Study On Thr Guarantee Mechanism Of Pollination And Fertilization Of Ipomaea Purpurea

Flowers,the reproductive organs of angiosperms,show a high degree of diversity in morphological structure and flowering habits to ensure pollination and fertilization of plants.Ipomoea purpurea L.,pertained Convolvulaceae,was an annual twining herbaceous plant with a long flowering period(June-October),and it had a short single flowering period(7-14 hours).The corolla of I.purpurea was trumpet-shaped,and there was an important relationship between floral organ changes and pollination during flowering.In this study,I.purpurea was used as experimental materials.Flowering phenology,single flower opening,and flower characteristic were observed.The pollen-ovule ratio,benzidine-hydrogen peroxide method,hybridization index,and bagging experiments were used to study the pollination type of I.purpurea.The changes of corolla morphology structure and effects of high temperature on pollen germination were studied by tissue anatomy,cytology.The purpose of this study was to investigate the causes of closure corolla of I.purpurea into inverted bell-shaped corolla and the reproductive biological significance of the inverted bell-shaped corolla for pollination and fertilization.The results were as follows:(1)The characteristics of flower development rhythm ensured that pollination of I.purpurea was the priority outcrossing and delayed self-crossing.The flowering process of I.purpurea was divided into six stages: bud,late budding,flowering,early flower senescence,late flower senescence,and full flower senescence.The corolla was trumpet-shaped in flowering stage,and the upper part of the corolla shrank inward after flowering,forming an inverted bell-shaped closed corolla,then wilting and falling off.The corolla was bright and full in flowering stage,the stigma secreted mucus,and the nectary secreted nectar.The anther-stigma distance was large and the stigma has a lower chance of receiving self-pollen in early flowering stage.The two stamens were elongated,the anther-stigma distance gradually becomed smaller,the stamens were elongated to the same height as the pistil,and the anthers contacted the stigma,providing opportunities for self-pollination.During the whole flowering period,the pollen viability and stigma receptivity showed "synchronicity",and the pollen viability and stigma receptivity were the strongest in flowering stage,with pollen viability of 83.1% and stigma receptivity lasting for 3h.The outcrossing index(OCI)was 3 indicating that I.purpurea was self-compatible and sometimes required pollinators.Pollen ovule ratio(P/O)was about 180,and the breeding system was judged to be facultative self-crossing.Bagging experiment showed that under natural conditions,there was no apomicosis in I.purpurea,and the sum of setting rate of self-pollination and cross-pollination was close to the setting rate of natural pollination.Ipomoea purpurea formed hybrid pollination and fertilization system with preferential outcrossing and delayed self-crossing.The stamen elongation and the characteristic of the flower suitable for the outcrossing made the hybrid mating system with priority outcrossing and delayed self-crossing.(2)The corolla of I.purpurea shrunk inwards about 6 hours after flowering and gradually formed an inverted bell-shaped closed corolla.In the process of corolla closure,the epidermis,parenchyma cells and vascular bundle of the upper part and the lower part of corolla were distorted in different degrees,and the parenchyma cells of the upper and lower epidermis were different.The parenchyma cells and vascular bundles near the upper epidermis of corolla disintegrated earlier and formed cavities,while the parenchyma cells near the lower epidermis aged later.The deformation degree of cells in the lower part of corolla was weak,and the lower part of corolla kept upright.DAPI staining showed that the nuclei of the cells in the upper part of the corolla were deformed and disintegrated earlier,the cells in the upper and lower parts of the corolla were not aging synchronously,and the cells in the upper part were aging earlier than those in the lower part.This causes the corolla to form an upright inverted bell shaped-closed corolla during the decay process.(3)The closed corolla can reduce the temperature inside the flower and ensure the pollen germination under high temperature in the field.The experiment of pollen germination showed that temperature affected the activity of pollen and the germination of pollen.When the temperature reached 35°C or above,the activity of pollen decreased and the germination of pollen was inhibited.When the stamens grown to the level of the stigma,the self-pollen fell on the stigma.At this time,the environment temperature was high,which was not conducive to the germination of pollen grains,but I.purpurea formed closed corolla to ensure the germination of self-pollen.The closed corolla leaded to the inside temperature always lower than the outside temperature,the average temperature difference between the inside and outside of the flower was 2-3°C,and the maximum temperature difference was 4.1°C.I.purpurea was in the middle of summer with high ambient temperature.The formation of closed corolla can enable it to cope with high temperature stress and ensure the completion of fertilization.To sum up,I.purpurea has the characteristics of bright flower color,full nectary and large pollen grain size,which are suitable for outcrossing and stamen elongation,forming a hybrid mating system of priority outcrossing and delay self-crossing.When delayed self-crossing occured,the germination of pollen grains was vulnerable to high temperature stress.Because of the morphological changes of different cell rhythms,I.purpurea formed an inverted bell-shaped closed corolla,which can reduce the temperature in the corolla and ensure the germination of self pollen.Therefore,the double guarantee mechanism of pollination and fertilization was formed.