The effects of GR24 and auxin on cotyledon senescence in sunflower (Helianthus annuus L.) seedlings grown under salt stress

Indole acetic acid (IAA) is a plant hormone that controls plant development and growth. GR24, a synthetic strigolactone, has recently been discovered and included in the growth regulator group due to its chemical structure and its effect on plant metabolism. Senescence is a programmed death process that involves the loss of some cells, tissues or organs of the plant during its developmental stage. The senescence process is affected by endogenous factors like genetic and hormonal, and exogenous factors that consist of various stress conditions. Increased salt content is one of the abiotic stress conditions. Salt stress prevents the growth and development process of plants due to toxic effects and accelerates the senescence process. The aim of this study is to investigate the effects of IAA and GR24 on senescence of cotyledons of seedlings grown under normal and salt stress conditions and to obtain information about senescence mechanism. For this purpose, sunflower seedlings (Helianthus annuus L.), which are frequently grown in Turkey and have the characteristic of sequential leaf senescence starting from cotyledon, were used in the study. One group of developing sunflower seedlings was irrigated with Hoagland solution and 150 mM NaCl (salt) solution was applied to the other group. IAA was applied exogenously by spraying to the surface of some of the seedlings which were irrigated with Hoagland solution and exposed to salt stress, and GR24 was applied to the other half. Senescence degrees of cotyledons of these seedlings, ie green area percentages were determined. On the day when the green area percentage of cotyledons of seedlings grown in Hoagland solution was 50, all cotyledons were harvested. After that fresh weight, pigment contents, total protein, malondialdehyde (MDA) and proline levels, peroxidase enzyme activities of cotyledons were determined. As a result of the analysis, senescence in cotyledons of seedlings grown in Hoagland medium was generally encouraged by IAA and partly by GR24. The application of IAA to the cotyledons of seedlings grown in salt medium significantly delayed the senescence and the GR24 application partially delayed the senescence. IAA and GR24 have generally been shown to mitigate the toxic effect of salt. In the light of all this information, the role of IAA and GR24 in the senescence process was investigated. It is thought that the basic information to be obtained if the senescence is delayed will help to increase the fruit and vegetable yield in agriculture.