Cooler Canopy Temperatures Research Articles

Deficit Irrigation with Silicon Application as Strategy to Increase Yield, Photosynthesis and Water Productivity in Lettuce Crops.

In regions where water is a limited resource, lettuce production can be challenging. To address this, water management strategies like deficit irrigation are used to improve water-use efficiency in agriculture. Associating this strategy with silicon (Si) application could help maintain adequate levels of agricultural production even with limited water availability. Two lettuce crop cycles were conducted in a completely randomized design, with a factorial scheme (2 × 3), with three irrigation levels (60%, 80% and 100%) of crop evapotranspiration (ETc), and with and without Si application. To explore their combined effects, morphological, productive, physiological and nutritional parameters were evaluated in the crops. The results showed that deficit irrigation and Si application had a positive interaction: lettuce yield of the treatment with 80% ETc + Si was statistically similar to 100% ETc without Si in the first cycle, and the treatment with 60% ETc + Si was similar to 100% ETc without Si in the second cycle. Photosynthetic rate, stomatal conductance, intercellular CO2 concentration, transpiration rate and total chlorophyll content increased under water-stress conditions with Si application; in the first cycle, the treatment with 80% ETc + Si increased by 30.1%, 31.3%, 7.8%, 28.46% and 50.3% compared to the same treatment without Si, respectively. Si application in conditions of water deficit was also beneficial to obtain a cooler canopy temperature and leaves with higher relative water content. In conclusion, we found that Si applications attenuate water deficit effects and provide a strategy to ameliorate the yield and water productivity in lettuce crops, contributing to more sustainable practices in agriculture.

Narrow root cone angle promotes deeper rooting, cooler canopy temperatures and higher grain yield in a rice (Oryza sativa L.) recombinant inbred line population grown under different water availabilities in aerobic production systems

Rice crops are one of the world’s major consumers of irrigation water. With water becoming more limited, water-saving aerobic production (AP) systems may provide an alternative to traditional flooded culture. Genotypes with narrow root cone angle (RCA) and subsequently deeper rooting systems are hypothesised as key for AP adaptation ensuring water uptake at depth. Using a recombinant inbred line (RIL) population and differential genotypes (narrow and wide RCA groups) derived from a Sherpa/IRAT109 cross, a glasshouse experiment and four field experiments with different water inputs in well-watered (WW, in two seasons), mild water deficit (MWD), and intermittent water deficit (IWD) conditions were carried out to characterise the relationship of RCA with grain yield (GY) and key physiological traits. Significant genotypic variation and group differences were found in root traits, canopy temperature and GY. Expression of genotypes with narrow RCA were found to be consistent across experiments and water availability conditions. Genotypes in the narrow RCA group produced a greater proportion of roots below 20 cm (7.4–17.7%), longer total root length (69.7%), cooler canopies (0.3–1.0 °C) and higher GY (14–62%) compared with wide RCA group. Two promising RILs were identified which expressed high GY in WW (12.0–13.4 t/ha) conditions while maintaining GY in IWD (up to 9.4 t/ha). Higher GY was obtained in genotypes with cooler canopies (r = −0.72) and narrow RCA (r = −0.39) in IWD. Narrower RCA was associated with higher total root length (r = −0.70) and higher stomatal conductance (r = −0.71) in the glasshouse and proportion of deep roots (r = −0.65) and cooler canopies (r = 0.66) in the field. To our knowledge, this was one of the foremost studies exploring and demonstrating such relationships in rice AP systems. Root traits determined in the field were congruent with the other root-related traits driving differences between RCA groups measured in the glasshouse providing confidence with the field phenotyping methods. This work demonstrated the advantage conferred by the development of narrow RCA in aerobic conditions, presented improved physiological understanding of RCA and with genomics assisted breeding, is considered valuable for the development of AP-adapted, sustainably produced rice.

