Mechanism By Which The Dark-Submergence Treatment Damages Photosynthetic Apparatus Of Plant Leaves

Submergence is a common environmental stress for plants. It seriously hampers growth and survival of plants. In the agricultural industry, submergence decreases crop yield by inhibiting the growth of crop, and even causes plant death. Studies have shown that the decline of the growth rate is due to the inhibition of photosynthesis and aggravating photoinhibition. Changes of photosynthesis and photochemical efficiency of plants under submergence stress have been widely studied. Most previous studies showed that the decline in activity of the photo synthetic apparatus is induced, firstly, by the inhibition of root respiration by low O2concentration in submerged soil, which decreases the absorptive capacity for water and nutrients; secondly, by the stomatal closure and leaf senescence caused by harmful metabolites and signals such as ABA and ethylene transported from hypoxia submerged roots of plants during continuous submergence of soil; thirdly, by the low CO2concentration in water in the presence of light inhibits photosynthetic carbon assimilation, triggering excess excitation energy and leading to accumulation of reactive oxygen species that cause photoinhibition. However, it has never been reported so far that whether mere leaf submergence affects photosynthetic apparatus when plant leaves are submerged in the water without the influence of the above mentioned factors. To address this question, we explored the function of the photosynthetic apparatus in detached leaves submerged in the dark (dark-submergence treatment, DST), which is a method of avoiding the influence of excess excitation energy and signals from the roots to the photosynthetic apparatus. The main results are as follows:(1) The photosynthesis rate and maximum quantum yield of PSII of leaves significantly decreased after dark-submergence treatment at room temperature, which indicates that mere submergence could damage the photosynthetic apparatus and decline the photochemical activity of leaves. It was referred as a "photoinhibition-like" phenomenon that is different from photoinhibition.(2) The process of dark-submergence treatment of detached leaves was accompanied by increase of relative variable fluorescence of J (2ms), K (300μs) L (100～120μs) steps in the OJIP transients and decline of the density of QA-reducing PSII reaction centers per cross section (RC/CSm), maximum quantum yield of PSII (Fv/Fm), efficiency of electron moves beyond QA-(Ψo) and PSI complex activity. This result indicates that the activities of donor side, reaction centre and acceptor side of PSII and PSI activities of leaves were inhibited during the dark-submergence treatment.(3) Similar to traditional photoinhibition induced by high light, the level of D1protein, the core protein of PSII, decreased obviously during the dark-submergence treatment. In addition, level of PsaA, the PSI complex core protein, also decreased.(4) After dark-submergence treatment at different temperature (15℃,25℃and35℃), the decline of photochemical activities of leaves was aggravated with increase of temperature, and the oxygen content in the water also changed similarly. In addition, the photochemical activities of leaves maintained unchanged when pumping air into the water simultaneously during the dark-submergence treatment; the declines of photosynthetic apparatus of leaves still occurred when leaves were placed in a humid N2condition in the dark to exclude the physical damage of water; the photosynthetic apparatus damages were recovered when pumping air into water after36h of dark-submergence treatment, but could not be recovered when pumping N2. Moreover, the chlorophyll content maintained unchanged and the H2O2content did not increase significantly during the dark-submergence treatment. The results above indicate that the damages to photosynthetic apparatus in leaves during the dark-submergence treatment were not caused by leaf senescence and over-accumulation of ROS, but due to hypoxia condition of the water.This study verified that the mere submergence can damage the photosynthetic apparatus of submerged leaves except root signals and excess excitation energy, and this damage is caused by hypoxia condition of the water. In other words, the hypoxia condition of water can damage the leaf photosynthetic apparatus directly when leaves are submerged in the water.