Assessing Rootstock Biology and Water Uptake through Proximal Sensing under Different Wetting/Drying Conditions

Considering the importance of the water status as a driver of the whole plant physiology at the vineyard scale, developing new technologies for the space-time measurement of roots distribution and water uptake is crucial for the development of efficient precision viticulture practices. This project (2017-2022) is devoted to the development of a non-invasive technique (electrical resistivity tomography, ERT), as a tool to compare dynamic changes in root growth and water uptake patterns, and to apply this technique to the study of several commercially available rootstocks under varied irrigation delivery methods (drippers, micro-sprinklers) and water regimes (sustained deficit irrigation, rain fed, fully watered). The project was conducted in a vineyard at UC Davis specifically planted for the study of the response of rootstocks to watering systems. According to the timetable of the funded proposal the first year consisted of two parallel objectives: i) the calibration of ERT to soil water and roots distribution iii) the physiological monitoring of different rootstocks under different water amounts and delivery methods.

In a vineyard  planted to Chardonnay on different rootstocks representing the most common parentage classes in a randomized complete block design, six soil pits were dug for the description of the soil profile, the sampling of the soil for chemical-physical analysis and the installation of TDR probes for the monitoring of soil moisture. The pits were then refilled, and the soil allowed to resettle for several months before the measurements commenced. The section of the field used for calibration purposes was not irrigated throughout the season in order to cover a full range of moisture levels for that soil (from 24% to 8% vol.). In close correspondence of these pits, at the soil surface 300 stainless steel electrodes were installed at a distance of 0.62 m (2 feet) for the ERT monitoring. A single ERT measurement was able to cover 2 plants on 2 different rootstock, which represented an experimental unit. The water status of the plants within the experimental units was monitored using infrared thermography and pressure chamber measurements. Photosynthesis and stomatal conductance were monitored through a pressure bomb. The electrical resistivity was calibrated to the soil water content obtained by TDR measurements, and two different pedoelectrical models were fitted to the data: Archie, in a canonical and linearized form, and Waxman and Smits models. Their performances were tested on 20% of the data from the whole dataset that were excluded from the model fitting and then used as a test set of unobserved data. The three modeling approaches gave similar results on the test set, with a slight decrease in performance from the log-log linearization (RMSE = 1.22% vol., R2 = 0.73) and the Archie law (1.22 %vol., R2 = 0.73), to the Waxman-Smits model (RMSE = 1.23% vol., R2 = 0.73). The Archie law was then chosen, and confirmed to be well adapted to predominantly sandy soils such as the experiment site. This calibration was used to transform the ERT images from soil resistivity to soil moisture maps under different rootstocks in 2D and 3D. This is the first report in the world that 3D images of soil moisture were developed in a vineyard, and the first time that this technique was used to compare rootstock physiology in general. Great differences were found between contiguous rootstocks in their spatial use of water. The presented data show how lateral heterogeneity in soil moisture could reduce the efficiency of spot measurements, as those obtained with soil probes, and soil moisture sensors in determining irrigation needs. In the experimental region, despite not irrigating for the whole summer, grapevines did not suffer from water stress  until it reached the harvest date where the midday stem water potential were at -0.7 MPa across all four rootstocks and no significant differences were found. Chardonnay/110R had lowest assimilation rate (P<0.0017) and stomatal conductance (P<0.05) but had greater berry weight, cluster weight and yield per vine, whereas Chardonnay grafted on 140Ru had the lowest yield among four rootstocks. At harvest Chardonnay /110R also had the greatest total soluble solids and lowest titratable acidity. The results of Ravaz Index (kg yield/kg of pruning weight) indicated that Chardonnay /110R and 140Ru achieved vine balance where the Ravaz Index were around 5. However, Chardonnay /101-14Mgt and 420A had  Ravaz Index lower than 3, indicating that excessive vegetative growth due to no water stress resulting in the unbalance of the grapevine. Our results suggested that although Chardonnay /110R had lower vigor, it had higher productivity and enhanced ripening of grapes, related to better vine balance under non-limiting water supply. Further information is needed to test the interaction between rootstocks and different irrigation regimes and their drought resistance. At the end of the season, the soil was sampled with an auger at different locations in the middle between contiguous ERT electrodes. The soil was sampled every 0.1 m, brought to the laboratory and oven-dried. Roots were physically separated from the soil, and their presence was assessed in a gravimetrically (mg of roots per g of soil). The presence of roots was negatively correlated to soil moisture obtained through ERT (r = 0.45), i.e. greater the amount of roots, lower the amount of water at the end of a dry period. This relations was then used to map the roots using ERT as ancillary variable for the spatialization, therefore enabling us to use the electrical images of the roots assess plant water use.

