Measurement Of Plant Water Status Research Articles

Impedance analysis of Labisia pumila plant water status

Labisia pumila (Kacip fatimah) is a popular medicinal plant in Malaysia. The constituents of this plant have been reported to possess anticancer, antioxidant and anti-inflammatory properties. The growth and production of L. pumila is greatly influenced by the plant water status. Current techniques to measure plant water status are generally based on the plant soil moisture, which apparently did not indicate the real water content inside the plant. There are other techniques to measure directly on the plant such as based on leaf water potential (LWP) and relative water content (RWC). However, these techniques are destructive and time consuming. In this study, four levels of evapotranspiration replacement (ER) treatment which were 100% ER, 75% ER, 50% ER and 25% ER was applied to 30 polybags of L. pumila plants. The plant water status was measured using an impedance spectroscopy technique. A pair of electrocardiogram (ECG) electrode connected to an impedance analyzer board was used to measure the impedance value of the leaf samples non-invasively. Plant water status parameters such as LWP, RWC, volumetric moisture content (VMC), and leaf thickness were measured using standard methods. The results show that after 20weeks of treatment, 25% ER had the highest impedance value ranged from 0.10MΩ to 0.15MΩ at the frequency of 70–100kHz. The resistance of 100% ER at 20weeks of treatment increased from 0.70kΩ to 1.23kΩ as the reactance decreased from 0.51kΩ to 0.28kΩ. Comparatively, the resistance of 25% ER increased from 1kΩ to 1.10kΩ as the reactance decreased from 0.88kΩ to 0.83kΩ. The polynomial regression of impedance measurements with plant water status parameters (VMC, leaf thickness, LWP and RWC) shows that LWP and RWC had the highest R2 (0.78, 0.73). The results show that impedance measurement technique is auspicious to evaluate plant water status.

Automatic irrigation system based on dual crop coefficient, soil and plant water status for Vitis vinifera (cv Godello and cv Mencía)

This research aims at testing an automatic control irrigation system, using a wireless sensor network, in traditional Galician vineyards of Vitis vinifera (L.) cv. ‘Godello’ and cv. ‘Mencía’ to determine the threshold values of soil water potential at which plant stress begins, calibrating crop coefficients, building soil–water characteristics curves and measuring plant water status. In the cv. ‘Godello’ trial, rain-fed and two irrigations systems (surface and subsurface drip irrigation) were conducted over two growing seasons (2012–2013); during the same seasons cv ‘Mencía’ was also studied, but only under rain-fed conditions. The SIMDualKc model, which estimates soil water balance by means of the dual Kc approach, was used to estimate crop evapotranspiration (ETc) by calibrating the full basal crop coefficient for the vine and cover crop (Kcb full), which represents the transpiration component of ETc, and a soil evaporation coefficient (Ke). The model was calibrated and validated by comparing model simulations with TDR observed soil water content data. Granular matrix sensor (GMS) was linked in a wireless sensor network; soil water potential measured with GMS, was used to correlate with TDR data. Leaf water potentials (LWP) – midday and stem – allowed us to obtain plant water status. A good fit was obtained between SIMDualKc model and TDR (r2>0.74), TDR and LWP (r2>0.65), TDR and GMS (r2>0.81), showing that continuous measures with GMS permit establishing a threshold value related with leaf water potential (midday or stem). For both cultivars, the threshold was Ψsoil=−0.1MPa. The process applied in this study proved to be useful for managing water in real-time in a vineyard; triggering the irrigation system when the threshold value was reached.

Global reprogramming of transcription and metabolism in Medicago truncatula during progressive drought and after rewatering

