Differentiation of germination characteristics in Scutellaria rubropunctata (Lamiaceae) associated with adaptation to rheophytic habitats in the subtropical Ryukyu Islands of Japan

Abstract

Seed germination characteristics of a facultative rheophyte Scutellaria rubropunctata, a perennial skullcap endemic to the subtropical Ryukyu Islands of Japan, were investigated at temperatures of 15 °C, 20 °C, 25 °C, and 30 °C. The rheophytic type of S. rubropunctata showed relatively simultaneous germination times and reached high germination rates under all temperatures tested. This characteristic may allow it to establish quickly without being influenced by temperature, giving a survival advantage in rheophytic habitats by avoiding seeds being washed away by the rising water. In contrast, the final germination rate of the terrestrial type of S. rubropunctata, as well as that of the congener S. indica var. parvifolia, was highest at 15 °C and declined drastically at 30 °C. Seeds of both of these plants that remained ungerminated at higher temperatures germinated soon after the temperature was changed to 15 °C, indicating that the germination suppression was a state of enforced dormancy that was released after transfer to the optimal temperature. These characteristics suggest an asynchronous germination which is thought to reduce the risk of disturbance in fluctuating environments. Our study may be the first to demonstrate the differentiation of seed germination characteristics in relation to adaptations to a rheophytic habitat. Because rheophytes occupy such a harsh environment under strong selective pressure, the adaptive differentiation of seed germination characteristics may occur in many other rheophytes.

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Acknowledgements

Seed sampling at Hiji River was conducted with the permission of Kunigami Village, Okinawa Prefecture. We are grateful to Mr. S. Taba and other stuff at the Kunigami Tourism Bussan Center (Kunigami Yui Yui) for supporting our field activities at Hiji Falls camping site. This work was supported by JSPS KAKENHI Grant Number 17K07539. We thank Sarah Williams, PhD, from Edanz Group (www.edanzeediting.com) for editing a draft of this manuscript.

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Correspondence to Tetsuo Denda.

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Yoshimura, H., Arakaki, S., Hamagawa, M. et al. Differentiation of germination characteristics in Scutellaria rubropunctata (Lamiaceae) associated with adaptation to rheophytic habitats in the subtropical Ryukyu Islands of Japan. J Plant Res 132, 359–368 (2019). https://doi.org/10.1007/s10265-019-01103-z

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Keywords

  • Endemic species
  • Enforced dormancy
  • Germination strategy
  • Rheophyte
  • Ryukyu Archipelago
  • Simultaneous germination