Nitrogen Source for Inflorescence Development in Phalaenopsis: I. Relative Significance of Stored and Newly Absorbed Nitrogen

Abstract

<jats:p><jats:italic>Phalaenopsis</jats:italic> orchid is a slow-growing crop that responds slowly to fertilization. In this study, we used <jats:sup>15</jats:sup>N-labeled Johnson’s solution to investigate the accumulation and use of fertilizer nitrogen (N) during the vegetative and reproductive growth stages of <jats:italic>Phalaenopsis</jats:italic> Sogo Yukidian ‘V3’ with a focus on the nitrogen source for inflorescence development. Labeling of fertilizer applied to mature plants 6 weeks before forcing or at 6 weeks into forcing showed that in the inflorescence, the ratio of N derived from fertilizer applied 6 weeks before forcing to the N derived from fertilizer applied 6 weeks into forcing was 31% to 69%, which shows the importance of newly absorbed fertilizer for supplying the N needed for inflorescence development. The fate of fertilizer N applied during the small, medium, or large plant stage of vegetative <jats:italic>Phalaenopsis</jats:italic> Sogo Yukidian ‘V3’ was traced separately with <jats:sup>15</jats:sup>N-labeling. The capacity of the plant to accumulate N after fertilizer application was different during the various stages of vegetative growth, with large plants having more N storage capacity as a result of their greater biomass. However, the percentage of the accumulated N that was later allocated to the inflorescence was similar regardless of the stage of fertilizer application: of the fertilizer N absorbed during various stages of the vegetative period, 6% to 8% was allocated to the inflorescence at the visible bud stage. This result highlights the mobility of N stored early on within the plant. By calculation, of the total N in the inflorescence at the visible bud stage, the N absorbed during the small, medium, and large plant stages contributed 7%, 11%, and 25%, respectively, whereas N applied after spiking made up the other 57%. This result indicates that both N stored during the vegetative stage and N applied during the reproductive stage contribute significantly to inflorescence development.</jats:p>

Nitrogen Source for Inflorescence Development in Phalaenopsis: II. Effect of Reduced Fertilizer Level on Stored Nitrogen Use

“…In plants receiving full-strength Johnson's solution, of the N accumulated in the inflorescence at the visible bud stage, the ratio of N derived from fertilizer applied 6 weeks before forcing to the N derived from fertilizer applied 6 weeks into forcing was 43% to 57%, respectively [i.e., 359:468 mg (calculated from Table 3)]. This is quite similar to the ratio that we found in another study (i.e., 31% to 69%), which was the corresponding ratio obtained from plants that were given ample fertilization (Susilo et al, 2014). When fertilizer level was reduced to 10%-strength Johnson's solution, the ratio of N derived from fertilizer applied 6 weeks before forcing to the N derived from fertilizer applied 6 weeks into forcing became 89% to 11% [i.e., 405:49 mg (calculated from Table 3)].…”

“…The 15 N content excess of the inflorescence differed little among the V 0%, V 10%, and V 100% treatments, but lower 15 N content excess was found in the newly grown leaves of plants receiving V 0% treatment (Table 3). This was because during the reproductive stage, the inflorescence is a stronger sink for N than leaves in Phalaenopsis (Susilo et al, 2014) and thus the allocation of stored N to the inflorescence in the present study was less affected by changes in the fertilizer level. In contrast, as a result of the decreasing sink strength of newly grown leaves during the reproductive stage (Susilo et al, 2014), plants allocated less N to newly grown leaves when they were given less fertilizer during the reproductive stage.…”

“…This was because during the reproductive stage, the inflorescence is a stronger sink for N than leaves in Phalaenopsis (Susilo et al, 2014) and thus the allocation of stored N to the inflorescence in the present study was less affected by changes in the fertilizer level. In contrast, as a result of the decreasing sink strength of newly grown leaves during the reproductive stage (Susilo et al, 2014), plants allocated less N to newly grown leaves when they were given less fertilizer during the reproductive stage. By the same reasoning, the dry weight of the inflorescence was unaffected by reduced fertilization during the reproductive stage, whereas the dry weight of newly grown leaves was lower (Table 1).…”

“…Between 3 and 4 mg of the dried, ground samples were packaged into 8 • 5-mm tin capsules (Elemental Microanalysis, Okehampton, U.K.). The packaged samples were sent to the Stable Isotope Facility at the University of California, Davis for total N and 15 N analysis using the same procedure as that described in Susilo et al (2014). The atom percent 15 N excess (atom% excess) was obtained by subtracting natural abundance of 15 N (0.366 atom%) from the results of the 15 N analysis.…”

“…In our previous study, we showed that this resilience to N deficiency is attributable to significant amounts of N being stored in the plants (Susilo et al, 2013). In another study, we quantified the contribution of fertilizer N stored throughout the vegetative stage of Phalaenopsis to the developing inflorescence when fertilizer supply was ample during forcing (Susilo et al, 2014). We hypothesize that reducing fertilizer level during the forcing period would increase the contribution of previously stored N to inflorescence development.…”