Infrared Thermal Imaging and Morpho-Physiological Indices Used for Wheat Genotypes Screening under Drought and Heat Stress

Bread wheat, one of the largest broadacre crops, often experiences various environmental stresses during critical growth stages. Terminal drought and heat stress are the primary causes of wheat yield reduction worldwide. This study aimed to determine the drought and heat stress tolerance level of a group of 46 diverse wheat genotypes procured from the Australian Grains Gene Bank, Horsham, VIC Australia. Two separate drought stress (DS) and heat stress (HS) pot experiments were conducted in separate growth chambers. Ten days after complete anthesis, drought (40 ± 3% field capacity for 14 days) and heat stress (36/22 °C for three consecutive days) were induced. A significant genotype × environment interaction was observed and explained by various morpho-physiological traits, including rapid, non-destructive infrared thermal imaging for computational water stress indices. Except for a spike length in DS and harvest index in HS, the analysis of variance showed significant differences for all the recorded traits. Results showed grains per spike, grains weight per spike, spike fertility, delayed flag leaf senescence, and cooler canopy temperature were positively associated with grain yield under DS and HS. The flag leaf senescence and chlorophyll fluorescence were used to measure each genotype's stay-green phenotype and photosystem II activity after DS and HS. This study identified the top ten best and five lowest-performing genotypes from drought and heat stress experiments based on their overall performance. Results suggest that if heat or drought adaptive traits are brought together in a single genotype, grain yield can be improved further, particularly in a rainfed cropping environment.

Leaf Pigments, Surface Wax and Spectral Vegetation Indices for Heat Stress Resistance in Pea

Pea is a grain legume crop commonly grown in semi-arid temperate regions. Pea is susceptible to heat stress that affects development and reduces yield. Leaf pigments and surface wax in a crop canopy make the primary interaction with the environment and can impact plant response to environmental stress. Vegetation indices can be used to indirectly assess canopy performance in regard to pigment, biomass, and water content to indicate overall plant stress. Our objectives were to investigate the contribution of leaf pigments and surface wax to heat avoidance in pea canopies, and their associations with spectral vegetation indices. Canopies represented by 24 pea cultivars varying in leaf traits were tested in field trials across six environments with three stress levels in western Canada. Compared with the control non-stress environments, heat stress reduced leaf lamina and petiole chlorophyll a, chlorophyll b, and carotenoid concentrations by 18–35%, and increased leaf lamina chlorophyll a/b ratio, anthocyanin and wax concentrations by 24–28%. Generally, greater leaf pigment and wax concentrations were associated with cooler canopy temperature and high heat tolerance index (HTI) values. Upright cultivars had higher HTI values, whereas the lowest HTI was associated with normal leafed vining cultivars. Vegetation indices, including photochemical reflectance index (PRI), green normalized vegetation index (GNDVI), normalized pigment chlorophyll ratio index (NPCI), and water band index (WBI), had strong correlations with HTI and with heat avoidance traits. This study highlights the contribution of pigments and wax as heat avoidance traits in crop canopies, and the potential application of spectral measurements for selecting genotypes with more heat resistant vegetation.

Quantification of water stress impacts on canopy traits, yield, quality and water productivity of onion (Allium cepa L.) cultivars in a shallow basaltic soil of water scarce zone

