|其他摘要||Girdling is defined as the removal of a ring of bark or phloem, mainly around the trunk or branch, which has the immediate effect of blocking the phloem transport pathway, thereby causing the accumulation of carbohydrates above the girdle and reduction of carbohydrates below the girdle. As a classic model in botany, girdling is easy to manipulate carbohydrate ratio in different organs. The research of effect of phloem girdling on plant physiological and ecological had been reported in many previous studies. However, studies of girdling on leaf water status and the mechanism of leaf senescence were rare, and the objective of our study was to provide data and basis for in this field. In this study, we chose desert plant Alhagi sparsifolia as our study materials, and we performed girdling at the base of A. sparsifolia. The aims of this study was to investigate the leaf water status and the process of leaf senescence which induced by phloem girdling, and to illustrate the chlorophyll fluorescence characteristics during senescence. Several correlated parameters of A. sparsifolia were measured, including soluble sugar content, starch content, relative water content, water potential, stomatal conductance, photosynthetic rate, transpiration rate, water use efficiency, abscisic acid content, chlorophyll content, carotenoid content, proline content, malondialdehyde content, chlorophyll fluorescence, root soluble sugar content, root starch content and root respiration rate. The results are as follows:
1. Phloem girdling first affected the accumulation of ABA in leaves and thus led to the reduction in stomatal conductance and transpiration rate, which subsequently resulted in the improvement of leaf water status. Phloem girdling then led to the accumulation of carbohydrate in the leaf and the reduction in carbohydrate in the leaves and thus resulted in the decline of root respiration rate. The decrease of water absorption by root and the accumulation of carbohydrate in leaves together may be the reason for decline of leaf water content and leaf water potential in girdled plants.
2. Phloem girdling not only induced leaf senescence of A. sparsifolia, but also accelerated the senescence process in the natural senescent leaves. In both natural senescence process and girdling induced senescence process, chlorophyll content, carotenoid content and photosynthetic rate decreased. One of the reasons for the decline of photosynthetic rate in girdled leaf may be the stomatal limitation, but from the 11th day, it was result from reduction in stomatal conductance, end-product feedback inhibition and leaf water stress. Natural senescence of A. sparsifolia was a tightly regulated process, which performed as the maintenance of activity of photosynthetic apparatus. However, when induced by phloem girdling, the process of leaf senescence cannot be effectively controlled, which was showed as the significant decline of chlorophyll a/b, the damage of oxygen-evolving complex in the donor side of photosystem II(PSII), and the significant reduction of maximum photochemical efficiency of PSII.
3. In the process of natural leaf senescence and girdling induced leaf senescence process, PQ sink which was in the acceptor side of PSII reaction centers (Sm) reduced, and then resulted in the ratio of the energy absorbed in the reaction center used for electron transfer (φEo) reduced. In addition, the degree of open access in the active reaction centers (ψo) reduced, which led to more light was used to restore QA, thereby resulted in overall performance and activity of PSII (PIabs) decreased. In the senescence process, the number of active reaction center in unit area (RC/CS) reduced, which resulted in the absorption of light energy in per reaction center (ABS/RC) increased. However, with the reduction of PSII activity, the energy which captured by reaction (TRo/RC) and the energy used for transfer of electron (ETo/RC) reduced, which led to more energy dissipated as the form of fluorescence and heat, shown as decline of DIo/RC. The increase of non-photochemical quenching leads to energy stress, which resulted in the increase of reactive oxygen species during senescence, and thus leads to membrane lipid peroxidation and production of malondialdehyde. During senescence, plants retain more carotenoids (Car) to absorb more surplus energy and quench reactive oxygen species so that prevent cells from membrane lipid peroxidation and maintain permeability of cell membrane. Compared to natural senescence, changes of PSII reaction centers appeared to be more severe in the girdling-induced senescence process. This result suggested that compared to the slowly conducted natural senescence process, girdling induced senescence is a more severe process.
In short, phloem girdling first improved the water status in A. sparsifolia leaves, and then the water status will decline. Girdling can induce A. sparsifolia leaf senescence, and in the process of leaf senescence, the energy absorbed by reaction center increased, however, most of the energy was used for heat dissipation, which resulted in a stress pressure and light stress in photosystem II.|