ABSTRACT
Seed germination in Melocactus and other cactus species is hampered by the occurrence of dormancy. However, most studies failed to achieve high seed germination rates, suggesting a complex mechanism of dormancy in Cactaceae. Thus, the objective of this study was to demonstrate that dormancy in Melocactus seeds may be associated with factors such as light and phytoregulators. Two consecutive experimental sets were designed: one with seed germination paper under different wavelengths and Photosynthetically Photon Flux Densities (PPFDs); and one in vitro experiment using a culture medium to evaluate the influence of different plant growth regulators, both in the germination of seeds of Melocactus zehntneri . The results obtained showed that seeds ofM. zehntneri are positive photoblastic. Red light and gradual increases in PPFD resulted in the highest germination rates and speed germination index. The experiment with phytoregulators showed a major effect of Ethephon to release the germination of dormant seeds of M. zenhtneri , totalizing 98% of seeds germinated under in vitro conditions, compared to 76% in control. The present study develops an efficient technique to break seed dormancy and the results can be useful to better understand cacti seed dormancy.
Key-words : cactus, dormancy breaking, in vitrogermination, wavelength, light intensity, ethephon
Introduction
The genus Melocactus comprises plants popularly known as ‘chapéu-de-frade cactus and Brazil is the country with the greatest diversity, with 55% total number of Melocactus species in the world, occurring throughout the Northeast region, and in parts of the North and Southeast regions of Brazil (Machado, 2009).
The demand for Melocactus species in the commercial sector, especially for ornamentation and collecting purposes, has had remarkable growth in recent years, due to its peculiar aesthetics, ornamentation, and easy cultivation. However, it is difficult to be precise about the exact numbers of the trade of these plants, since the illegal trade is difficult to quantify (Cavalcante and Vasconcelos, 2016). The fact is that many Melocactus populations have been drastically reduced (Martinelli and Moraes, 2013).
Consequently, some Melocactus species are already under protection, as a way of controlling the high extraction for trade (Cites, 1994; Cites, 2019). The species M. zehntneri is classified as a least concern species, but its population has also shown a recent and constant decline, being recently placed under protection (Bravo Filho et al. 2018a; Machado, 2017). In part, its population reduction can be attributed to two main factors: the continuous deforestation of the Caatinga Biome and the replacement of wild species with agriculture and livestock activities and, the recent increase of commercial exploitation, resulting in the extraction of individuals for the production of food, traditional medicines, and animal fodder (Fabricante et al. 2010; Zappi et al. 2011), but especially for ornamental purposes (Correia et al. 2018a).
Melocactus present in its apex a peculiar structure to the genus called cephalium, which occurs only when the cactus reaches reproductive maturity, at approximately ten years of age, mainly due to very slow vegetative growth (0.04-4.7 cm per year) (Lafite and Salimon, 2020). The natural reproduction is exclusively by seeds due to limited vegetative propagation since they do not develop segmentations as observed in other cactus species (Bravo Filho et al. 2018a).
In addition, seed germination in different species of Melocactusis hampered by dormancy. Different studies have reported the use of chemical scarification to increase germination rates, and associated dormancy with the rigid seed coat. Also, phytoregulators, such as Gibberellic Acid (GA3), have been used to increase the germination rate in Melocactus . In M. azureus , the imbibition of seeds in water or gibberellic acid (GA3) for 2 hours resulted in an increase of 14 and 20% seeds germinated, compared to only 3% germinated seeds in the control (Bárbara et al. 2015).
Also, the use of in vitro germination has been proposed as an alternative to increase germination rates in Melocactus . Successful in vitro germination was reported for M. glaucescens (68.1%), M. sergipensis (64%), M. zehntneri(58.7%), and M. violaceus (59.3%) (Santos, 2019). Additionally, previous results obtained by our group have demonstrated similar percentages of seeds germinated (50% on average) for M. zehntneri using the chemical scarification of seeds (Magnani and Cardoso, 2022). Thus, the results obtained with different species ofMelocactus showed that a substantial part of the seeds (40-85%) continues with some unknown type of dormancy, with no germination.
Thus, the present study aimed to test and determine the main factors that can affect the germination of M. zehntneri seeds, such as the wavelength, light intensity, and phytoregulators. The experiments, from a practical point of view, can solve and discover the main causes of dormancy in Melocactus seeds that do not germinate, enable in vitro conservation and serve as a method for propagation ofMelocactus species, and for further studies on seed dormancy breaking in Cactaceae.
Material and Methods
Plant material
For the experiment, Melocactus zehntneri was used, from the germplasm collection of the Center of Agrarian Sciences, UFSCar, catalog number ABBC280 (Fig. 1) by the Sistema Nacional de Gestão do Patrimônio Genético e Conhecimento Tradicional Associado(SisGen/Brazil). Three adult plants in full fruiting were used as a source of seeds, and fruit were collected at the moment they were detached from the cephalium (Fig. 1).