The species is dioecious, with a clear distinction between male and female plants. Female plants have only a few branches to a growth height of 1.5 m on single stems compared to more multi-branched stems from male plants (Rebelo 2001; Carolus & Porter 2008). The leaves of the species are elliptic and slightly larger in female plants, reaching a length of 14–57 mm and width of 7–21 mm, whereas leaves from male plants grow up to 13–49 mm long and 5–19 mm wide (Rebelo 2001). Both male and female plants have yellow involucral leaves which overlap the base of the flower head. Flowers appear from July to September and the fruits and cones develop in February (Hall & Veldhuis 1985; Carolus & Porter 2008).
The species mainly reproduces from seeds in its natural habitat. The pungent smell of the male and female flower heads indicates that they are insect-pollinated by flies and beetles and the main pollinator is the monkey beetle (
Germination of the seed is fire-dependent to ensure that the species reproduction cycle is completed. Proteaceae fynbos must burn at an age of 6 to 45 years to propagate (Brown & Botha 2004). Some species are either resprouters or reseeders, meaning they either grow again from an existing plant or they grow from seed after a fire. The reseeders either keep their seeds safe in a cone until a fire occurs or drop them after a few months, and ants bury them in the ground to keep them safe from rodents and consumption by fire (Brown & Botha 2004; Holmes & Newton 2004).
Considering the economic potential of both male and female cut flowers, there have not been many attempts to cultivate these sexes individually. Earlier reports from Laubscher and Ndakidemi (2008a, b) indicated that there is no scientific evidence that
The experiment was conducted from 23 May until 24 September 2019 in the propagation greenhouse on the Cape Peninsula University of Technology, Bellville Campus South Africa. This part of the greenhouse is a semi-controlled environment (Viljoen et al. 2021). The sides of the greenhouse are made of insect-proof netting to allow for natural airflow and moisture penetrating the building; however, the roof prevented rain from above. Temperatures ranged between 15 and 26 °C while the relative humidity ranged from 90 to 96% (Faber et al. 2020). During the rooting period, plant bags were placed on hotbeds with a temperature of 22 °C and received intermitted mist irrigation alternating every 40 minutes for 21 seconds (Nichols 2005; Malan 2012; Reinten 2014). The amount of water received per interval of irrigation measured 500 ml during the rooting stage. Once the cuttings had rooted, they were moved to a hardening-off section where they received a mist spray every 50 minutes for 18 seconds; the amount of water received per interval of irrigation measured 150 mL.
The plant material of
The treatments consisted of three rooting hormones, namely indole-3-acetic acid (IAA), 1-naphthaleneacetic acid (NAA), and indole-3-butyric acid (IBA) (Sigma Aldrich) at concentrations of 2000 ppm, 4000 ppm, and 6000 ppm. The treatments were laid out in a randomized block design with ten female and ten male plants to allow for 10 repetitions for each treatment. Treatments were done with cuttings dipped in the liquid for ten seconds at a depth of one to two centimeters. The control cuttings were not treated with growth regulators.
Cuttings were continuously monitored for the stress of overwatering and treated weekly with a preventative fungicide spray. A ratio 1:1 of a mixture of Captab (4 g·L−1), and 5 mL·L−1 Kelpak, fertilizer containing seaweed extract, manufactured by Kelp Products (Pty) Ltd. (Simon’s Town, South Africa) was sprayed on the cuttings at 12 pm weekly (Laubscher & Ndakidemi 2008a; Xego 2017). Overwatering, signs of blacking on leaves and stems, and rotting were monitored during the rooting period and yellow and dead leaves were removed from cuttings. At week 13, the cuttings were moved to the hardening-off condition, where they received 150 ml of water via mist sprayers every 50 minutes for 18 seconds. The plants were sprayed weekly with a mixture of Captab (4 g·L−1) and Kelpak (5 mL·L−1).
At week 18, the cuttings were removed, the substrate was carefully rinsed off the roots, and measurements were done. The callusing percentage was determined on the number of cuttings that developed successful callus tissue of parenchyma cells which covered the wounded area at the basal cut end and node of each cutting. The rooting result was presented as the number of cuttings that developed adventitious roots from the callused tissue, the number of roots, and the length of the longest root. Rooting time could vary but cuttings should root between six to sixteen weeks and develop roots that were visible on the sides of the bag as an indication that they are ready to be planted out (Brown & Duncan 2006; Reinten 2014).
