The effect of epibrassinolide on senescence in horizontal sunflower (Helianthus annuus L.) seedlings

This study examined the effect of epibrassinolide (eBL) on senescence occurring in cotyledons of sunflower ( Helianthus annuus L.) seedlings, which were kept in vertical and horizontal positions. 10 -11 M and 10 -9 M eBL and 2,3,5-triiodobenzoic acid (TIBA), an inhibitor of auxin transport, were sprayed to the 14 days old seedlings. From the 15 th day on, some of the seedlings from the control and the experimental groups were kept in a horizontal position, and senescence process of the cotyledons of these seedlings was observed. Applications of eBL (especially 10 -9 M) were found to have induced senescence both in horizontally and vertically positioned plants. When it was applied with TIBA, a marked delay of senescence was noted in the lower cotyledons of the horizontally positioned plants. Total chlorophyll and protein amounts and peroxidase activity were determined. In case brassinosteroids are applied with TIBA, they do not affect senescence, implying that they accelerate senescence in the presence of auxin. Since auxin is distributed asymmetrically, eBL and auxin accumulated in the lower cotyledons may lead to accelerated senescence. This study showed that eBL and auxin may act as a senescence signal in the vertically and horizontally positioned seedlings, and earlier death of the lower cotyledons than the upper ones may be a result of asymmetrical auxin distribution. In this study, it was revealed that promoting effect of eBL on senescence occured only in the presence of auxin. Keywords: eBL – Epibrassinolide, TIBA - 2,3,5-triiodobensoic acid, Auxin, Senescence, Sunflower ( Helianthus annuus L.). *Corresponding Author: Serap Saglam-Cag (e-mail:sercag@istanbul.edu.tr) (Received: 13.09.2012 Accepted: 04.03.2013)