LOW-TEMPERATURE PRETREATMENT OF THE ROOT-SYSTEM OF BRASSICA-RAPA L PLANTS - EFFECTS ON THE XYLEM SAP EXUDATION AND ON THE NITRATE ABSORPTION RATE

Brassica rapa plants were exposed for a 52h period (as pretreatment) to a differential temperature (DT) between roots (5 degrees C) and shoots (20 degrees C), while control plants were maintained with both shoot and roots at 20 degrees C (warm grown=WG). Measured at 20 degrees C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration Bows were similar in pretreated and control plants. Both transpiration and exudation Bows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate Bow from the pretreated roots was associated with a two-fold increase in nitrate uptake rate. The relationship of the cold-induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.