Medicago truncatula is a model legume forage crop native to the arid and semi-arid environments of the Mediterranean. Given its drought-adapted nature, it is an ideal candidate to study the molecular and biochemical mechanisms conferring drought resistance in plants. Medicago plants were subjected to a progressive drought stress over 14 d of water withholding followed by rewatering under controlled environmental conditions. Based on physiological measurements of plant water status and changes in morphology, plants experienced mild, moderate and severe water stress before rehydration. Transcriptome analysis of roots and shoots from control, mildly, moderately and severely stressed, and rewatered plants, identified many thousands of genes that were altered in expression in response to drought. Many genes with expression tightly coupled to the plant water potential (i.e. drought intensity) were identified suggesting an involvement in Medicago drought adaptation responses. Metabolite profiling of drought-stressed plants revealed the presence of 135 polar and 165 non-polar compounds in roots and shoots. Combining Medicago metabolomic data with transcriptomic data yielded insight into the regulation of metabolic pathways operating under drought stress. Among the metabolites detected in drought-stressed Medicago plants, myo-inositol and proline had striking regulatory profiles indicating involvement in Medicago drought tolerance.Global transcriptional and metabolic responses to drought and rewatering were investigated in Medicago truncatula, a naturally drought-adapted model legume species. Integration of metabolomic and transcriptomic data yielded insights into the regulation of metabolic pathways underlying drought-stress adaptation. Many genes and metabolites with expression tightly coupled to drought intensity were identified, suggesting active involvement in Medicago drought resistance. These could prove useful targets for future translational approaches to improve closely related crop plants such as common bean, soybean and pea.

Normal vector based dynamic laser speckle analysis for plant water status monitoring

The analysis of dynamic laser speckle is widely known as an effective non-contact technique for detecting micro-activity on a sample surface. However, the analysis obtained using conventional statistical methods is heavily influenced by the properties of the sample, laser beam and ambient lighting. Recently, we proposed a new normal vector based dynamic speckle analysis method and demonstrated its ability to remove interference due to non-uniform reflectivity, illumination, and time-varying ambient lighting. In this paper, the normal vector based statistical technique for dynamic speckle analysis is further justified by the application to leaf surface micro-motion detection for monitoring plant water status. We carried out experiments for surface activity detection of attached leaves to monitor variation of leaf water status. The presented results highlight the advantage of the normal vector based analysis over image intensity based methods and demonstrate the potential of measuring plant water status via dynamic laser speckle analysis.

Water Relations of Well-watered Citrus Exposed to Cold-acclimating Temperatures

This study was conducted on well-watered citrus to determine changes in water relations during cold acclimation independent of drought stress. Potted sweet orange and Satsuma mandarin trees were exposed to progressively lower, non-freezing temperatures down to 10/4 °C, light/dark temperatures, respectively, for 9 weeks in environmental growth chambers to promote cold acclimation. The trees were watered twice daily and three times on the day water relations data were collected to minimize drought stress. Although soil moisture was higher and non-limiting for plants in the cold than in the warm chamber, cold temperatures promoted stomatal closure, higher root resistance, lower stem water potential (Ψstem), lower transpiration, and lower leaf ψS. Leaf relative water content (RWC) was not different for cold-acclimated trees compared with the controls. Cold acclimation promoted stomatal closure at levels only observed in severely drought-stressed plants exposed to warm temperatures and where Ψstem and RWC are typically much lower than what was found in this study. Ψstem continued to decline the last 4 weeks of the experiment although air temperature, leaf ψS, RWC, stomatal conductance (gS), and transpiration were constant. The results of this experiment indicate that water relations of citrus during cold acclimation vary from those known to occur as a result of drought stress, which have implications for using traditional measures of plant water status in irrigation scheduling during winter.

A biophotonic sensing method for plant drought stress

Physiologically-based drought stress evaluation is very desirable for sustainable plant production, but effective measures and technologies are lacking. Drought stress limits photons utilized for photosynthetic reactions and delayed fluorescence (DF) generated by photosystem II (PSII). In this research, DF is measured as an output variable of the PSII phototransduction system and its dependence on photon utilization rate as affected by water status is modeled and analyzed. This yields an effective way to define and measure plant water status or deficiency (drought stress) according to PSII photon utilization rate. Water deficiency is determined by the deficit from the maximum photon utilization rate achievable through rehydration. The method was validated experimentally with drought-stressed plant samples. Analysis and experiments show that this drought-dependent deficit in photon utilization rate can be effectively evaluated from measured DF emissions and it varies with drought severity.

