Effect of darkening the cotyledons on the growth and curvature of the sunflower hypocotyl: evidence of hydraulic signalling

When both cotyledons of light-grown sunflower seedlings (Helianthus annuus L.) were darkened by covering them with aluminium foil, the resulting increase in the rate of elongation of the hypocotyl was closely proportional to the associated reduction in seedling transpiration and to the increase in xylem water potential (ψPX. Covering only one cotyledon induced curvature away from that side. This response was associated with a higher ψPX in the vascular bundles connected directly with the covered cotyledon than in those connected with the illuminated cotyledon. The water content of peripheral tissues of the hypocotyl below the covered cotyledon was also higher than that of similar samples from below the illuminated cotyledon. These results are consistent with the hypothesis that the effect of darkening the cotyledons on the growth and curvature of the hypocotyl is mediated by hydraulic signalling, characterized by transmission to the hypocotyl of an increase in ψPX resulting from the reduction in cotyledon transpiration.