ORIGINAL ARTICLES
Effect of N Source on Growth and N Uptake of Hippeastrum Using 15N Tracers
2022 Volume 91 Issue 1 Pages 85-93
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2022 Volume 91 Issue 1 Pages 85-93
Understanding the mechanism of N uptake is of key importance to manage N utilization efficiency. Hippeastrum is a popular geophyte, but its N absorption and translocation characteristics are still not well understood. Therefore, the objective of this study was to assess the effect of different N sources on growth, N uptake and N distribution in Hippeastrum. The experiment was set up in a completely randomized design (CRD) with four different N sources of equal N concentration, i.e., 1) 2.5 mM 15NO3− + 2.5 mM NH4+ as treatment 1 (T1), 2) 2.5 mM NO3− + 2.5 mM 15NH4+ as treatment 2 (T2), 3) 5 mM 15NO3− as treatment 3 (T3), and 4) 5 mM 15NH4+ as treatment 4 (T4). A 15N-labelled 30 mL nutrient solution was drenched on plant pots (per time per pot). Plants were supplied with the 15N solution every two weeks. Plant samples were divided into three stages according to plant growth stage, i.e., Stage 1 (emerging stage—1 WAP), Stage 2 (flowering stage—3 WAP), and Stage 3 (vegetative stage—15 WAP). The results indicated that plants fed with a combined N fertilizer and nitrate had a higher total dry weight than plants supplied solely with ammonium. At Stage 3, the plants supplied with 5 mM NO3− (T3) had a higher total N content than plants supplied with a mixed form of N (T1, T2) or solely NH4+ (T4). In addition, plants supplied solely with NO3− (T3) had a higher N distribution in leaves than plants supplied with a mixed N source (T1, T2) and solely NH4+ (T4). At 15 weeks after planting (Stage 3), a higher 15N use efficiency (15NUE) was observed in plants supplied with a mixed N source (T1, T2) and 5 mM 15NO3− (T3) in comparison with those supplied solely with NH4+ (T4). Scales were a major site of 15N distribution in Hippeastrum at Stage 1. Nevertheless, most 15N at Stage 2 and 3 was found in the roots + basal plate and leaves, respectively.
Hippeastrum (amaryllis) is an ornamental bulbous flowering plant that belongs to the Amaryllidaceae family. It is native to Central and South America and is easily grown in tropical and subtropical regions. It is commonly grown as a potted plant, as cut flowers in greenhouses, or grown outside as a garden flower or ornamental plant in warm to mild climate areas (Rees, 1985, 1992; Okubo, 1993). De Hertogh and Le Nard (1993) reported that Hippeastrum is a minor flower bulb crop with a production area of 100 to 900 ha, whereas the major flower bulbs such as Gladiolus, Hyacinthus, Iris, Lilium, Narcissus, and Tulipa have a production area of more than 900 ha. In 2017, 39 million stems of Hippeastrum were sold as cut flowers in the Netherlands, with sales figures (clock and direct sales) of 24.8 million EUR (Hübner, 2018).
In bulbous plants, nutritional requirements are an important factor affecting plant growth and yield. El-Malt et al. (2006) indicated that treating Hippeastrum vittatum plants with chemical fertilizer (NPK) at 5 g/pot improved both vegetative and flowering growth. Moreover, El-Naggar and El-Nasharty (2009) found that fertilizing Hippeastrum vittatum with mineral fertilizer at 5 g/plant had the maximum beneficial effect on vegetative growth characteristics, flowering, bulb, and bulblet production.