An artificial neural network approach to the estimation of stem water potential from frequency domain reflectometry soil moisture measurements and meteorological data

Stem water potential seems to be a sensitive measure of plant water status. Nonetheless, it is a labour-intensive measurement and is not suited for automatic irrigation scheduling or control. This study describes the application of artificial neural networks to estimate stem water potential from soil moisture at different depths and standard meteorological variables, considering a limited data set. The experiment was carried out with ‘Navelina’ citrus trees grafted on ‘Cleopatra’ mandarin. Principal components analysis and multiple linear regression were used preliminarily to assess the relationships among observations and to propose other models to allow a comparative analysis, respectively. Two principal components account for the systematic data variation. The optimum regression equation of stem water potential considered temperature, relative humidity, solar radiation and soil moisture at 50cm as input variables, with a determination coefficient of 0.852. When compared with their corresponding regression models, ANNs presented considerably higher performance accuracy (with an optimum determination coefficient of 0.926) due to a higher input-output mapping ability.

Photosynthesis of barley awns does not play a significant role in grain yield under terminal drought

In order to determine the importance of awn photosynthesis on grain yield under terminal drought, six two-rowed cultivars of barley representing high yielding commercial releases from 1961 to 2006 were studied in a glasshouse experiment at CSIRO, Western Australia. The cultivars were grown with and without watering from anthesis. Detailed measurements of plant water status, awn, flag and penultimate leaf photosynthetic rate were made from anthesis. At final harvest, biomass, yield and yield components were measured. Awn net photosynthesis rate (Pn) was lower than penultimate and flag leaf and decreased gradually after anthesis. The rate of decreasing Pn was rapid under terminal drought. There was no difference in awn Pn among cultivars under well-watered conditions, but under terminal drought the awn Pn of barley cultivars Baudin and Clipper decreased faster than Vlaming, Gairdner and Stirling. Surface area of awns in each cultivar was higher than the flag and penultimate leaf. Thus, total awn photosynthesis under both well-watered and terminal drought conditions was higher than flag leaf photosynthesis. In Vlaming, total awn photosynthesis was higher than penultimate leaf photosynthesis. Grain yield of the cultivars Baudin, Covette and Gairdner was affected by terminal drought, but grain yield did not correlate with awn total photosynthesis. Under well-watered conditions awn Pn had a significant negative correlation with ear weight. We suggest that under terminal drought, higher awn area does not lead to higher grain yield because sink size may be the factor limiting grain yield in barley.

ELECTRICAL SIGNAL MEASUREMENTS IN AVOCADO TREES: A POTENTIAL TOOL FOR MONITORING PHYSIOLOGICAL RESPONSES TO SOIL WATER CONTENT?

Monitoring soil water content coupled with phytomonitoring techniques have been shown to be good tools for irrigation management in avocado orchards. There are many well-tested devices for monitoring soil moisture in orchards but options for measuring plant water status are limited. The objective of this study was to determine whether measuring voltage differences between roots and leaves can be used as a technique to measure plant water status related to soil water content. Root and shoot voltage differences were monitored in young ‘Hass’ avocado trees grafted onto clonal rootstocks grown in containers, and in young seedling ‘Mexicola’ avocado trees grown hydroponically, both experiments carried out under laboratory conditions. In container-grown ‘Hass’ avocado trees, root and shoot voltage differences were initially measured for about 2 h to determine steady state conditions. Plants were then exposed to cycles of soil drying and re-watering. In seedling ‘Mexicola’ trees, electrical signals were measured for at least 75 min in control plants and plants exposed to simulated drought stress. The extracellular electrical potential difference between the base of the trunk and the leaf petiole was continuously monitored after exposing plants to treatments. Results indicated that in ‘Hass’ avocado trees, a change in soil water content induced by root drying and re-watering was accompanied by a slow change in the electrical signal at the leaf petiole which was greatest after 52 and 32 min for root drying and re-watering, respectively. This was related to a decrease in stomatal conductance (gs) of plants exposed to drought. Also, in hydroponic conditions, significant voltage differences occurred in plants exposed to drought simulated by the addition of polyethylene glycol (PEG 6000) to the hydroponic medium resulting also in a decrease in gs. These results suggest that it may be possible to use voltage differences measurements as a technique for monitoring physiological responses to soil water content.

Can changes in leaf water potential be assessed spectrally?

