Water status is a key regulator of many growth, yield, quality and sustainability. So, what factors affect the water status of plants? The most obvious answer is the amount of water in the soil, and that can be a critical limitation when the soil dries. However, there are many other factors that can affect the plant water status and interact with other factors that do also. As the figure below shows there are many important factors which the plant integrates to give its water status at any time.
It is best understood as water supply versus demand. Just like our own economics, when the supply is inadequate for the demand, there is stress. The supply is via the soil from stored water, rainfall or irrigation. But not all the water in the soil is available to the plant as it may be too deep or the roots may not explore all the soil. Some of the water from rainfall or irrigation evaporates or may be lost by leaching. Knowing just how much water is available to the plant and how difficult it is to extract the soil is a problem, especially in large deep-rooted perennial plants.
The demand is more complicated as it depends on the plant (the leaf area and how it is displayed, stage of development, mineral nutrition status, health, crop level in some cases), on the weather (sunlight, temperature, humidity, wind), and for perennial crops the competition from weeds or cover crops. Also, due to the dependence on weather, the water status changes during the day and can change very quickly.
Clearly, due to all these factors the actual water status of a plant at any time is difficult to know without measuring it directly.
As discussed earlier, there are many factors affecting the water status at any time. But there are two general factors that are most important for healthy plants: soil water availability and weather-caused variations. The soil is the baseline limit to how well-watered the plant is, but the weather can cause rapid changes, always causing more stress during the day when the plant loses most water.
The figure shows the pattern of daily cycling of water stress. Physics defines the water stress as making it more difficult to have the maximum water required. So the numbers are negative, i.e. the lower or more negative the number, the more stress. During the middle of the day the plant is losing the most water via transpiration while it is taking up CO2 for photosynthesis. This rapid loss of water exceeds the ability of the soil and the plants hydraulic system to supply that water, leading to the most stress in the afternoon (the lowest points on the curve).
Conversely, in the early morning around dawn the plants lose much less water. This lower demand can be met by the soil water supply so the plant water generally reaches the same stress as the soil water. As the soil water availability decreases, the curves show gradually more and more stress especially in mid-day, as seen in the first 3 days. When irrigated (blue arrow) the stress is relieved and another drying cycle begins. This is common in sunny conditions in arid zones, but the pattern can be more complex in more humid zones where the weather is more variable day to day.