Understanding and mitigating the adverse impacts of water stress on agriculture production have become global concern in water scarce environments. Particularly, Deccan plateau, the largest semi-arid zone of peninsular India, is confronted with severe water resources conflicts and extensive pressure for food production under climate change scenario. For sustainable agriculture, it is critical to quantify the impact of water stress on major crops like winter onion (Allium cepa L.) which is grown under diverse water management practices on shallow basaltic soils of this region. We conducted a field experiment during two growing seasons (2017–18 and 2018–19) for evaluating responses of different cultivars of red (B. Raj, B. Kiran, B. Super, B. Red and Local-Fursangi) and white (B. Shubra, B. Sweta and B. Safed) onion to variable water stress created using a line source sprinkler (LSS) system. There was a 1.1, 1.6, 3.9, 9.2 fold decline in total bulb yields (TBY) with increasing water deficit i.e. when depth of irrigation water (IW) applied equalled 0.60, 0.40, 0.20 and 0.00 times the cumulative open pan evaporation (CPE) as compared with the recommended practice (IW:CPE 0.80). The TBY increased between 86.2 and 106.6 Mg ha−1 in red cultivars when irrigation was scheduled at excess to optimum water regimes (IW: CPE 0.80–1.00), while it was improved by 5.0–20.4% for white cultivars under medium (0.40–0.59) to severe (0.0–0.39) water deficit stress. Further, highest (4.3–16.2%) improvement in TBY of Local cultivar was reported irrespective to different water stress levels. Local and white cultivars could have better tolerance to water stress by keeping cooler canopy temperatures, higher leaf relative water content and modulating the stomatal opening. Lower yield losses due to rotting (0.30–1.81%) and splitting (4.4–6.5%) of bulbs were observed in red and white cultivars. This study further endorses the appropriate selection of onion cultivars reduces water requirements by 37.5–61% to attain similar water productivity (16.0 kg m−3) as with recommended irrigation level. Increased in water stress levels though reduced quality of physical traits of harvested bulbs viz., mean weight, average dia., shape index (SI), harvest index (HI) and firmness, these improved the storage quality. Water deficit also increased accumulation of soluble solids, total phenolics, protein, dry matter and elevated the pH and rehydration quality besides potentially affecting the enzymatic activity (SOD and POD) of onion cultivars. It was concluded that white onion cultivars like B. Shubra with higher WP under medium and severe water deficit stress and ‘Local’ red cultivar should be preferred for efficient utilisation of water resources both in terms of the productivity and post-harvest quality of onion under drought prone and shallow basaltic regions of Deccan plateau.

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.

Comparative conventional and phenomics approaches to assess symbiotic effectiveness of Bradyrhizobia strains in soybean (Glycine max L. Merrill) to drought

Symbiotic effectiveness of rhizobitoxine (Rtx)-producing strains of Bradyrhizobium spp. in soybean (cultivar NRC-37/Ahilya-4) under limited soil moisture conditions was evaluated using phenomics tools such as infrared(IR) thermal and visible imaging. Red, green and blue (RGB) colour pixels were standardized to analyse a total of 1017 IR thermal and 692 visible images. Plants inoculated with the Rtx-producing strains B. elkanii USDA-61 and USDA-94 and successive inoculation by B. diazoefficiens USDA-110 resulted in cooler canopy temperatures and increased canopy greenness. The results of the image analysis of plants inoculated with Rtx-producing strains were correlated with effective nodulation, improved photosynthesis, plant nitrogen status and yield parameters. Principal component analysis (PCA) revealed the reliability of the phenomics approach over conventional destructive approaches in assessing the symbiotic effectiveness of Bradyrhizobium strains in soybean plants under watered (87.41–89.96%) and water-stressed (90.54–94.21%) conditions. Multivariate cluster analysis (MCA) revealed two distinct clusters denoting effective (Rtx) and ineffective (non-Rtx) Bradyrhizobium inoculation treatments in soybean. Furthermore, correlation analysis showed that this phenotyping approach is a dependable alternative for screening drought tolerant genotypes or drought resilience symbiosis. This is the first report on the application of non-invasive phenomics techniques, particularly RGB-based image analysis, in assessing plant-microbe symbiotic interactions to impart abiotic stress tolerance.

Genetic diversity and phenotypic variation for drought resistance in alfalfa (Medicago sativa L.) germplasm collected for drought tolerance

Drought is a major environmental factor hampering alfalfa productivity worldwide. Gene banks provide an array of trait diversity, frequently consisting of specific seed collection projects that focused on acquiring germplasm adapted to specific traits such as drought tolerance. These subsets provide a logical starting point to identify useful breeding resources. In the present study, we investigated the genetic diversity and evaluated 18 agronomic, physiological and quality traits associated with drought tolerance in a subset of 198 alfalfa accessions collected for their potential drought resistance. Significant differences were found among accessions for most of the traits evaluated. Water deficit significantly decreased biomass dry yield by 61.9% compared with well-watered control. A positive correlation was found between relative water content (RWC) and drought susceptible index, while negative correlations were found between RWC and leaf senescence (LS), and between RWC and canopy temperature (CT). Alfalfa accessions with high RWC showed relative lower yield reduction, cooler CT and delayed LS. Genetic variation for forage quality was also investigated and the correlation between agronomic and quality traits was analyzed. Biomass yield under drought was negatively correlated with total protein, relative feed value and relative forage quality, suggesting that the efforts to improve yield under drought tend to negatively affect forage quality. Therefore, maintaining forage quality should be considered for breeding alfalfa with drought resistance. Alfalfa accessions with high drought resistance scores and minimum loss of quality were identified in this study and can be used as genetic resources for developing alfalfa varieties with improved drought resistance and water use efficiency while maintaining forage quality.

