Investigating the Nitrogen Sink–Source Relationship in Phalaenopsis by Using Stable Isotope 15N As a Tracer

Phalaenopsis is an important floriculture crop characterized by slow growth, slow response to fertilization, and resilience to drought and nutrient starvation. Little has been published about the partitioning of nitrogen among the various organs of Phalaenopsis. In this study, ¹⁵N-labeled Johnson’s solution containing K¹⁵NO₃ was applied to Phalaenopsis Sogo Yukidian ‘V3’ plants in different experiments, to study the partitioning of fertilizer nitrogen shortly after application and after it is stored within the plant. Shortly after fertilizer application, fertilizer nitrogen was not sufficient to supply the requirement of expanding leaves, and nitrogen was sourced elsewhere from within the plant. Over a 2-month period, fertilizer nitrogen was preferentially accumulated in the two youngest leaves rather than older leaves. Following application of ¹⁵N-labeled fertilizer to roots or leaves, young leaves and newly-grown roots contained the highest ¹⁵N concentrations among the distant organs to which the label has been translocated. Following reduction of endogenous nitrogen pool in Phalaenopsis through root excision, the amount of stored nitrogen allocated to the inflorescence and young leaf did not decrease, while the extent of utilization of stored nitrogen in mature leaves increased as the size of nitrogen pool decreased. Export of fertilizer nitrogen previously accumulated in mature leaves and roots was evident as Phalaenopsis plants shifted from vegetative to reproductive growth stage, even though absorption of fertilizer nitrogen through the roots still continued. These experimental results indicate that new leaves, new roots, and new inflorescences are strong sinks of nitrogen in Phalaenopsis, while mature leaves and roots have the capacity to serve as sources of nitrogen.