A Study Of The Pollination Accuracy In Tristylous Lythrum Salicaria (Lythraceae)

Heterostyly is one of plant sexual polymorphisms in which two(distyly)or three(tristyly)different floral morphs occur within one species.Tristyly is a rare form in heterostylous plants.Darwin proposed that "The pattern of heterostylous style,in which pistil and stamen are mutually complementary,can promote accurate pollination between heterostylous types",namely the hypothesis of promotion of pollination accuracy between different floral morphs.To achieve pollination accuracy,pollen grains from different floral morphs are needed placing on different parts of the pollinator body,and then picked up by the corresponding stigma at the similar height of pollen-donor morph.In this study,Lythrum salicaria,a tristylous plant with pollen color polymorphism,was used to explore how tristylous plants maintain the accuracy of pollination,to test Darwin’s hypothesis.The three floral morphs in L.salicaria are:long-,medium-and short-styled.Each floral morph has two rounds of stamens of different lengths,correspondingly stamens can be categorized into long,middle and short forms according to their length.Pollen grains in long stamens are green,but yellow in other forms.Our observations of floral visitors in two field populations in Liaoning and Hubei Province showed that the most frequent visitor groups were Lasioglossum,Megachile,Nomia and Ceratina.Among them,Lasioglossum and Ceratina had pollen grooming behavior,and the yellow pollen amount in the pollen basket was much higher than the green pollen.To test whether pollen of different colors could be susceptive to visiting insects,we measured the reflection of pollen color of all anther forms in the studied tristylous species.Our results showed that green pollen grains from the long forms were less likely to be susceptive by bee visitors than yellow pollen grains from the middle and short forms.The possibility of legitimate pollen grains deposited on stigmas by visitors could be speculated from the proportion of different color pollen grains carried by the body parts of visiting insects contacting with stigmas.Our results showed that the pollination accuracy was higher of Megachile than of Lasioglossum.Comparison of the pollen removal and deposition in a single visit to the long-styled morph by Megachile and Lasioglossum,also showed that the proportion of green(legitimate)pollen grains deposited by Megachile(28.68%)was significantly higher than by Lasioglossum(6.48%).To explore breeding system in L.salicaria,we compared the fruit set and seed set of artificial(intermorph and intramorph)pollination between two morphs.The result showed that the compatibility between anthers and stigmas at the similar height levels was the strongest,which was consistent with the heteromorphic incompatibility in heterostyly.However,self-and illegitimate cross-pollinations exhibit various degrees of compatibility.In conclusion,our experiments showed that the heteromorphic incompatibility system was not strict in tristylous L.salicaria.Compared with the yellow pollen grains,the green pollen grains were not easy to be groomed by bees.The polymorphism of pollen color of L.salicaria may help of facilitating accurate intermorph pollination and reducing the pollen mixture from different morphs during pollen transport.