The Role of Inorganic Nitrogen Ions in Plant Adaptation Processes

Plant growth rate, biomass production, and yield are correlated with the capacity of the plant to transport nitrate from the root to the shoot. Nitrate transport to the shoot depends on K-shuttle operation, which provides coordination of nitrate uptake and nitrate utilization while promoting rapid growth rates. Some, but not all, trees have a limited K-shuttle activity and exhibit slow growth rates. Stresses such as salinity or drought block K-shuttle activity, inhibiting the transport of nitrate and cytokinin to the shoot, while enhancing reduction of nitrate in the root and the assimilation of the resulting ammonium. Salinity therefore shifts the main location of nitrate assimilation from the shoot to the root, which is followed by a corresponding reduction in growth rate. Ammonium assimilation under saline conditions is accompanied by enhanced CO₂ fixation by phosphoenol pyruvate carboxylase in the roots to generate oxaloacetate. Xylem loading of cytokinin and ABA are involved in the regulation of massive nitrate fluxes through xylem parenchyma cells, which regulate xylem loading and transport of amides and nitrate to the shoot. The balance of cytokinin/ABA in the xylem sap was correlated to changes in the expression of the Mo-enzymes, aldehyde oxidase and nitrate reductase. Ammonium and stress increased the level of aldehyde oxidase in the roots while nitrate depressed it. The amides/nitrate ratio is correlated with assimilate allocation to vegetative or reproductive sinks, thus determining the harvest index of crops.