 

Determining the Impacts of Dormant Pruning Methods and Nitrogen Fertilization on Pinot Noir Bud Fruitfulness and Yield

This research evaluated the impacts that dormant pruning and nitrogen (N) fertilization have on bud fruitfulness and yield of Pinot noir through two separate experiments over two growing seasons and three dormant periods. The research was conducted to better understand yield limitations and potentially improve yield uniformity across years, as this is an issue for producers in the cool climate of Oregon’s Willamette Valley. Oregon Pinot noir producers use primarily cane rather than spur pruning, as they believe Pinot noir does not have fruitful basal buds and will result in low yields. With the increasing need for mechanization, growers are interested in spur pruning. A ten-year-old vineyard was used for an experiment comparing cane- and spur-pruned vines for dormant bud fruitfulness, fruitfulness in spring, canopy growth, yield, fruit ripeness at harvest, and dormant pruning weights from Feb 2017 to Feb 2019. Results show that basal buds of Pinot noir are fruitful and that cane and spur-pruned vines had similar vine growth, pruning weights, yield, and fruit composition at harvest. However, spur-pruned vines had ~20 g smaller clusters, but there were no differences in berry size. Results of this study show that spur pruning Pinot noir is possible without yield loss or differences in fruit ripeness at harvest.

A separate experiment was conducted to evaluate N fertilization compared to control (no fertilization) in two vineyard blocks in 2017 and 2018. One block (Block 1) was used to evaluate legacy effects of N-fertilization applied two years prior to the data collection year compared to control vines that were not fertilized during that time. A second vineyard block (Block 2) was evaluated for concurrent effects of N-fertilization from Jan 2018 to Feb 2019 on a lower N status block that had been treated with N fertilization (or control) in the 2017 and 2018 crop years. The evaluation of the legacy block (Block 1) found greater numbers of floral primordia and greater primordia size in canes of higher vigor, regardless of N treatment after two years of N supplementation. However, there were no clear differences in vine growth or yield as a result of the prior years of N treatment. The concurrent analysis of N-fertilization on yield potential (Block 2) showed greater floral primordia numbers and size in buds of the N treatment compared to the control. There were also increases in fruitfulness in spring and larger inflorescence size (more flowers per inflorescence) in the N treatment. Véraison leaf blade N correlated with fruitfulness and inflorescence primordia size in Block 2. Yield may have increased in the block with concurrent N application; however, there were no differences in yield by harvest due to commercial thinning practices. These results suggest that modest N-fertilization of vineyards with moderate-low N status may improve yield potential without causing excessive vine growth.

Understanding the Yield-Quality relationship in Cool Climate Pinot Noir and Chardonnay

The long-term crop load study continued during 2018 for the seventh season of the project. Ten companies conducted research in 12 vineyards during 2018, including 11 Pinot noir vineyards and one Chardonnay vineyard. The report focuses on the results of the Pinot noir sites. The Pinot noir yields in 2018 were high, similar to 2015 and 2017, all high-yielding years for western Oregon. Average yield across all crop thinning treatments and sites was 1.16 lb/ft, which is higher than the 7-year mean of 1.02 lb/ft. The 2018 season was warm and dry and led to lower pruning weights and a higher yield to pruning weight ratio than in prior years of the study. Despite the higher yields and higher crop load, the 2018 season had fewer sites showing differences in fruit composition at harvest than in the six years prior with only 55% of sites having some crop level effect on fruit composition at harvest. The most common differences by crop level were higher total soluble solids and total anthocyanin concentration with lower crop level; however, the differences were not always consistent across the years or vineyards. Wine sensory results of past vintages indicate no consistent differences in sensory perception by crop level. However, in-house evaluations of wines by collaborators indicate that there are differences but it is difficult to identify a clear preference by crop level. Industry collaborators shared their observations about the study through interviews, reporting that higher crop levels have not affected vine health, and they report that fruit and wine quality is not as different as they expected it to be. Collaborators are starting to evaluate their target yield each year, rather than using the same target yield every year. Many reported that the study gave them confidence in increasing crop levels by 10-25% and helped them evaluate their farming practices with more data collection and interpretation. They want to encourage others in industry to learn from their experiences from this study and to think “smarter” about yield management.

