Leaf Temperature Measurements Research Articles

Leaf Temperature Measurement Using Low-Resolution Thermal Camera Based on Thresholding and Clustering Techniques

Leaf temperature can indicate photosynthetic rates, leaf water status, and stomata conductance. Leaf temperature can be measured using thermal resistance sensors, thermocouple devices, infrared thermometers, or infrared thermal imaging devices. Additionally, measuring leaf temperature using a thermal camera is simple and efficient. Therefore, the research proposes a leaf temperature measurement method using AMG8833, a low-resolution (64 pixels) thermal camera. The proposed system adopts an image segmentation technique to extract the leaf area from a thermal image. The leaf temperature is then calculated by averaging the temperature values on the leaf area. The proposed system aims to utilize a low-cost and low-resolution thermal camera for measuring the leaf temperature. The proposed approach is evaluated using real images of the Dieffenbachia plant, a popular ornamental plant that can be easily planted. In the experiments, fourteen segmentation methods consisting of eight thresholding techniques and six clustering techniques are evaluated. The experimental findings on the Dieffenbachia plant indicate that the most accurate leaf temperature measurements are obtained using local thresholding with an absolute error of 0.0109 and k-means clustering with an absolute error of 0.0134. The proposed method provides a simple, effective, and low-cost leaf temperature measurement system compared to the existing systems which employ high-cost commercial thermal cameras and complex measurement methods.

Modelling Canopy Temperature of Crops With Heterogeneous Canopies Grown Under Solar Panels

With global warming and the increase of heatwaves frequencies, it has become urgent to protect crops. Agrivoltaic systems tackle this issue by shading plants with photovoltaic panels to lower the temperature of canopies. However, a permanent shading would lead to an important loss of carbon for plants. For this reason, dynamic agrivoltaic systems (AVD) emerged with panels which could be steered in real time according to the needs of plants. Shading at the right time is not that easy with the risk to either miss a hot event and cause serious and irreversible injuries to the plants or shade too often, and impact carbon production. In this paper we present first an experiment with measurements of leaf temperature at different positions of grapevine canopy for two summer days in 2020 and 2021. Then, the energy balance sub-model part of a crop model that simulate plant growth for fruit trees and vines grown in heterogeneous AVD environments is presented. Finally, after having evaluated the coherence of the model with experimental results, the relevance of a mechanistic model to steer solar panels and protect plants from heat is illustrated through several examples. The heterogeneity of temperature within the canopy observed in the field experiments related with different variables such as air and ground temperature, leaf orientation and self-shading was correctly reproduced by the model. This work indicated that canopy temperature could be more integrative than a unique threshold of air temperature to take decisions on panel orientation to protect plants from heat stress.

Practical Considerations and Limitations of Using Leaf and Canopy Temperature Measurements as a Stomatal Conductance Proxy: Sensitivity across Environmental Conditions, Scale, and Sample Size.

Stomatal conductance (gs) is a crucial component of plant physiology, as it links plant productivity and water loss through transpiration. Estimating gs indirectly through leaf temperature (Tl) measurement is common for reducing the high labor cost associated with direct gs measurement. However, the relationship between observed Tl and gs can be notably affected by local environmental conditions, canopy structure, measurement scale, sample size, and gs itself. To better understand and quantify the variation in the relationship between Tl measurements to gs, this study analyzed the sensitivity of Tl to gs using a high-resolution three-dimensional model that resolves interactions between microclimate and canopy structure. The model was used to simulate the sensitivity of Tl to gs across different environmental conditions, aggregation scales (point measurement, infrared thermometer, and thermographic image), and sample sizes. Results showed that leaf-level sensitivity of Tl to gs was highest under conditions of high net radiation flux, high vapor pressure deficit, and low boundary layer conductance. The study findings also highlighted the trade-off between measurement scale and sample size to maximize sensitivity. Smaller scale measurements (e.g., thermocouple) provided maximal sensitivity because they allow for exclusion of shaded leaves and the ground, which have low sensitivity. However, large sample sizes (up to 50 to 75) may be needed to differentiate genotypes. Larger-scale measurements (e.g., thermal camera) reduced sample size requirements but include low-sensitivity elements in the measurement. This work provides a means of estimating leaf-level sensitivity and offers quantitative guidance for balancing scale and sample size issues.

