Diurnal variation in the radial reflection coefficient of intact maize roots determined with the xylem pressure probe

Xylem pressure and its relative response to the imposition of an external osmotic stress (the so-called radial reflection coefficient) were recorded in roots of intact maize plants using the xylem pressure probe technique. Consecutive insertion of two probes into the same xylem vessel or into adjacent vessels of intact roots of plants exposed to high light intensity and salt stress under laboratory conditions showed that the xylem tension was not changed by vessel probing. It was also shown by using the double probe approach that the plants were capable of overcoming artificially induced leakages. This and other evidence reported in the literature convincingly demonstrated that the probe accurately reads xylem pressure and xylem pressure responses to osmotic stress. Additional experiments were performed on plants grown in a greenhouse at a subtropical latitude. Under these conditions the plants were exposed to strong diurnal fluctuations in light intensity, relative humidity and temperature. The results showed that the absolute xylem pressure in the roots of untreated plants decreased with increasing transpiration rate from positive values in the early morning to negative values around noon (average value 0.15 MPa; maximum negative value —0.57 MPa). As the day progressed and the transpiration rate decreased, xylem pressure increased again to positive values. Correspondingly, the radial reflection coefficient for NaCI increased from about zero in the early morning to about unity at noon when transpiration reached its highest value and decreased again to very low values towards the evening. The data raise questions concerning conclusions about the mechanism of water transport in intact roots drawn from the low radial reflection coefficients measured on excised roots using the root pressure probe.