Nitrogen in citrus: Signal, nutrient, and use efficiency

Abstract

Nitrogen (N) is the most important nutrient for growth, fruit yield, and quality of citrus plants. In order to reduce both the requirements for costly nitrogen fertilizers and environmental pollution of soil and water, the improvement of the nitrogen use efficiency (NUE) on citrus plants is fundamental in sustainable agriculture. In this chapter, a critical overview on the definitions of NUE and its components, nitrogen uptake (NUpE), and nitrogen utilization efficiency (NUtE) was provided, together with current knowledge and future challenges to understand and manipulate NUE in citrus plants. Further, the different N fertilizer use strategy in combination with irrigation to increase the NUE in citrus species was explained. The nitrogen content, the removal and the partitioning among the citrus organs, and the N availability in citrus soils provided a comprehensive picture of the N economy in citrus trees and soil orchards, and the basis of the NUE. However, an important approach for improving the NUE in citrus plants was to understand the regulation of the morpho-physiological and molecular mechanisms controlling plant nitrogen economy such as nitrogen uptake, translocation, assimilation, and remobilization. This approach accompanied by new techniques in molecular biology, root biology, plant-soil interactions, and modeling will provide an accurate criteria to discriminate between the nitrogen-efficient and inefficient citrus plants. Finally, the future challenges for improving NUE in citrus species considering both the agronomic and physiological approaches were discussed.