N is an essential constituent of proteins, amino acids, nucleic acids, chlorophyll, and numerous secondary substances such as alkaloids (Bergmann, 1992; Cresswell and Weir, 1997). C. alismatifolia responds to N fertilizers in terms of flower quality and rhizome yield (Ruamrungsri and Apavatjrut, 2003; Ohtake et al., 2006). Silberbush et al. (2003) studied the effect of nitrogen on the growth of Hippeastrum when applied at the following concentrations: 0, 5, 10, 15, 20, 25 mM N (as NH4NO3). The results showed that bulbs grew best when fertilized with 25 mM N. Nitrogen-deficient plants were stunted and the quality of their flowers and rhizomes was significantly reduced. Moreover, Ohtake et al. (2006) found that an increase in N concentration from 0 to 50 mg·L−1 increased the number of flowering shoots and consequently the number of rhizomes, but depressed the growth of storage roots. Usually, plants can uptake N as nitrate (NO3−) and ammonium (NH4+), but some prefer one source or another depending on the plant species, and this is generally related to the physiological adaptations of plants to natural ecosystems (Marschner, 1995). In soilless culture, the major form of N in nutrient solutions is NO3−, but the addition of some NH4+ appears to be beneficial to plant growth. Modification of the NH4+:NO3− ratio in the nutrient solution is an important means of controlling the relative uptake of nutrients (Sonneveld, 2002). Plants such as wheat, cucumber, and bell pepper prefer nitrate as the major source of N (Heuer, 1991; Osorio et al., 2003). In the French bean, an increase in the ratio of NH4+ to NO3− in the root zone impairs growth and reduces yield (Guo et al., 2002). However, while toxicity is observed in many plant species when NH4+ is supplied by itself, this can be alleviated by co-provision of NO3−-N (Britto and Kronzucker, 2002). For species such as cotoneaster (Cotoneaster dammeri Schneid., a woody ornamental) and rudbeckia (Rudbeckia fulgida Ait., a herbaceous perennial), NH4+-N or a combination of NH4+-N and NO3−-N resulted in better growth than NO3−-N alone (Kraus and Warren, 2002). Wang and Below (1992) demonstrated that growth and yield were enhanced when wheat plants were provided with mixtures of NO3− and NH4+, compared with either form alone. Similarly, in potatoes, mixed forms of N increased total and tuber dry weights, plant size, leaf area, and specific leaf area as compared to either NH4+ or NO3− alone, and the enhanced growth was greatest at 8% to 20% NH4+-N (Cao and Tibbitts, 1993).
Although N is the major limiting factor in agricultural production, there is no information regarding the characteristics of N uptake in Hippeastrum. Therefore, the purpose of this study was to determine the effects of different N sources on growth and N uptake in roots and basal plate, scales, leaves, and inflorescences of Hippeastrum at each growth stage. Such basic information can be used to guide nitrogen management with the aim of improving bulb production in the future.
This experiment was conducted in northern Thailand at part of the Mae-Hia Agricultural Research, Demonstrative and Training Centre, Faculty of Agriculture, Chiang Mai University. Bulbs of Hippeastrum ‘Apple Blossom’ with an average circumference of 22 cm, average bulb fresh weight of 178 g, and 3.8 Newtons of bulb firmness were planted in 12-inch plastic pots (one bulb per pot, three replicates per treatment) using perlite and vermiculite at a ratio of 1:2 as growing media. Plants were grown in a plastic greenhouse that allowed natural ventilation via side openings. During the experiment, the temperature of the greenhouse ranged from 28–30°C, under full sun, and relative humidity was 60–65%. The plants were watered daily with deionized water except on days when they were fed fertilizer. Pests and diseases were completely controlled by chemical treatments.
