, OSAVI, and WI/NDVI were the most sensitive indices to distinguish water stress levels in tomato plants. This study provides an insight into the feasibility of using spectral vegetation indices to monitor water stress in tomato crops for precision irrigation water management.

Sensitivity of olive leaf turgor to air vapour pressure deficit correlates with diurnal maximum stomatal conductance

Effective study and management of crops and forests would benefit greatly from useful plant-based indicators of the biological controls on evapotranspiration, and particularly stomatal conductance (gs). Given the strong influence of gs on bulk leaf water potential and turgor pressure (P), in vivo measurement of P may provide useful information about diurnal or seasonal dynamics of gs. Moderate plant water stress affects the diurnal dynamics of P as leaf-to-air vapour pressure deficit (D) varies, and these dynamics correlate to gs. Here, we explored relative changes in P in response to changes in D under mild drought conditions, and how these changes are linked to stomatal behaviour, and specifically to diurnal maximum gs (gs,max), one of the best indicators of plant water stress. We monitored ecophysiological and environmental variables, as well as a relative proxy for P, during three consecutive seasons in a hedgerow olive orchard where trees were supplied with different irrigation treatments to create well-watered and moderately water-stressed conditions. Our results demonstrated that the sensitivity of P to D correlated well with gs,max reached by the trees within a range in which variations in gs are the main diffusional limitation to photosynthesis. We further showed that this correlation held under a wide range of meteorological conditions and soil water availability. This turgor proxy measurement, which is much easier to measure than gs, can facilitate the use of gs,max as an indicator of plant water stress and evapotranspiration in agriculture and plant science research.

A comprehensive stress indicator for evaluating plant water status in almond trees

This study evaluated a comprehensive plant water stress indicator that integrates the leaf temperature and the environmental conditions that can provide feedback on the plant response to irrigation scheduling. This quantity—Comprehensive Stress Indicator (CSI)—is based on the reformulation of the leaf energy balance equation. Specifically, CSI is the ratio of the temperature difference between a dry leaf (i.e., a leaf with a broken stem) and a live leaf (on the same tree) [i.e., Tdry-Tleaf] and the difference between the vapor pressure inside the stomatal cavity at saturation and the vapor pressure of the air at ambient temperature [i.e., es(TL)-e(TA)]. The required measurements to compute CSI include dry leaf temperature, live leaf temperature, relative humidity, and air temperature at the tree being monitored. In this study, all measurements were obtained using a sensor suite called the leaf monitor. Leaf monitors were connected to a wireless mesh network to communicate the data to a website. The leaf monitor included two thermal infrared sensors, one for the dry leaf and one for the live leaf, both of which were housed in the same unit with almost identical environmental conditions due to a diffusing hemispherical dome that enclosed these sensors. The CSI is a dynamic indicator with a value for every data sample collected. For a single indicator representing each day, the CSI was evaluated in two ways. First, the CSI was integrated with respect to time from 10 A.M. to 6 P.M. to obtain the Integrated Comprehensive Stress Indicator (ICSI). Second, the CSI was averaged from 1 P.M. to 3 P.M. to obtain the Average Comprehensive Stress Indicator (ACSI). Both ICSI and ACSI were compared to other stress indicators, including the existing Crop Water Stress Index (CWSI) and Integrated Degrees Above Non-Stressed (IDANS). Results show that ICSI and ACSI are satisfactorily correlated with midday stem water potential (SWP). The ICSI and ACSI indicators may be more convenient than other stress indicators because they require measurements only at the tree being monitored.

Plant Water Status Indicators for Irrigation Scheduling Associated with Iso- and Anisohydric Behavior: Vine and Plum Trees

Plant water status indicators have been increasingly used for scheduling irrigation. Different variables may be used to do this, depending on personal preferences and the resources available. Many studies have suggested that selection of an indicator should take into account plant behavior in relation to isohydricity. In two Iberian studies, deficit irrigation (DI) was applied in a vineyard and in a plum orchard while plant water status and fluxes were monitored. These case-studies are discussed with special focus on the use of predawn leaf water potential (Ψpd) versus stem water potential (Ψst) to determine whether Ψst performed better and would therefore be the preferred stress indicator for plants exhibiting anisohydric behavior. In contrast, in plants with isohydric behavior, Ψpd would be generally preferred. This hypothesis seems to be supported by the present results obtained and by prior studies. The cultivars used and the intensity of stress applied have an important influence on the results. This suggests that, if no specific information is available from the existing literature, daily preliminary studies would be recommended prior to application in order to select the most appropriate plant water stress indicators.

