Introduction
Hydrangea macrophylla (family: Hydrangeaceae; order: Cornales) is a deciduous shrub mainly distributed in East Asia, including Korea, Japan, and China, with its origin known to be China and Japan (Griffiths, 1994; Larson, 2012). Its flowers mainly bloom from late spring to fall, and the flower’s colors, referring to the color of the sepal, are diverse, including red, blue, pink, and white. Recently, hydrangeas have been popular with consumers in South Korea, as well as around the world, and are widely sold as cut, potted, and garden flowers (Yeon et al., 2022). Sales of hydrangeas as cut flowers in 2022 amounted to 3.96 billion won, a 3.5-fold increase from 10 years ago (MAFRA, 2023).
The sepal color of hydrangea cultivars changes depending on cultivation conditions. It was reported that the color becomes blue in acidic soil and pink in alkaline soil (Allen, 1943; Kodama et al., 2016). It was also found that aluminum ion concentration plays an important role in changing the sepal color. Since aluminum ions are easily soluble in acidic soil, they are absorbed through the roots and move to the vacuoles of the sepals, making them blue. On the other hand, in neutral or alkaline soil, they precipitate as Al(OH)3 to be insoluble, making sepals pink or red (Allen, 1943; Takeda et al.,1990; Schreiber et al., 2011). As such, it is known that the change in sepal color is determined by the pH of the soil, as well as by the concentration of aluminum.
The reason why the sepals of hydrangea are pink, red, and blue is because they contain delphinidin-3-glucoside, one of the anthocyanins (Asen and Siegelman, 1957), and the research has been conducted on the role of this pigment in relation to the color change. Allen (1943) reported that aluminum in the soil is absorbed into the sepals of hydrangea and turns blue when combined with delphinidin-3-glucoside. Moreover, the role of copigments is also important in the expression of blue color. In hydrangea cultivars with blue sepals, the blue color is formed by the delphinidin-3-glucoside-aluminum-3-caffeoylquinic acid complex, which combines the copigments of 3-caffeoylquinic acid, aluminum, and anthocyanin (Takeda et al., 1985a, b; Kondo et al., 2005). Yuan et al. (2023) also found that hydrangea cultivars that turn blue in acidic soil had a high ratio ofdelphinidin-3-glucoside, delphinidin-3-arabinoside, and delphinidin-3-arabinoside-5-glucoside of anthocyanins. Notably, the ratio of delphinidin-3-glucoside was found to be high, indicating that it plays the most important role in forming the blue complex of hydrangea sepals.
Depending on the pH and aluminum concentration of the soil, not all sepal color of hydrangea cultivars changes in the same way. When in an acidic medium, the soil was treated with AS, the sepal color of some hydrangea cultivars changed from pink to blue, while other cultivars were maintained as pink (Yuan et al., 2023). In addition, Kodama et al. (2016) suggested that ‘Ruby Red’, a hydrangea cultivar with red sepals, maintained its red color regardless of whether it was in alkaline soil or acidic soil treated with aluminum, while ‘Frau Yoshimi’ has red in alkaline soil but turns blue in acidic soil. As such, the sepal color of hydrangeas appears to depend both on the pH and aluminum concentration of the soil and on their cultivars.
Most hydrangeas in South Korea are grown from cultivars bred overseas. To date, 15 hydrangea cultivars bred in S. Korea have been registered, but those distributed through auction houses are limited to a few cultivars, including ‘Morning Star’ and ‘Pink Ari’, which have pink sepals (KAFFTC 2023). To commercially produce hydrangea cultivars for cut or potted flowers, the stable sepal color desired by growers is required. However, there is a lack of research on changes in sepal color according to soil pH and aluminum treatment targeting cultivars bred in South Korea.
Therefore, this study was conducted to examine changes in medium pH, stem growth, flowering characteristics, and sepal color by AS treatments, and to find out whether it could be used as cut or potted flowers in Korean hydrangea cultivars ‘Pink Ari’, ‘Morning Star’, and ‘Green Ari’.