15N feeding and analysisThe experiment was set up in a completely randomized design (CRD) with four different N sources, i.e., 1) 2.5 mM 15NO3− + 2.5 mM NH4+ as treatment 1 (T1), 2) 2.5 mM NO3− + 2.5 mM 15NH4+ as treatment 2 (T2), 3) 5 mM 15NO3− as treatment 3 (T3), and 4) 5 mM 15NH4+ as treatment 4 (T4). The 15NO3− was prepared from 60 atom% (Na15NO3) and 15NH4+ was derived from 60 atom% (15NH4)2SO4. The concentrations of other nutrients were held constant at 5 mM K, 3 mM P, 2.5 mM Ca, and 4 mM Mg. This labelling technique was adapted from Ruamrungsri et al. (2014). The 15N-labelled 30 mL culture solutions were top-dressed on the plant pots (per time per pot). Plants were supplied with 15N solution nine times every two weeks [supplied at 0, 2, 4, 6, 8, 10, 12, and 14 weeks after planting (WAP)]. Plant sampling was divided into three stages according to plant growth stage, i.e., Stage 1 (emerging stage—1 WAP), Stage 2 (flowering stage—3 WAP), and Stage 3 (vegetative stage—15 WAP) (Fig. 1). N uptake and N translocation were investigated in various organs. Plants were separated into roots and basal plates (R), bulb scales (S), leaves (L), and inflorescences (F) (Fig. 1), and then carefully washed three times with deionized water. Samples were ground into a fine powder, and about 5 mg of ground samples were packed into a tin capsule. The N concentration and 15N uptake were analyzed in three replicates per treatment using an elemental analyzer (Flash EA1112; Thermo Electron, Milan, Italy) coupled to an isotope-ratio mass spectrometer or IRMS (Delta Plus XP; Thermo Fisher Scientific, Bremen, Germany).
The appearance and plant parts of Hippeastrum at each growth stage.
15N use efficiency (15NUE) was calculated from the ratio between the amount of labelled N in plants and the amount of labelled N supplied and expressed as a percentage (adapted from Sandrock et al., 2005):
Plant growth in terms of plant height, number of leaves per plant, and leaf color intensity (SPAD units) were recorded. Flower quality in terms of stalk length and flower width were measured at the flowering stage (3 WAP). Bulb quality in terms of bulb circumference was recorded, and bulb firmness was determined at growth stage 1, 2, and 3 (1, 3, and 15 WAP, respectively) by manually puncturing the bulb surface using a hardness tester (Fruit Hardness Tester, 5 kg; Fujiwara Scientific, Tokyo, Japan).
Statistical analysisThe experiment was performed in a completely randomized design. Results are expressed as means of values measured from at least three replicates using Statistix 8 analytical software (SXW Tallahassee, FL, USA). The least significant difference at P < 0.05 was used to determine significant differences in growth parameters, total N content per plant, 15N concentration, absorbed 15N, percentage 15N increment, and 15N use efficiency.
Different N sources affected the plant height of Hippeastrum. At Stage 3 (15 WAP), plants supplied with both NH4+ and NO3− forms (T1, T2) were taller than plants supplied with NO3− (T3) or NH4+ (T4) alone (Table 1; Fig. 2). Kraus and Warren (2002) reported that in cotoneaster (Cotoneaster dammeri Schneid., a woody ornamental) and rudbeckia (Rudbeckia fulgida Ait., a herbaceous perennial), a combination of NH4+-N and NO3−-N resulted in better growth than NO3−-N alone. Wang and Below (1992) demonstrated that growth and yield were enhanced when wheat plants were provided with mixtures of NO3− and NH4+, compared with either form alone. Similar results were found in potatoes, in which mixed forms of N increased total and tuber dry weights, plant size, leaf area, and specific leaf area as compared to either NH4+ or NO3− alone (Cao and Tibbitts, 1993). However, there were no significant differences in the number of leaves per plant or leaf color intensity (SPAD unit) when plants were supplied with different N sources. In addition, there were no significant differences in flower quality in terms of stalk length, flower width (Table 1; Fig. 2), or bulb quality in terms of bulb firmness and bulb circumference (Table 2; Fig. 2) when plants were supplied with different N sources. This suggests that nutrient supply during the pre-flowering stage did not affect the flower quality of Hippeastrum. Moreover, the flower quality of Hippeastrum may be influenced by the initial bulb size (all bulbs started with the same circumference), which reflects the amount of food reserves in the bulb. Le Nard and De Hertogh (2002) reported that large bulbs generally produce vigorous plants with large and/or many flowers. In addition, Hippeastrum bulbs with a circumference greater than 20 cm are generally planted for flower production (De Hertogh, 1994).
Plant height (cm), number of leaves per plant, leaf colour intensity, and flower quality of Hippeastrum supplied with different N sources.