Crop water status assessment in controlled environment using crop reflectance and temperature measurements

Crop water status is an important parameter for plant growth and yield performance in greenhouses. Thus, early detection of water stress is essential for efficient crop management. The dynamic response of plants to changes of their environment is called ‘speaking plant’ and multisensory platforms for remote sensing measurements offer the possibility to monitor in real-time the crop health status without affecting the crop and environmental conditions. Therefore, aim of this work was to use crop reflectance and temperature measurements acquired remotely for crop water status assessment. Two different irrigation treatments were imposed in tomato plants grown in slabs filed with perlite, namely tomato plants under no irrigation for a certain period; and well-watered plants. The plants were grown in a controlled growth chamber and measurements were carried out during August and September of 2014. Crop reflectance measurements were carried out by two types of sensors: (i) a multispectral camera measuring the radiation reflected in three spectral bands centred between 590–680, 690–830 and 830–1000 nm regions, and (ii) a spectroradiometer measuring the leaf reflected radiation from 350 to 2500 nm. Based on the above measurements several crop indices were calculated. The results showed that crop reflectance increased due to water deficit with the detected reflectance increase being significant about 8 h following irrigation withholding. The results of a first derivative analysis on the reflectance data showed that the spectral regions centred at 490–510, 530–560, 660–670 and 730–760 nm could be used for crop status monitoring. In addition, the results of the present study point out that sphotochemical reflectance index, modified red simple ratio index and modified ratio normalized difference vegetation index could be used as an indicator of plant water stress, since their values were correlated well with the substrate water content and the crop water stress index; the last being extensively used for crop water status assessment in greenhouses and open field. Thus, it could be concluded that reflectance and crop temperature measurements might be combined to provide alarm signals when crop water status reaches critical levels for optimal plant growth.

A cost-effective canopy temperature measurement system for precision agriculture: a case study on sugar beet

Increasing agricultural efficiency in a sustainable manner will contribute to feed a growing population under limited land, nutrient and water resources. Water scarcity and the increasing social concern for this resource are already requiring more sophisticated irrigation and decision-support systems. To address the heterogeneity in crop water status in a commercial field, precision irrigation requires accurate information about crops (e.g., crop water status), soil (e.g., moisture content) and weather (e.g., wind speed and vapor pressure deficit). Numerous studies have shown that plant canopy temperature can be used to derive reliable plant water stress indicators, thus making it a promising tool for irrigation water management. However, efficient and cost-effective measurement techniques are still lacking. This paper assesses the potential of infrared thermometry and thermal imaging for monitoring plant water stress in a commercial sugar beet field by comparing canopy temperature data acquired from a conventional thermal camera with an inexpensive infrared sensor, both mounted on a rotary-wing unmanned aerial vehicle (UAV). Measurements were taken at various phenological stages of the sugar beet growing season. Laboratory tests were performed to determine the key features for accurate temperature measurements and flight altitude. Experiments were conducted in 2014 and 2015 in experimental and commercial sugar beet fields in Southwestern Spain to (i) develop an affordable infrared temperature system suitable for mounting on a UAV to obtain thermal information, (ii) compare sugar beet canopy temperature measurements collected with the low-cost platform with those obtained from a conventional thermal camera, both mounted on a rotary-wing UAV, (iii) identify the factors that will limit the use of the low-cost system to derive temperature-based water stress indices. To accomplish these objectives, well-watered and deficit irrigated plots were established. Results indicated that the lightweight canopy temperature system was robust and reliable, although there were some constraints related to weather conditions and delimitation of the area covered by the infrared sensor.