Feasibility study on Lepton 3.5 in terms of accuracy for measuring leaf temperature of crops

The precise monitoring of leaf temperature is becoming more important as crop leaf temperature is utilized more frequently for different uses such as irrigation, disease and pest detection. This study aims to explore the potential usage of a cost-effective Lepton 3.5 camera to measure the crop canopy temperature. The accuracy of the Lepton 3.5 will be compared to a FLIR E8-XT thermal camera and an MLX90614 infrared thermometer. With the usage of three devices: a custom Lepton 3.5 camera, an Implexxio LT-1T thermistor and an MLX90614, the temperature of the target leaf of a laboratory plant was automatically measured every five minutes. The data would then be recorded on a private cloud server and manually measured with a handheld FLIR E8-XT. The performance of these three devices was evaluated to the standard of a highly accurate Implexxio LT-1T thermistor using the mean absolute error and root mean squared error. Among the non-contact sensors- MLX90614, Lepton 3.5 and FLIR E8-XT- the MLX90614 sensor showed the highest accuracy. However, the Lepton 3.5 module had an accuracy of less than ±2°C, which was similar to FLIR E8-XT and much better than the error value specified for the Lepton. The low-cost Lepton 3.5 can be used to periodically measure leaf temperature with an accuracy comparable to that of an intermediate-level thermal imager.

Relationships between leaf temperature, stomatal conductance and architecture: potential impact on leaf burning among a range of genotypes in grapevine

In the context of climate change, extreme heatwaves are often observed. The consequences of these events led to leaf and grape burning, as observed in June 2019 in the South of France. Previous observations showed that genotypic variability exists in response to these heatwaves. One of the main hypotheses to explain the differences is that genotypes could differentially regulate their leaf temperature. This temperature is closely associated with stomatal conductance and the amount of energy absorbed by the leaves. This amount of energy is known to be a consequence of plant architecture that determines light interception. This study was performed on a set of 33 genotypes that were selected with different leaf-burning sensitivities to high temperatures. Functional (i.e., stomatal conductance, photosynthesis) and architectural traits (internode length, leaf area and leaf elevation angles) were measured to compute their heritabilities and to determine correlations between these traits. Measurements of stomatal conductance and leaf temperature were performed during 30 measurement periods in 2021 and 2022. Architectural traits were extracted from 3D digitizing. High heritability in architectural traits were observed (around 0.8). Heritability of functional traits, although lower, were not negligible (higher than 0.6) and were partly dependent on the weather conditions during the measurements. A clustering of genotypes based on mean values of their architectural and functional traits revealed that both types of traits could be combined independently. New combinations of traits and their impact on leaf temperature were then examined. Stomatal conductance appeared to be associated with the intensity of the burning symptoms than architectural traits. The genotypes with high stomatal conductance also displayed low leaf temperature in accordance with the evaporative cooling effect. However, these genotypes were also the most sensitive to leaf burn. This likely suggests that leaf burn resulted from a high transpiration rate that could cause embolism under hot and dry weather conditions. For future works, modelling approaches could be of major interest to quantify the relative impact of architectural and functional traits on leaf temperature. Nevertheless, our study shows that leaf temperature is not completely associated with the observed leaf-burning symptoms and that other processes are involved.