The data was analyzed by using a two-way analysis of variance (ANOVA), with the computing software program STATISTICA 13 and MINITAB 17. The occurrence of statistical differences were determined by using the Fisher's least significant difference (LSD) at values of p ≤ 0.05 levels of significance.
Cuttings that were not treated with auxin did not form callus or roots, whether they were from male or female plants, and 50% of them died, also regardless of their origin. The value of rooting parameters depended on the type and concentration of auxin. Overall, the male plant cuttings took root in a higher percentage, but it was dependent on the type and concentration of auxin.
The auxin IAA at 4000 ppm was highly successful with an 80% callusing although it was not statistically different from other treatments except for NAA 600, IBA 400 and 600 ppm (female), and IAA 6000 and NAA 6000 (male). The treatments of the 2000 and 4000 ppm IAA were most effective for callusing of male and female cuttings (Fig. 2).
The cuttings’ ability to form roots was a reflection of their ability to form callus. The greatest number of female cuttings (80%) formed roots after treatment with IAA at a concentration of 4000 ppm (Fig. 3). After treatment with IBA 4000 or 6000, approximately 20% and 10% of the female cuttings took root and the female cuttings treated with NAA 6000 ppm did not take root at all. Male cuttings took root in 70% after treatment with IAA 4000 and 6000 ppm, NAA 4000 and IBA 6000 ppm. Significantly fewer cuttings rooted after the IAA 6000 ppm treatment (20%) and after the 6000 ppm NAA treatment (40%).
The greatest number of roots on female cuttings (over 30) was formed after the IAA treatment of 4000 ppm (Fig. 4). Similar results, from 15 to 20 roots, were obtained on female cuttings treated with 2000 and 6000 ppm IAA, 2000 and 4000 ppm NAA and 2000 ppm IBA. About 5 roots formed on female cuttings treated with IBA 4000 and 6000 ppm. Male cuttings rooted significantly less after treatment with IAA 6000 ppm, NAA 6000 ppm, and IBA 6000 ppm (from 2 to 6 roots). The remaining auxin combinations produced a similar number of roots (14–17).
The longest roots, from 85 to 102 mm, were found among rooted female cuttings treated with IAA and IBA of 2000 ppm (Fig. 5). Roots from 50 to 70 mm were produced on female cuttings treated with NAA 2000 and 4000 ppm, while those rooted after IBA 4000 and 6000 ppm treatments were 18 to 30 mm long. The roots of the male cuttings were generally longer than that of the female ones, measuring from 60 to 100 mm; only the roots after IAA treatment at 6000 ppm were just over 20 mm.
The best results of callusing of female and male cuttings of
In this study, the cuttings of the female gender rooted the best with the use of IAA, with a maximum at 4000 ppm. The male cuttings accepted IBA better, especially at 2000 and 4000 ppm. According to reports of Perry and Trueman (1999) NAA should be avoided as auxin for rooting of
IBA has been reported as the most popular auxin used for cuttings propagation with various degrees of success documented in Proteaceae. The concentration IBA of 2000 ppm (Faruchi et al. 1997; Reinten 2014) was recommended for rooting of
As reported by Pérez-Francés et al. (2001), the cuttings of
As can be seen from the above results, IAA at 4000 ppm consistently yielded the best results for the female cuttings. These results support Laubscher and Ndakidemi’s (2008a) studies, who observed a significant difference in the root length and number of roots by IAA concentration. The male cuttings did not show consistent results; the only certain result was that IBA was the best hormone for them in all concentrations with the exception of root number, where 2000 ppm was better than other concentrations. In the previous research by Laubscher and Ndakidemi (2008b), it was shown that IBA at concentrations of 4000 ppm and higher suppressed rooting, and similar results were shown with all hormones used in this experiment.
Due to availability and access to cutting material, it was difficult to complete this experiment during autumn, as was recommended as highly efficient by Ben-Jaacov and Silber (2006), Brown and Duncan (2006), Malan (2012), and Reinten (2014). Although the influence of seasons was not studied in our experiment, it is interesting to note that the collection of cuttings later in the season provided an extended period for cutting propagation of
This study established that