11:00 AM - 1:00 PM
[HTT18-P04] Evaluation of photosynthetic function and pollution level on leaf surface of mid-shrub street trees at collection sites with different traffic volumes in Kyoto City
Keywords:carbon stable isotope ratio, environmental stress, photosynthesis
[Objective]
Street trees are expected to perform various functions such as air purification by adsorbing air pollutants, carbon dioxide fixation, and prevention of temperature increase by forming green shade. However, street trees planted along roads are constantly exposed to a variety of stresses. Therefore, it is conceivable that if stress from the urban environment is applied for a long period of time, physiological aspects such as photosynthesis will be suppressed, and street trees will not be able to fulfill their expected functions. However, since there are large interspecies differences in plant stress tolerance, it is expected that some tree species are more tolerant to the various stresses that occur in urban environments than others.
In this study, we evaluated the physiological characteristics such as photosynthetic rate of street trees in the sampling sites with different traffic volumes and aimed to help select tree species with expected functions as street trees.
[Materials and Methods]
We focused on exhaust gas and dust from vehicle traffic as a factor of environmental stress in the city. Leaves of street trees were collected at four sampling points: two sites with a daytime traffic volume of about 1,0000 vehicles, one site with a traffic volume of about 24,000 vehicles, and one site with a traffic volume of more than 40,000 vehicles. The samples were collected from June to August, and the experimental plants used were Rhododendron pulchrum and Rhaphiolepis indica var. umbellata. Photosynthetic rate (A400) and stomatal conductance (gs) were measured on the leaves of the street trees collected at the study site. From the measured data, A-Ci curves were constructed and the maximum carboxylic acceleration (Vcmax), electron transfer rate of thylakoid membranes (J) and water use efficiency (WUEi) were calculated. In addition, the degree of contamination and injury on the leaf surface and the long-term water use efficiency by carbon stable isotope fractionation were calculated. The gs is an index of stomatal openness, with larger gs indicating more open stomata. WUEi is a parameter that indicates the amount of carbon dioxide gained relative to the amount of water lost by transpiration. WUEi is an indicator of stress, since it is high due to the strong effect of stomatal closure caused by stress and other factors that reduce the amount of transpiration.
[Results]
In azalea, A400 and WUEi were lower, and carbon stable isotope fractionation was higher at high traffic sites in June. In yaddo hawthorn, Vcmax, J and carbon stable isotope fractionation in June were higher at sites with higher traffic. In azalea, the degree of leaf surface contamination and injury tended to increase with increasing traffic, but this trend was not observed in yaddo hawthorn.
In the August measurements, there were no parameters that showed a changing trend with the increase in traffic for either tree species.
[Discussion].
There were many parameters that showed different results among the study sites in June for both tree species for physiological aspects. Photosynthetic rate of azalea was negatively correlated with traffic volume, suggesting the effect of vehicle traffic on photosynthetic function. However, by August, the parameters were no longer significantly different. In June, A400 and WUEi of azalea became lower as the traffic increased, suggesting that the biochemical function of azalea may be damaged in the sampling sites with high traffic. Leaf surface staining and damage was only observed in azaleas with trichomes, suggesting that trichomes may be one of the factors that negatively affect the physiological aspects of azaleas.
Street trees are expected to perform various functions such as air purification by adsorbing air pollutants, carbon dioxide fixation, and prevention of temperature increase by forming green shade. However, street trees planted along roads are constantly exposed to a variety of stresses. Therefore, it is conceivable that if stress from the urban environment is applied for a long period of time, physiological aspects such as photosynthesis will be suppressed, and street trees will not be able to fulfill their expected functions. However, since there are large interspecies differences in plant stress tolerance, it is expected that some tree species are more tolerant to the various stresses that occur in urban environments than others.
In this study, we evaluated the physiological characteristics such as photosynthetic rate of street trees in the sampling sites with different traffic volumes and aimed to help select tree species with expected functions as street trees.
[Materials and Methods]
We focused on exhaust gas and dust from vehicle traffic as a factor of environmental stress in the city. Leaves of street trees were collected at four sampling points: two sites with a daytime traffic volume of about 1,0000 vehicles, one site with a traffic volume of about 24,000 vehicles, and one site with a traffic volume of more than 40,000 vehicles. The samples were collected from June to August, and the experimental plants used were Rhododendron pulchrum and Rhaphiolepis indica var. umbellata. Photosynthetic rate (A400) and stomatal conductance (gs) were measured on the leaves of the street trees collected at the study site. From the measured data, A-Ci curves were constructed and the maximum carboxylic acceleration (Vcmax), electron transfer rate of thylakoid membranes (J) and water use efficiency (WUEi) were calculated. In addition, the degree of contamination and injury on the leaf surface and the long-term water use efficiency by carbon stable isotope fractionation were calculated. The gs is an index of stomatal openness, with larger gs indicating more open stomata. WUEi is a parameter that indicates the amount of carbon dioxide gained relative to the amount of water lost by transpiration. WUEi is an indicator of stress, since it is high due to the strong effect of stomatal closure caused by stress and other factors that reduce the amount of transpiration.
[Results]
In azalea, A400 and WUEi were lower, and carbon stable isotope fractionation was higher at high traffic sites in June. In yaddo hawthorn, Vcmax, J and carbon stable isotope fractionation in June were higher at sites with higher traffic. In azalea, the degree of leaf surface contamination and injury tended to increase with increasing traffic, but this trend was not observed in yaddo hawthorn.
In the August measurements, there were no parameters that showed a changing trend with the increase in traffic for either tree species.
[Discussion].
There were many parameters that showed different results among the study sites in June for both tree species for physiological aspects. Photosynthetic rate of azalea was negatively correlated with traffic volume, suggesting the effect of vehicle traffic on photosynthetic function. However, by August, the parameters were no longer significantly different. In June, A400 and WUEi of azalea became lower as the traffic increased, suggesting that the biochemical function of azalea may be damaged in the sampling sites with high traffic. Leaf surface staining and damage was only observed in azaleas with trichomes, suggesting that trichomes may be one of the factors that negatively affect the physiological aspects of azaleas.