Leaf water potential (LWP) is an important indicator of plant water status. However, its determination via classical pressure-chamber measurements is tedious and time-consuming. Moreover, such methods cannot easily account for rapid changes in this parameter arising from changes in environmental conditions. Spectrometric measurements, by contrast, have the potential for fast and non-destructive measurements of plant water status, but are not unproblematic. Spectral characteristics of plants vary across plant development stages and are also influenced by environmental factors. Thus, it remains unclear whether changes in leaf water potential per se can reliably be detected spectrometrically or whether such measurements also reflect autocorrelated changes in the leaf water content (LWC) or the aerial plant biomass. We tested the accuracy of spectrometric measurements in this context under controlled climate chamber conditions in series of six experiments that minimised perturbing influences but allowed for significant changes in the LWP. Short-term exposure of dense stands of plants to increasing or decreasing artificial light intensities in a growth chamber more markedly decreased LWP than LWC in both wheat and maize. Significant relationships (R2-values 0.74-0.92) between LWP and new spectral indices ((R940/R960)/NDVI; R940/R960) were detected with or without significant changes in LWC of both crop species. The exact relationships found, however, were influenced strongly by the date of measurement or water stress induced. Thus, global spectral relationships measuring LWP probably cannot be established across plant development stages. Even so, spectrometric measurements supplemented by a reduced calibration dataset from pressure chamber measurements might still prove to be a fast and accurate method for screening large numbers of diverse lines.

Evaluating water stress in irrigated olives: correlation of soil water status, tree water status, and thermal imagery

Irrigation of olive orchards is challenged to optimize both yields and oil quality. Best management practices for olive irrigation will likely depend on the ability to maintain mild to moderate levels of water stress during at least some parts of the growing season. We examined a number of soil, plant and remote sensing parameters for evaluating water stress in bearing olive (var. Barnea) trees in Israel. The trees were irrigated with five water application treatments (30, 50, 75, 100 and 125% of potential evapotranspiration) and the measurements of soil water content and potential, mid-day stem water potential, and stomatal resistance were taken. Remote thermal images of individual trees were used to alternatively measure average canopy temperature and to calculate the tree’s crop water stress index (CWSI), testing empirical and analytical approaches. A strong non-linear response showing similar trends and behavior was evident in soil and plant water status measurements as well as in the CWSI, with decreasing rates of change at the higher irrigation application levels. No statistically significant difference was found between the analytical and the empirical CWSI, suggesting that the relative simplicity of the analytical method would make it preferable in practical applications.

The large-effect drought-resistance QTL qtl12.1 increases water uptake in upland rice

Drought stress is the most important abiotic factor limiting upland rice yields. Identification of quantitative trait loci (QTL) conferring improved drought resistance may facilitate breeding progress. We previously mapped a QTL with a large effect on grain yield under severe drought stress ( qtl12.1) in the Vandana/Way Rarem population. In the current paper, we present results from a series of experiments investigating the physiological mechanism(s) by which qtl12.1 affects grain yield under drought conditions. We performed detailed plant water status measurements on a subset of lines having similar crop growth duration but contrasting genotypes at qtl12.1 under field (24 genotypes) and greenhouse (14 genotypes) conditions. The Way Rarem-derived allele of qtl12.1 was confirmed to improve grain yield under drought mainly through a slight improvement (7%) in plant water uptake under water-limited conditions. Such an apparently small increase in water uptake associated with this allele could explain the large effect on yield observed under field conditions. Our results suggest that this improvement of plant water uptake is likely associated with improved root architecture.

The potential of high spatial resolution information to define within-vineyard zones related to vine water status

The goal of this study was to test the usefulness of high-spatial resolution information provided by airborne imagery and soil electrical properties to define plant water restriction zones within-vineyards. The main contribution of this is to propose a study on a large area representing the regions’ vineyard diversity (different age, different varieties and different soils) located in southern France (Languedoc-Roussillon region, France). Nine non-irrigated plots were selected for this work in 2006 and 2007. In each plot, different zones were defined using the high-spatial resolution (1 m2) information provided by airborne imagery (Normalised Difference Vegetation Index, NDVI). Within each zone, measurements were conducted to assess: (i) vine water status (Pre-dawn Leaf Water Potential, PLWP), (ii) vine vegetative expression (vine trunk circumference and canopy area), (iii) soil electrical resistivity and, (iv) harvest quantity and quality. Large differences were observed for vegetative expression, yield and plant water status between the individual NDVI-defined zones. Significant differences were also observed for soil resistivity and vine trunk circumference, suggesting the temporal stability of the zoning and its relevance to defining vine water status zones. The NDVI zoning could not be related to the observed differences in quality, thus showing the limitations in using this approach to assess grape quality under non-irrigated conditions. The paper concludes with the approach that is currently being considered: using NDVI zones (corresponding to plant water restriction zones) in association with soil electrical resistivity and plant water status measurements to provide an assessment of the spatial variability of grape production at harvest.

