Osmoregulation in water stressed roots: Responses of leaf conductance and photosynthesis

Kober 5 BB vines were subjected to either moderate and slow soil dehydration or to repeated, severe and rapid stress and irrigation cycles. Moderate soil dehydration to 50 % of the soil water capacity led to a small but significant decrease of the osmotic potential at RWC = 100 %, i.e. to osmoregulation in root tips, but not in other parts of the roots. Osmoregulation was associated with the maintenance of a high water status in the root tips and high rates of leaf gas exchange. In a second experiment three severe and rapid drying cycles led to a decrease of the osmotic potential at RWC = 100 % in root tips as well as in unsuberised and suberised roots, the maximum rate being 2.3 bar. In this experiment osmoregulation in roots contributed to a partial increase of the root water status. The observation that, despite a low soil moisture content, leaf conductance and rate of photosynthesis had slightly recovered is discussed