ROOT-TO-SHOOT COMMUNICATION IN FLOODED PLANTS - INVOLVEMENT OF ABSCISIC-ACID, ETHYLENE, AND 1-AMINOCYCLOPROPANE-1-CARBOXYIC ACID

Nonhydraulic communication between roots and shoots of flooded plants is reviewed. Contrary to reports for roots of droughted plants, those of flooded tomato plants (Lycopersicon esculentum L.) did not export increased amounts of abscisic acid (ABA) to the shoots, even stomata closed in association with increased foliar ABA, and in the absence of marked losses in leaf hydration. Flooded tomato root systems did export more of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene formed in the leaves by oxidation of the ACC to ethylene and the epinastic leaf curvatures the ethylene induced were found to be inhibited in tomato plants transformed with an antisense gene to the ethylene-forming enzyme ACC oxidase. Our estimates of the concentration and delivery of ABA and ACC in the xylem sap as it passes into the shoot from the roots avoided artifacts associated with variable sap flow rates and temporary contamination from wounded tissue. Measurements of ethylene production by individual roots of intact seedlings of maize (Zea mays L.) by highly sensitive photoacoustic laser detection confirmed earlier work linking enhanced ethylene evolution with more extensive aerenchyma development in roots that are partially deficient in oxygen. The development of aerenchyma enhances oxygen movement from shoots to roots.