Physiological And Ecological Adaptation And Mechanism Of Submerged Plant Vallisneria Natans To Changes In Water Level

As important primary producer in the freshwater ecosystem,submerged plants play an important role in maintaining the structure and function of the ecosystem.It has become a key measure for ecological restoration to recover submerged plants in eutrophic shallow lakes.Water level is one of the most important factors that affects the growth,development and reproduction of submerged plants.In the natural state,the water level of shallow lakes often fluctuates seasonally,but global warming and human activities have excessively disturbed the natural water level of shallow lakes,resulting in abnormal water levels of shallow lakes.Therefore,it is necessary to explore the response of submerged plants to the process of water level variation as well as the adaptation strategies and mechanisms to different water levels.It also has important guiding significance for the reconstruction and management of submerged plants in freshwater lake ecosystems.For upright or canopy-forming submerged plants,the distance between the leaves and the water can be shortened by elongating the stem to obtain sufficient light as the water depth increases.However,the rosette-forming submerged plant has no erect stems and cannot simply adapt to the water depth by changing the length of the stem.How to adapt to extreme water level environments through physiological and ecological adjustments has more ecological research value.In addition,there is currently a lack of corresponding functional molecular level assessments of the ecological adaptation mechanisms of submerged macrophytes in shallow lakes under different water levels.Functional genomics can reveal the adaptation mechanisms of plants in specific environmental conditions at different scales,which can further deepen our understanding of the adaptability of submerged plants under different water levels Vallisneria natans,the common submerged plant in shallow lakes in the middle and lower reaches of the Yangtze River,was selected as the research species.An in situ experimental platform was constructed in a lake,V.natans were planted in flowerpots and hung in the water.By adjusting the hanging height of the flowerpots,different water depths and water level changes are simulated to explore the effects of water levels(water depth gradients)and water level fluctuations on the growth,leaf functional traits,physiological traits,differential gene expression and metabolites of V.natans,in order to clarify how the growth and physiological traits respond to different water levels and water level fluctuations.In addition,this study attempts to reveal the physiological and ecological adaptation strategies and mechanisms of V.natans in the environment of extreme water level and water level fluctuations.The main conclusions are as follows:(1)In the rapid growth stage(starting from the second week),most of the growth traits of V.natans began to show a steady response trend that gradually increased with the weakening of underwater light intensity(increasing water depth).Among them,the number of leaves,plant height,leaf thickness and leaf area of V.natans gradually increased with the decrease of underwater light intensity during the rapid growth stage,and the maximum value was all at 5% of underwater light intensity(360 cm water depth).The number of ramets showed a trend of increasing first and then decreasing.The physiological characteristics of V.natans showed different response patterns to underwater light intensity(water depth)in different periods.In the seedling stage(the first week),the chlorophyll of V.natans increased with the weakening of underwater light intensity,and there was no significant oxidative stress.In the rapid growth stage(starting from the second week),chlorophyll first decreased and then increased with the decrease of underwater light intensity(water depth increased),while the activities of antioxidant enzymes were on the contrary.With the increase of the cultivation time,the growth traits of V.natans showed a gradually stable response trend,indicating that the adaptation of growth traits is a continuous process.The physiological traits of V.natans are more sensitive to the environment and represent the response of plants in a short period of time.(2)The most suitable underwater light intensity for the growth of V.natans is 40%-10%,too strong underwater light(≥85%,extremely shallow water)or too weak(≤5%,deep water environment)are not conducive to the growth of V.natans.In the range of40%-10% underwater light intensity,it has relatively high total biomass and aboveground biomass,and there is little variance of most of the growth and physiological traits of V.natans in this range during different periods of the experiment.In addition,at the end of the experiment,there is no oxidative stress reaction in the leaves in this range.Based on the points above,the range of 40%-10% underwater light intensity is the most suitable condition for the restoration of V.natans in shallow lakes in the middle and lower reaches of the Yangtze River.In addition,for water level fluctuations,we should mainly focus on the process of the water level from low to high.