Effects of NO2and O3Exposure Alone or in Combination on Kidney Bean Plants: Amino Acid Content and Composition

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Abstract
The effects of NO2 and O3 exposure alone or in combination were investigated with respect to the amino acid content and composition in kidney bean. The short-term exposure (up to 8 h) to NO2 at a concentration of 4.0 ppm alone or in combination with O3 at a concentration of 0.4 ppm induced a rapid increase in the total amino acid content among which glutamine accounted for most of the part. Total amino acid content was also increased by O3 exposure at 0.4 ppm after 2 hours’ lag period. Ammonium level became higher in the case of combined exposure to NO2 and O3, while it remained constant in the case of exposure to NO2 and O3 alone. When the exposure period was extended to 2 to 7 days (long-term exposure), the increase in the content of the total amino acids was observed in most of treatments. Roots of the plants exposed to various concentrations of NO2 and O3 showed the most remarkable increase in the content of total amino acids. Asparagine, in place of glutamine, became a major amino acid. The percentage of asparagine was especially increased by the mixed exposure to NO2 and O3 These results indicate that glutamine which accumulates considerably in the early phase of the gas exposure (short-term exposure) seems to be gradually converted into other amino acids, mainly asparagine. The correlation between the content of each amino acid, ammonium and total amino acids was calculated using data from the above experiment. Most of the amino acids in the primary and trifoliate leaves showed a high correlation with the total amino acids, suggesting that the changes in the amount of total amino acids caused by the air pollutants may be reflected not only by a particular amino acid, but also by an individual amino acid composing soluble metabolite pool. A high correlation was obtained among amino acids belonging to the serine family such as glYCine, serine, and cysteine.