A New Embedded Sensor for Continuous Monitoring of Stem Water Potential in Grapevines

This project is to extend the application of a recently developed microtensiometer (MT) device, which measures water tension, to the measurement of stem water potential (SWP) in grapevines under field conditions. The objectives of this project were to test the accuracy of this device using standard laboratory methods, test the performance of sensors compared to pressure chamber measured SWP under field conditions, and help optimize sensor packaging and installation for accuracy and robustness in the field. Laboratory and greenhouse testing indicated that the sensors responded correctly to water potential, although in some cases, sensor response to changes in water potential was relatively slow and temperature sensitivity high, both of which might limit sensor performance in the field. Improvements in the sensor and in handling/installation have been made and are ongoing, but near-commercial prototypes were installed in field vines starting in August, 2018. A total of 14 sensors were installed on 6 mature Cabernet Sauvignon vines in the RMI vineyards at Davis, CA. These vines were minimally drip-irrigated, had not been irrigated for about 3 weeks, and did not receive any irrigation once the sensors were installed. Essentially all sensors exhibited similar daily patterns in SWP that were comparable to patterns shown by the pressure chamber, but there was large sensor-to-sensor variation in the range of water potential values reported. For these vines, the relative values of the sensors indicated that the highest SWP did not occur at predawn (prior to 6:00am in August), as is commonly assumed, but rather about an hour after dawn. The midday minimum water potential (maximum stress) occurred around 3:00 pm, about 2h after peak reference evapotranspiration (ETo). These findings are consistent with previous in-situ measurements in other woody perennial species. Each MT was calibrated against the corresponding pressure chamber measured SWP on an individual vine basis, giving consistent MT measured SWP values for the 5 (of 6) vines in the study that had enough calibration points. The average values from these vines showed a very clear pattern of overnight recovery to about -3 bars from August to November, when rains caused an increase to above -2 bars. Midday SWP values were around -7 bars, also recovering with normal seasonal leaf loss as well as rains. Midday MT measured SWP also fluctuated in parallel with the weather-related baseline (fully irrigated) SWP for about 3 months, indicating the possibility for continuous monitoring over relatively long periods. These data illustrate that automated and sensitive monitoring of SWP in grapevine is possible with these sensors, and should provide useful information for irrigation management as well as for physiological studies.

Study on the use of source-sink-reserves relations to manipulate grapevine nitrogen use efficiency, aroma and grape maturity and its relationship to yields and wine composition

Canopy management and fruit load control aim to keep a balance between vine’s sources and sinks. In fact, balanced vines may produce more consistent yields and have a more even ripening. This study aims to study the relationship between source–sink ratios and important parameters for production logistics and grape quality, such as progress of ripening and grape composition at harvest. After homogenizing all vines by removing laterals and adjusting the number of shoots to 20, we tested three levels of canopy density and fruit load combined in a factorial design (3 by 3). This is, 3 canopy levels, 100%, 66% and 33% of the leaves combined with 3 fruit loads, 100%, 66% and 33% of the fruit corresponding to 30, 20 and 10 clusters per vine, respectively. Carbon fixation rates were transiently higher in plants with a 33% of the canopy mediated by higher chlorophyll content, although this did not compensate for their smaller leaf area. Onset of ripening was sequentially delayed in 66% and 33% canopy treatments. The progress of ripening, accumulation of soluble solids and loss of acidity (increase in pH and decrease in total acidity), also occurred slower in 66% and 33% canopy treatments compared to 100% of the canopy. In fact, the time to reach commercial maturity (>25°Brix) was delayed 6 weeks for the 33% canopy level. Surprisingly, fruit load did not have a significant effect on the progress of ripening. When comparing all treatments at commercial maturity, the treatment maintaining 100% of the canopy had the highest total acidity and lowest pH. Contrarily, Anthocyanin content was slightly lower in this treatment. These results provide the basis for the control of the speed of ripening, aiming to coalesce variability within a vineyard or optimize the tank capacity through sequential ripening.

