XVI International Botanical Congess

It was seen that sequential leaf senescence in sunflower seedlings may be stimulated by IAA (Sa_lam and Okatan, 1990). In order to be sure and to get more information about the role of this hormone, we have tried to control the stabilization of the IAA by the zinc (Skoog, 1940) in the seedlings that were grown Hoagland's solution with or without zinc. Results have significantly show that senescence took place in the cotyledons of the seedlings, which were grown in the solution without zinc, before than of the seedlings those grown in the solution with zinc. On the other hand, seedlings that were grown in the solution without zinc had gotten significantly taller than the seedlings that were grown in the solution with zinc. These results have shown that the amount of the zinc, which is naturally found in aken (especially in pericarp), good enough for seedlings development, and more zinc has been offered by Hoagland's solution, may somewhat be toxic for growth process of the seedlings, even it has caused delay the senescence.