Appearance of Hippeastrum when supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5 mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage [A = Stage 1 (1 WAP), B = Stage 2 (3 WAP), and C = Stage 3 (15 WAP)].
Bulb quality of Hippeastrum supplied with different N sources at each growth stage.
Total plant fresh and dry weight did not differ significantly between treatments at growth stages 1 and 2 (1 and 3 WAP) (Fig. 3). At Stage 1, the total fresh and dry weights of Hippeastrum were in the range of 167–174 g and 200–211 g, respectively. At Stage 2, total fresh and dry weight increased by 21.6% and 29.5%, respectively (compared with Stage 1). However, there were significant differences in fresh and dry weight at growth stage 3 (15 WAP), for which the results showed that plants supplied with a mixed form of N, 2.5 mM NO3− + 2.5 mM NH4+ (T1, T2) had higher total fresh weights than plants supplied with a single form of 5 mM NO3− (T3) or 5 mM NH4+ (T4) (Fig. 3). These results suggest that a mixed form of NO3− and NH4+ is favored by Hippeastrum. Similar results were reported by Vazquez et al. (2015), who found that the total weight of Hippeastrum bulbs was increased when they were supplied with a high rate of nitrogen in mixed form as NO3−: NH4+ (ratio; 50:50 and 70:30) or supplied with 100% nitrogen as NO3−. Zhang et al. (2005) reported that enhanced growth was also observed in spinach when both NO3− and NH4+ were supplied compared with just NO3− or NH4+ alone. Khuankaew et al. (2010) also reported that Curcuma preferentially absorbed N from the combination of NH4+ and NO3−. Furthermore, this research found that plants supplied with NH4+ alone (T4) had the lowest plant dry weight (17.9 g) (Fig. 3). NH4+ at high concentrations can be toxic, causing a deficiency in nutrients, acidification of the root zone, alterations in the osmotic balance, changes in phytohormone levels, and impairment of N metabolism (Gerendas et al., 1997; Lorenzo et al., 2000).
Total plant fresh and dry weights of Hippeastrum when supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage [Stage 1 = 1 week after planting (WAP), Stage 2 = 3 WAP, and Stage 3 = 15 WAP].
The total N content in Hippeastrum did not differ significantly among treatments at growth stages 1 and 2 (1 and 3 WAP) (Fig. 4). At Stage 1, the total N content of Hippeastrum in all treatments was an average of 429 mg/plant, and at Stage 2, the total N content in all treatments increased to 466 mg/plant and declined in Stage 3 to 409 mg/plant. The reduction in total N from Stage 2 to 3 may have occurred because some N was lost by flower senescence at Stage 3. However, there were significantly different total N contents at Stage 3 (15 WAP), where plants supplied with 5 mM NO3− (T3) had a higher total N content than plants supplied with a mixed form of N, 2.5 mM NO3− + 2.5 mM NH4+ (T1, T2) or 5 mM NH4+ (T4) (Fig. 4). In addition, total N in the mixed N source and NH4+ alone treatment was decreased when compared with solely NO3− at Stage 3. This result could be attributed to the fact that nitrogen in NO3− form has a higher remobilization capacity than mixed N and NH4+ forms. At Stage 3, there was significantly higher total nitrogen in the scales of plants supplied solely with NO3− (248 mgN/organ) compared with plants supplied with mixed N (170 mgN/organ) and NH4+ alone (114 mgN/organ) (Fig. 4). This may be due to the high N remobilization from source (senescent inflorescence in Stage 2) to sink (scale in Stage 3) in plants supplied solely with NO3−. Guiboileau et al. (2013) reported that N remobilization from source to sink mainly occurs in response to stress and during senescence. Ruamrungsri et al. (2000) reported that in Narcissus, the presence of ammonium in the nutrient solution depressed the uptake of nitrate when plants were supplied with both forms of N. Marschner (1995) explained that compared with NH4+, NO3− has the advantage of being a storage form in plants with no need to be assimilated in the roots. One possibility is that the amount of energy consumed is lower for the assimilation of NH4+ than that of NO3−. In fact, the energy required for the assimilation of NO3− is 20–21 ATP·mol−1, compared to only 5 ATP·mol−1 for the assimilation of NH4+ (Traore and Maranville, 1999).