Assessing accuracy over warm-up time of Lepton 3.5 thermal imaging for measuring leaf temperature of crops

Accurately monitoring leaf temperatures is becoming more and more critical as more studies use a crop's leaf temperature for irrigation, disease, and pest detection. In the present study, the accuracy of a module for camera warm-up times of 30 sec, 1 min, 2 min, 3 min, 3.5 min, and 4 min was compared when leaf temperature was periodically measured using a Lepton 3.5 module, a low-cost thermal imaging camera. The experiment was conducted for a plant in a laboratory, and a high-accuracy LT-1T thermistor sensor was used together to compare module accuracy. The power consumption of both sensors decreased by using the sleep mode of the module, and all measurements were conducted in intervals of five minutes. The accuracy was compared using the R-Squared, MAE, and RMSE of the two values measured by LT-1T and Lepton 3.5. As a result of the experiment, the accuracy was the highest when the warm-up time was 3.5 minutes showing the mean absolute error (MAE) and the root means squared error (RMSE) values of 0.56 ? and 0.59 ?, respectively. The accuracy was 2.5%, which was substantially higher than the 5% accuracy of the device specification.

Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis).

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37°C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30±0.5°C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4±0.38°C for HEAT and 48.2±0.24°C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30±0.5°C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.

Impacts of zebra chip disease and irrigation on leaf physiological traits in potato

Zebra chip disease (ZC) is caused by the fastidious, phloem-limited, bacterial pathogen ‘Candidatus Liberibacter solanacearum (Lso), which is transmitted from plant to plant by the potato psyllid (Bactericera cockerelli (Sulc)). Understanding how ZC impacts potato (Solanum tuberosum L.) physiology, could help growers in making more informed crop management decisions. Measurements of instantaneous leaf physiological responses, such as photosynthetic CO2 uptake and transpiration on the leaf surface, can be used not only for fast screening of affected plants in the field but also for optimizing irrigation management. Over the 2019 and 2020 field seasons, we characterized time-course photosynthetic physiological responses of potato plants infested by potato psyllids (B. cockerelli (Sulc)) carrying the Lso haplotypes A+B. Potato plants were subjected to different variable-rate irrigation (VRI) treatments (100%, 80%, and 60% of field capacity of the soil) through a center-pivot sprinkler system to examine the impact of the disease on key physiological parameters of photosynthesis and transpiration. Leaf and air temperatures, and hyperspectral profiles of the canopy were also measured and compared. The measurements were made during midday weekly from 25 to 50 days after plant infestation (DAI) with bacteriliferous psyllids. The results showed that many of the measured variables, including stomatal conductance, photosynthesis rate, transpiration rate, quantum yields, and normalized difference in vegetation index started to decrease beginning approximately 28–35 DAI, gradually worsening until 50 DAI, in both 2019 and 2020, as the infection proceeded. The decreases in stomatal conductance in infected plants led to decreases in photosynthesis and transpiration. In turn, reduced transpiration resulted in increased leaf temperature due to decrease in evaporative cooling on the leaf surface. Higher leaf temperatures under hot and dry conditions with high light intensity during the daytime would further reduce photosynthetic light harvesting, which is supported by our data, indicating the damage to the photosynthetic pigment formation and machinery. These findings support the previous report that increased leaf temperature in infected plants may have been derived from the closure of stomata in hypersensitive reactions to infection. These stomatal responses were detected within 28 DAI, a week earlier than the differences in hyperspectral profiles observed 35 DAI, and could be implemented in early disease detection strategiesusing measurements of leaf temperature.

