Enhancement of the inhibitory activity for greening in xylem sap of squash root with waterlogging

Squash plants are used as rootstock for cucumber plants to prevent soil-borne disease. The chlorophyll content in the cotyledons of cucumber was observed in the present study to decrease in response to grafting onto squash rootstock. The decrease in chlorophyll content was enhanced by the waterlogging of squash root. To prove the hypothesis that root-derived chemical signals are involved in the shoot behavior of waterlogged plants, the xylem sap collected from waterlogged squash root was analyzed for activity related to the greening of etiolated cucumber cotyledon. When the fractions of xylem sap separated on a Sephadex G-25 column were subjected to the greening assay, a much stronger inhibition of greening was observed in the xylem sap obtained from waterlogged squash plants than in that from control plants at the position corresponding to an apparent molecular mass of 1 400. These results suggest that a novel higher molecular mass chemical signal in xylem sap might mediate between shoots and roots in waterlogged plants.