(3)In an extremely low water level of 20 cm,the underwater light intensity is 85%.In order to avoid photosynthetic damage caused by high light intensity,the plant height,leaf area and photosynthetic pigment content in the leaves(chlorophyll a,chlorophyll b,carotenoids)are low,while the antioxidant enzyme activity and soluble sugar content are high,indicating that extremely low water levels can cause peroxidative damage to V.natans.In addition,the leaf dry matter content of V.natans under extremely low water level environment is high,indicating that V.natans has invested a lot of material energy for physical defense.There were 2028 differentially expressed genes of V.natans in the extremely shallow water level(underwater light intensity of 85%,water depth of20 cm)compared with the optimum water level(underwater light intensity of 40%,water depth of 110 cm),and the differentially expressed genes in the process of photosynthetic electron transport chain,photosynthesis,and carotenoid biosynthesis pathways were the most significant.In addition,the relative value of carbohydrates of V.natans in the extreme low water level of 20 cm is high,indicating that the underwater light is strong(underwater light intensity is 85%)and plants need to invest more materials for photosystem protection.(4)In an extremely high water level environment(water depth of 360 cm,underwater light intensity of 5%),V.natan increased plant height,leaf area,number of leaves,number of ramets and pigment content(chlorophyll a,chlorophyll b,carotenoids)to improve the efficiency of photosynthesis and maintain normal growth.The activity of antioxidant enzymes in V.natan was low in the extremely high water level environment,but the content of malondialdehyde was significantly higher than other water levels,indicating that V.natan suffered strong oxidative damage at the extremely high water level.Compared with the suitable water level,the extremely deep water level(underwater light intensity of 5%,water depth of 360 cm)has 1536 differentially expressed genes,the differential gene expression is the most significant in the process of synthesis,metabolism of polysaccharides,plant circadian rhythm,and photosynthesis pathways.However,the circadian rhythm pathway is significantly expressed in the extreme deepwater environment,which helped the biological clock to match the changes of daily irradiation level to weigh photosynthetic efficiency.(5)The gradual weakening of underwater light intensity is the main stress of V.natans caused by rising water levels.As the water level increases,in order to adapt to the gradually weakening underwater light environment,the plant height,specific leaf area,and photosynthetic pigment content(chlorophyll a,chlorophyll b,carotenoids)of V.natan increased significantly,while the leaf dry matter content decreased significantly.It shows that the distribution of material and energy follows the optimal distribution strategy,that is,the material and energy are allocated first for resource acquisition(high photosynthesis efficiency),rather than physical defense.In addition,the activities of superoxide dismutase and catalase are significantly increased to eliminate excess reactive oxygen free radicals.The water level increase from 110 to170 cm.There are 1744 differentially expressed genes in V.natans,which are significantly expressed in the process of photosynthesis-light capture,photosynthetic electron transport chain and carbohydrate metabolism,and are mainly enriched in the photosynthesis-antenna protein pathway.It shows that the increase of water level affects not only the photosynthesis related process of V.natans,but also carbohydrate metabolism.(6)The decrease of water level gradually increased the underwater light intensity and improved the habitat of V.natans.As the water level decreases,the total biomass,aboveground biomass,leaf number,leaf thickness,specific leaf area and dry matter content of V.natans are non-significantly different from those of the control,indicating that V.natans does not need to allocate material energy to enhance the defense mechanism of the leaves.The antioxidant enzyme activity and the concentration of malondialdehyde are also no different from the control,indicating that the V.natans is not damaged by peroxidation,so the process of water level drop would not stress the V.natans.There are 857 and 105 differentially expressed genes in V.natans to water depth rising from 80 to 20 cm and from 110 to 50 cm.These genes are most prominently expressed in the process of chloroplast RNA,photosynthesis-light capture,and are mainly enriched in the carotenoid synthesis and photosynthesis pathways.