Germination responses to light were studied in the upper and lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). The lower seed was dark-germinating and negatively photoblastic; the upper one had a red-light (R) requirement and was positively photoblastic. Germination of the lower seeds was inhibited by a prolonged single irradiation with R, blue (B) or far-red (FR) light applied during imbibition. The maximal inhibitory effect of a single irradiation occurred 9 h and 13 h after the start of soaking at 33 °C and 23 °C, respectively. However, the inhibitory effect of R differed from that of B and FR, by only delaying germination. A single exposure to B or FR light could be replaced by intermittent B or FR irradiation, and their effects were repeatedly reversible by the following R irradiation. If the upper seeds were not exposed to R during imbibition, they failed to germinate even at 33 °C which was optimal for germination, and the promotive effect of R increased with delay of its application time. The photoperceptive locus in cocklebur seeds was the axial tissue for all B, R and FR. Light received by the cotyledonary tissue had little effect. Germination dimorphism in response to light is discussed with respect to the phytochrome content and the ageing of axial tissues.
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