Abscisic acid-induced senescence in light- and dark-incubated detached leaves of barley

Abscisic acid (ABA)-induced foliar senescence was examined using detached leaf segments of light-grown Hordeum vulgare L. cv. Dyan seedlings, incubated with or without exogenous ABA in either total darkness or under continuous illumination. In darkness, the chlorophyll (Chl) content declined rapidly, ABA levels increased sharply whereas carotenoid levels were sustained, and declined gradually during the course of senescence. By comparison, untreated leaf segments exposed to continuous illumination displayed enhanced loss of Chl and carotenoids and slower rates of ABA accumulation. In response to exogenous ABA, Chl and carotenoid content of light-incubated leaf segments declined markedly and the rate of decline increased with an increase in exogenous ABA concentration. In dark-incubated leaf segments, exogenous ABA, in the range 0.1–100μM, had little or no significant effect on the rate of Chl loss, and calculation of the Chl : carotenoid ratio revealed that dark-incubated leaf segments were insensitive to exogenous ABA. At 1mM ABA, Chl loss was delayed. It is proposed that the inability of exogenous ABA to accelerate Chl breakdown in dark-incubated leaves was caused by a transient accumulation of carotenoids, and in particular xanthophylls, possibly related to the rapid elevation in endogenous ABA levels. Furthermore, it is suggested that retardation of carotenoid breakdown is a contributing factor accounting for differences in ABA-induced Chl loss between light- and dark-incubated senescing leaves