Effect of Planting Time and Corms Treatment with Gibberellic Acid on Growth, Flowering, and Vase Life of Freesia hybrida ‘Corona’

The experiment was arranged as a two factorial with randomized complete block design (RCBD) with three replications. Each experimental treatment consisted of 5 corms (5 pots), in each replication (n = 15). The corms of freesia were soaked in a solution of GA₃ at 0, 75, and 150 mg·L⁻¹ for 60 min. before planting (the first factor). After the treatment, corms were left to dry for 2 hours and then planted. The second factor was represented by two dates of planting, 1st and 15th December.

Three growth parameters were recorded: number of days to sprout, number of leaves (measured when the first floret colored), and plant height (measured from the soil surface to the top of floret head). Flowering parameters (taken at the beginning of flowering) included a number of days from planting to flowering (to the appearance of inflorescence stem), number of days to the first floret coloring, inflorescence stem length, inflorescence stem diameter, floret head diameter, florets number per inflorescence, and fresh weight of inflorescences. After the appearance of leaves yellowing, corms and cormels were removed, and corms were cleaned after drying. Corm yield parameters included the mother corm diameter, fresh weight of the mother corm, fresh weight of the cormels, and their number per mother corm.

The water uptake and vase life were measured in the laboratory, at a temperature of 27 ± 2 °C and humidity of 65 ± 5%. The inflorescence stems were excised in the morning at the stage of opening and coloring of the first buds (Zencirkiran 2002), transferred to the laboratory, recut to 30 cm, and placed in separate vases containing 300 ml distilled water that was changed every 3 days.

Solution uptake was measured every 3 days, and vase life was taken from the time of the first floret opening to the time when more than 50% of the florets were visibly senescing and petals wilted.

All plants were utilized for measuring all parameters, and then the results were analyzed statistically using the analysis of variance (ANOVA) procedure with SAS software (9.0) in the randomized complete block design. The values of the means were compared by Duncan's multiple range test at the significant level p ≤ 0.05.

All plants under experiment sprouted. The planting time had a significant effect on the time of sprouting (Table 1). The results showed a significant difference between corms planted on December 15th that needed only 20.9 days and on December 1st that needed 31 days. A temperature of 18–22 °C is required for good germination, which in December is about 16–21 °C during daytime (Fig. 1), while 10 °C is adequate for slow growth, and 12–15 °C is suitable for faster growth (Mansour 1968).

The later planting shortened the time to flower by 17 days but did not affect the time of the first florets’ coloration. High temperatures delayed the process of flower initiation in freesia after planting in the temperature range of 9–15 °C (Berghoef et al. 1986). Also, the number of days to flower decreased with increasing greenhouse temperature, but the response varied somewhat among the cultivars; for example, Aurora cultivar flowered earlier at 15 °C than at 20 °C (Wulster et al. 1989). To delay flowering until May, Anderson (2006) planted corms from December 1st to December 20th. These results are in concurrence to that of Aftab et al. (2007), which reported a decrease in the time to flower from 145–146 days to 134–137 days when delayed planting date from 15th October to 30th October.

The GA₃ application did not affect the time of sprouting (Table 1). GA₃ corm-soaking did not influence the number of days to sprout but shortened the time to flower and delayed significantly flower coloration when was applied in higher concentrations. The sprouting time is connected with day and night temperature. Application of GA₃ speeds flowering in gladiolus (Baskaran et al. 2017; Baskaran & Abirami 2016). Speeding of freesia flowering with GA₃ treatment was significant only at the later corm planting. Application of GA₃ delayed also the first floret coloring. The results of Cocozza Talia (1985) showed that GA₃ treatment in freesia induces advanced flowering without changing the qualitative characteristics of the flowers. The improving effect of GA₃ on flowering characteristics may be due to their stimulatory effect on cell division, elongation, and differentiation. Earlier flowering of plants treated by GA₃ might be due to their vital role in the production and regulation of floral stimulus (Taha 2012).

The time of planting did not influence plant height contrary to the GA₃ (Table 1). Plants, which corms were soaked in the GA₃ solution 150 mg·L⁻¹, were 13.1% longer over the non-soaked control. The promotion of plant height with GA₃ in freesia was earlier reported by Żurawik and Placek (2013). This treatment was more effective (higher by 21.5%) at earlier planting. The main function of GA₃ is cell elongation and division, and it is known to be involved in various processes of plant development. In various plants, exogenous gibberellin promotes shoot elongation and plant height. This was confirmed in gladiolus (Padmalatha et al. 2014; Chopde et al. 2015; Baskaran et al. 2017; Baskaran & Abirami 2016), Iris (Al-Khassawneh et al. 2006), and Tuberose (Asil et al. 2011).

Contrary to plant height, the leaf number was regulated by the time of planting but not by GA₃ application (Table 1). This finding confirmed the results of Thompson et al. (2011) as they reported the higher number of leaves in Watsonia (Iridaceae) in plants growing under long-day and higher temperatures. Thus, it is desirable to grow freesia at 12–15 °C for obtaining 6–8 leaves prior to flowering, which is required for breaking dormancy of the corms and optimal quality and yield of freesia flowers (Dole 2003).