Plant water stress detection based on aerial and terrestrial infrared thermography: a study case from vineyard and olive orchard

Irrigation scheduling is critical for vineyards and olive orchards, since it directly affects yield and fruit composition. Regulated deficit irrigation (RDI) strategies have been applied on both crops with positive results in the past. However, to successfully regulate stress levels, it is necessary to have accurate measurements of plant water status, which is usually achieved using a pressure chamber. In this regard, canopy temperature (Tc) has been shown to be an accurate indicator of plant water stress. Therefore, the objective of this study was to evaluate the accuracy of water stress detection based on aerial and terrestrial infrared thermography for a vineyard and an olive orchard. Lateral infrared thermal images were obtained using a handheld infrared camera and nadir-view infrared thermal images were obtained using an unmanned aerial vehicle (UAV). In addition, measurements of midday stem water potential (Φstem) from olives trees (Olea europaea L. 'eArbequina'f) and grapevines (Vitis vinifera L. 'eCarmenere'f) were taken under different RDI strategies during the 2013- 2014 growing season. The image analysis was performed using a customized code written in MathlabR defining thresholds to exclude non-plant elements. Results showed that the use of infrared thermal images aided in recognizing the differences in the water availability for irrigation treatments when the plants were under stress conditions.

The dynamics of radial sap flux density reflects changes in stomatal conductance in response to soil and air water deficit

Water scarcity in semiarid regions of Europe threatens the sustainability of fruit tree orchards unless irrigation water is optimized and scheduled in deficit irrigation strategies. Stomatal conductance (gs) is one of the best indicators of plant water stress, since it is placed in the crossroad between water and CO2 fluxes at the leaf level. Unfortunately, it is not possible to measure gs automatically and continuously, which reduces its potential for irrigation scheduling. In this work we examined the use of sap flux density (Js) in the outer rings of the sapwood of olive trees as a surrogate of gs. The working hypothesis was that as olive trees are well-coupled to atmosphere because of their small leaves, the ratio of Js to air vapor pressure deficit (D) should correlate well with the dynamics of gs in the canopy. It was also expected that current year, sun exposed leaves were mainly connected to the outer rings of the sapwood, and the oldest, shaded leaves to the inner rings. This was tested by measuring gs in new, sun-exposed leaves vs gs in old, shaded leaves. Both hypotheses were contrasted and our results confirmed that gs can be estimated from Js/D (R2 of the relationships were always higher than 0.8). A wide range of estimated gs values (0.350–0.025molm−2s−1) were derived from Js measurements in an olive orchard under three different irrigation regimes. Results were satisfactory and open the possibility of applying this method to estimate gs and use it either as a reliable water stress indicator or in transpiration and photosynthesis models applied to fruit tree orchards under a wide range of water stress conditions.

Implementing deficit irrigation scheduling through plant water stress indicators in early nectarine trees

A three-year experiment on early nectarine (Prunus persica L. Batsch cv. Flanoba) trees was carried out with the aim of increasing water use efficiency through applying a sustained irrigation treatment, in a commercial orchard in southern Spain. Experiments compared irrigation scheduling using conventional micrometeorology (110% of crop evapotranspiration, ETc) as a control treatment (TCTL), a treatment based on the normal practice of the farmer (TFARMER) and a regulated deficit irrigation treatment (TRDI), which involved irrigating the crop at the same level as the control (TCTL) during the critical periods of the first year (second rapid fruit growth period and 2 months after harvest) and at 60% TCTL during postharvest. In the last two years (2010 and 2011), the irrigation was scheduled to maintain the signal intensity (SI) of the maximum daily shrinkage of the trunk (MDS, SI=MDSTRDI/MDSTCTL) at different water stress levels depending on the phenological stage SI=1.0 (non-water stress) and SI=1.4 (moderate water stress). Most of the time that irrigation scheduling was based on MDS SI, this parameter varied only slightly around the pre-established threshold values. The information given by the stem diameter sensors and stem water potential (Ψstem) gave −1.5MPa and MDS SI 1.5 as threshold values not to be exceeded during postharvest, since MDS and Ψstem SI values were only linear down to 1.5. The water saved amounted to 17, 15 and 37% of the amount used in the control in the three seasons, respectively. In contrast, the TFARMER treatment applied more water (about 20 and 5% more than TCTL) during the first 2 years, and 10% less than TCTL during the third season.