Ecological relevance of minimum seasonal water potentials

The minimum seasonal water potential a plant experiences, ψmin, provides an important measure of plant water status, as it reflects the maximum water deficit that leaves and xylem must tolerate to maintain physiological activity. ψmin also acts as a selective force on xylem structure which, in turn, generates correlations between ψmin and numerous hydraulic traits. This review focuses on the ecological relevance of ψmin as a reflection of overall plant hydraulic strategy. The focus is on plant functional strategies with respect to soil drought, but we conclude with preliminary findings on the role of atmospheric drought.

Suitability of existing and novel spectral indices to remotely detect water stress in Populus spp.

Undetected water stress within poplar plantations can result in high economic losses. Although remotely sensed spectral measures associated with variations in plant water status have been developed for non- Populus species, little is known about the suitability of such spectral indices to identify water stress in Populus species. Our experiments assessed whether such spectral indices acquired at the leaf and canopy level were suitable for detecting water stress in Populus. Relationships between four spectral indices and four common measures of plant water status were analyzed for both low to moderate, and highly water stressed trees. The proposed maximum difference water index (MDWI) exhibited significant relationships ( P < 0.001) with changes in plant water status at both the leaf and canopy levels. At the leaf level, statistically significant, though poorer relationships were obtained between each of the normalized differential water index (NDWI), the red edge inflection point (REIP), the water index (WI), and the plant water status measures. At the canopy level, statistically significant relationships were obtained between MDWI and the measure of relative water content (RWC), and the equivalent water thickness (EWT) ( r 2 > 0.56, P < 0.001), while WI produced reasonable relationships with these water status measures ( r 2 > 0.42, P < 0.001). On exclusion of the highly stressed plants, only MDWI exhibited low sensitivity to RWC ( r 2 = 0.37, slope = 0.64) and leaf water potential at the canopy level ( r 2 = 0.42, slope = 9.89) whereas no other indices exhibited sensitivity to changes in plant water status. MDWI, which incorporates short-wave infrared (SWIR, 1300–2500 nm) spectral bands, is more suitable to detect more changes in plant water status as compared to NDWI, REIP, and WI that incorporate near infrared (NIR, 700–1300 nm) spectral bands. These results suggest that such remotely sensed indices are not a viable option to detect low and moderate levels of water stress at a scale of a Populus spp. plantation, although further research is warranted to assess the broader applicability of MDWI and other SWIR wavelength incorporating indices to detect water stress in other species.

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h−1) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.

Phosphorus nutrition and tolerance of cotton to water stress: II. Water relations, free and bound water and leaf expansion rate

In previous experiments, increased leaf-Phosphorus (P) content with increasing P supply enhanced the individual leaf expansion and water content of fresh cotton leaves in a severely drying soil. In this paper, we report on the bulk water content of leaves and its components, free and bound water, along with other measures of plant water status, in expanding cotton leaves of various ages in a drying soil with different P concentrations. The bound water in living tissue is more likely to play a major role in tolerance to abiotic stresses by maintaining the structural integrity and/or cell wall extensibility of the leaves, whilst an increased amount of free water might be able to enhance solute accumulation, leading to better osmotic adjustment and tolerance to water stress, and maintenance of the volumes of sub-cellular compartments for expansive leaf growth. There were strong correlations between leaf-P%, leaf water (total, free and bound water) and leaf expansion rate (LER) under water stress conditions in a severely drying soil. Increased soil-P enhanced the uptake of P from a drying soil, leading to increased supply of osmotically active inorganic solutes to the cells in growing leaves. This appears to have led to the accumulation of free water and more bound water, ultimately leading to increased leaf expansion rates as compared to plants in low P soil under similar water stress conditions. The greater amount of bound and free water in the high-P plants was not necessarily associated with changes in cell turgor, and appears to have maintained the cell-wall properties and extensibility under water stressed conditions in soils that are nutritionally P-deficient.