Using an IR camera to improve leaf area and temperature measurements: A new method for increasing the accuracy of photosynthesis-related parameters

Accurate measurements of leaf temperature and area are crucial to calculate leaf gas exchange parameters when using portable gas analysers such as the LI-6400 XT portable photosynthesis system (LI‐COR Inc., Lincoln, USA). The importance of leaf temperature data is well known for robust measurements, but there are still technical limitations in measuring it with full accuracy. Here we present a tentative solution by incorporating leaf temperature measurements with thermal imaging. An infrared (IR) camera (IRay T3 Pro, Shandong, China) was combined with a LI-6400XT to measure mean leaf temperatures while avoiding leaf thermal heterogeneity and to accurately calculate leaf area while avoiding destructive sampling. The thermal heterogeneity of Helianthus annuus and Zea mays leaves were studied. We then evaluated the accuracies of leaf area and temperature measurements obtained for 9 species and discussed the influences of these measurements on the resulting photosynthesis-related parameters. Our results provide direct visual evidence of the thermal heterogeneity of leaves in an infrared gas analyzer (IRGA) chamber. The temperature differences across the leaf surface were as high as 0.28°C∼1.38°C, causing large errors in stomatal conductance (gs, -5.07%∼-55.02%) and intercellular CO2 concentrations (Ci, -0.41%∼-6.72%). The IR camera reduced the area measurement error (-0.71∼0.29 cm2) and greatly increased the accuracy by -11.12%∼17.60% for the net photosynthesis (Pn) and transpiration rate (E) and by -11.24%∼17.69% for gs calculations. Our results showed that combining an IR camera with LI-6400XT can improve leaf area measurements, avoid thermal heterogeneity in leaves and thus dramatically improve the accuracy of photosynthesis-related parameter calculations under semi controlled and moderate climate conditions.

Tradeoffs between leaf cooling and hydraulic safety in a dominant arid land riparian tree species.

Leaf carbon gain optimization in hot environments requires balancing leaf thermoregulation with avoiding excessive water loss via transpiration and hydraulic failure. The tradeoffs between leaf thermoregulation and transpirational water loss can determine the ecological consequences of heat waves that are increasing in frequency and intensity. We evaluated leaf thermoregulation strategies in warm-(>40°C maximum summer temperature) and cool-adapted (<40°C maximum summer temperature) genotypes of the foundation tree species, Populus fremontii, using a common garden near the mid-elevational point of its distribution. We measured leaf temperatures and assessed three modes of leaf thermoregulation: leaf morphology, midday canopy stomatal conductance and stomatal sensitivity to vapour pressure deficit. Data were used to parameterize a leaf energy balance model to estimate contrasts in midday leaf temperature in warm- and cool-adapted genotypes. Warm-adapted genotypes had 39% smaller leaves and 38% higher midday stomatal conductance, reflecting a 3.8°C cooler mean leaf temperature than cool-adapted genotypes. Leaf temperatures modelled over the warmest months were on average 1.1°C cooler in warm- relative to cool-adapted genotypes. Results show that plants adapted to warm environments are predisposed to tightly regulate leaf temperatures during heat waves, potentially at an increased risk of hydraulic failure.

Evidence for efficient nonevaporative leaf-to-air heat dissipation in a pine forest under drought conditions.

The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present. Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leaf-to-air temperature differences (ΔTleaf-air ) of pine needles during drought. On mid-summer days, ΔTleaf-air remained < 3°C, both in trees exposed to summer drought and in those provided with supplemental irrigation, which had a more than 10-fold higher transpiration rate. The nonevaporative cooling in the drought-exposed trees must be facilitated by low resistance to heat transfer, generating a large sensible heat flux, H. ΔTleaf-air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but was dependent on canopy structure and within-canopy turbulence that enhanced the H. Nonevaporative cooling is demonstrated as an effective cooling mechanism in needle-leaf trees which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy-scale 'convector effect'.