Fruit thinning is an important cultural practice in a premium vineyard as it directly affects yields. From fruit set through veraison, wine (especially red) grape growers can remove up to 2/3 of the initial crop borne by the plant at fruit set. This practice is performed aiming for a vine balance between leaf area (sources) and fruit load (sinks). A vine with a higher leaf area-to-fruit ratio (LA/F) is able to ripen (increase °Brix) the crop faster (Kliewer and Dokoozlian 2005; Parker et al. 2014), and presumably, achieve better quality. Contrarily, when there is too much fruit for a given leaf area, ripening (increase in °Brix) can be slower and even stop when is pushed into late October. Under normal circumstances, grape berry accumulates sugars and anthocyanins, while losing astringency (mainly proanthocyanins) and green aromas throughout ripening. Therefore, lack of ripening can be assessed as a slower accumulation of TSS and anthocyanins or retaining high amounts of proanthocyanidins and green aroma compounds. It is well understood that although a balanced wine is the target, there is greater penalization for under ripe wine characteristics than over ripe (Bindon et al. 2014), and in poor sugar accumulation is intimately related to vintage failure (Jones and Davis 2000). Therefore, there is a risk associated with carrying more crop in the grapevine as this can result in an insufficient ripening and quality loss. As grape price or wine retail price is a greater multiplier of revenue than yield in wine grape, dropping fruit is always the decision taken by premium wine grape growers. In addition, when there is a contract between grape growers and wineries, there are great economic interest plead for or against fruit thinning, without any certainty of the true pros and cons of fruit thinning. Given the stability of 2 summer weather in California, a better understanding of how source-sink-reserves affect plant fitness, growers could reduce the amount of fruit that they need to drop and still safely achieve an adequate ripening.

Nitrogen applications are a delicate matter as in their deficiency may result into poor growth and in excess may inhibit the production of grape anthocyanins (Soubeyrand et al. 2014). This study aims to determine the impact of nitrogen use efficiency and make recommendations about the application of nitrogen in combination with canopy and fruit load management decisions. Nitrogen is the most frequently deficient nutrient in vineyards, and thus the most often supplemented by fertilizers. Grapevines lose approximately 2.76 lbs of N per ton of grapes harvested, and this value serves as an indication of the minimum amount of N that should be replenished annually. However, that value is an estimate based on the average of data from several studies showing a range from 4.2 to 1.8 lbs of N per ton of grapes (see references in Mullins et al., (1992)). In addition, a great amount of nitrogen is invested in the growth of trunk, roots, leaves and shoots. Although a fraction of this nitrogen is reabsorbed into permanent structures when plants go dormant, a significant amount is lost with leaf fall and dormant pruning, leading to additional demand. Given the significant storage of nitrogen perennial crops in permanent structures, the use of a fertilizer containing labeled nitrogen (15N) allows an estimation of the fertilizer use efficiency, and therefore, determine if the amount of nitrogen prescribed needs to be modified according to cultural practices canopy or crop load management.

Evaluation of Ten Vitis vinifera cv. Pinot gris Clones in the San Joaquin Valley of California.

In June of 2017, 25 vines per Pinot gris clone from Foundation Plant Service, including FPS 01, 04, 05, 09, 10, 11, 52 and 08, grafted on Freedom rootstock as green potted vines, were planted at the commercial site located at Firebaugh, Fresno County, CA (36°45’22.2″N 120°28’12.6″W). Five vines of clone FPS 12, and ten vines, clone FPS 457, with the same rootstock were planted at the same date, due to lack of cuttings. Stakes with 24-inch horizontal cross arm and cordon wires, established to 54-inch height above vineyard floor, and irrigation tubing were installed after the planting. Irrigation and fertilization were carried out according to the vineyard cooperator based on the commercial industry standards for that area. Each experimental vine was flagged and labeled after the planting with the information of variety, rootstock, planting date and clone number on it. Vines were field checked regularly to assess the canopy growth, count the missing vines, and scout disease/pest. After the summer of 2017, extra vines with a total of 64 vines plus Freedom rootstocks have been ordered from Foundation Plant Service and cuttings will be collected in the dormant season of 2017. Cooperators from local nursery will help the grafting and prepare the green potted vines for replacing the missing vines in the field by the late spring of 2018.