Efeitos da rega e do regime hídrico em olival super intensivo no Alentejo

This study characterizes the effects of a typical farmer irrigation management and one of deficit irrigation imposed to a hedgerow olive orchard (1, 35 x 3,75 m) in the region of Evora, Southern Portugal on a Eutric Cambisol soil (FAO, 1998). The two irrigation regimes and their effects on tree transpiration, soil water content and stem and leaves water potential, a physiological parameter indicator of plant water stress, are analysed. With deficit irrigation DI imposed after pit fruit hardening, it was observed a progressive reduction on the values of tree transpiration, stem and leaf water potential and soil water content predominantly towards the end August and September. For the normal irrigation treatment (FI) transpiration values slightly declined for the months of July and August, although less than the ones for treatment DI. Results obtained with the “big leaf” model of Penman-Monteith (Jones, 1992) and bulk canopy conductance model of Orgaz et al. (2007) indicate that once validated they can be used to predict the transpiration of non-stressed hedgerow olive orchards in Alentejo. Predicting transpiration with the remotely sensed vegetation index NDVI proved difficult for lack of a strong correlation between the two parameters.

Scheduling peach orchard irrigation in water stress conditions: use of relative transpiration and predawn leaf water potential

Introduction . Plant water stress indicators have become valuable for moving towards deficit irrigation strategies and saving water. In this case evapotranspiration (ET ) is below its maximum value for the crop and stage (ETc ), and a stress coefficient (Ks ) is applied to obtain actual ET (ETa ). Predawn leaf water potential (Yp ) can be related to relative transpiration (RT ), the ratio between transpiration of a stressed plot (T ) and transpiration of a well-irrigated plot (Tm ). Estimating RT from Yp allows calculating ETa for determination of irrigation amounts, if deficit irrigation practices are used, as RT corresponds approximately to Ks . Materials and methods . RT and Yp were measured with the aim of establishing a relationship to estimate RT under moderate water stress for irrigation scheduling, in a peach orchard in south Portugal. RT was calculated using sap flow measurements (heat balance method) in two plots, one well-irrigated (daily drip irrigation amounts calculated for Tm ) and another temporarily without irrigation. Results and discussion . A high correlation was found between RT and Yp , allowing the estimation of RT for the studied conditions. Significant differences regarding the relationship RT- Y p obtained for another peach orchard in the same region and similar soil conditions were found for Yp in the range between –0.11 and –0.45 MPa. The results suggest that the differences resulted from the different irrigation systems: drip and micro-sprinkling, as they determine different temporal and spatial water distribution and therefore different geometry of root systems. A formerly proposed equation to estimate RT from Yp with a general form for different fruit tree species was tested, proving to be adequate within some limits: when RT is lowered to 0.7, the error was below 9%. The threshold value RT = 0.7 was considered a minimum as it was successfully tested in deficit irrigation practices for peach orchards.

Evaluation of the response of maximum daily shrinkage in young cherry trees submitted to water stress cycles in a greenhouse

In the semiarid zones of the Mediterranean area, such as the eastern coast of the Iberian Peninsula, as in many others in the world, water scarcity and the increase of water use in many socio-economical sectors, suppose an urgent need to improve irrigation management. To assess the feasibility of using maximum daily shrinkage (MDS) of the tree trunk as a plant water stress indicator for irrigation scheduling in cherry trees (Prunus avium), an experiment was carried out in 2009 under greenhouse conditions where the relationship of MDS with several environmental and physiological variables was studied. Two years-old cherry trees planted in plastic pots were submitted to several irrigation withholding cycles. Fully irrigated and stressed trees presented differences in all measured physiological variables: leaf stomatal conductance (gs), midday leaf water potential (Ψmd), and daily sap flow (Qd), as expected; and also in MDS which, among the variables measured in the plant, correlated best with Ψmd (r2=0.49***). Also a strong (r2=0.65***), two phase relationship was found between MDS and substrate matric potential (Ψs) with a linear response of MDS with some scattering till Ψs reached about −33kPa, followed by a stationary phase with a MDS value about 0.30mm, a potential threshold for irrigation scheduling of young cherry trees. This value corresponded to Ψmd of −2.3MPa and gs of 50mmolm−2s−1, indicative of water stress. Finally, MDS (which can be considered an adequate indicator of cherry tree water status given its strong relation with other indicators of plant water status) was the most sensitive and early indicator of water stress (highest signal to noise ratio) of the measured physiological variables. This high sensitivity makes it a potential candidate indicator for irrigation scheduling in young cherry trees, if the threshold obtained in this work can be validated or any other can be established.