Evaluación del índice de estrés hídrico de cultivos (IEHC) en ají (Capsicum) bajo riego por goteo en las condiciones áridas de la costa norte del Perú

As irrigation water decreases, crop transpiration also decreases, resulting in an increase in leaf temperature and an increase of the crop water stress index (CWSI). Therefore, in the study, the response of Capsicum to different irrigation regimes was evaluated with periodic measurements of leaf temperature, stomatal conductance and estimation of the CWSI. The treatments consisted of a complete drip irrigation (RO) and deficit irrigation (RD-78 and RD-58) that received 78% and 58% of the complete irrigation, applying a total volume of water of 3363, 2618 and 1956 m3 ha-1 for RO, RD-78 and RD-58, respectively; and obtaining maximum yields of 30.2, 13.9 and 12.9 t ha-1 with CWSI of 0.50, 0.62 and 0.54 in C. bacatum, C. annumm and C. chinense, respectively. The highest stomatal conductance values ​​ranged from 724 to 887 mmol s-1 m-2, with CWSI from 0.1 to 0.3. The CWSI value of 0.3 (soil moisture from 25 to 33%) results in a good indicator of the time of irrigation, and it might be a criterion to be incorporated to save water and improve the management of Capsicum irrigation programs.

'Dual-reference' method for high-precision infrared measurement of leaf surface temperature under field conditions.

Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high-precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote-sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg . A novel 'dual-reference' method was developed to increase the accuracy of infrared needle-leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf-to-air temperature differences of needle-leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively. Routine high-precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf-scale processes and their dynamics.

A Review of Evapotranspiration Measurement Models, Techniques and Methods for Open and Closed Agricultural Field Applications

Detailed knowledge of energy and mass fluxes between land and the atmosphere are necessary to monitor the climate of the land and effectively exploit it in growing agricultural commodities. One of the important surface land fluxes is evapotranspiration, which combines the process of evaporation from the soil and that of transpiration from plants, describing the movement of water vapour from the land to the atmosphere. Accurately estimating evapotranspiration in agricultural systems is of high importance for efficient use of water resources and precise irrigation scheduling operations that will lead to improved water use efficiency. This paper reviews the major mechanistic and empirical models for estimating evapotranspiration including the Penman–Monteith, Stanghellini, Priestly–Taylor, and Hargreaves and Samani models. Moreover, the major differences between the models and their underlined assumptions are discussed. The application of these models is also reviewed for both open and closed field mediums and limitations of each model are highlighted. The main parameters affecting evapotranspiration rates in greenhouse settings including aerodynamic resistance, stomatal resistance and intercepted radiation are thoroughly discussed for accurate measurement and consideration in evapotranspiration models. Moreover, this review discusses direct evapotranspiration measurements systems such as eddy covariance and gas exchange systems. Other direct measurements appertaining to specific parameters such as leaf area index and surface leaf temperature and indirect measurements such as remote sensing are also presented, which can be integrated into evapotranspiration models for adaptation depending on climate and physiological characteristics of the growing medium. This review offers important directions for the estimation of evapotranspiration rates depending on the agricultural setting and the available climatological and physiological data, in addition to experimentally based adaptation processes for ET models. It also discusses how accurate evapotranspiration measurements can optimise the energy, water and food nexus.

Estimation of Aerodynamic and Canopy Resistances in a Mediterranean Greenhouse Based on Instantaneous Leaf Temperature Measurements

Aerodynamic and canopy resistances have long been considered to be of key interest in model equation parameterization, particularly for the accurate estimation of crop evapotranspiration. However, model parameters applied in greenhouses showed variation affected by the micrometeorological environment. Three experiments were carried out in a plastic greenhouse to evaluate microclimate effects on resistances of a soilless cucumber crop. The regression analysis of canopy-to-air temperature (Tc − Ta) difference on air vapor pressure deficit (VPD) was substituted into the energy balance equation for the estimation of aerodynamic and canopy resistance values. As expected, a fan and pad evaporative cooling system proved to be the more efficient method of decreasing crop temperature (Tc) compared to the forced air ventilation system. The estimated transpiration by the Penman–Monteith model based on calculated aerodynamic and canopy resistance values successfully validated values measured with lysimeters in different growing periods. In this article, we report for the first time the calculation of aerodynamic and canopy resistance values inside a greenhouse based on equations for an open field that were found in the literature. Results may be helpful in Mediterranean greenhouses for direct determinations of plant water evaporative demand and smart climate control systems.

