XYLEM AND CELL TURGOR PRESSURE PROBE MEASUREMENTS IN INTACT ROOTS OF GLYCOPHYTES - TRANSPIRATION INDUCES A CHANGE IN THE RADIAL AND CELLULAR REFLECTION COEFFICIENTS

Xylem probe measurements in the roots of intact plants of wheat and ba rley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflectio n coefficients (sigma(r)) depended on the transpiration rate. Under ve ry low transpiration conditions (darkness and high relative humidity), sigma(r) assumed values of the order of about 0.2-0.4. The sigma(r) v alues of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For trans piring plants (light intensities at least 10 mu mol m(-2) s(-1); relat ive humidity 20-40%) the response was nearly 1:1, corresponding to rad ial reflection coefficients of sigma(r) = 1. Further increase of the l ight intensity to about 400 mu mol m(-2) s(-1) resulted in a slight bu t significant decrease of the sigma(r) values to about 0.8. Similar me asurements on maize roots confirmed our previous results (Zhu et al. 1 995, Plant, Cell and Environment 18, 906-912) that, in intact transpir ing plants at low light intensities of about 10 mu mol m(-2) s(-1) and at relative humidities of 20-40% as well as in excised roots, the xyl em pressure response was much less than expected from the external osm otic pressure (sigma(r) values 0.3-0.5). In contrast to wheat and barl ey, very high light intensities (about 700 mu mol m(-2) s(-1)) were ne eded to shift the radial reflection coefficients of maize roots to val ues of about 0.9. Osmotically induced xylem pressure changes were appa rently linked to changes in turgor pressure in the root cortical paren chyma cells, as shown by simultaneous measurements of xylem and cell t urgor pressure. In analogy to the sigma(r) values of the respective gl ycophytes, the sigma(c) values of the root cortical cells of wheat and barley were close to unity, whereas sigma(c) for maize was significan tly smaller (about 0.7) under laboratory conditions. When the light in tensity was increased up to about 700 mu mol m(-2) s(-1), the cellular reflection coefficient of maize roots increased to about 0.95. In con trast to the sigma(r) values, the sigma(c) values of the three species investigated remained almost unchanged when the leaves were exposed t o darkness and humidified air or when the roots were cut. The transpir ation-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophyt es apparently resulted from (flow-dependent) concentration-polarizatio n and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly in terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21-31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradient s under transpiring conditions, and for the non-linear flow-force rela tionships in roots found by other investigators are discussed.