Research Methods
Plant materials
This experiment was conducted in a glass greenhouse at the trial fields of the Jeollanam-do Agricultural Research & Extension Services (JARES) located in Naju-si, Jeollanam-do. The rooted cuttings of three cultivars bred and registered by JARES were used as experimental materials, including ‘Pink Ari’ with deep pink sepals, ‘Morning Star’ with pink ones, and ‘Green Ari’ with pink and green ones. After collecting the apical bud from a 3-year-old mother plant on August 1, 2021, a 6 cm long cutting with 3 leaves was cut, and rooted cuttings were collected 60 days after cutting.
AS treatment and cultivation management
Rooted cuttings (with a plant height of 12±2 cm and 6 leaves) were planted on October 1 in plastic pots with a diameter of 15 cm (1.2 L) to which 300 g of horticultural medium (Punong Sangto, Punong Co., Ltd., Gyeongju, Korea) was added, and 3 g, 9 g, and 15 g of AS (Al2(SO4)3· 18(H2O), Duksan Chemicals, Incheon, Korea) per pot were mixed with horticultural media. Twenty pots per AS treatment were used for this experiment. The rooted cuttings planted in the pots were cultivated under a sub-irrigation system, and the nutrient solution was supplied by mixing 5[Ca(NO3)2·2H2O]NH4NO3 148 g, KNO3 76 g, NH4NO3 16 g, KH2PO4 32 g, MgSO4·H2O 60 g, FeEDTA (12.5%) 2 g, H3BO3 0.252 g, CuSO4.5H2O 0.04 g, ZnSO4.7H2O 0.202 g, MnSO4·H2O 0.166 g, and Na2MoO4·2H2O 0.026 g per 200 L. When the daytime temperature was above 30°C, it was managed with ventilation fans and fog spray, and the nighttime temperature was managed under natural conditions. The temperature and integrated solar radiation in the glass greenhouse during the cultivation period are shown in Fig. 1.
Thirty days after treatment with aluminum sulfate, medium was collected from a depth of 5 cm at the pots of ‘Pink Ari’ and ‘Morning Star’, and the pH was measured three times using a pH meter (Orion 3 Star, Thermo Fisher Scientific Inc., Massachusetts, USA). The growth was examined on June 8, the flowering season of the three cultivars. Plant height, leaf length and width, stem diameter, chlorophyll content, inflorescence width and height, number of inflorescences, and sepal color were surveyed. Plant height was measured from the highest part of the plant to the soil surface of pots, while leaf length, leaf width and stem diameter were measured on leaves and stems of second lower node from the apex. Chlorophyll content was measured using a chlorophyll meter (SPAD-503, Konica Minolta, Tokyo, Japan). Inflorescence width and height were measured for the largest inflorescence. Sepal color was examined by measuring Hunter ‘L’, ‘a’, and ‘b’ values with a colorimeter (Chroma Meter CR-400, Konica Minolta, INC., Tokyo, Japan).
Statistical processing
A one-way analysis of variance was performed on the research data on the pH of bed soil, stem growth, and flowering characteristics using IBM SPSS (Version 22.0 software, IBM Corp., 2019). The means between all AS treatments were compared using Duncan’s multiple range test at a confidence level of 95 %.
Results
Growth and flowering characteristics of hydrangea
‘Pink Ari’ The pH of the medium by amount of AS treatment was surveyed 30 days after planting in hydrangea ‘Pink Ari’. When not mixed with AS, the pH was 5.1, but the higher the amount of AS mixed, the lower the pH 3.9 when mixed with 3 g; and 3.4 and 3.3 when mixed with 9 g and 15 g, respectively (Fig. 2a).
The shoot growth of the hydrangea ‘Pink Ari’ was surveyed during the flowering season after AS treatment. The plant height was the longest at 54.2 cm in those untreated with AS, and as the amount of AS for the treatment increased, it became shorter, with those treated with 15 g being shortest at 17.4 cm. There was no significant difference in leaf length and width or in stem diameter between control and treatment with 3 g of AS, and the higher the amount of AS, the shorter they were (Table 1). The chlorophyll content of leaves was lowest in those treated with 15 g at 39.2, and there was no statistically significant difference between the other treatments. In addition, the mortality rate was the highest in those treated with 15 g at 25%, the largest amount of AS, but there was no mortality for other treatments.
