5:15 PM - 6:30 PM
[HTT16-P13] Effect of flooding stress on photosynthesis of aquatic plants
Keywords:flooding stress, carbon stable isotope method, mesophyll conductance
Introduction
How to efficiently take in CO2 from the air through photosynthesis is an important issue for plants to grow and survive in various environments on earth. However, in the natural environment, there are various environmental stresses such as high light stress, low temperature stress, and water stress, which cause a decline in photosynthetic function. Among the water stresses is the flooding stress caused by floods. Aquatic plants are subjected to significant changes in the degree of flooding, and the area in contact with the air fluctuates, resulting in large fluctuations in the amount of CO2 they can take up. Therefore, photosynthetic function is also expected to change significantly due to flooding stress. However, although many ecological studies have been conducted on aquatic plants in response to flooding stress, it is not well understood how photosynthetic function is affected. Therefore, the purpose of this study was to investigate how flooding stress affects the photosynthetic function of aquatic plants.
Material & Method
Two types of grasses were used as plant material: rice(Oryza sativa), a crop, and reeds(Phragmites australis), a wild plant that grows in colonies in lakes and marshes. The height of the grass is about 60 cm for rice and about 150 cm for reeds. Plant seedlings were planted in plastic pots and flooded to fix the water level at 60 cm, and the flooding response of photosynthetic function was investigated at three stages: 5, 10, and 15 days later.
The LI-7000 infrared gas analyzer was used to measure the following three parameters of leaf photosynthetic function: the saturation photosynthetic rate (A400) at a CO2 concentration of 400 ppm, the stomatal conductance (gs), which indicates how open the stomata were, and the mesophyll conductance (gm), which indicates how easily the CO2 taken up through the stomata passed through the mesophyll cells. The leaf conductance is a measure of carbon stability. Leaf conductance was measured using the carbon stable isotope method. In addition, the number of pores per 1 mm2 of leaf area (stomatal density) was measured using the secondary replica method.
Results
In both rice and reed species, all three parameters of photosynthetic function were significantly reduced by flooding. In other words, rice and reed showed strong responses to flooding stress, such as reduced photosynthetic rate, stomatal closure, and limited CO2 diffusion in the mesophyll cells. The rate of decrease was higher in reed than in rice for all parameters. The stomatal density of reed was 661, which was about 16 times higher than that of rice.
Discussion
The reason for the significant decrease in the three parameters of photosynthetic function in plants subjected to flooding stress is that the exposure of the plant to water caused problems in the regulation of stomatal opening and closing, which inhibited CO2 uptake. In addition, some morphological changes in the leaves may have restricted CO2 diffusion in the mesophyll cells. The reason why each photosynthetic parameter was significantly reduced in reed than in rice was because the stomatal density was greater than that of rice, and thus the limitation of CO2 uptake by flooding was greater. These results indicate that flooding is a strong stress on aquatic plants and that the degree of response of photosynthetic functions varies among plant species.
How to efficiently take in CO2 from the air through photosynthesis is an important issue for plants to grow and survive in various environments on earth. However, in the natural environment, there are various environmental stresses such as high light stress, low temperature stress, and water stress, which cause a decline in photosynthetic function. Among the water stresses is the flooding stress caused by floods. Aquatic plants are subjected to significant changes in the degree of flooding, and the area in contact with the air fluctuates, resulting in large fluctuations in the amount of CO2 they can take up. Therefore, photosynthetic function is also expected to change significantly due to flooding stress. However, although many ecological studies have been conducted on aquatic plants in response to flooding stress, it is not well understood how photosynthetic function is affected. Therefore, the purpose of this study was to investigate how flooding stress affects the photosynthetic function of aquatic plants.
Material & Method
Two types of grasses were used as plant material: rice(Oryza sativa), a crop, and reeds(Phragmites australis), a wild plant that grows in colonies in lakes and marshes. The height of the grass is about 60 cm for rice and about 150 cm for reeds. Plant seedlings were planted in plastic pots and flooded to fix the water level at 60 cm, and the flooding response of photosynthetic function was investigated at three stages: 5, 10, and 15 days later.
The LI-7000 infrared gas analyzer was used to measure the following three parameters of leaf photosynthetic function: the saturation photosynthetic rate (A400) at a CO2 concentration of 400 ppm, the stomatal conductance (gs), which indicates how open the stomata were, and the mesophyll conductance (gm), which indicates how easily the CO2 taken up through the stomata passed through the mesophyll cells. The leaf conductance is a measure of carbon stability. Leaf conductance was measured using the carbon stable isotope method. In addition, the number of pores per 1 mm2 of leaf area (stomatal density) was measured using the secondary replica method.
Results
In both rice and reed species, all three parameters of photosynthetic function were significantly reduced by flooding. In other words, rice and reed showed strong responses to flooding stress, such as reduced photosynthetic rate, stomatal closure, and limited CO2 diffusion in the mesophyll cells. The rate of decrease was higher in reed than in rice for all parameters. The stomatal density of reed was 661, which was about 16 times higher than that of rice.
Discussion
The reason for the significant decrease in the three parameters of photosynthetic function in plants subjected to flooding stress is that the exposure of the plant to water caused problems in the regulation of stomatal opening and closing, which inhibited CO2 uptake. In addition, some morphological changes in the leaves may have restricted CO2 diffusion in the mesophyll cells. The reason why each photosynthetic parameter was significantly reduced in reed than in rice was because the stomatal density was greater than that of rice, and thus the limitation of CO2 uptake by flooding was greater. These results indicate that flooding is a strong stress on aquatic plants and that the degree of response of photosynthetic functions varies among plant species.