T. McBurney, P.A. Costigan
A method of continuously monitoring plant water potential in the field is necessary to improve understanding of causes of variation in yield between plants growing in different soils and to improve estimates of irrigation requirements of crops. The most promising approach is to use methods employing electronic sensors. Two techniques are described.

In situ leaf psychrometers, used by some workers, have limitations because of difficulties in attaining the necessary conditions of thermal and vapour equilibrium between the plant tissues and the sensor. However, attaching an in situ psychrometer to the plant stem even enabled monitoring of rapid oscillations of plant water potential. The method has been extended to permit its use in the field by incorporating a means of temperature control into the sensor.

Another technique, which could enable monitoring of plant water potential indirectly, is the displacement transducer used to measure small changes in plant stem diameter. A major difficulty with this technique is how to account for any growth component in stem diameter measurements. Measurements made in the laboratory on young cabbage plants suggested that readings can be corrected for growth on the basis of the relationship:

D = I + KT + alphapsiS

where D = stem diameter, T = time, psiS = the water potential in the stem, and I, K and alpha are constants where I is the initial stem diameter, K the rate of increase in stem diameter due to growth and is the bulk elasticity of the stem (slope of D/psiS relationship). The results also established that stem shrinkage is a sensitive indicator of changes in water potentials within the stem.

It was concluded that the psychrometer and stem diameter techniques may provide new opportunities for investigating the influence of water stress on plant growth and the requirements of crops for irrigation.

McBurney, T. and Costigan, P.A. (1988). CONTINUOUS MEASUREMENT OF PLANT WATER STRESS. Acta Hortic. 228, 227-234
DOI: 10.17660/ActaHortic.1988.228.26

Acta Horticulturae