Flowering characteristics by amount of AS for the treatment were surveyed. The number of inflorescences were the highest at 3.6 in the treatment with 3 g of AS. As the amount of AS increased, the number of inflorescences decreased, with the fewest at 0.3 for the treatment with 15 g (Table 2). The width and height of inflorescences were found to be larger in control and the treatment with 3 g than in those with 9g and 15g. The sepal color was deep pink in control, where the values of Hunter ‘L’, ‘a’, and ‘b’ were 57.2, 26.5, and −1.8, respectively. Under treatment with 3 g of AS, the ‘L’ value, which indicates brightness, increased, changing the sepal color to a brighter color, and the ‘a’ value, which indicates the degree of redness and greenness, decreased (22.1), making the color turn lighter pink than control. Under treatment with 9g of AS, the ‘L’ value increased (64.7) compared to control; the ‘a’ value decreased to 16.0; and the ‘b’ value, which represents the degree of yellowness and blueness, also decreased significantly to −8.5, increasing the level of blueness, but the sepal color appeared to be light pink. Under treatment with 15 g, the ‘a’ value decreased further, reducing the level of redness further, and the color appeared to be light pink (Fig. 3).
Growth and flowering characteristics of hydrangea ‘Morning Star’
The pH of the medium in control was 5.0 in hydrangea ‘Morning Star’, but it decreased as the amount of AS treatment increased. It was 4.1 in the treatment with 3 g of AS, and 3.4 and 3.3 in the treatment with 9 g and 15 g, respectively, which was similar to the results of ‘Pink Ari’ (Fig. 2b).
The plant height by AS treatment was the longest under control, at 88.4 cm. As the amount of AS treatment increased, it became shorter, being shortest at 14.6 cm under treatment with 15 g (Table 3). Its leaf length and width and its stem diameter were larger under control and treatment with 3 g than under other treatments, and became smaller as the amount of AS increased. The chlorophyll content of leaves was lowest at 30.8 in the treatment with 15 g, and there was no statistically significant difference between the other treatments. The mortality rate was 5% and 75% under treatment with 9 g and 15 g, respectively.
The number of inflorescences according to the amount of AS was 1–1.3 in control and the treatment with 3 g of AS, which were more than in the treatments with 9 g and 15 g. The width and height of inflorescences also showed similar results (Table 4). The sepal color appeared to be pink under control and treatment with 3 g, and there was no statistically significant difference in the ‘L’, ‘a’, and ‘b’ values between the two treatments. Meanwhile, under treatment with 9 g, the ‘a’ value decreased to 5.0, and the ‘b’ value, which represents the degree of blueness, decreased significantly to −23.0, causing the sepal color to change to blue. Under treatment with 15 g, the ‘L’ and ‘b’ values increased to 69.9 and −12.8, respectively, which were higher than under treatment with 9 g, and the ‘a’ value decreased to −0.7, visually showing a light blue color (Fig. 4).
Growth and flowering characteristics of hydrangea ‘Green Ari’
When ‘Green Ari’ hydrangea was treated with AS mixed into the medium, the plant height was the largest at 46.1–58.4 cm under control and treatment with 3g of AS. On the other hand, it was significantly decreased, to 32.1 cm and 20.1 cm, following the treatments with 9 g and 15 g. There was no statistically significant difference in leaf length and width or in stem diameter between control and treatment with 3 g, while they all were significantly decreased under treatments with 9 g and 15 g (Table 5). The chlorophyll content was lowest under treatment with 15 g at 36.6, while there was no statistically significant difference between the other treatments. Moreover, there was no mortality in the treatment with 0–9 g, and a 20% mortality rate was found in the treatment with 15g.
The number of inflorescences was highest at 2.2 under treatment with 3 g AS, and was lowest at 0.4 under treatment with 15 g AS. There was no difference in the width and height of inflorescence between control and treatment with 3 g, and they were smallest under treatment with 15 g (Table 6). The ‘Green Ari’, with multi-colored sepals of pink and green, had Hunter ‘L’, ‘a’, and ‘b’ values of 63.6, 18.6, and 1.2, respectively, under control. Under treatment with 3 g of AS, the ‘L’ value increased to 71.9, which was higher compared to control, and the ‘a’ value was decreased to 11.3, changing the sepal color to light pink. Under treatment with 9 g and 15 g, the ‘L’ value slightly decreased to 66.8 and 68.8, respectively, which was lower compared to treatment with 3 g, and the ‘a’ (7.1 and 5.0, respectively) and ‘b’ value (−9.5 and −8.4, respectively) decreased, showing a light blue sepal color. Notably, for the treatment with 9 g, the green color of sepals gradually became lighter, and for the treatment with 15 g, most of the green color disappeared (Fig. 5).
