LEAF ILLUMINATION AND ROOT COOLING INHIBIT BEAN LEAF EXPANSION BY DECREASING TURGOR PRESSURE

Phaseolus vulgaris plants with expanding primary leaves were subjected to dark-light or light-dark transition at a root temperature of 25 de grees C, or to root cooling to 10 degrees C. Illumination or darkening caused rapid changes in water flux through the plants and in epiderma l turgor pressure when analysed by pressure probe. However, these were not concurrent with variations in bulk leaf water potential and turgo r pressure as determined by the pressure chamber method. In addition, the turgor pressure of epidermis measured with the pressure probe was invariably 0.05 to 0.15 MPa lower than that measured in bulk tissue wi th the pressure chamber. Cooling roots to 10 degrees C induced water s tress and wilting. Both techniques indicated a decrease of turgor pres sure, but a 20-30 min lag was observed with the pressure chamber. Due to stomatal closure and decreased transpiration, root-cooled plants re gained cell turgor after 5-7 h of cooling, but bulk tissue and epiderm al turgor (as well as leaf growth rate) remained significantly lower t han control levels. These findings indicate that changes in turgor pre ssure as the result of hydraulic signalling are sufficient to explain the rapid changes in growth rate following illumination or cooling rep orted in earlier work (Sattin et al., 1990). They also indicate that d ata obtained by use of the pressure chamber must be treated with cauti on.