Comparison of osmotic adjustment, leaf proline concentration, canopy temperature and root depth for yield of juncea canola under terminal drought

AbstractTwo glasshouse and two field experiments were conducted in 2013 and 2014 to compare the relative importance of four physiological traits: osmotic adjustment (OA), leaf proline concentration, canopy temperature depression (CTD) and root depth on drought performance of canola quality B. juncea (juncea canola). Glasshouse experiments were conducted at The University of Melbourne, Parkville, and field experiments were conducted at Horsham, Victoria. The experiments used juncea canola hybrids and their parental lines and were laid out in a randomised complete block design with three replications. The glasshouse experiments consisted of two treatments, well watered and water deficit from first open flower to maturity, whereas the field experiments were sown at a site that received 266 mm annual rainfall in 2014. In the glasshouse, canopy temperature depression was the only trait to show a positive and consistent association with drought performance of juncea canola. Cooler canopy temperature was also associated with improved yield in field experiments. Root depth was positively correlated with CTD in 2014 in glasshouse, whereas no correlation of root depth with OA and leaf proline was observed. The results indicated that CTD was the only reliable trait among those tested to screen juncea canola for drought tolerance. Root depth of juncea canola hybrids was a constitutive trait and probably was a result of hybrid vigour.

Genetic variation in physiological responses of mungbeans (Vigna radiata (L.) Wilczek) to drought

Mungbean is a relatively drought tolerant leguminous crop with a short life cycle. Using leaf water loss (LWL) as a screen for drought tolerance, two mungbean genotypes exhibiting more than two–fold variation in leaf water loss were explored for the genetic variation in their physiological and molecular responses to drought. Efficient stomatal regulation together with better photosynthetic capacity constituted an important trait combination for drought adaptation in water saving low LWL genotype. The stomatal closure under drought was accompanied with a concomitant down-regulation of farnesyl transferase gene. However, cooler canopy temperature, a well branched root system coupled with a relatively higher proline accumulation in water spending high LWL genotype constituted another set of adaptive traits operating when exposed to deficit soil moisture conditions. We report drought induced down-regulation of proline dehydrogenase and the presence of 118 base pair intron in this gene. The high seed yield of low LWL genotype despite a hotter canopy might be attributed to higher net assimilation and quantum yield recorded under drought in this genotype. Thus, these interlinked features contribute to adaptive mechanisms of mungbeans which is widely grown in harsh environments exposed to drought and high temperatures.

Heat Stress Adaptation in Elite Lines Derived from Synthetic Hexaploid Wheat

ABSTRACTThe contribution of synthetic hexaploids in spring wheat (Triticum aestivum L.) breeding has been documented under drought stress, but not previously under heat stress. A set of six advanced wheat lines derived from synthetic hexaploid wheat (ASD) was compared to their conventional hexaploid (Conv‐Hex) and synthetic derivative (Syn‐Der) parents under three different temperature scenarios in the field (temperate or non‐stress, heat‐stress environment, and late‐ or extreme heat environment). The ASD lines showed a yield advantage under heat and extreme heat stress compared to the best parent (Syn‐Der) by on average 15 and 13%, respectively, while the average yield advantage under temperate conditions was just 5%. A similar pattern to yield was observed for grain number, while individual kernel weight of ASD lines was similar to the best parent (Syn‐Der) in all three environments. The ASD lines expressed on average 12% more final biomass than the best parent (Syn‐Der) under heat environment, but similar biomass to them at temperate and extreme heat environments, respectively. Physiological traits related to heat tolerance included higher crop growth rate, increased water‐soluble carbohydrates (WSC) storage in stems, cooler canopy temperature, and spectral indices which are related to pigment composition, photo‐protective mechanisms, and radiation use efficiency. These traits enabled a larger number of grains to be set, in addition to growth of taller stems with a greater WSC storage capacity that was significantly related to kernel weight. Results reinforce the positive impact of using synthetic wheat in plant breeding for climate change.