Discussion
The color of hydrangea sepals is known to change based on soil acidity, aluminum concentration of the medium, and content of inorganic elements. Allen (1943) reported that the color of hydrangea sepals was blue when soil has a pH of 5.5 or less, and an intermediate color between pink and blue at a soil pH of 5.5–6.2; while at a soil pH 6.7 or higher, the blue color in the sepals completely disappeared and turned pink. However, the ‘Blue Sky’ hydrangea’s sepal color remained blue even at a soil pH of 6.5 or 5.5 (Kodama et al., 2016). Even for the ‘Pink Ari’ hydrangea in this study, the sepal color appeared pink and did not change under acidic conditions where the soil pH was 5.0 (control), or 3.9 and 3.4 (treatment with 3 g and 9 g of AS, respectively). These results indicate that soil pH affects the sepal color change differently depending on hydrangea cultivars.
In this study, as the amount of AS increased, plant height, leaf length and width, stem diameter, and number and size of inflorescences decreased in hydrangeas ‘Pink Ari’, ‘Morning Star’, and ‘Green Ari’. Notably, under treatment with 15 g of AS, the plant height was significantly reduced, to 14.6–20.1 cm, and the mortality ratio also increased significantly, to 20–75%. When ‘Early Blue Rose’ hydrangea was treated with 0–40 g of AS per pot with a diameter of 15cm, the plant height was reported to decrease significantly, from 27 cm to 16 cm, while the plant width was reduced from 40 cm to 26 cm (Landis et al., 2019). In addition, when hydrangea cultivars ‘Curly Sparkle’ and ‘Hor Tivoli’ were treated with AS, the dry weight of the plants was decreased by 29% and 51%, respectively, under treatment with 15 g of AS compared to control, and the size of inflorescence was also significantly reduced (Landis et al., 2021a, b). When the soil pH is less than 5.0, the Al3+ form of aluminum in the soil increases and is absorbed into the plant, showing strong toxicity. It mainly inhibits the elongation and differentiation of roots, hindering the absorption of nutrients and water, and limits plant growth and development by inhibiting chlorophyll synthesis and reducing photosynthetic efficiency (Ma et al., 2001; Kochian et al., 2004; Panda et al., 2009). Therefore, even for the three cultivars of hydrangeas targeted in this study, as the amount of AS for the treatment increased, it seems that the amount of Al3+ absorbed into the plant increased, resulting in a reduction in chlorophyll content, and an inhibitory effect on the growth of stems and the development of inflorescences. And, when treated with more than 9 g of aluminum sulfate, the plant heights of the three hydrangea cultivars were shortened to less than 34.7 cm, making them unsuitable for cultivation as cut flowers.
The color of hydrangea sepals varies depending on the concentration of aluminum in the soil. In cultivating the hydrangea cultivar ‘Hot Red’, the pH of medium leachate that was not treated with AS was 4.7, but the pH of medium leachate treated with 3 g·L−1 of AS was reduced to 3.5, making the color change to blue (Landis et al., 2021a). Moreover, for the ‘Early Blue’ and ‘Hor Tivoli’ hydrangeas, which have blue sepals, as the aluminum concentration increased, the soil pH decreased and the sepal color turned deep blue (Landis et al., 2021b). Schreiber et al. (2011) surveyed the aluminum content according to the color of hydrangea sepals: 0–10 μg/g for red sepals, 1–40 μg/g for purple ones, and 40 μg/g or more for blue ones, suggesting that the critical aluminum content required to change red sepals to blue is about 40 μg/g. In this study, the pH of medium became more acidic as the amount of AS applied for the treatment was increased. Under treatment with 9g of AS, the sepals of ‘Morning Star’ changed from pink to blue, and the multi-colored sepals of ‘Green Ari’ changed from a combination of pink and green to one of light blue and green. As such, when the medium is treated with AS, the medium pH is lowered and becomes acidic. In acidic soil, the solubility of aluminum increases. As the amount of aluminum absorbed into the plant increases, the color of hydrangea sepals changes.
