Abstract:Gas exchange and chlorophyll a fluorescence kinetics were used to explore photosynthetic characteristics during complete submergence and photoinhibition after subsequent re-emergence in Echinodorus amazonicus seedlings. After submergence, leaf area and stomata size of the leaves developed before submergence (referred to as “aerial leaves”) showed no difference in comparison with that of control. But “submerged leaves” developed during submergence were enlarged and its stomata size was smaller than normal plants. However, the total number of stomata in submerged leaves was almost the same as that in control. In the submerged leaves, most stomata were distributed on the upper epidermis, thus the stomata conductance and transpiration rate increased gradually. During the submergence, carbon assimilation capacity, photochemistry efficiency and chlorophyll content decreased sharply in aerial leaves but increased gradually in submerged leaves with the development and maturation. After re-emergence, the relative water content (RWC) and the maximum photochemistry efficiency of photosystem Ⅱ (Fv/Fm) dropped sharply caused by strong irradiance both in aerial and submerged leaves; but all these parameters showed little changes under the weak light. In vitro, with sufficient water supply, serious photoinhibition still occurred both in aerial and submerged leaves when exposure to strong irradiance for 6 hours and this photoinhibition was not fully recovered even in the weak light. Based on above results, we suggested that keeping relative high carbon assimilation capacity and large leaf area in submerged leaves were the key causes to survive under submergence for Echinodorus amazonicus seedlings. For aerial and submerged leaves, strong light resulted in serious photoinhibition; excessive water loss result from increased stomata conductance and transpiration rate aggravated photoinhibition under high light. The combination of the two causes lead to the death of aerial and submerged leaves after re-emergence under natural conditions.