A tillering inhibition gene influences root-shoot carbon partitioning and pattern of water use to improve wheat productivity in rainfed environments.

Genetic modification of shoot and root morphology has potential to improve water and nutrient uptake of wheat crops in rainfed environments. Near-isogenic lines (NILs) varying for a tillering inhibition (tin) gene and representing multiple genetic backgrounds were phenotyped in contrasting, controlled environments for shoot and root growth. Leaf area, shoot and root biomass were similar until tillering, whereupon reduced tillering in tin-containing NILs produced reductions of up to 60% in total leaf area and biomass, and increases in total root length of up to 120% and root biomass to 145%. Together, the root-to-shoot ratio increased two-fold with the tin gene. The influence of tin on shoot and root growth was greatest in the cv. Banks genetic background, particularly in the biculm-selected NIL, and was typically strongest in cooler environments. A separate de-tillering study confirmed greater root-to-shoot ratios with regular tiller removal in non-tin-containing genotypes. In validating these observations in a rainfed field study, the tin allele had a negligible effect on seedling growth but was associated with significantly (P<0.05) reduced tiller number (–37%), leaf area index (–26%), and spike number (–35%) to reduce plant biomass (–19%) at anthesis. Root biomass, root-to-shoot ratio at early stem elongation, and root depth at maturity were all increased in tin-containing NILs. Soil water use was slowed in tin-containing NILs, resulting in greater water availability, greater stomatal conductance, cooler canopy temperatures, and maintenance of green leaf area during grain-filling. Together these effects contributed to increases in harvest index and grain yield. In both the controlled and field environments, the tin gene was commonly associated with increased root length and biomass, but the significant influence of genetic background and environment suggests careful assessment of tin-containing progeny in selection for genotypic increases in root growth.

Common genetic basis for canopy temperature depression under heat and drought stress associated with optimized root distribution in bread wheat.

Key message QTL related to cooler canopy temperatures are associated with optimal root distribution whereby roots proliferate at depth under drought or near to surface under hot, irrigated conditions. Previous research using a bread wheat RIL population of the Seri/Babax cross showed that common QTL were associated with cooler canopies under both drought and heat-stressed conditions. A subset of RIL was grown under water-limited and hot-irrigated field environments to test how cooler canopies are related to root development. Eight sisters and the two parents were used in the study with genotypes grouped as COOL or HOT according to their respective QTL for canopy temperature and previous phenotypic data. Root mass production and residual available soil moisture were measured around anthesis at four depth profiles (from 0 to 120 cm depth). When considering different root profiles, there was a clear interaction of QTL with environment. Under water stress, the COOL genotypes showed a deeper root system allowing the extraction of 35 % more water from the 30–90 cm soil profile. The strategy under heat was to concentrate more roots at the surface, in the 0–60 cm soil layer where water was more available from surface irrigation. Since COOL genotypes showed better agronomic performance, it can be concluded that their QTL are associated with more optimal root distribution in accordance with water availability under the respective stresses. The study demonstrates the importance of root development under both water-limited and hot-irrigated environments, and shows a common genetic basis for adaptation to both stresses that appears to be associated with sensitivity of roots to proliferate where water is available in the soil profile.

