Floral Biology Characteristics And Mating System Of Vitex Negundo L.var. Heterophylla(Franch.) Rehder

Vitex negundo Linn.var.heterophylla(Franch.)Rehd.(Lamiaceae)is a common wild deciduous shrub in northern China.It is also a valuable melliferous plant and an important tree species for soil and water conservation,with good ecological characteristics and economic development value.A comprehensive study of the plant breeding system is conducive to understanding the reproductive strategies and adaptive evolution of the species,and providing a theoretical basis for developing scientific and feasible conservation and management strategies and better utilization of germplasm resources.However,the reproductive ecology of V.negundo var.heterophylla and even Vitex species remains to be elucidated.In this paper,for V negundo var.heterophylla,the floral phenological observations,morphological measurements,and nectar production analysis were conducted in the field.Pollen viability and stigma receptivity at different flowering stages were measured by biochemical methods.Finally,genetic analysis based on microsatellite(SSR)molecular markers was used to reveal the mating system,and manipulative pollination experiment,pollen-ovule ratio(P/O),and hybridization index(OCI)were used to assist the analysis.The main results and conclusions are as follows:(1)Flowering lasted 3 months(June to August),and reached a peak by mid-July.The terminal panicle bloomed from the bottom to the top,and approximately 20 days were required for all flowers to open.The blooming time was from approximately 9:00 to 11:00.The duration of a single flower was roughly 1-2 days according to our observation,and this period could be extended in rainy and cloudy days,which can be regarded as a mechanism for reproductive assurance.(2)Flowers are hermaphroditic,zygomorphic,with didynamous anthers and the stigma exposed in the air.Pollen activity and stigma receptivity were higher within 6 h of anthesis and almost completely lost after 24 h,indicating homogamous flowers.V.negundo var.heterophylla flowers are partial herkogamy(stigma situated higher than anthers).We hypothesize that the spatial difference in position between stigma and anthers might be the product of unstable selective pressures during mating system evolution.(3)These flowers produced small amounts of concentrated nectar at the base of corolla tube mainly within 24 h of anthesis,and the nectar secretion was 0.478 ± 0.028?L/flower/day,concentrated in the middle and later stage of anthesis.The sugar concentration in nectar was 46.6 ± 8.60%(from 33%to 58%),and the total sugar content is 0.268±0.016 mg/flower/day.The main flower-visiting insects in the natural population are bees and wasps,and occasionally ants and butterflies stop to feed(4)Considering the floral morphology(morphological characteristics of outcrossing and selfing are coexist,P/O(1015±218),and OCI(3 or 4),we give it a conclusion that the mating system of V.negundo var.heterophylla belongs to the mixed mating system dominated by outcrossing,with self-compatible,unable to self-pollinated spontaneously.The effect of wind pollination is weak,and the success of reproduction depends on the service level of pollinators.Inbreeding depression is apparently severe in the early stage of seed ripening and germination with selfed seed never survive to adulthood.(5)In the parental analysis of V.negundo var.heterophylla based on SSR,we found that there was no bias in the evaluation of outcrossing rate by two different models(MLTR and BORICE)when the maternal genotypes were known.The outcrossing rate of natural population was about 95%.The high level of outcrossing rate proves that this species has a wide range of reproductive sensitivity and genetic variation,which is important for evolution.This study provides an overview of the reproductive traits of V.negundo var.heterophylla mainly by morphological observations and SSR markers experiment,which made up for the lack of research in China.These reproductive traits presented here provide theoretical background and technical reference for subsequent studies of pollination biology and population genetic structure,and provide scientific basis for the protection of plant resources,the effective use of slope protection function,and the breeding of wild varieties of V negundo var.heterophylla,so as to better serve the soil and water conservation work and the construction of ecological civilization.