3.1 Effects of light intensity and wavelength on germination
Variations in the PPFD of different LEDs wavelengths were reported. The highest PPFD values were observed in the Red > White > Blue > Red and Blue LEDs (Table 1).
The percentage of seeds germinated was affected by wavelength and PPFD, but there was no interaction between these two factors. The highest percentage of germination range from 60.8% to 61.7% and was reported in the white, red, and red/blue LEDs. The use of only blue LEDs resulted in a drastic reduction in seed germination to 37.5% (Fig. 3A) and the lowest GSI values (Fig. 3B).
About the PPFD, the gradual increases of light PPFDs resulted in a positive and significant correlation with the percentage of germinated seeds of M. zenhtneri (Fig. 3C). The GSIs values were also increased in the higher PPFDs (II and III), compared to the lowest PPFD used (Fig.3B).
The germination of the first seeds was reported four days after seeding (DAS) in the red/blue and white LEDs and at seven DAS in the other wavelengths. The blue light resulted in the late beginning of germination, at nine DAS, with the lowest average germination speed (AGS) (Fig. 3D). For the red, white, and red/blue LEDs, the maximum AGSs occurred between DAS 9 and 14 (Fig. 3D). Increases in PPFD were those resulting in the highest values of AGS.
The different wavelengths also affected the color of embryos after germination. In the red light, 100% germinated embryos had a light green color, related to chlorophyll biosynthesis, while in the blue LED and white LED, the red-purple color was predominant in the embryos. Intermediately, the presence of green and purple embryos in the red/blue LED was also observed (Fig. 2).
The use of darkness for the germination of M. zehntneri seeds drastically reduced the germination rate from 63.75% (control using light) to 11.3, 2.5, and 3.8% when seeds were cultivated in darkness for 10, 20, and 30 days, respectively (Table 2). Interestingly, seeds subjected to darkness for these short-period treatments, followed by exposure to light conditions were not able to germinate, for up to 12 months, suggesting the acquisition of secondary and light-irreversible dormancy.