EFFECTS OF SYNTHETIC PLANT-GROWTH RETARDANTS AND ABSCISIC-ACID ON ROOT FUNCTIONS OF BRASSICA-RAPA PLANTS EXPOSED TO LOW ROOT-ZONE TEMPERATURE

Possible interactions of two synthetic plant-growth retardants during the short-term response of Brassica rapa L. ssp. oleifera (DC.) Metzge r plants to low root-zone temperature were investigated by pretreating with mefluidide or paclobutrazol. Water and solute transfers were stu died by measuring xylem sap volume flow (under root pressure exudation ) and ion flow from the roots. Relations with nitrate uptake rate were also considered. Root pretreatment with paclobutrazol strongly restri cted the cold-inducible processes which normally restore water and sol ute flow from the root xylem. Paclobutrazol decreased the rates of nit rate uptake and exudation flow from the root xylem (principally by red ucing root hydraulic conductivity) with dramatic consequences for ion flow, especially that of nitrate. The effects of root ABA pretreatment on plant response to root cooling were then studied separately or in association with a pretreatment with paclobutrazol. Despite a slight d ecrease in nitrate uptake rate, ABA pretreatment of the roots enabled the plant to develop rapid mechanisms for adaptation to cold constrain t at the root level. Moreover, this action of exogenous ABA greatly re duced the effect of a simultaneous paclobutrazol pretreatment and part ly restored water and solute flows. Thus, the improvement of plant res istance to cold conditions brought about by treatments with mefluidide and paclobutrazol (previously shown in long-term experiments) cannot simply be explained by their short-term effects.