The result showed that later corm planting and soaking in GA₃ affected inflorescence lengthening (Table 2). Stem diameter was larger in plants in which corms were planted earlier and GA₃ did not affect this trait. These results are inconsistent with the results of Aftab et al. (2007), who reported that the freesia inflorescence length decreased with the planting delayed from 15th to 30th October. GA₃ increases inflorescence stem length through cell enlargement and cell division (Hartmann et al. 1990; Daykin et al. 1997; Kumar et al. 2013), which might have elongate the stalk length. Several workers have already reported an increase in the length of the stalk in Iris (Al-Khassawneh et al. 2006), in Tuberose (Asil et al. 2011; Shanker et al. 2011), and in Gladiolus (Baskaran et al. 2017; Baskaran & Abirami 2016; Sajjad et al. 2014) due to GA₃ treatment. Larger stem and floret head diameter and fresh weight of inflorescence were correlated with leaf number that provides more metabolites that can strengthen the plant.

Floret's head diameter was influenced by both studied factors. Different planting dates and various concentrations of GA₃ and interaction of these factors had a significant effect on floret head diameter. The diameter of the flower head and fresh weight of inflorescence were greater in plants treated with GA₃ (Table 2). The opposite results were obtained by Żurawik and Placek (2013), working with Easy Pot Freesias, and confirmed results of Kumar and Misra (2012) on tulip.

The number of florets per inflorescence did not depend on the time of corms planting but was higher in plants that corms were soaked in 150 mg·L⁻¹ GA₃ (Table 2). The least number of florets, 6.5 per spike, was noticed in control plants. According to Żurawik and Placek (2013), treating Easy Pot Freesia with gibberellic acid increases the number of flowers depended on the concentration of the gibberellic acid. The obtained results are in line with those of Pogroszewska et al. (2007) on Allium, Kumar and Misra (2012) on tulip, Baskaran et al. (2017) and Baskaran and Abirami (2016) on Gladiolus.

There were no significant differences in water up-take due to GA₃ application to corms (Table 2). The amount of water uptake increased by increasing GA₃ concentration. The interaction of planting date × GA₃ was significant. The best option was planting the corms on December 15th coupled with GA₃ at 150 mg·L⁻¹, which caused a maximum solution uptake of 8.6 ml.

It is worth noting that the maximum vase life was 6.8 days that was recorded for the corms planted on December 1st, compared to 5.3 days for the flowers obtained from corms planted on December 15th. The results in Table 2 show that treatment of GA₃ significantly extended the vase life of freesia cut flowers compared to control. The longest vase life, 7.3 days, was obtained by planting corms on December 1st and using 150 mg·L⁻¹ GA₃; while, the vase life of flowers from untreated corms was only 5 days. This result is in line with Ramzan et al. (2014) and Kumar et al. (2013) who found longer vase life of tulip obtained on plants treated with 400 mg·L⁻¹ gibberellic acids compared with the control plants. This result could be due to the role of gibberellin in controlling the transferring of nutrients from the leaves toward the flowers (Ahmed et al. 2016).

The planting dates did not affect the corm yield parameters except the fresh weight of cormels (Table 3). Delay of planting decreased the fresh weight of cormels by 30%. Soaking of cormels in GA₃, especially at 150 mg·L⁻¹, caused an increase in mother corm weight and diameter. Similar findings concerning the increase in bulb diameter were reported for gladiolus. The increase of weight of corms with the application of GA₃ can be attributed to its ability to increase the yield of photosynthetic products, which was reported in freesia by Żurawik and Placek (2013) and in gladiolus by Arora et al. (1992), Siraj and Al-Safar (2006), and Kumar and Misra (2012).

From the results of our investigation, it should be concluded that the choice of planting time of corms between 1st and 15th December could have significant consequences for freesia production. To the positives of earlier planting, anyone can account for more leaves, greater inflorescence and floret head diameter, greater fresh weight of inflorescence and cormels, and longer vase life. On the other hand, plants need more time for sprouting and flowering and produced shorter inflorescences. Flowers are produced almost at the same time as from corms planted two weeks later but inflorescences are stronger what influenced positively vase life. It has been suggested that an appropriate planting date can lower production costs by reducing the production period from 14–17 weeks to 12–13 weeks (AL-Sawaf & Alwan 2010; Jasim 2009) and enable the production of flowers of higher quality and greater market value by offering products on the local market in early February and mid-March, when the supply of blooming flowers is low.

On the basis of the presented results, it can be concluded that soaking corms in 150 mg·L⁻¹ GA₃ had a positive effect on increasing the plant height, and it also caused the earliest stalking with a higher quality of flowers, increasing such parameters as the inflorescence stem length, floral head diameter, florets number per inflorescence, fresh weight of inflorescence, longer vase life, as well as greater corm diameter and weight. Therefore, we recommend the planting of Freesia hybrida ‘Corona’ corms soaked in 150 mg·L⁻¹ GA₃ on December 1st in unheated greenhouses.