ASHS 2024 Conference

In California, avocado is primarily grown in Southern and Central parts of the state, typically in regions tempered by coastal climates and fine or course sandy loam soils. These regions face uncertain water supplies, mandatory reductions of water use, and the rising cost of water, while efficient use of irrigation water is one of the highest conservation priorities. Moreover, due to increasing salinity in water sources, effective irrigation is more critical to ensure optimal yield and high-quality avocado fruits. A two-year study was conducted in 12 mature avocado sites in California. Extensive field measurements and surveys were conducted to better understand the current water management practices, to acquire and develop relevant information on crop water use (ET) and crop coefficients, and to assess the performance of satellite-based OpenET tool for irrigation management in avocados. Surface renewal and eddy covariance equipment was used to measure actual evapotranspiration (ETa) in each site. The results illustrated considerable variability in avocado crop water consumption both spatially and temporally. The crop coefficients curves were developed for each site. Across the avocado research sites, the average seasonal crop coefficient values varied from 0.6 to 0.76. The findings demonstrated that canopy features, soil types and conditions, pruning practices, soil surface cover, and row orientations need to be considered to perform effective water management in avocado orchards. Ground shading percentage and row orientations provide a good estimation of canopy size/volume and the amount of light that it can intercept are likely the most important drivers influence crop water needs. The RMSE of the measured ETa from eddy covariance equipment and estimated ETa from Ensemble OpenET varied from 0.53 to 1.37 mm d-1. The preliminarily findings indicated that the Ensemble OpenET estimates ETa relatively well in some sites and could be an effective irrigation management tool in the future for avocado orchards, however more evaluations are required.

Almond and Walnut are the major irrigated crops in the Northern San Joaquin Valley (NSJV) of California. The recurring droughts and climate change in California will likely increase the uncertainty in water supply to almond, walnut, and other specialty crops. Site-specific irrigation is critical to cope with these challenges. Knowing the water consumption of these water use intensive crops is imperative for optimizing irrigation management since it affects nut quality, productivity, and composition. This requires accurate estimates of crop water use (Evapotranspiration, ET). Traditional methods for estimating crop water use are spatially limited, whereas satellite remote sensing of ET offers the advantage of large-scale coverage and is increasingly adopted in irrigated agriculture. This study compares OpenET models, an open-source database providing ET estimates, against calculated ET from weather stations that are commonly used by growers in their irrigation management. Evaluation of OpenET against estimated ET might provide a good opportunity for growers to improve water use efficiency. Such improvements could lead to the adoption of publicly available irrigation management tools and ensure healthier tree development, better resource utilization, and more resilient orchards in the face of climate change. This presentation delves into the preliminary findings of the OpenET evaluation against calculated ET from weather stations in estimating water use for almonds and walnuts, while also examining the potential and challenges associated with each approach for implementation in growers' fields.

The rapidly changing climate is creating challenges for the selection and management of woody perennial crops. For North American (NA) cultivars of hazelnut (Corylus avellana), there is insufficient information on water stress management to maintain physiological performance and optimize productivity under limited soil water availability. Current plantings of NA hazelnuts are predominantly comprised of cultivars resistant to biotic stress (e.g., Eastern Filbert Blight) such as ‘Jefferson’ and ‘Yamhill’ cultivars, but their responses to abiotic stressors exacerbated by climate change is unknown. Our research objectives were to: 1. identify cultivar-specific physiological thresholds in response to water stress such as negligible leaf gas exchange (i.e., stomatal closure) and onset of leaf wilting (i.e., cell turgor loss) for phenotyping in greenhouse conditions; and 2. relate vapor pressure deficit to plant water status in order to generate a water-potential baseline capable of differentiating between atmospheric and soil moisture impacts on water stress in field conditions. Using the water potential (Ψ) curve (WPC) method, stomatal closure was initiated at less negative Ψ in ‘Jefferson’ (-0.85 MPa) compared to ‘Yamhill’ (-1.1 MPa). Similarly, turgor loss was found to occur at less negative Ψ in ‘Jefferson’ (-1.26 MPa) compared to ‘Yamhill’ (-1.48 MPa). These cultivar-specific differences were confirmed with direct measurements of stomatal conductance using a porometer and an evaluation of turgor loss point using the pressure-volume curve method. In the field, we established a water potential baseline to distinguish between the effects of soil moisture and vapor pressure deficit on Ψ. Our field results found a deviation from baseline of -1.0 MPa resulted in stomatal closure in Yamhill, which was consistent with our prediction from the WPC. ‘Yamhill’ trees that had Ψ on average -0.68 MPa below baseline over the growing season were also observed to have 34% smaller nuts, 46% higher shell-to-kernel ratio, and an estimated 50% of total in-shell yield. Upcoming research will seek to replicate results experimentally with both cultivars. In summary, our results indicate that the WPC is a valid tool for physiological phenotyping and preliminary results suggest that thresholds from the WPC provide viable cultivar-specific targets for improving irrigation management in hazelnuts. These results highlight methods to help determine sustainable irrigation management targets that can help conserve water resources strained by climate change while also maintaining plant productivity.