In the fall of 2017, decision has been made to prune the vines back to two buds in the dormant period due to the non-uniform growth across vines. Therefore, vine trunk and cordon will be trained in 2018 and additional 64 vines will be planted to replace the missing vines in the late spring of 2018. The first harvest will be expected in 2019.

Determining the Impacts of Dormant Pruning Methods and Nitrogen Fertilization on Pinot Noir Bud Fruitfulness and Yield

This project is designed to provide scientifically tested information to growers about the impacts of two commercial management practices that can influence vineyard productivity: dormant pruning and nitrogen (N) fertilization. The Oregon industry uses primarily cane pruning for Pinot Noir, as they believe spur pruning will result in low yields (due to low bud fruitfulness) and reduce wine quality. Objective 1 of this study tests this question in a commercial vineyard, and we have been able to show with year 1 data that spur-pruned vines have fruitful basal buds, and that fruit composition and vine growth are equitable to that of cane-pruned vines. Although cluster size is smaller in spur-pruned vines, it was not enough to cause differences in whole vine yield. Results from our N-fertilization trial show little effect on bud and actual fruitfulness and yield after two seasons. Both experiments highlight the impact of vine vigor on bud fruitfulness, with greater fruitfulness observed with higher cane weights in the nitrogen trial and higher internode diameters in both pruning and fertilization trials. Additional years of this research will help determine the impacts of these management methods on yield potential over different seasons and vineyards to help growers adopt new pruning practices and/or enhance their vineyard N-management.

A Researcher-Industry Partnership to Understand the Yield-Quality Relationship in Cool Climate Pinot noir and Chardonnay Production

The long-term crop load study continued during 2017, the project’s sixth season. Eleven companies conducted the research on-site in 12 vineyards during 2017. Yields were the second highest in the six years of the study, second only to 2015. Average yield across all crop thinning treatments and sites was 1.13 lb/ft, which is higher than the 6-year mean of 0.93 lb/ft. Seasonal heat unit accumulation for 2017 was the lowest since 2012, and harvest was later than in recent years. However, fruit had sufficient ripening with total soluble solids (TSS) ranging from 21.0 – 25.0 °Brix. Analysis of harvest data across all sites in 2017 shows that fruit composition was affected by vineyard site and crop level. Treatment effects were tested within each vineyard site, and results show that only half of the sites (50%) had some treatment effect on fruit composition; however, the effects varied by site. There was no single fruit parameter that was affected by crop level at all sites, and the most common differences found by treatment were for TSS and pH in 2017. However, only 25% of vineyard sites had a difference in TSS with cluster thinning. Multiple regression analysis of data across the first five years (2012-2016) shows that crop load (yield: pruning weight, Y:PW) was related to TSS more than yield alone. Yield, however, was related to anthocyanin, pH and TA, with higher yields predicting lower pH, TA, anthocyanin, and tannin but higher TA. Fruit YAN was best predicted by pruning weights and Y:PW, not yield. Expert evaluation of wines from the 2012-2014 vintages show no yield effect on wine sensory perception, as wines did not group by yield level for descriptive analyses. According to industry collaborators surveyed, 43% had sufficient confidence in the study’s findings that they adopted higher yield targets in their vineyards beyond the research project, citing the ability to increase yield without compromising quality, and in recent warm years higher yields were preferred by winemakers.

Assessing Rootstock Biology and Water Uptake through Proximal Sensing under Different Wetting/Drying Conditions

Considering the importance of the water status as a driver of the whole plant physiology at the vineyard scale, developing new technologies for the space-time measurement of roots distribution and water uptake is crucial for the development of efficient precision viticulture practices. This project (2017-2022) is devoted to the development of a non-invasive technique (electrical resistivity tomography, ERT), as a tool to compare dynamic changes in root growth and water uptake patterns, and to apply this technique to the study of several commercially available rootstocks under varied irrigation delivery methods (drippers, micro-sprinklers) and water regimes (sustained deficit irrigation, rain fed, fully watered). The project was conducted in a vineyard at UC Davis specifically planted for the study of the response of rootstocks to watering systems. According to the timetable of the funded proposal the first year consisted of two parallel objectives: i) the calibration of ERT to soil water and roots distribution iii) the physiological monitoring of different rootstocks under different water amounts and delivery methods.

