No abstract

Senescence constitutes the final stage of a plant organ and tissue development and is a subject to gene control and strict regulation. By the late growing season, when Alhagi sparsifolia entered the natural senescence period, a girdling treatment was conducted on the phloem to increase the sugar content in leaves and to investigate carbohydrate-induced leaf senescence. After the semi-girdling and full-girdling treatment, organic matter could not leave leaves due to the destruction of sieve tubes. This led to constantly increasing sugar contents in leaves. Girdling was shown to greatly accelerate the senescence of plants. In girdled leaves, chlorophyll (Chl) a, Chl b, carotenoids (Car), and both ratios of Chl a/b and Chl/Car were significantly reduced. On the donor side of PSII, the oxygen-evolving complex was damaged under high concentrations of carbohydrates, which was manifested as the emergence of the K phase in fluorescence kinetic curves. On the acceptor side of PSII, the high carbohydrate content also led to the disruption of electron transport and reduced light-use efficiency, which was manifested as a reduction in numerous fluorescence parameters. We believe that the emergence and development of plant senescence was not necessarily induced by the high content of carbohydrates, because even a decrease in the carbohydrate concentration could not arrest the senescence process. Although the high content of carbohydrates in plants was capable of inducing plant senescence, this kind of senescence was likely a pathological process, including degradations of physiological functions.