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Interactions between NO2 absorption and nitrogen metabolism in soyabean plants

Qiao, ZhengDao (1998) Interactions between NO2 absorption and nitrogen metabolism in soyabean plants. PhD thesis, Murdoch University.

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NO2 is a common atmospheric pollutant. It can enter plants through leaf absorption and participate in general N metabolism to ameliorate N-deficiency in plants. However, NO2 exposure sometimes exerts detrimental effects on plant growth. The hypotheses to explain the mechanism of these detrimental effects have remained inconsistent.

Besides formation of nitrate and nitrite as an additional N supply to plants, NO2 absorption by leaves also brings H+ into plants. Almost every process of nitrogen uptake and assimilation, (such as uptake of nitrate and ammonium, fixation of N2, absorption of NO2, reduction of nitrate and nitrite, and synthesis of amino acids from ammonium), involves production or consumption of H+. Therefore NO2 absorption and N metabolism may interfere with each other through changes in concentration of H+, which take part in reactions of N metabolism and influence activity of the enzymes catalyzing biochemical reactions in N metabolism of plants.

This study aims to specify the effects of NO2 exposure on nitrogen uptake and metabolism in soybean plants, and to explore the mechanistic links between the acidity changes caused by NO2 absorption and N metabolism, and the effects of NO2 on N metabolism and growth of plants.

The study was conducted mainly through exposure experiments. Plants were supplied with ammonium or 15N-labelled nitrate, and exposed to NO2 or clean air. H+ concentrations and concentrations of the products of N uptake and metabolism in plants and growth mediums were measured and compared between exposed plants and control plants.

These exposure experiments have explored the effects and relevant mechanisms of NO2 exposure on uptake and metabolism of nitrate and ammonium, on translocation and distribution of N within plant organs, on activity of N2-fixation, as well as the effects of N supplies to roots on NO2 absorption by leaves. In addition, two new calculation methods were proposed to improve the estimation methods for NO2 absorption by plants.

Exposing soybean plants, supplied with nitrate, to high levels of NO2 (1.1 µl 1-1) caused accumulations of ammonium and nitrate in leaves, decreases in nitrate uptake by roots and in the amount of organic N per plant, and increases in the acidities of both leaves and nutrient solution. These results suggest that an increase in H+ from NO2 absorption in exposed plants may inhibit ammonium assimilation, and convert HCO3- to H2CO3 in roots, causing the decrease in nitrate uptake by roots. The H+ from NO2 absorption may be neutralized by the OH- produced from nitrate reduction, or it may exude into the growth medium through roots. If the H+ cannot be promptly removed from exposed plants, the H+ accumulation may inhibit general assimilation and uptake of nitrate.

NO2 absorption, as a supplement to N supply, can partly meet the N demand of plants; and the N derived from NO2 absorbed by leaves was mainly located in leaves. Consequently NO2 exposure of plants supplied with nitrate usually decreases the nitrate transport from roots to leaves and the redistribution of N within plants.

Exposing the soybean plants, grown with ammonium, to 0.2 - 0.25 µl 1-1 of NO2 increased ammonium concentration in leaves, slowed down the rise in leaf acidity from ammonium assimilation, and decreased ammonium uptake by roots. These results suggest that reduction of the nitrate and nitrite, produced from N02 absorption, consumes H+ and competes for energy with ammonium assimilation which produces H+. It inhibits the increase in concentration of H+ from ammonium assimilation in exposed plants. As plants take up ammonium through exchange of the H+ in roots with the ammonium in nutrient solution, the lower H+ concentration in exposed plants decreases ammonium uptake by roots, compared with control plants. This inhibition of ammonium uptake may further restrict ammonium assimilation and H+ increase in exposed plants.

Long-term exposure of soybean plants to 0.2 µl 1-1 of NO2 increased concentrations of nitrite and H+ in root nodules, and inhibited activity of nitrogenase. Nitrate supply to roots intensified the nitrite accumulation, decreased the acidity in nodules, and alleviated the inhibition of nitrogenase activity. These results suggest that the H+ from N02 absorption accelerates reduction of nitrite to NO and conversion of LHb to LHbNO, resulting in inhibition of nitrogenase activity. Reduction of nitrate in plants consumes H+, which may be the reason why nitrate supply can alleviate the inhibitory effects of NO2 on nitrogenase activity.

Leaf pH and absorption rate of NO2 of the plants growing in a nitrate solution were higher than those of the plants growing in ammonium. The absorption rates of NO2 of both sets of plants decreased with increasing the duration of NO2 exposure. These results suggest that accumulation of H+ in leaves, caused by NO2 absorption and ammonium assimilation, may slow down the dissolution of NO2 in extracellular fluid of leaves, and result in the decrease in absorption rate of NO2. The decline in NO2 absorption with increasing exposure time, which is analogous to a decline in exposure concentration of NO2, may be one of the reasons why plants show some acclimation to long-term exposure.

The popular methods for quantitative estimation of NOx absorption by leaves of plants supplied with 15N-labelled fertilizer, the 15N dilution method and the difference method, were compared with the classical mass balance (MB) method and/or the 15NOx method. Neglect of the influence of NOx exposure on N uptake by roots results in an overestimation in NO2 absorption calculated using the 15N dilution method. Two new alternative calculation methods, the TNF method and the TWF method, are proposed on different prerequisites. The TNF method can reduce the error of 15N dilution method, and give closer results to the MB method or 15NOx method. The TWF method is suitable to calculate NOx-N in organs (leaf, stem, root) of exposed plants. The estimation error of these methods are found to increase with decreasing concentration of 15N-labelled fertilizer supplied to roots.

The results of these exposure experiments suggest that the acid-alkali balance plays a fundamental role in determining the interactions between NO2 absorption and N metabolism in exposed plants. H+ not only take part in the reactions of uptake and assimilation of various forms of nitrogen, it also affects the activity of the enzymes catalyzing reduction of oxidized N and synthesis of organic N. NO2 and other forms of N interact on each other through production or consumption of H+ in their uptake and assimilation. Whether the concentration of the H+ produced from NO2 absorption and N assimilation in exposed plants can be kept at a suitably low level, may determine whether the effects of NOx exposure on plants are beneficial or detrimental.

According to the role of H+ in uptake and metabolism of N, two conclusions are possible:
1. Susceptibility of plants to NO2 exposure may depend on the acidity of the growth medium.
2. Velocities of uptake and assimilation of nitrogen in plants may fluctuate cyclically.

Further experiments to test these inferences are recommended.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Murray, Frank
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