Research on plant hydraulics helps explain the effect of drought stress on shade trees

By Sadie Keller, Rebecca Sheridan, Scout Dahms-May, Carolyn Scagel and Lloyd Nackley

In Oregon’s Willamette Valley, the heart of the nursery industry, rainfall is scarce during the summer and humidity is low. The plant stress resulting from the low soil moisture, high heat and low relative humidity has been exacerbated in recent years by the increasing frequency of heatwaves.

Drought and heat stress can scorch the canopies and reduce growth, leading to decreased plant quality and economic losses for shade tree growers. Scientists at the Oregon State University Horticulture Department and the USDA-ARS are using principles of plant hydraulic physiology to understand how shade trees respond to drought and heatwaves.

Plant hydraulic physiology is a way to understand how water moves through plants, similar to the way blood moves through the body. Imagine a big drinking straw running from the roots to the leaves of a plant; this is similar to the stem, which is made up of thousands of tiny tubes that help the plant pull water from the roots to the leaves. The water evaporates from the leaves, goes into the air, and the plant soaks up more water to repeat the cycle, ensuring the plant gets all the water and nutrients it needs.
The water moving through the plant is under tension, and as air and soil dry, the tension increases. If the tension becomes too great during drought, air bubbles can form inside the plant, causing cavitation. This can reduce growth and eventually lead to branch death and plant death.

To prevent hydraulic failure, the plant closes its stomata to reduce water loss in transpiration, but this also decreases the carbon available for photosynthesis. The plant must balance between drying out and a lack of carbon dioxide during drought.

Understanding individual drought response

The pattern of water use varies by species, cultivar, environmental and soil conditions, and drought severity, and is the result of coordination and tradeoffs among various traits along the entire water transport pathway. Seedlings and saplings have different drought responses than mature plants, so understanding the specific responses of nursery-grown plants is important for irrigation management.

The current research at Oregon State University focuses on the hydraulic responses of Red Sunset® maple (Acer rubrum ‘Franksred’) and red oak (Quercus rubra) to increased xylem tension. Red maple is a deciduous tree native to the eastern half of North America and is known for its fall foliage. Red oak is also a deciduous tree native to the eastern and central United States and easily recognizable by its deeply lobed leaves.

These species were selected because they are important to the Oregon shade tree industry and have evolved in environments different from the Willamette Valley. Their native ranges are hot and humid in the summer, whereas Oregon is hot and dry.

The water balance of a plant is crucial for its survival and growth. Too little water can cause wilting and death, while too much water can lead to anaerobic soil conditions and root rot. The water content of a plant’s cells, leaves and tissues can change in response to environmental conditions, and the water balance is regulated by the movement of water into and out of the plant in response to changes in the environment, such as temperature, light intensity or water availability.

The water balance responses of plants are described on a hydric continuum from isohydric to anisohydric. Isohydric plants maintain a constant water potential across their leaves, even under changing environmental conditions, and are well-adapted to withstand periods of drought. Anisohydric plants, on the other hand, show a greater range of water potential across their leaves and are more flexible in their water balance; they can quickly adjust the water potential in their leaves in response to changing environmental conditions.

During the summer of 2022, graduate student Sadie Keller conducted weekly measurements of Red Sunset maple and red oak transpiration and water balance. She used a leaf porometer to measure the concentration of water vapor in the air around a leaf to determine the amount of water lost through the leaf over time, and to understand how environmental factors such as light, temperature and humidity influence the process.

Keller also used a pressure chamber to measure the water potential of plants. Her research showed that under ideal conditions, oaks use more water and grow faster than maples, which have a more conservative hydraulic strategy and restrict water movement at an earlier onset of drought.

Measuring the flow of water

Graduate students Keller and Scout Dahms-May have constructed an experimental device, the Stem Hydraulics Research Device (SHREDder), at the USDA Horticultural Crops Research Lab in Corvallis, Oregon. The device measures the flow of water through plant stems by applying a pressure difference across a stem segment and measuring the flow rate of water.

This information is used to calculate the hydraulic conductivity of the stem, which is a measure of its ability to transport water. By measuring the hydraulic conductivity of plant stems, researchers can better understand how plants respond to changes in their environment, such as variations in water availability or temperature.

A SHREDder was previously used by Dr. Rebecca Sheridan to investigate safe limits for cold storage, with results published in the February 2021 of Digger (“The cold shoulder season”). The current research on field-grown shade trees suggest that growers should aim to keep soil moisture pressure above –20 bar to avoid growth-reducing cavitation.

In the absence of other information, –15 bar to –20 bar can be a useful threshold to stay above since the wilting point for other tree crops is also around –15 bar to –20 bar. Note though that wilting points will vary based on the water status of the soil, the growth stage of the plant, and environmental conditions such as temperature and light.

To improve irrigation efficiency and achieve water conservation benefits without sacrificing plant quality, growers need to know how their plants respond to water deficits. However, species, life stages and production methods differ in their sensitivity to water stress, making it difficult to determine an ideal set-point for irrigation scheduling.

Our research group at OSU will continue to determine plant stress thresholds for shade trees, new shrub varieties and plants developed by the OSU Ornamental Plant Breeding Lab. Keep up with our work by visiting the blog and on Instagram (@Nackley.Lab).

About the authors

Sadie Keller is a graduate research assistant working in the Nackley and Scagel Labs at Oregon State University and USDA-ARS. She can be contacted at [email protected].

Rebecca Sheridan is a nursery scientist in the Strategy and Technology Division, Production Forestry, Western Timberlands, at Weyerhauser. She can be contacted at [email protected].

Scout Dahms-May is a graduate research assistant at Oregon State University. She can be contacted at
[email protected].

Carolyn Scagel is a plant physiologist at the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS). She can be contacted at [email protected].

Growing Knowledge from the April 2023 issue of Digger magazine | Download PDF