Compensation of Root Suction Force within a Single

A modified refractometric method (compensation in sucrose solution) was used for measuring the suction force (water potential) of active roots of Fraxinas excelsior L. in pot experiments and in the field under natural humidity conditions in two soil types. It was shown that if one part of the root system was in soil with low humidity as compared with the remaining predominant part, the suction force of roots in the “dry” soil did not rise in proportion with the rise of the suction force of the drying soil but, due to gradients of suction forces between these root parts, water was translocated into roots in the “dry” soil and thus their suction force was decreased to the relatively lowest value of suction force within the whole root system. The suction force of roots surrounded by soil of humidity below the availability limit was in these cases very low or else its value changed in parallel with changes of the suction force of the remaining part of the root system. It was completely independent of the soil water content in which it existed. The root system is thus a hydrodynamic unit, the individual parts of which do not respond to changes in soil humidity separately by changes in their suction force, but rather in mutual relationship which is brought about by gradients in suction force. These gradients are the cause of water translocation between individual branches of the root system.