EFFECT OF DARKENING THE COTYLEDONS ON THE GROWTH AND CURVATURE OF THESUNFLOWER HYPOCOTYL - EVIDENCE OF HYDRAULIC SIGNALING

When both cotyledons of light-grown sunflower seedlings (Helianthus an nuus L.) were darkened by covering them with aluminium foil, the resul ting increase in the rate of elongation of the hypocotyl was closely p roportional to the associated reduction in seedling transpiration and to the increase in xylem water potential (psi(PX)). Covering only one cotyledon induced curvature away from that side. This response was ass ociated with a higher psi(PX) in the vascular bundles connected direct ly with the covered cotyledon than in those connected with the illumin ated cotyledon. The water content of peripheral tissues of the hypocot yl below the covered cotyledon was also higher than that of similar sa mples from below the illuminated cotyledon. These results are consiste nt with the hypothesis that the effect of darkening the cotyledons on the growth and curvature of the hypocotyl is mediated by hydraulic sig nalling, characterized by transmission to the hypocotyl of an increase in psi(PX) resulting from the reduction in cotyledon transpiration.