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).