J Integr Plant Biol. ›› 2017, Vol. 59 ›› Issue (12): 881-894.DOI: 10.1111/jipb.12598

Special Issue: Sexual Reproductions

• Molecular Ecology and Evolution • Previous Articles     Next Articles

Stigmatic exudate in the Annonaceae: Pollinator reward, pollen germination medium or extragynoecial compitum?

Jenny Y. Y. Lau, Chun-Chiu Pang, Lawrence Ramsden and Richard M.K. Saunders*   

  1. School of Biological Sciences, the University of Hong Kong, Hong Kong, China
  • Received:2017-08-25 Accepted:2017-09-05 Published:2017-09-06
  • About author:*Correspondence: E-mail: Richard M.K. Saunders (saunders@hku.hk)

Abstract:

Although “dry-type” stigmas are widely regarded as ancestral in angiosperms, the early-divergent family Annonaceae has copious stigmatic exudate. We evaluate three putative functions for this exudate: as a nutritive reward for pollinators; as a pollen germination medium; and as an extragynoecial compitum that enables pollen tube growth between carpels. Stigmatic exudate is fructose dominated (72.2%), but with high levels of glucose and sucrose; the dominance of hexose sugars and the diversity of amino acids observed, including many that are essential for insects, support a nutritive role for pollinators. Sugar concentration in pre-receptive flowers is high (28.2%), falling during the peak period of stigmatic receptivity (17.4%), and then rising again toward the end of the pistillate phase (32.9%). Pollen germination was highest in sugar concentrations <20%. Sugar concentrations during the peak pistillate phase therefore provide optimal osmolarity for pollen hydration and germination; subsequent changes in sugar concentration during anthesis reinforce protogyny (in which carpels mature before stamens), enabling the retention of concentrated exudate into the staminate phase as a pollinator food reward without the possibility of pollen germination. Intercarpellary growth of pollen tubes was confirmed: the exudate therefore also functions as a suprastylar extragynoecial compitum, overcoming the limitations of apocarpy.

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