QTL on wheat (Triticum aestivum L.) chromosomes 1B, 3D and 5A are associated with constitutive production of leaf cuticular wax and may contribute to lower leaf temperatures under heat stress

Cooler canopy temperatures and glaucousness have both been identified as adaptive traits for improving abiotic stress tolerance in wheat. The objective of this study was to determine if glaucousness resulting from the constitutive production of leaf cuticular waxes was associated with cooler leaf and spike temperatures under heat stress and to identify associated quantitative trait loci (QTL). A set of 121 recombinant inbred lines (RILs) derived by crossing the heat tolerant wheat cultivar ‘Halberd’ and heat susceptible wheat cultivar ‘Karl92’ was utilized for QTL mapping. Flag leaf cuticular wax was extracted and quantified using a colorimetric technique at 10 days after pollination prior to initiation of the heat stress. The parental cultivars and RIL population were then subjected to a heat treatment of 38 °C and temperature depression of both leaf and spike was measured. The parental cultivar Halberd had high flag leaf wax content, and cooler leaf and spike temperatures compared to Karl92, with the RIL population showing significant genetic variation for these traits. QTL identified for leaf and spike temperature depression and leaf waxes explained 8–12 % of the phenotypic variation. Stable QTL for leaf wax content were located on chromosomes 1B and 5A with the 5A QTL region showing localization with QTL for leaf and spike temperature depression, indicating a genetic link between these traits. The results suggest that common genetic loci may influence both of these adaptive traits and could be targeted to improve adaptation to high temperature stress.

Genetic Yield Gains and Changes in Associated Traits of CIMMYT Spring Bread Wheat in a “Historic” Set Representing 30 Years of Breeding

ABSTRACTThe genetic yield progress of 26 spring wheat (Triticum aestivum L.) advanced lines released by the International Maize and Wheat Improvement Centre (CIMMYT) in the period from 1977 to 2008 was evaluated in the selection environment in Mexico as well as at a set of target environments in Asia and Africa. In Mexico, grain yield progress was significantly linear and about 0.7% yr−1 and yield was associated with fewer days to heading, cooler canopy temperature at grain filling, and increased stay‐green and thousand kernel weight. These results suggested that changes in the former traits at least partially explain the yield progress. When genetic yield progress was measured in subsets of sites in Asia and Africa grouped into high, intermediate, and low yielding, the genetic yield progress was 0.9, 0.7, and 0.5% yr−1, respectively. We conclude that there is no evidence that genetic gains to increase yield have slowed down in wheat lines released by CIMMYT.

Partitioning of assimilates to deeper roots is associated with cooler canopies and increased yield under drought in wheat

Dehydration avoidance through cooler canopy temperature (CT) has been shown to explain over 60% yield variation in a random progeny derived from a Seri/Babax cross. A near ‘isomorphic’ subset of Seri/Babax progeny and parents encompassing a restricted range of height and phenology were used for detailed characterisation of drought-adaptive trait expression under contrasting water regimes. Under drought, five of the six progeny out yielded the best parent Babax by up to 35%. The main physiological attributes associated with drought adaptation were increased root dry weight at depth, transpiration rate – evidenced by grain carbon isotope discrimination (Δ13C) – grain filling duration and decreased CT during grain filling. Furthermore, increased root mass at depth was associated with reduced levels of stem water soluble carbohydrates (WSC) when comparing genotypes. It is concluded that differences in rooting depth expressed among iso-morphic wheat sister lines explains superior adaptation to drought. These effects can be detected in season using remote sensing. In addition, the data suggest that accumulation of stem carbohydrates and deep rooting may be two alternative strategies for adapting to drought stress, the latter being beneficial where water is available at depth.

A thermal-based remote sensing technique for routine mapping of land-surface carbon, water and energy fluxes from field to regional scales

