Background Floral morphs are characterized differentiations in reciprocal positions of sexual organs and ancillary floral traits in heterostylous plants. However, it remains unclear how differential floral morphs ensure reproductive success between morphs using the same pollinator. Results Measurements of floral traits in white-flowered Tirpitzia sinensis with long corolla tubes indicated that it is typically distylous, long-styled (L-) morph producing more but smaller pollen grains per flower than short-styled (S-) morph. Both morphs secreted more nectar volume at night than in the day and the sugar composition was rich in sucrose, potentially adaptive to pollination by hawkmoths (Macroglossum spp.) which were active at dusk. A bumblebee species functioned as the nectar robber in both morphs and a honeybee as the pollen feeder in the S-morph. The L-morph secreted more nectar volume but relatively lower sucrose/hexose ratio than the S-morph. Floral visitation rate by hawkmoths was higher but its pollination efficiency was lower in the S-morph than the L-morph. Hand pollination treatments indicated self-incompatibility in T. sinensis and seed set of open-pollinated flowers did not differ between morphs. Conclusions Our findings suggest that the two morphs differ with respect to traits relevant to pollination. The L-morph, with its exserted stigma, has more pollen grains per anther and a greater volume of nectar, which may prolong the foraging time and increase the pollination efficiency of hawkmoths. The S-morph has a higher sucrose/hexose ratio in its nectar which can be more attractive to hawkmoths and increase the visit rates. Ancillary polymorphic floral traits between two morphs are adaptive to hawkmoth and ensure reproductive success in distylous plant T. sinensis.

The differentiation of reproductive characteristics not only exists between different populations, but also may exist within populations. In this work, the differences between the central and peripheral populations were experimentally compared and analyzed in terms of biodiversity index, plant traits, anthesis, pollen germination, floral visitors, seed setting rate, and ploidy. The results showed that the diversity and richness of other plant species, in the central population were significantly lower than those in the peripheral population, but the plant density was much higher than in the peripheral population. The plant anatomical traits, anthesis, pollen germination, floral visitors, seed setting rate, and ploidy were significantly different between central population and peripheral populations. The term increasing rate (IR) is proposed as a means of comparing morphologies in different organs. IR differences in vegetative characteristics were more stable, while those in reproductive characteristics differed significantly. For the central population, the effect of the intraspecific reproductive competition and pollinator selection on plants may significant, and morphology was differentiated in terms of reproductive characteristics. Plants in the peripheral populations were visited by many more pollinators than in the central population, and all pollinators visited infrequently. The reproductive characteristics of plants in the peripheral populations may therefore only be weakly affected by pollinator selection. The reproductive characteristics of plants in the peripheral population may weakly affected by the selection of pollinators and the variation was small. In conclusion, morphological differentiation among the different populations was associated with differences in vegetative and reproductive characteristics.

Aims:The adaptive significance of heterostyly is to improve the transfer of compatible pollen, so as to promote outcrossing. Nevertheless, many studies have found that a large number of incompatible pollens are deposited on the stigmas. At present, few studies have focused on whether the stigmas promote the compatible pollen germination and pollen tube elongation and provide evidences for the pollination accuracy hypothesis by emasculating intraflower and intraplant pollen. Methods: Tirpitzia sinensis (Linaceae) was taken as materials, and the number of long-styled morph (L-morph) and short-styled morph (S-morph) of T. sinensis were investigated in the separate populations. The pollens and stigmas size of different morphs were measured and their surface ornamentations were observed. The viability of stigma and pollen

Background: Heterostylous plants are commonly associated with pollinators promoting disassortative pollination. How different morphs adapt to pollinators to ensure reproductive success and whether floral characters (such as pistil, stamen and nectar traits) are relevant to this process remain unclear. Results: Tirpitzia sinensis is distylous flowers. Its floral characters were measured. Field observation of effective pollinator and pollination efficiency to different morphs was conducted, and breeding systems were examined by hand pollination treatments. Our results showed that L-morph produced more but smaller pollen grains per anther than S-morph. T. sinensis secreted more nectar at night as a possible adaptation to hawkmoths (Macroglossum spp.) active at dusk. L-morph produced much nectar due to its large secretion volume at night. The nectar was rich in sucrose. The sucrose/hexose ratio was higher in the nectar of S-morph than in that of L-morph. S-morph had higher visit rate but lower pollination efficiency of hawkmoths compared with L-morph. T. sinensis was self-incompatible. Seed sets in nature did not differ between morphs. Conclusions: Our findings suggest that L-morph increases its pollination efficiency through outstanding stigma, many pollen grains per anther, and high amount of nectar. S-morph attracts many hawkmoths to pollinate due to its high sucrose/hexose ratio in its nectar.