However, the change in hydrangea sepal color according to the concentration of aluminum in the soil varies depending on the cultivars. For the hydrangea cultivars ‘Saxon Kleiner Winterberg’ and ‘Clarissa’ with white sepals, the color of the sepals did not change, even in soil treated with 20 g of AS (pH 5.0–5.5). However, the hydrangea cultivars ‘Early Blue’, ‘Baby Blue’, ‘Bailmer’, ‘Bela’, and ‘Atlantic’, which have pink sepals, turned blue in soil treated with AS, while ‘Saxon Sonnenstein’ and ‘Duro’, which have pink sepals, did not change to blue and remained pink. Moreover, hydrangea ‘Ruby Red’, which has deep pink sepals, maintained its color even in alkaline soil with a pH of 6.5 or acidic soil with a pH of 5.5 treated with AS, and ‘Blue Sky’, which has blue sepals, also remained blue as it was. On the other hand, hydrangeas ‘Frau Yoshiko’ and ‘Frau Yoshimi’, which have pink sepals, maintained their pink color in alkaline soil of pH 6.5, but turned blue in acidic soil of pH 5.5 treated with AS. In this study, the hydrangea cultivar ‘Pink Ari’, which has deep pink sepals, maintained its pink color even in soil with a pH that was acidified to 3.3–3.9 through treatment with 3–15 g of AS; whereas in soil treated with 9g of AS, the sepal color of ‘Morning Star’ changed to blue and that of ‘Green Ari’ changed to light blue, showing that the effect of AS varies depending on hydrangea cultivars.
Conclusion
When cultivating the hydrangea cultivars ‘Pink Ari’, ‘Morning Star’, and ‘Green Ari’ by mixing AS sulfate into the medium, it seems that the soil pH was greatly lowered and acidified, resulting in increased aluminum absorption into the plants. Accordingly, when treated with 15 g of AS, the height of all three cultivars was decreased to 14.6–20.1 cm due to damage caused by the excessive absorption of aluminum, and the size of leaves, stem diameter, and number and size of inflorescences also decreased. In addition, when treated with 15 g of AS, the mortality rates of ‘Pink Ari’, ‘Morning Star’, and ‘Green Ari’ were 25%, 75%, and 20%, respectively, and in particular, ‘Morning Star’ reacted sensitively to AS. When treated with less than 3 g of AS, the sepal colors of three cultivars did not change. But, when treated with 9 g of AS or more, the color of ‘Pink Ari’, which has pink sepals, changed to light pink, while the pink sepals of ‘Morning Star’ and ‘Green Ari’ were changed to blue and light blue, respectively.
For the hydrangea ‘Pink Ari’ that is mainly cultivated for cut flowers, the height of those treated with 3 g of AS was slightly shorter (41.6 cm) than the height of control. However, it seems that they have sufficient value as a potted product since the number of inflorescences with pink sepals was 3 to 4, blooming more than control. Meanwhile, those treated with 9 g of AS had a short height of 24 cm, and the number of inflorescences was few at 1.3, making them unsuitable even as potted flowers.
For the hydrangea ‘Morning Star’ cultivated for cut flowers, the height of those treated with 3 g of AS was 56.7 cm, which was shorter than those of control. However, there was no statistical difference between them in stem growth (e.g., leaf size and stem diameter) or in flowering characteristics (e.g., inflorescence and sepal color). Meanwhile, in those treated with 9 g of AS, the sepal color was changed from pink to blue, but the height was shortened to 34.7 cm and the number of inflorescences was 0.5, indicating low marketability which makes them unsuitable as a cultivated product. Therefore, it is necessary to conduct further experiments on the ‘Morning Star’ by treating them with AS within the range of 3–9 g to determine the concentration that would maintain marketability while changing the sepal color.
When growing the hydrangea ‘Green Ari’, those treated with 3 g of AS had a higher number of inflorescences (2.2) than those of control, and there was no statistical difference between them in stem growth or in sepal color. As such, they appeared to have a higher commercial value as cut or potted flowers than those of control. The height of those treated with 9 g of AS was shortened to 32.1 cm, but the number of inflorescences was 1.5, and the multi-colored sepals changed from a combination of pink and green to a combination of light blue and light green. Therefore, it seems to be desirable to treat 9 g of AS for cultivation of potted plants with bicolor sepals of light blue and light green.
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