Phalaenopsis (the moth orchid) is currently the most important indoor potted flowering plant worldwide (Royal Flora Holland, 2017; U.S. Department of Agriculture, 2016). Growers normally produce phalaenopsis at high temperatures (>28 °C) to promote vegetative growth (Chen et al., 1994; Yoneda et al., 1991). With sufficient light (Wang, 1995), mature plants can then be forced to flower at lower temperatures (20 to 25 °C) (Lee and Lin, 1984; Yoneda et al., 1992). Although flowering of phalaenopsis is mainly controlled by low temperature, mineral nutrition also plays an important role in flowering (Lee and Lin, 1987; Poole and Seeley, 1978; Sheehan, 1961; Tanaka et al., 1988).
Ratio of tissue C and N is called C/N ratio. In the literature, high tissue C/N promotes reproductive growth; low C/N increases vegetative growth and may inhibit flowering (Klebs, 1918). In Pharbitis nil, seedlings with high tissue C/N were more sensitive to the inductive dark periods, which resulted in a higher flowering rate (Wada and Shinozaki, 1985). In Oncidium Gower Ramsey, there was a positive correlation between tissue C/N and flowering quality (Wang, 2011). Tissue analysis can directly reveal plant nutrient status, but sampling position and the effects of plant developmental stages must be considered to appropriately interpret the results. However, there has not been a significant amount of research on the distribution of tissue C and N and their partitioning during the development of phalaenopsis. The literature on how C and N affect growth and flowering of phalaenopsis is also limited.
The N concentration of mature leaves in phalaenopsis was between 1.93% DW and 2.83% DW (Poole and Sheehan, 1974). N concentration is affected by the location of leaf, cultivars (Poole and Sheehan, 1974), and N level in fertilizer (Wang and Konow, 2002). N concentration decreases as leaves became more mature in phalaenopsis (Poole and Sheehan, 1974). Susilo et al. (2013) compared N concentrations at different leaf locations in Phalaenopsis Sogo Yukidian ‘V3’ and found that new leaf had the highest N concentration of 2.31% and the sixth leaf had the lowest value of 1.16%. Kataoka et al. (2004) took five pairs of leaf discs, where each pair was symmetrically positioned with respect to the midrib, at five different distances from the leaf tip for sugar analysis. The second, third, and fourth leaf disc pairs from the leaf tip had almost the same amount of sugar, but the first and fifth pairs had different values. The literature indicates that C and N are not evenly distributed in the leaves of phalaenopsis (Kataoka et al., 2004; Poole and Sheehan, 1974; Susilo et al., 2013). Appropriate sampling position is thus crucial for correctly interpreting the results of tissue analysis.
Many orchids have enlarged stems called pseudobulbs, which can store water, carbohydrate, and minerals (Ng and Hew, 2000). Phalaenopsis does not have pseudobulbs, but its succulent leaves and roots have been proved to have storage function of N (Susilo et al., 2013). With 90% reduction in fertilizer during reproductive stage, flowering quality was only slightly reduced compared with the full-fertilization treatment in phalaenopsis and stored N became the major N source for inflorescence development (Susilo and Chang, 2014). Susilo et al. (2014) compared N concentrations of whole plant at five developmental stages in Phalaenopsis Sogo Yukidian ‘V3’ and found the N concentrations of young plant, just-matured plant, well-matured plant, plant with flower bud visible, and plant with 2/3 flowers opened were 2.00%, 2.35%, 2.18%, 1.82%, and 1.84%, respectively. Their data implied that a natural change, in the form of decreasing N concentration during development, might exist among cultivars in Phalaenopsis. By contrast, Lee and Wang (1997) did not find obvious change in tissue N concentration during 7 months of transition stage (between juvenile and mature stages) of a hybrid white-flowered phalaenopsis. Lei (2007) used fully mature white-flowered phalaenopsis (Phalaenopsis amabilis) as plant material and found no significant changes in N concentration during reproductive stages. Whether N concentration would naturally decrease or not during development among phalaenopsis cultivars needs to be further understood.
This study aimed to reveal the nature of tissue N and C in phalaenopsis and their progressive changes with plant growth and development; the similarities and differences between standard-type and miniature-type phalaenopsis in those regards were compared. From the data obtained, we hope to acquire important information regarding precise sampling and how to appropriately interpret the results of tissue analysis for phalaenopsis.
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