In a vineyard planted to Chardonnay on different rootstocks representing the most common parentage classes in a randomized complete block design, six soil pits were dug for the description of the soil profile, the sampling of the soil for chemical-physical analysis and the installation of TDR probes for the monitoring of soil moisture. The pits were then refilled, and the soil allowed to resettle for several months before the measurements commenced. The section of the field used for calibration purposes was not irrigated throughout the season in order to cover a full range of moisture levels for that soil (from 24% to 8% vol.). In close correspondence of these pits, at the soil surface 300 stainless steel electrodes were installed at a distance of 0.62 m (2 feet) for the ERT monitoring. A single ERT measurement was able to cover 2 plants on 2 different rootstock, which represented an experimental unit. The water status of the plants within the experimental units was monitored using infrared thermography and pressure chamber measurements. Photosynthesis and stomatal conductance were monitored through a pressure bomb. The electrical resistivity was calibrated to the soil water content obtained by TDR measurements, and two different pedoelectrical models were fitted to the data: Archie, in a canonical and linearized form, and Waxman and Smits models. Their performances were tested on 20% of the data from the whole dataset that were excluded from the model fitting and then used as a test set of unobserved data. The three modeling approaches gave similar results on the test set, with a slight decrease in performance from the log-log linearization (RMSE = 1.22% vol., R2 = 0.73) and the Archie law (1.22 %vol., R2 = 0.73), to the Waxman-Smits model (RMSE = 1.23% vol., R2 = 0.73). The Archie law was then chosen, and confirmed to be well adapted to predominantly sandy soils such as the experiment site. This calibration was used to transform the ERT images from soil resistivity to soil moisture maps under different rootstocks in 2D and 3D. This is the first report in the world that 3D images of soil moisture were developed in a vineyard, and the first time that this technique was used to compare rootstock physiology in general. Great differences were found between contiguous rootstocks in their spatial use of water. The presented data show how lateral heterogeneity in soil moisture could reduce the efficiency of spot measurements, as those obtained with soil probes, and soil moisture sensors in determining irrigation needs.

At the end of the season, the soil was sampled with an auger at different locations in the middle between contiguous ERT electrodes. The soil was sampled every 0.1 m, brought to the laboratory and oven-dried. Roots were physically separated from the soil, and their presence was assessed in a gravimetrically (mg of roots per g of soil). The presence of roots was negatively correlated to soil moisture obtained through ERT (r = 0.45), i.e. greater the amount of roots, lower the amount of water at the end of a dry period. This relations was then used to map the roots using ERT as ancillary variable for the spatialization, therefore developing the first electrical image of root distribution in a vineyard.

The Impacts of Cluster-Thinning and Cluster-Zone Leaf Removal on the Hormone Content of Pinot Noir Grape Berries

During the third year, we confirmed that the relative developmental stage of a berry was not fully a function of the flowering time, but was more associated with seed development. Therefore, our study reinforced the use of a new seed index (SI), also named SB or seed weight-to berry weight ratio to describe berry developmental stage. We have identified and optimized a selection method for selecting individual fruits of cluster at discrete stages using multivariate statistical tools in order to minimize inherent biological variability existing within a grape cluster. The analysis of bioactive forms of auxin, abscisic acid, cytokinin, brassinosteroid, gibberellin, jasmonic acid, and salicylic acid revealed a clear spatial and temporal distribution for most of them during the major critical steps of berry development.

Indeed, we were able to correlate the accumulation dynamics of most of them with major physiological events occurring during berry development. We were also successful in quantifying the effects of two common viticulture practices, cluster-thinning and cluster-zone leaf removal, on two major plant hormones responsible for the ripening initiation (ABA and Auxin). The analysis of the conjugate forms, precursors of, and catabolites of those bioactive analytes revealed specific regulatory pathways. These observations will lead to develop new working hypothesis to explain their mode of regulation in a tissue and developmental context. Based on our results, we will investigate in a near future other layers of biological information including gene expression. Encouraging results during the second year showing a peak ethylene during the ripening will need further investigation in order to prove the real contribution of endogenous ethylene production during grape berry development. From this study, we expect to have at least two peer-reviewed papers published as a result of this funded research and we will propose the results for oral presentation at the next conference of the American Society of Enology and Viticulture.