Effects of shade on germination traits of the endangered cycad Dioon edule (Zamiaceae)
The endangered cycad Dioon edule requires shade provided by filtered sunlight under the canopy of trees or maternal plants during initial growth stages. It is known that germination improves under shade, but there is no report of radiation conditions. In order to understand how photosynthetic photon flux density (PPFD) affect germination traits, we evaluated some germination indexes. A sample of three mature strobili and 200 viable seeds per strobilus were selected to evaluate seed size (length, width, and fresh weight). Two experimental treatments were established simulating shade under the oak forest canopy with PPFD 81 µmol m?2 s?1 (PPFD81), and under maternal plant canopy with PPFD 17 µmol m?2 s?1 (PPFD17), as measured previously in the study site. Means of response variables (germinability, velocity of germination, synchronization, mean germination time and relative frequency of germination) for the two treatments were compared using a t-test and a correlation analysis. No significant correlation between seed size and germination traits was detected. Germinability was higher at PPFD17 (89 %) than PPFD81 (39 %), but mean germination time was similar across treatments. The velocity of germination was greater under PPFD17 but synchronization was the opposite. The low PPFD stimulated D. edule germination, but also the spectral composition must be evaluated.
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