Use of infrared thermography imaging for assessing heat tolerance in high and low iron pearl millet lines

In the arid regions of Asia and Africa, pearl millet serves as a staple source of dietary energy and mineral micronutrients for millions of people. These regions are more vulnerable to increased temperature. The availability of rapid and efficient screening tools based on the relevant non-destructive quantifiable traits would facilitate pearl millet improvement for heat tolerance. The objective of this study was to evaluate pearl millet lines with contrast micronutrients for heat tolerance using infrared thermal imaging, a rapid proxy-canopy (panicle and flag leaf) temperature measurement. Results showed the highly significant genotypic differences between high-Fe and low-Fe genotypes for grain Fe and Zn densities and agronomic traits. Both high-Fe and low-Fe group genotypes differed significantly for panicle temperature depression (PTD) during high- vapor deficit (VPD) at stigma stage (3.0 to 6.73°C). PTD values were positive across all genotypes during stigma stage and were very low or negative during the low-VPD. Cooler canopy temperature (high-PTD) was observed during stigma stage rather than seed-set stage at higher-VPD in both high-Fe and low-Fe genotypes. The cooler temperature achieved by panicle might be helpful in maintaining stigma receptivity for longer periods in the female parents, whereas in male parents it might be helpful in maintaining pollen viability for longer periods. Flag leaf temperature (FTD) was cooler than PTD at both high-VPD and low-VPD as well in both stigma (less by 2.1°C) and grain-filling stage (less by 2.7°C), again signifying that the reproductive parts are more prone to heat stress as compared to vegetative parts. Since, thermal imaging discriminates the heat stress and non-stress canopies, this can serve as a proxy canopy temperature tool for heat stress tolerance screening in pearl millet.

When a cuvette is not a canopy: A caution about measuring leaf temperature during gas exchange measurements

Plant gas exchange systems are widely used to study leaf physiological processes and properties such as stomatal function and the maximum carboxylation rate of Rubisco. Increasingly, these systems are used to assess how leaf gas exchange varies with temperature in order to better understand how rising temperature will impact plant function. Examples of such studies include variation in optimal temperatures of photosynthesis as a function of species and growth environment, and respiratory acclimation to higher temperatures. Leaf thermoregulation leading to homeothermy has been reported based on leaf gas exchange measurements spanning a large (∼25 °C) temperature range. However, as we show here, the design of a popular gas exchange system used for temperature-response measurements can lead to biased measurements of leaf temperature. We demonstrate this with an example showing that apparent leaf thermoregulatory behavior can arise even in empty cuvettes. More broadly, our results have implications for other temperature manipulations in similar gas exchange systems.

Sensitivity analysis of four crop water stress indices to ambient environmental conditions and stomatal conductance

Crop water stress indices (CWSIs) quantify plant water status based on measurement of plant temperature. The goal of CWSI formulation is to normalize measured leaf temperatures based on reference temperatures to remove sensitivity to ambient environmental conditions (e.g., air temperature, humidity, radiation), while retaining sensitivity to plant water status as reflected by stomatal conductance. This study sought to better understand the sensitivity of these temperatures to ambient environmental conditions, and ultimately how they influence various CWSIs. The surface energy balance was modeled to simulate the impacts of input parameter variation on leaf temperature and reference surface temperatures used to calculate four different CWSIs. The performance of the CWSIs were assessed based on their ability to maximize sensitivity to stomatal conductance while minimizing the relative sensitivity to ambient environmental conditions.The sensitivity analyses indicated that all four CWSIs performed poorly in shaded conditions, as they had relatively low sensitivity to stomatal conductance and were sensitive to all environmental parameters. Two CWSIs had high sensitivity to stomatal conductance, and low sensitivity to all environmental parameters except wind speed. None of CWSIs could remove sensitivity to all environmental parameters while retaining sensitivity to stomatal conductance.