Irrigation scheduling: advantages and pitfalls of plant-based methods.

This paper reviews the various methods available for irrigation scheduling, contrasting traditional water-balance and soil moisture-based approaches with those based on sensing of the plant response to water deficits. The main plant-based methods for irrigation scheduling, including those based on direct or indirect measurement of plant water status and those based on plant physiological responses to drought, are outlined and evaluated. Specific plant-based methods include the use of dendrometry, fruit gauges, and other tissue water content sensors, while measurements of growth, sap flow, and stomatal conductance are also outlined. Recent advances, especially in the use of infrared thermometry and thermography for the study of stomatal conductance changes, are highlighted. The relative suitabilities of different approaches for specific crop and climatic situations are discussed, with the aim of indicating the strengths and weaknesses of different approaches, and highlighting their suitability over different spatial and temporal scales. The potential of soil- and plant-based systems for automated irrigation control using various scheduling techniques is also discussed.

Continuous measurement of plant and soil water status for irrigation scheduling in plum

The usefulness of continuous measurement of soil and plant water status for automated irrigation scheduling was studied in a drip-irrigation experiment on plum (Prunus salicina Black Gold). Two levels of water restriction were imposed at different phenological periods (from pit-hardening to harvest, post-harvest) and compared with a well irrigated control treatment. Soil matrix water potential (ψsoil) was measured with granular matrix sensors (Watermark); and short-period trunk diameter variation (TDV) was measured with linear variable displacement transformers. The Watermark sensor readings were in reasonable agreement with the irrigation regime and showed a good indication of plant water status across the season (r2=0.62), although they were a better predictor of stem water potential (ψstem) in the dry range of ψsoil. Nonetheless, the most important drawback in their use was the high variability of readings (typical CV of 35–50%). From TDV measurements, maximum daily shrinkage (MDS) and trunk growth rate (TGR) were calculated. Their performance was also compared with ψstem, which had the lowest variability (CV of 7%). During most of the fruit growth period, when TGR was minimum, MDS was higher in the less-irrigated treatment than in the control and correlated well (r2=0.89) with ψstem. However, after harvest, when TGR was higher, this correlation decreased as the season progressed (r2=0.73–0.52), as did the slope between MDS and ψstem, suggesting tissue elasticity changes. Later in the season, TGR was better related to plant water status. These observations indicate some of the difficulties in obtaining reference values useful for irrigation scheduling based exclusively on plant water status measurements.

Use of Spectral Radiance to Estimate In-Season Biomass and Grain Yield in Nitrogen- and Water-Stressed Corn

Current technologies for measuring plant water status are limited, while recently remote sensing techniques for estimating N status have increased with limited research on the interaction between the two stresses. Because plant water status methods are time-consuming and require numerous observations to characterize a field, managers could benefit from remote sensing techniques to assist in irrigation and N management decisions. A 2-yr experiment was initiated to determine specific wavelengths and/or combinations of wavelengths indicative of water stress and N deficiencies, and to evaluate these wavelengths for estimating in-season biomass and corn (Zea mays L.) grain yield. The experiment was a split-plot design with three replications. The treatment structure had five N rates (0, 45, 90, 134, and 269 kg N ha−1) and three water treatments [dryland, 0.5 evapotranspiration (ET), and full ET]. Canopy spectral radiance measurements (350–2500 nm) were taken at various growth stages (V6–V7, V13–V16, and V14–R1). Specific wavelengths for estimating crop biomass, N concentration, grain yield, and chlorophyll meter readings changed with growth stage and sampling date. Changes in total N and biomass in the presence of a water stress were estimated using near-infrared (NIR) reflectance and the water absorption bands. Reflectance in the green and NIR regions were used to estimate total N and biomass without water stress. Reflectance at 510, 705, and 1135 nm were found for estimating chlorophyll meter readings regardless of year or sampling date.