Total N content of Hippeastrum when supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5 mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage (Stage 1 = 1 WAP, Stage 2 = 3 WAP, and Stage 3 = 15 WAP).
After plants were supplied with different N sources for one week (one application of N treatment; Stage 1), most of the N in all treatments was in the bulb scales (S), with a range of 42–50% (167–218 mgN/organ), followed by leaves (23–38%, 77.1–184 mgN/organ), roots and basal plates (19–28%; 92.5–122.3 mgN/organ), respectively. Plants supplied solely with NO3− (T3) had a higher N distribution in the leaves (38%, 181 mgN/organ) than plants supplied with mixed N sources (T1, T2) and solely NH4+ (T4) (Fig. 5). Based on this result, it can be assumed that leaves are major organs in Hippeastrum where NO3− accumulation takes place. Meyer and Stitt (2001) reported that nitrate reduction can take place in both roots and shoots but is spatially separated between the cytoplasm where the reduction takes place and plastids/chloroplasts where nitrite reduction occurs.
Percentage distribution of total N in different organs of Hippeastrum supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5 mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage (Stage 1 = 1 WAP, Stage 2 = 3 WAP, and Stage 3 = 15 WAP).
At three weeks after planting (two applications of N treatment; Stage 2), most of the N in all treatments was in the scales (S), similar to the distribution in Stage 1 with a range of 46–52% (204–244 mgN/organ), followed by inflorescences (17–25%, 82.8–110 mgN/organ), roots, basal plates (14–25%, 65.6–121 mgN/organ) and leaves (10–13%, 48.2–61.6 mgN/organ), respectively. Plants supplied solely with NH4+ (T4) had a higher N distribution in the inflorescences (25%, 110 mgN/organ) than plants supplied with mixed N sources (T1, T2) and solely NH4+ (T4) (Fig. 5). Therefore, it may be that the NH4+ form was preferred as the translocation form of N to the inflorescence organ.
At 15 weeks after planting (nine applications of N treatment; Stage 3), the results showed that distribution of N varied by N source. In T1, T2, and T3, most of the N was found in the scales (46–54%, 170–248 mgN/organ), followed by leaves (28–35%, 128–129 mgN/organ), roots and basal plates (17–19%, 69.2–81.0 mgN/organ), respectively. However, in T4, most of the N was found in the leaves (50%, 175 mgN/organ), followed by scales (33%, 114 mgN/organ) and roots and basal plates (17%, 59 mgN/organ), respectively (Fig. 5). Thus, leaves were a major organ for N accumulation when plants were supplied solely with NH4+ as the N source. Panjama et al. (2018) reported that in Vanda, the distribution of 15N was higher in the leaves and slightly decreased in roots, regardless of the N source supplied, after 30 days of fertilizer treatment, clearly indicating that N absorbed in the roots was transported to the leaves. This result suggests that the leaves may be potential sink organs, especially young leaves, and have the highest sink strength during the vegetative growth phase of Vanda.
Effects of N source on total uptake of fertilizer N in HippeastrumThe total 15N contents in Hippeastrum were affected by N source and growth stage. At Stage 1, there were no significant differences in 15NUE between the different 15N treatments. However, at Stage 2 (3 WAP), plants supplied with mixed N sources (T1, T2) had higher total 15N contents than plants supplied with 5 mM 15NO3− (T3). And after plants had been supplied with N for 15 weeks (nine applications of N), the results clearly showed that plants supplied solely with 5 mM 15NO3− (T3) and mixed N sources (T1, T2) had significantly higher total absorbed 15N (3.53 and 3.377 mg 15N/plant, respectively) than plants supplied solely with NH4+ (Fig. 6). This result clearly indicates that Hippeastrum prefers NO3− or mixed N forms as its N source.
