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.