ROLE OF THE COTYLEDONS IN THE MAINTENANCE OF HYPOCOTYL GROWTH IN HELIANTHUS-ANNUUS L

The effect of cotyledon excision on hypocotyl growth was investigated in sunflower seedlings (Helianthus annuus L.) that were grown in continuous white light. Removal of both cotyledons resulted in an inhibition of hypocotyl elongation and cessation of primary leaf development. Cell osmotic pressure, the amount of soluble sugars and wall plasticity declined in the hypocotyls without cotyledons and the accumulation of cell wall material stopped. In contrast, the excision of one cotyledon did not affect hypocotyl extension and the development of the primary leaves. Cell osmotic pressure was unaffected in the growing region of the stem, in spite of the fact that the accumulation of soluble sugars was reduced by 50 %. After the removal of one cotyledon a transient inhibition of the accumulation of cell wall material occurred, but no effect on cell wall extensibility was detectable. It is concluded that cell elongation and maintenance of wall plasticity can occur without net increase in cell wall material.