Pecans have high economic importance in the US. Nonetheless, as one of the top pecan producers, there is little research on water use of pecan trees in the Southeast of the US. The water status of the tree impacts the yield, mostly during the kernel filling period (August and September). The knowledge gap of pecan water requirements stems largely from the Southwest. There, pecan tree needs in the hot and arid climate of the Southwest contrast sharply with those of the long, hot and humid Southeastern climate. Furthermore, the Southwest management practices use flood irrigation in contrast with most Georgia orchards which use micro-irrigation. This paper reports on the development of a crop coefficient specifically addressing the pecan tree needs in the Southeastern US. This study uses an eddy-covariance system and micro-lysimeter to determine the actual evapotranspiration of pecans. The potential evapotranspiration is determined using nearest local weather station data. This paper discusses the behavior of the crop coefficient throughout the different physiological stages of the tree from budbreak to harvest. Results of the crop coefficient obtained throughout the season differs from the Southwest, where the actual evapotranspiration during the growing season is significantly higher than the one observed in the Southeast. The daily and monthly crop coefficient throughout the growing period from 2019 through 2023 respectively are discussed. The year-to-year variability is also discussed. These results should support pecan growers and researchers alike to more tailored irrigation schedule in Southeast pecan orchards.

Thermal cameras can easily determine plant canopy temperature, and the resulting data can be used for irrigation scheduling in addition to other water management tools. This study aimed to develop a method to use thermal imaging for canopy temperature measurements in one-year-old citrus plants to assess citrus water status. We evaluated the influence of five water levels (25%, 50%, 75%, 100%, and 125%) based on the crop evapotranspiration replacement of two citrus species [‘Red Ruby’ grapefruit (Citrus paradisi) and ‘Valencia’ sweet orange (Citrus sinensis)] for 48 days in a greenhouse. To determine the irrigation requirements for the treatment 100%, we estimated the water loss from pots by calculating the difference in soil moisture between the day before and the day of the measurement. We irrigated the pots when the soil moisture was close to the maximum allowable water depletion, keeping the soil moisture between the field capacity and the maximum allowable depletion. A portable thermal camera was used to take images that were later analyzed using open-source software. We determined the canopy temperature, leaf photosynthesis and transpiration, and plant biomass. A positive relationship between the amount of water applied and the temperature response of plants exposed to different water levels was observed. Grapefruit and sweet orange plants that received less water presented water restrictions and reached 6 °C higher canopy temperatures than the air. The thermal images easily identified water-stressed plants. This study allowed quick measuring of the canopy temperature using readily available equipment and can be used as a tool to assess water status in citrus plants in greenhouses. An automated routine to process the thermal images in real-time and remove the background weeds to determine the canopy temperature can potentially allow using it for irrigation management.

The growing demand for affordable and healthy food to feed the growing population necessitates multilayered strategies to meet food demand and supply features: excessive irrigation application to overcome the impact of erratic rainfall, which imposes pressure on groundwater withdrawals, adversely affecting crop failure and sustainability. The objective of the study was to determine the impact of varying irrigation levels on tree growth, leaf nutrient concentrations, and water relations at selected citrus tree densities. The experiment was carried out on Malabar fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) in a commercial citrus grove near Immokalee, FL, USA from 2019 to 2022. Mature thirteen-year-old ‘Valencia’ (Citrus sinensis) citrus trees grafted on Carrizo (a hybrid of Washington Navel orange and Poncirus trifoliata) planted in tree densities of 360, 485, and US-897 (Citrus reticulata Blanco x Poncirus trifoliata (L.) Raf.) citrus rootstock with 920 trees ha-1. Significant water distribution and movement were detected along the soil profile in response to the irrigation rates with higher volumetric water content on the grower standard highest irrigation. As a result, significant fibrous root length densities (FRLD) and median lifespan were observed in the three-row and two-row experiments with the deficit (50%-crop evapotranspiration, ETc) and moderate (78%-ETc) as compared with the grower standard highest (100%-ETc) irrigation regimes, respectively. Stomata conductance and stem water potential ( manifested less tree water stress when trees received moderate irrigation in the low and moderate tree densities than the highest tree density. This significantly impacted the FRLD in the soil and leaf area index (LAI) above the ground tree growth. Moderate irrigation triggered FRLD and improved root survival probability and lifespan. Meanwhile, nutrient uptake from the soil significantly affected leaf nutrient concentration when trees received moderate irrigation than deficit or highest irrigation rates. As a result, irrigation management improved water relations, leaf nutrient concentration, and tree growth across the varying irrigation regimes.