In situ measurement of plant water potentials by equilibration with microdroplets of polyethylene glycol 8000

Microdroplets (3-5 nanoliters) of polyethylene glycol 8000 solution were allowed to equilibrate with plant water potential by placing the microdroplet on an abraded surface and covering it with mineral oil to prevent evaporation. Osmolality was followed by cryoscopic measurements, accurate to about ±0.1 bar, on subnanoliter samples.

Under constant environmental conditions, apparent equilibrium between microdroplet and plant water potentials was attained in about 1 to 2 hours. Microdroplet osmolality responded promptly to treatments (illumination, excision, osmotica) which changed plant water status. The values obtained for plant water potentials appeared to be physiologically reasonable. However, comparison with values obtained by other means (dewpoint hygrometry, treatment of tissue with polyethylene glycol solutions, calculation from turgor and osmotic pressures) suggest that they might be somewhat more negative than the actual tissue water potential.

Aside from the advantage of providing in situ measurements of plant water status, the method is not temperature sensitive and requires only about 10 square millimeters of surface area, which allows its use on even small structures with little interference by shading or with gas exchange.