65 Water Channels in Strawberry, and Their Role in the Plant’s Response to Water Stress Michael M. Blanke David T. Cooke Obstversuchsanlage Klein-Altendorf School of Biological Sciences Universität Bonn University of Bristol Meckenheimer Str. 42 Fenswood Road D-53539 Rheinbach Bristol BS48 Germany United Kingdom Keywords: Fragaria × ananassa Duch., salinity, stomata, transpiration, water transport, water potential Abstract The objective of the present work was to demonstrate the presence of water channels in strawberry (Fragaria × ananassa Duch.) and to identify their role in the adaptation of the plant to water stress. Drought stress decreased strawberry leaf water potential from –1 to -2 MPa with a concomitant reduction in transpiration from 5.6 to 3.4 mmol H 2 O m -2 s -1 . However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from flooded strawberry plants showed no change in water channel activity, indicating that water channels remained open. By contrast, water channel activity was reduced in drought stressed strawberry plants. The effect of flooding on water relations of strawberry was less pronounced than that of drought, a result that cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry than drought stressed strawberry, despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. In flooded plants, turgor may be preserved by maintaining root pressure and an electrochemical and ion gradient, assuming water channels remain open. The lesser effects of flooding could be explained in terms of water channel activity. In conclusion, strawberry plants appear to be more affected by drought than by flooding. If water channels act as a hydrostatic signal, under flooding conditions, water channels might be maintained in the open position, whereas under drought, they are likely to close. Closure of the water channels may be the first stress signal, followed by ABA, to cause stomatal closure in plants including strawberry. INTRODUCTION Strawberries are often exposed to various forms of water stress such as flooding in Finland, Holland, Germany or England near rivers; and drought or salt stress e.g. in California or Australia. The absence of water stress is particularly relevant during fruit development to maintain good fruit quality (Blanke, 2003) and during vegetative propagation i.e. runner development after fruit harvest. Water transport within a plant is driven by a combination of root pressure, cohesion, and suction forces generated by evaporation. The latter relies on the presence and functioning of stomata and water vapour pressure deficit (VPD) between the plant and the ambient atmosphere. Strawberries react to all three forms of water stress, i.e. flooding, salt and drought, by closing their stomata, which occur at a density of 320-360 stomata/mm 2 on the lower leaf surface (Blanke, 2003) and reducing transpirational water loss (Blanke and Cooke, 2004). Abscisic acid (ABA) has long been thought to be the primary signal for this stomatal closure, i.e. apoplastic ABA transport to the guard cells, followed by delivery of ABA in the xylem from the roots where it is newly synthesised. However, recent work on ABA has revealed an unidentified indigenous compound in plants which also binds in the ABA radioimmunoassay (M.B. Jackson, pers. commun., 2004). Furthermore, stomata of flooded Proc. V th Int. Strawberry Symposium Ed. G. Waite Acta Hort. 708, ISHS 2006