4.2 Light strongly influenced seed germination in
Melocactus zehntneri
Regarding the wavelengths, except for monochromatic blue light that
reduced the germination percentage of M. zehntneri seeds (38%),
all other wavelengths resulted in germination percentages between 61.0%
and 62.0%. Except for monochromatic blue, all other LEDs tested in this
study contained in their spectral composition, emission peaks in the red
wavelength range, such as the monochromatic red LEDs, the red/blue LEDs,
and the white LEDs. Red light seems to be the most important wavelength
that accelerates, promotes, and increases the germination of seeds ofM. zehntneri seeds.
Red light is an important wavelength mediating seed germination. The
photomorphogenic responses, which involve light receptors like
phytochromes, control the plant development through the presence or
absence of light, and also the information and interpretation of
different wavelengths in the environment, which guides the most
appropriate development, including seed germination (Kami et al. 2010;
Neff et al. 2009).
Cho et al. (2012) also demonstrated that germination ofArabidopsis seeds is promoted by red light-enriched environments
by the activation of Phytochrome B, which results in a gradual increase
in the levels of gibberellins that trigger germination. Interestingly,
different studies in Cactaceae species sought to correlate the presence
of light with increasing GA3 in seeds due to germination
gains with these treatments (Ortega-Baes and Rojas-Aréchiga, 2007).
However, there was no strong evidence and no effectiveness of the
application of external GA3 which leads to the release
of a large number of seeds from dormancy in this plant family (Barrios
et al. 2020). The percentage of germinated seeds under different
treatments rarely exceeds the average values observed in the present
study with M. zehntneri .
The monochromatic blue light substantially reduced the seed germination
of M. zenhtnerii seeds. The reduction of seed germination in
response to blue light is not exclusive to the family Cactaceae and it
is frequently reported to inhibit the germination of dormant seeds in
cultivated grasses (Hoang et al. 2013; Jacobsen et al. 2013). Some
studies suggest that the interaction of blue light with cryptochrome 1
results in an increased concentration of Abscisic Acid (ABA) that
inhibit germination (Barrero et al. 2014; Hofmann 2014).
The majority, if not all, Cactaceae species are
positive photoblastic
(Rojas-Aréchiga and Garcia-Morales, 2022). Flores et al. (2007) reported
that from a total of 28 cactus species, all were considered positive
photoblastic, and these authors also described the occurrence of
secondary dormancy as a consequence of seed exposure to a period of
darkness during germination. The same result was observed with M.
zehntneri in the present study, in which light was necessary for seed
germination and exposure of seeds to dark conditions, even for short
periods (10, 20, and 30 DAS), significantly reduced the percentage of
seed germination (Table 2). Interestingly, even after a short period of
darkness (10 days), seeds are unable to germinate even after up to 12
months under the same light conditions that promote germination, with
the acquisition of irreversible secondary dormancy.
Under light conditions, the percentage of germinated seeds of M.
zehntneri was, on average, 40-60% (Magnani and Cardoso, 2022), showing
their photoblastic positive response. Otherwise, 40-60% of seeds are
not able to germinate under light conditions, suggesting another type of
dormancy not directly associated with light. However, the studies
carried out so far, even under light conditions and when applying other
types of treatments to break seed dormancy, fail to indicate the main
cause(s) of non-germination of seeds that remains dormant.