Osmotic adjustment leads to anomalously low estimates of relative water content in wheat and barley

Relative water content (RWC) is used extensively to determine the water status of plants relative to their fully turgid condition. However, plants often adjust osmotically to salinity or water deficit, which maintains turgor pressure and obscures the definition of 'full turgidity'. To explore this problem, turgor was measured by isopiestic psychrometry in mature leaf blades of barley (Hordeum vulgare) and durum wheat (Triticum turgidum ssp. durum) salinised to 150 mM NaCl, or bread wheat (Triticum aestivum) grown in soil dehydrated to varying degrees. Osmotic adjustment maintained turgor in all the plants but despite full maintenance in some of the salinised plants, their leaf RWC decreased substantially. This occurred because excess water was absorbed while the samples were floated on water as part of the RWC measurement. The absorption falsely increased the weight of the 'fully turgid' condition, causing RWC to be anomalously low by 10-15%. Cell solution was secreted into intercellular spaces and was seen under a microscope, which is a test encouraged for all RWC measurements. Several alternate methods are suggested for rehydrating tissues while minimising excess water absorption, but no simple definition of 'full turgidity' seems possible. In general, direct measurements of osmotic adjustment and turgor are preferred.