Absorbed 15N per plant and 15N use efficiency of Hippeastrum supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5 mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage (Stage 1 = 1 WAP, Stage 2 = 3 WAP, and Stage 3 = 15 WAP).
Nitrogen use efficiency (NUE) is an important key to improving N use in plants, which mainly involves the assimilation of NO3− and NH4+ and recycling organic N. In this study, 15NUE in Hippeastrum was affected by growth stage and N source. 15NUE was higher at Stage 3 than Stage 1 and 2. This may have been due to the different frequency of 15N supply in each treatment, as at Stage 1 and 2 plants had already received one and two applications of 15N (2.19, 4.38 mg of total 15N supply), respectively, whereas at Stage 3 plants had received nine applications of 15N (17.5 mg of total 15N supply). Moreover, more 15N was absorbed at Stage 3 than at Stage 1 and 2 (Fig. 6). This could be due to the highly changeable N uptake during crop development. In our study, the results clearly showed low N uptake during seedling and flower development (Stage 1 and 2, respectively), while there was high N uptake during leaf development (Stage 3). Okubo (1993) reported that Hippeastrum does not require high nutrition in the early growth stage. The different N sources resulted in different 15NUE levels at each stage. Regardless of N form, there was no significant difference in 15NUE at plant growth stage 1 (1 WAP). However, plants supplied with 5 mM 15NO3− (T3) had the lowest 15NUE at the flowering stage (Stage 2; 3 WAP). At 15 weeks after planting (Stage 3) higher 15NUE was found in plants supplied with 5 mM 15NO3− (T3) (20.2%) and mixed N forms (T1, T2) (19.2%), than in plants supplied with 5 mM 15NH4+ (T4) (10.6%) (Fig. 6). This suggests that Hippeastrum preferentially absorbs NO3− or mixed N forms rather than NH4+. Similar results were reported by Panjama et al. (2018) who found that in Vanda, fertilizer solutions composed of combinations of 5 mM NO3− + 5 mM NH4+ showed the highest 15N use efficiency, followed by 10 mM NO3−, with the lowest N use efficiency in the 10 mM NH4+ treatment. Rouphael et al. (2016) suggested that nitrate-based fertilizer containing 14 mM NO3 and 1 mM NH4 is optimum for potted Hippeastrum due to higher WUEs and crop profitability.
Effects of N source on 15N concentrations in Hippeastrum organs15N concentrations were affected by the N source and growth stage of Hippeastrum. At Stage 1, plants supplied with 5 mM 15NO3− (T3) and 5 mM 15NH4+ (T4) had lower concentrations of 15N in the scales than plants supplied with mixed N forms (T1, T2), but there was no significant difference in 15N concentration in roots and the basal plate between N source treatments (Table 3). At the flowering stage (Stage 2; 3WAP), plants supplied solely with NH4+ had the highest 15N concentration in the scales and inflorescences (3.68 and 4.59 μg 15N·g−1DW, respectively). These results demonstrate that at the flowering stage, NH4+ (T4) is preferentially distributed to scales and inflorescences (as a strong sink organ) rather than combined forms (T1, T2) and NO3− (T3). At Stage 3 (15 WAP), plants supplied with 5 mM 15NO3− alone (T3) had the highest 15N concentration in roots + basal plates and scales, with 416 and 106 μg 15N·g−1DW, respectively (Table 3). This may indicate that when NO3− (T3) is the sole nitrogen source it is preferentially distributed to the roots + basal plates and scales, more so than NH4+ (T4) and combined forms (T1, T2). Lin et al. (2008) explained that after being taken up by root cells, NO3− must be transported across several cell membranes and distributed in various tissues. The AtNRT15 gene, located on the plasma membrane of root pericyclic cells close to the xylem, is involved in the long-distance transport of NO3− from the root to the shoot. Panjama et al. (2018) also reported that in Vanda, plants supplied with 15NO3− alone had the highest 15N concentration in roots, followed plants supplied with 15NH4+ and the combined forms.
