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Nitrogen Uptake and Translocation in Vanda Orchid after Roots and Leaves Application of Different Forms 15N Tracer

Vanda is an economically important orchid that is widely produced in Thailand. Usually, growers apply large amounts of fertilizer throughout the plant, covering the leaves and roots to ensure good quality products. Nitrogen fertilizer, in terms of ammonium (NH4+) and nitrate (NO3−), is generally used as an N source. In addition, nitrogen organic fertilizer (glutamine) is increasingly being used to promote rapid growth in some plants. However, the absorption efficiency of organic N compared with the inorganic form (NH4+ and NO3−) via the roots or leaves of Vanda has not been evaluated. Therefore, this research aimed to compare the fate of organic N (in glutamine form) and inorganic N in Vanda using a 15N tracer. Vanda ‘Patchara Delight’ was grown in a plastic greenhouse under a 50% shading net at an average temperature of 25 °C and 80% relative humidity (RH). The plants were sprayed weekly via roots or leaves with 100 mL of 15N solution, 2.5 mM 15NO3− + 2.5 mM NH4+ (N1), 2.5 mM NO3− + 2.5 mM 15NH4+ (N2), and 2.5 mM glutamine (15N2)(N3) for 4 weeks. The plants were then sampled and separated into leaves and roots, and 15N abundance was analyzed using an elemental analyzer coupled with an isotope-ratio mass spectrometer or IRMS. The plants that received only glutamine via roots showed the highest 15N use efficiency (15NUE) of about 28.19% at 30 days after the first feeding (DAF), whereas 15NH4+ resulted in the lowest 15NUE among 15N sources. Regardless of the application site, plants supplied with 15NH4+ showed a lower labeled N concentration and labeled N content in stems and leaves than those fed with a combination of 15NO3− or a sole application of 15N-glutamine. The largest labeled N concentrations in stems, leaves and roots were found in plants supplied with sole glutamine via roots. At 30 DAF, 15N solution either combined with 15NO3− or solitary 15N-glutamine did not affect the labeled N concentration in leaves. Therefore, supplying organic N in glutamine form to Vanda can provide a 4–7% higher NUE than inorganic N, especially when supplying the solution to the roots.