IRIS HEXAGONA HORMONAL RESPONSES TO SALINITY STRESS, LEAFMINER HERBIVORY, AND PHENOLOGY

Abstract

We used field, common garden, and laboratory studies to investigate the influence of environmental stress on a native wetland species, Iris hexagona (Iridaceae), and an iris leafminer, Cerodontha iridiphora (Diptera: Agromyzidae). There were strong effects of salinity, herbivory, and phenology on jasmonic acid (JA) and salicylic acid (SA), two important signal phytohormones. Within 48 h, foliar JA increased an order of magnitude, and SA declined 50% in plants grown under salinity stress compared to freshwater controls. Leafminer performance declined on plants exposed to the salinity treatment. Both herbivory and salinity stress induced JA and suppressed SA, indicating that plants can respond to abiotic and biotic stress through shared hormonal pathways. Surprisingly, leafminer pupae contained JA and SA, and we discuss the implications of this novel discovery. Unrelated to salinity, there was a sharp decline in foliar JA between April and September. Despite its strong impact on hormonal signaling in plants, the abiotic environment is often ignored in ecological studies of induced plant defenses. Importantly, natural seasonal cycles and/ or environmental stress can induce stronger changes in phytohormones than herbivory, and should be incorporated into the design of ecological experiments whenever possible, for a balanced understanding of phytohormonal ecology.