Robust yet simple remote sensing methodologies for mapping instantaneous land-surface fluxes of water, energy and CO 2 exchange within a coupled framework add significant value to large-scale monitoring networks like FLUXNET, facilitating upscaling of tower flux observations to address questions of regional carbon cycling and water availability. This study investigates the implementation of an analytical, light-use efficiency (LUE) based model of canopy resistance within a Two-Source Energy Balance (TSEB) scheme driven primarily by thermal remote sensing inputs. The LUE model computes coupled canopy-scale carbon assimilation and transpiration fluxes, and replaces a Priestley–Taylor (PT) based transpiration estimate used in the original form of the TSEB model. In turn, the thermal remote sensing data provide valuable diagnostic information about the sub-surface moisture status, obviating the need for precipitation input data and prognostic modeling of the soil water balance. Both the LUE and PT forms of the model are compared with eddy covariance tower measurements acquired in rangeland near El Reno, OK. The LUE method resulted in improved partitioning of the surface energy budget, capturing effects of midday stomatal closure in response to increased vapor pressure deficit and reducing errors in half-hourly flux predictions from 16 to 12%. The spatial distribution of CO 2 flux was mapped over the El Reno study area using data from an airborne thermal imaging system and compared to fluxes measured by an aircraft flying a transect over rangeland, riparian areas, and harvested winter wheat. Soil respiration contributions to the net carbon flux were modeled spatially using remotely sensed estimates of soil surface temperature, soil moisture, and leaf area index. Modeled carbon and water fluxes from this heterogeneous landscape compared well in magnitude and spatial pattern to the aircraft fluxes. The thermal inputs proved to be valuable in modifying the effective LUE from a nominal species-dependent value. The model associates cooler canopy temperatures with enhanced transpiration, indicating higher canopy conductance and carbon assimilation rates. The surface energy balance constraint in this modeling approach provides a useful and physically intuitive mechanism for incorporating subtle signatures of soil moisture deficiencies and reduced stomatal aperture, manifest in the thermal band signal, into the coupled carbon and water flux estimates.

Soil water depletion by sunflower and sorghum under rainfed conditions

Grain sorghum [ Sorghum bicolor (L.) Moench] and sunflower ( Helianthus annuus L.), were grown under rainfed conditions in 1988 and 1989 near Manhattan, Kansas, on a Muir silt loam (Cumulic Haplustoll, fine-silty, mixed, mesic) to determine if the two crops exploited water at different depths and at different rates in the soil. Soil water content was measured weekly to a depth of 3.20 m with a neutron probe to ascertain depletion rate and depth. Transpiration, stomatal resistance, canopy temperature, and plant water potential also were analyzed to help explain any differences in soil water depletion that might be observed. Sunflower depleted water to a deeper depth, and had a higher rate of depletion at lower depths, than sorghum. In 1988 and 1989, sunflower depleted water to 2.55 and 2.70 m, respectively. About 90% of the total water used by sorghum was from the 0 to 1.65 m and 0 to 1.50 m depth in 1988 and 1989, respectively. Sunflower had a higher rate of depletion compared to sorghum below the 1.20 m depth (1988) and the 1.65 m depth (1989). Sunflower had a higher transpiration rate, lower stomatal resistance, cooler canopy temperature, and higher leaf water potential compared to sorghum, perhaps because, in part, sunflower could exploit more water at deeper soil depths. The results suggested that sunflower might be planted in rotations with shallow-rooted crops or after irrigated crops to take advantage of water at depth.

Canopy temperature and growth of differentially irrigated alfalfa

Plant temperature of alfalfa ( Medicago sativa L.) grown with different amounts of irrigation water has not been reported. The objective of this experiment was to determine if progressive differences in canopy temperature existed among plots of alfalfa (ev. Cody) subjected to 7 graded watering treatments. Irrigation water (0, 2.5, 5.1, 7.6, 10.2, 12.7, 15.2 cm) was added after each of three harvests in 1980 and 1981. Extremes in weather between the summers of 1980 and 1981 enabled comparison of data from a stressed season (1980) with those from a non-stressed season (1981). Throughout the growth period in both years, canopy temperatures, leaf area, stem dry weight, leaf dry weight and total dry weight were determined. Canopy temperature was measured with an infrared thermometer. The relationship between canopy-minus-air temperature ( T c — T a) versus vapor-pressure deficit ( VPD) was determined on well watered alfalfa for 1980 and 1981. Differences in canopy temperature, leaf area index, leaf dry weight, stem dry weight, and total dry weight, due to treatments, were evident in the dry year (1980), but not in the wet year (1981). In the dry year, the irrigated plots generally had cooler canopy temperatures and higher dry weights than the dry land plots, but differences due to the level of water added were not apparent. In both the dry year and the wet year, ( T c — T a) was inversely related to VPD. Also, in both years, and for all treatments, leaf dry weight was about equal to stem dry weight.