Abstract:

Heteracia szovitsii (Asteraceae) is a common ephemeral annual species occurring only in desert regions of northern Xinjiang in China, with short-lived florets and achenes within a single infructescence (capitulum) having three different morphs. To explore the relationship between morphological differences in florets and the polymorphic fruits and reproductive characteristics, we compared floral traits, flowering pattern, and flower visitors to the three floral morphs in H. szovitsii, and reproductive efforts by hand pollination treatments. The results showed that: Peripheral and intermediate florets did not have pappus, while central florets did. Number of florets and length and width of ligules were significantly different among the three floral morphs in a single capitulum. Further, the length of stigma lobes of peripheral florets was significantly greater than that of central florets, the length of ovary beaks of central florets was significantly greater than that of peripheral and intermediate florets. Differences in morphology with or without pappus, width of ovaries, length of ovary beaks among three morphs of florets were consistent with those of three morphs of achenes. These results indicated that the numbers of three morphs of achenes and their morphology had differentiated during the development period of three kinds of florets. The concentrated flowering pattern of blooming in the morning of three kinds of florets within capitulum, made the capitulum act as functional units, i.e. like a flower, thereby increasing flower display and attracting pollinators. Pollinator visits may facilitate outcrossing during the short flower longevity. Pollen grains of the three floral morphs could germinate and produce pollen tubes on their stigma lobes, indicating that this species is self-compatible. The three kinds of florets bagged without emasculation all can produce achenes, but fruit-set was all significantly lower than that of natural pollination, suggesting that this species could be autogamous and cross-pollination could increase fruit set. Due to the protandrous and the pump/bush mechanism of secondary pollen presentation, pollen of three floral morphs was present at the apex of the stigma and on the brush of hairs of both the upper part of style and outside lateral of the closed stigma lobes. This character prolonged the duration of pollen presentation (male stage), and reduced the interference between male and female functions within flower, promoted cross pollination, thereby improving male/female fitness. Meanwhile, it could allow the pollen deposition on the lobes when the stigma lobes expanded to complete self-pollination autonomously. Halictus sexnotatulus was the most frequent floral visitor, and the duration time among insects visiting, inflorescence opening, highest pollen viability and the highest stigma receptivity had a high degree of synchronicity. This strategy facilitated the output of pollen at the male stage and receipt of pollen on the stigma at the female stage, thus ensuring that pollination was completed quickly and effectively in a short time after flowering and that outcrossing was successful. In the desert spring environment of the northern Xinjiang, H. szovitsii with short-lived florets can not only provide reproductive assurance via autonomous self-pollination quickly under the condition of the lack of pollinators and/or limited activity due to low temperatures and windy conditions, but also can provide the opportunities for outcrossing, through concentrated flowering, protandry and secondary pollen presentation when environmental conditions became favorable for pollinator activities.

Key words: Heteracia szovitsii, heteromorphic florets, short-lived florets, autonomous self-pollination, secondary pollen presentation, pollination