The PMS at any given time reflects the plant’s interaction with the water supply and the demand for water placed upon the plant by its environment. Since these factors are almost always changing, PMS is nearly always changing. The time of measurement therefore requires careful consideration.
PMS varies throughout the day as well as throughout the year. It is necessary to understand what causes these changes and what information is desired from the measurements before the best time to sample can be chosen.
PREDAWN MEASUREMENTS are the most common because that is the time when PMS is changing slowly and is usually at a minimum for the day. Under most conditions plants are likely to be close to equilibrium with the soil moisture at that time. For many purposes the predawn measurement is the most useful single measurement that can be made.
MID-DAY measurements are likely to indicate the maximum PMS for the day and can be helpful for scheduling irrigation or water deficit irrigation. Since the environment may change from one day to the next, a series of measurements is required to establish clearly the time of maximum PMS for any given day. The maximum PMS indicates whether a plant is under moisture stress sufficient to impair physiological processes, such as cell elongation or photosynthesis. For an accurate diurnal (daily) curve, sampling should be more intensive when the PMS is changing rapidly, as in the early part of the day.
SEASONAL changes in PMS can be measured by taking a series of daily measurements. For an accurate seasonal pattern, sampling should be more intensive when PMS is changing rapidly, such as during the onset of drought; at other times bi-weekly or even monthly measurement will suffice for most purposes. A COMBINATION of sampling times may be appropriate for some purposes. Consider, for example, the following schedule for measuring PMS of a crop. The results of these measurements would be used to schedule irrigation.
- Weekly predawn measurements from germination until harvest.
- Bi-weekly maximum (midday) measurements during the growing season.
- Three diurnal curve measurements during the growing season to establish the time of maximum PMS during the day.
- Other selected measurements to determine the effects of irrigation, high temperatures, etc.
This information would help a manager determine when to irrigate, the appropriate quantity of water to apply, the effectiveness of irrigation, and how to minimize the use of water and equipment. Since the level of PMS is often important in processes such as flowering, fruit set, fruit development and expansion, dormancy, frost hardiness, and resistance to insects and disease, the successful management of the water regime can make the critical difference between profit and loss for the crop. Wise choice of times to measure PMS is fundamental to water management.