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.