15N concentration (μg 15N·g−1DW) in each organ of Hippeastrum supplied with different N sources.
The distribution of labelled N content in Hippeastrum was affected by both N source and growth stage. At Stage 1 (one treatment with 15N; 1 WAP), the majority of labelled N in all treatments was in the scales (S), ranging from 37–60% (11.8–15.6 μg 15N/organ). 15NH4+ in mixed N (T2) was more rapidly translocated to the scales than the single 15NH4+ form (T4). Moreover, plants supplied solely with NO3− (T3) had more labelled N in the leaves (35%, 14.3 μg 15N/organ) than plants supplied with mixed N sources (T1, T2) and solely NH4+ (T4) (Fig. 7). This suggests that scales and leaves are major organs of Hippeastrum for NO3− accumulation. Furthermore, the plants supplied solely with NH4+(T4) had more labelled N in the roots and basal plates (40%, 15.2 μg 15N/organ) than plants supplied with mixed N sources (T1, T2) and solely NO3− (T3) (Fig. 7). Hence, roots, basal plates and scales are major organs for NH4+ accumulation in Hippeastrum.
Percentage distribution of labelled N content in different organs of Hippeastrum supplied with different N sources (T1 = 2.5 mM 15NO3− + 2.5 mM NH4+, T2 = 2.5 mM NO3− + 2.5 mM 15NH4+, T3 = 5 mM 15NO3−, and T4 = 5 mM 15NH4+) at each growth stage (Stage 1 = 1 WAP, Stage 2 = 3 WAP, and Stage 3 = 15 WAP).
At Stage 2 (two treatments with 15N; 3 WAP), labelled N from mixed N sources (T1, T2) was abundant in roots and basal plates, differing from Stage 1 with a range of 46–64% (18.9–50.0 μg 15N/organ), followed by scales (21–32%, 13.6–37.1 μg 15N/organ), leaves (8–10%, 4.45–6.67 μg 15N/organ) and inflorescences (6–13%, 6.30–15.5 μg 15N/organ), respectively. This result indicates that at the flowering stage, roots actively uptake N from solution to use for flowering and that some of the N in scales is also translocated to the inflorescences. Therefore, the 15N abundance in scales decreases. However, plants supplied solely with NO3− (T3) contained more labelled N in the scales (50%, 37.1 μg 15N/organ) than plants supplied with a mixed N source (T1, T2) and solely NH4+ (T4) (Fig. 7).
At Stage 3 (nine treatments with 15N; 15 WAP), the majority of labelled N from mixed N sources (T1, T2) and solely NH4+ (T4) was found in the leaves, unlike in Stage 1 and 2, in which the major accumulation sites were the scales or roots and basal plates, respectively (Fig. 7). This result indicates that the leaves are a strong sink at this stage. At T3 (sole NO3−), a high percentage of labelled N was found in the roots, basal plates and scale organs. This suggests that some of the 15N in leaves is translocated to the roots, basal plates and scales. Khuankaew et al. (2010) reported that in Curcuma alismatifolia (bulbous plant) 41% of 15N was recovered in new rhizomes and 17% in new storage roots at the harvest stage.
ConclusionsThis research found that a combination of NO3− and NH4+ and NO3− alone promote plant dry weight in Hippeastrum. The distribution of labelled N differed by growth stage and N source. At Stage 1 (1 WAP) scales were a major site of labelled N (37–60%). At Stage 2 (3 WAP), roots and the basal plates contained the most labelled N (32–64%). At Stage 3 (15 WAP), leaves contained the most labelled N (26–44%). Hippeastrum prefers NO3− by itself or mixed N forms over NH4+, with these treatments resulting in the highest 15N uptake and 15N use efficiency (15NUE), which will benefit food reserves in bulb scales for plant growth in the subsequent season.
This research work was partially supported by Chiang Mai University. We thank H.M. and the King’s Initiative Centre for Flower and Fruit Propagation, Chiang Mai, Thailand for their kind support.
