Membrane Degradation, Accumulation of Phosphatidic Acid, Stimulation of Catalase Activity and Nuclear DNA Fragmentation during 2,4-D-Induced Leaf Senescence in Mustard

We investigated 2,4-D-induced leaf senescence in young mustard seedlings. A set of morphometric, biochemical and molecular parameters were analyzed to characterize senescence markers. In accordance with earlier reports, chloroplast-membrane degradation marked the early phase of leaf senescence based on the analysis of the galactolipid fraction. Degradation of grana occurred earlier to that of the envelope, as revealed by the relative level of their specific galactolipids, namely, monogalactosyl diglyceride and digalactosyl diglyceride. Phospholipids showed extensive degradation resulting in the accumulation of Iyso-derivatives of major phospholipids and phosphatidic acid (PA) in senescing leaves. Catalase activity was stimulated by 2,4-D and reflected scavenging of reactive oxygen species. Nuclear DNA degradation, a previously known death signal that represented a point of no return from progression of senescence, occurred late on the 4th day subsequent to 2,4-D supplementation. $AgNO_3$, an inhibitor of ethylene biosynthesis, inhibited leaf senescence by ca. $54\%$ based on PA content. Involvement of 2,4-D, ethylene and abscisic acid in leaf senescence is discussed in relation to hormonal interplay.