Finetuning the Grapevine Stress-Response Picture for Irrigation

By Abby Hammermeister, 2022 NGRA Fellow

Water scarcity already threatens sustainable agriculture in arid regions of the western U.S., and changing climatic conditions will make matters worse. Reduced water availability forces growers to employ conservation techniques like deficit irrigation. This practice induces water stress in plants that decreases growth and yield, but is often used purposefully in vineyards to control grapevine vigor, improve fruit quality or facilitate harvest. To effectively implement deficit irrigation strategies, whether on its own or as part of a precision farming regime, grape growers need reliable real-time data on crop evapotranspiration (ET) and water stress. Ground-based and remote sensing platforms can deliver this information, but still require refinement and rigorous testing under realistic commercial settings to better link to vine function for irrigation decisions. In part, that’s because grapevines’ response to stress is complicated.

Grapevine leaves represent a complex exchange system, ideally maximizing carbon capture and limiting water loss. Under well-watered conditions and moderate temperatures, leaves can utilize sunlight effectively and have ample water to maintain optimal internal water pressure, or turgor, for the stomata to capture CO2. These ideal conditions can change dramatically under stress imposed by drought and high temperatures. But the expression of these changes can be incremental and nuanced.

The McElrone Lab has been studying the processes by which leaves exchange light, CO2 and water, and how stressful conditions can induce structural changes inside the leaves that impact how they absorb, transmit and reflect light. Remote sensing uses reflectance data to generate information about leaves’ response to stress as it corresponds to spectral indices. For irrigation management, however, this data may not yet yield the clearest picture.

For example, I recently utilized X-ray microCT scanning to evaluate how grapevine leaf petioles and lamina are altered by drought stress. In doing this, my collaborators (Drs. Caetano Albuquerque and Andrew McElrone) and I discovered that the range of physiological responses to mild drought stress may not be accounted for in current models of how water moves through the grapevine. That is, the variable responses of leaf structures within the canopy can lead to mixed stress signal detection. Incorporating such dynamic responses into existing models should improve ET and stress-detection efforts. As a graduate student in biophysics, I feel uniquely positioned to utilize ongoing research in our lab to better link leaf-level physiological responses with whole canopy and vineyard scale measurements to improve our understanding of signal output from proximal and remote sensors.

With my dissertation research, I will use a variety of approaches in field and controlled conditions including plant water relations, pressure volume curves, gas exchange physiology, fluorescence and spectral imaging, and modelling efforts at the leaf scale to finetune the stress-response picture. I also will utilize existing datasets and the experimental sites established by the GRAPEX variable-rate irrigation project to link changes in stress physiology with those detected with proximal and remotely sensed systems. The McElrone Lab recently developed a new wavelet method to track vineyard water use, but more work is needed to interpret stress signals with this technique. This innovation represents a timely opportunity to hone my research and ultimately improve irrigation strategies in ever hotter, drier vineyards.

Currently, I am in the early stages of my project planning and would welcome the insights of industry collaborators to better refine this work for applied outcomes, and will continue to explore collaboration with other research labs working in this space.

Abby Hammermeister is a Ph.D. candidate in the McElrone USDA-ARS Plant Physiology Lab at UC Davis.