Effect of Gibberellic Acid on the Senescence of Leaf Discs of Nasturtium (Tropaeolum majus).

Aging in leaves is characterized by a decrease in chlorophyll content and an accompanying loss of protein and ribonucleic acid. It is well known that these symptoms of senescence occur at an accelerated rate in excised mature leaves and in leaf discs floated on water. In recent years it has been established that several chemical substances retard the senescence of leaves. Among the compounds which retard senescence are the kinetins and related structures (4, 6, 8, 10, 12) and atuxins (3, 9). It is significant that the retardation of senescence by these substances varies from species to species. Thtus, auxins delay senescence in Prunus leaves but are without effect on Xanthium leaves. In contrast kinetin retards leaf senescence in Xanthium but is ineffective in Prunus (5). Recently it has been reported (2) that the gibberellins can rctard leaf senescence. However, the results are again species specific. Thus in 17 species studied, only Taraxacumn officinaile demonstrated a gibberellic acid regulation of senescence (2). During investigations in this laboratory into the senescence of leaf discs of 7ropaeolum majus it was noted that senescence could be prevented by gibberellic acid treatment. Thus the Nasturtiumn must be added to the species in which senescence is regulated by gibberellin. Although the mode of action of gibberellic acid in controlling senescence remains to be eluicidated, the response of leaf discs to gibberellin treatment has been established and is reported here. Nasturtiumr seed Tropaeolum majus (Gloriotus Gleam) were sown in a vermictulite: peat: soil mixtture (1: 1: 1) and germinated and grown in the greenhouse with a minimum temperature of 180 and natural illumination. Leaves of approximately 6 cm diameter were collected from these seedlings at intervals as required. One-cm discs were punched from the leaves, and 30 discs were floated in 20 ml of either water or gibberellic acid soluttion in petri dishes. The gibberellic acid (GA) tused