Rain’s role in pine reproductive biology

Rain-mediated reproduction, or hydrophily, is present in only 0.1% higher plant taxa. Pinus spp. is included on this list so here we present a synthesis illustrating three roles for rain in pine reproductive biology: pollen transport, pollen delivery and pollination. Pine pollen has been shown to survive long-range transport beneath and inside rain clouds after which germination still occurs. Pine pollen is captured inside raindrops so rain delivers pine pollen back to the earth’s surface and this pollen can also germinate. Rain is the primary pollination mode for Pinus taeda. The pollination drop only appears later if rain does not fall. Pine pollen does not appear to burst into subpollen pieces (SPP) upon water contact. For these reasons, wind and rain are vectors of pollen transport, deposition and pollination. Accordingly, research gaps abound and we formulated these as three testable hypotheses: (1) wetted pollen has aerodynamic properties which deter transport, (2) rain delivers its own load of pollen and (3) rain contributes to long-distance gene flow among populations within a species. Rain acts as a fluid medium contributing to Pinus spp. reproduction.