In vivo human-like robotic phenotyping of leaf traits in maize and sorghum in greenhouse

In plant phenotyping, leaf-level physiological and chemical trait measurements are needed to investigate and monitor the condition of plants. The manual measurement of these properties is time consuming, error prone, and laborious. The use of robots is a new approach to accomplish such endeavors, enabling automated monitoring with minimal human intervention. In this paper, a plant phenotyping robotic system was developed to realize automated measurement of plant leaf properties. The robotic system comprised of a four Degree of Freedom (DOF) robotic manipulator and a Time-of-Flight (TOF) camera. A robotic gripper was developed to integrate an optical fiber cable (coupled to a portable spectrometer) for leaf spectral reflectance measurement, and a thermistor for leaf temperature measurement. A MATLAB program along with a Graphical User Interface (GUI) was developed to control the robotic system and its components, and for acquiring and recording data obtained from the sensors. The system was tested in a greenhouse using maize and sorghum plants. The results showed that leaf temperature measurements by the phenotyping robot were significantly correlated with those measured manually by a human researcher (R2 = 0.58 for maize and 0.63 for sorghum). The leaf spectral measurements by the phenotyping robot predicted leaf chlorophyll, water content and potassium with moderate success (R2 ranged from 0.52 to 0.61), whereas the prediction for leaf nitrogen and phosphorus were poor. The total execution time to grasp and take measurements from one leaf was 35.5 ± 4.4 s for maize and 38.5 ± 5.7 s for sorghum. Furthermore, the test showed that the grasping success rate was 78% for maize and 48% for sorghum. The phenotyping robot can be useful to complement the traditional image-based high-throughput plant phenotyping in greenhouses by collecting in vivo leaf-level physiological and biochemical trait measurements.

A new indicator of leaf stomatal conductance based on thermal imaging for field grown cowpea

ABSTRACTCompared with a porometer, a thermal camera can be easily applied to large plant populations comprising a set of varieties, treatments, and replications, whereby, leaf temperature-based indicators are widely used to estimate stomatal conductance (gs); however, a major difficulty in applying these indicators is their vulnerability to meteorological conditions. In this study, a new indicator of gs (GsI) was developed with a modified theoretical equation of gs that was highly simplified by means of several assumptions. GsI calculation uses leaf and air temperature, relative humidity, and solar radiation measurements. To validate and compare GsI values with other thermal indicators as leaf-air temperature difference and crop water stress index, glasshouse and field experiments were conducted. Leaf temperature of cowpea plants was measured using a low-cost thermal camera to ensure a cost-friendly method. GsI proved to be more stable than other indicators, relative to the measured gs, irrespective of solar radiation, air temperature, and relative humidity conditions. As no reference temperature is needed for the calculation of GsI, it easily applies to large plant populations, although the GsI is most accurate in the range from moderate to high gs values (approximately, >0.2 mol m−2 s−1). We used GsI to evaluate a cowpea germplasm collection consisting of 248 accessions, and elucidated that most accessions with higher GsI, which expected to have higher gs, are originated in West-Africa. As GsI is available regardless of varying meteorological conditions, it is a useful indicator of gs, especially in field studies involving multilocation and time-course evaluations.Abbreviations: Cp: specific heat of the air; CWSI: crop water stress index; DAS: days after sowing; DT: saturated vapor pressure at temperature T; ea: vapor pressure of the air; es: vapor pressure at leaf surface; G: heat flux to the ground; ga: boundary layer conductance; gs: stomatal conductance; gv: total conductance; RH: relative humidity; Rn: net radiation; Rs: short-wave radiation; S: heat flux to the leaf; Ta: air temperature; Tdry: dry reference temperature; Ts: leaf surface temperature; Twet: wet reference temperature; VPD: vapor pressure deficit; γ: psychrometric constant; λE: latent heat flux; ρ: air density.