How to Estimate or Measure the Water Status of Plants in the Field?

Many different interacting factors affecting supply (soil moisture), demand (weather) or plant factors (leaf area, canopy display) that combine to affect water stress in plants. Since there are so many indirect and interacting factors involved, the plant integrates all of them. So, measuring any one is inadequate. So how best to estimate or measure it?

Soil Water - Since the plant water status is limited ultimately by soil water, a very common method is to measure the amount of soil water to estimate the soil water availability to the plant. This seems straightforward for crops like many annuals that develop a very dense shallow root system that explores every area of that layer of soil. Perennial plants however, tend to have large but not necessarily intensive root systems that may be very deep but erratic in distribution. Clearly, we cannot be sure exactly what any soil moisture reading means in terms of plant stress.

WeatherWeather affect potential water use that could lead to stress if water supply is limiting, but it does not estimate plant water stress itself. Since the weather strongly affects the demand, it is common to estimate water use, or evapotranspiration (ET) by the planting by models such as the Penman-Monteith equation that relies on estimates of the plants such as leaf area and sunlight interception. Weather data can be useful to give relative water demands, but again it is only one component that affects plant stress.

Plant-Based Methods - Consequently, it makes sense to measure the plant itself, whether the water status directly or some plant characteristic that correlates well to water status stress (growth of organs, shrinking/swelling of plant parts, changes in optical characteristics, etc.).

Growth and Shrinking/Swelling – Growth of plant leaves, stems and fruit is dependent on adequate water for the turgor needed for cells to expand. Even woody stems shrink and swell daily as they lose some water in the middle of the day and re-hydrate at night. Growers observe slowed shoot growth or wilting as a general indicator of stress. However it is only semi-quantitative and often too late for best timing of irrigation after shoots stop growing. More recently, quantitative measurements of growth or shrinking and swelling of stems, fruit or leaves have been related to direct water potential measurements. These methods are related and are often referred to generally as “dendrometers”. The advantage is that they are easily automated to provide monitoring over days and weeks. However, again these measurements are not of the stress itself, but of the tissue response to stress, and the correlation of tissue response to water status often changes during the season. For example, woody stems grow in the spring with large thin-walled cells but change to thicker-walled cells in the summer, giving the common tree rings.

Direct measurements of water status – Though there are some research tools to measure the water status directly, but the only rugged, practical field method is the Scholander Pressure Chamber. Invented in the 1960’s, informally called by users the “pressure bomb”, this method measures the water status as tension in the plant’s vascular system, called water potential. If exposed leaves are measured, it is called leaf water potential. However, if a leaf is enclosed in a shaded bag to stop water loss by transpiration, the leaf equilibrates with the internal water potential, called the stem water potential. This is the major disadvantage of this manual method where typically growers only obtain one or two readings per week at any site.

If the stem potential is measured pre-dawn, it is considered to be an estimate of both the plant but also of the effective wettest soil water potential that re-hydrates the plant at night. In mid-day the stem potential integrates the effective soil water potential, additional stress due to hydraulic resistance in the path from root-to-top, and the effect of water loss by transpiration driven by evaporative demand. It integrates all the variable leaf stresses due to sun and shade and provides the base water potential for the growth of fruits, roots and shoots.

There have been many studies relating water potential to the responses of growth, photosynthesis, water loss, and in specialty crops the product quality (e.g. apple size, wine quality). One recent report from 13 research groups in Spain evaluated >78,000 types and times of water potential measurements in irrigation trials. They found that stem water potential during the day was the best measure grapevine water stress.

Since the water potential of the plant varies rapidly with the weather, it is important to have many measurements over time to relate those variations to plant performance and guide precision water management. This is the great advantage of the continuous monitoring of stem potential by the FloraPulse microtensiometer.

Monitoring stress with the FloraPulse microtensiometer. With continuous monitoring of the plant stem water potential, we can begin to closely observe a key regulator of growth and fruit or nut quality. The lowest, most negative, values show the most severe stress that generally occurs in mid-late afternoon when water loss rates are the highest. Conversely, the highest, least negative, values estimate the plant’s equilibration with the wettest soil. Observing these daily values along with the weather gives valuable information to help us better understand how much stress is reached and how irrigation and drying cycles affects plant stress.

Example of daily variations in almond stem water potential with irrigation (arrows) in uniform sunny conditions in California.

With such information, and knowledge of the target stresses desired for product quality or harvest management (e.g. hull split in almonds), water use can be optimal and sustainable.