Measurement of Inner Bark and Leaf Osmolality

Sugar transport in the phloem is driven by turgor pressure gradients, which are created by osmotic gradients resulting from sugars loaded into the phloem at the source tissue and unloaded at the sink tissue. Therefore, osmolality is a key parameter that can be used to evaluate sugar status and to indicate the driving force for phloem transport. Here, we describe how osmotic concentration measurements from inner bark and needles of trees can be made. This protocol presents the procedure used by Lintunen et al. [15] and Paljakka et al. [16], and extends it with practical advice and discussion of potential errors and caveats. We describe how to implement this procedure for gymnosperm and angiosperm trees. This method uses mechanical sap extraction with a centrifuge from inner bark and leaf samples that have undergone deep-freeze treatment and thawing. The osmotic potential of these samples is then analyzed using a freezing-point or vapor-pressure osmometer. The aim of these measurements is to study the spatial and temporal dynamics of phloem function.