Effects of NO2and O3Alone or in Combination on Kidney Bean Plants (Phaseolus vulgarisL.): Growth, Partitioning of Assimilates and Root Activities

Abstract

Ten-day old kidney bean plants (Phaseolus vulgaris L. cv. Shin-edogawa) were exposed to 2.0 and 4–0 parts 10⁻⁶ NO2, and 0.1, 0.2, and 0.4 parts 10⁻⁶ O3 alone or in combination for 2, 4, and 7 d. The effects of these air pollutants were examined with respect to the growth, partitioning of assimilates, nitrogen uptake, soluble sugar content, and root respiration. Decreased dry matter production was significant with all treatments except 2.0 parts 10⁻⁶ NO₂ and 0.1 parts 10⁻⁶ O₃. Exposure to mixtures of the gases produced more severe suppression of growth than exposure to the single gases. Root/shoot ratio was significantly lowered at 7 d by the gas treatments other than 2.0 parts 10⁻⁶ NO₂ and 0.1 parts 10⁻⁶ O₃. The total nitrogen content of plants was increased by all treatments; the higher percent of nitrogen found with O₃ exposure will result from the growth retardation which increases the concentration of nitrogen in the plants because the absorption of nitrogen by roots was unaffected. The combination of O₃ with NO₂ significantly decreased the assimilation of NO₂ by the plants. The concentration of soluble sugars in roots was decreased by the gas treatments. There was a strong positive correlation between soluble sugar content and dry weight of the roots harvested at 7 d. Root respiration was relatively unchanged until 5 d and then decreased significantly at 7 d by 2.0 parts 10⁻⁶ NO₂ and 0–2 parts 10⁻⁶ O₃. Retarded growth of the roots and the decreased root respiration may be due to diminished translocation of sugars from leaves to roots caused by exposure to air pollutants. The uptake of soil nitrogen was not closely related with root respiration in the case of O₃ exposure.