Apple Fruitlet Ethylene Evolution and MdACO1, MdACS5A, and MdACS5B Expression after Application of Naphthaleneacetic Acid, 6-Benzyladenine, Ethephon, or Shading

Abstract

Abscission of apple (Malus ×domestica Borkh.) fruitlets is associated with increased ethylene evolution, although the role of ethylene is not clear. Fruitlet ethylene evolution and expression of ethylene-related MdACO1, MdACS5A, and MdACS5B genes were followed in the abscission zone (AZ) and fruit cortex (FC) of king and lateral fruitlets of ‘Golden Delicious’/M.9 after spraying with 15 mg·L−1 naphthaleneacetic acid (NAA), 150 mg·L−1 6-benzyladenine (BA), 500 mg·L−1 ethephon, and after 3 days 96% shading, all applied at a mean 10 mm fruitlet diameter. Lateral fruitlets (LF) were more susceptible to abscission after treatment with chemical thinners or shading compared with king fruitlets (KF) in which only ethephon-treated KF abscised later. On KF, only ethephon increased fruitlet ethylene evolution, MdACO1 expression in FC and AZ 8 days after treatment (DAT), and abscission of KF 10 DAT, but the other treatments did not. Although ethephon increased MdACO1 expression in FC of KF 8 DAT, the expression of MdACS5A and MdACS5B remained unchanged. On LF, all treatments at 8 DAT increased ethylene evolution and MdACO1 expression in FC and enhanced abscission 10 to 22 DAT. Expression of MdACS5A and MdACS5B in FC of LF 8 DAT was expressed at a lower level compared with MdACO1. In the AZ of both KF and LF, only ethephon increased expression of MdACO1, MdACS5A, and MdACS5B.