Physiological mechanisms influencing plant nitrogen isotope composition

Whole-plant and leaf nitrogen isotope composition are determined by the iso tope ratio of the external nitrogen source and physiological mechanisms wit hin the plant. Whole-plant isotope composition can reflect that of the nitr ogen source when plant demand exceeds nitrogen supply, Uptake by mycorrhiza e can cause the isotope ratio of the plant to deviate from the source. Intr a-plant variation in isotope composition can be caused by multiple assimila tion events, organ-specific loss of nitrogen, and resorption and reallocati on of nitrogen. Future work must address acquisition of organic nitrogen fr om the soil solution, the role of mycorrhizae, and internal transformations within the plant.