Plant–plant interspecific competition via pollinators occurs when the flowering seasons of two or more plant species overlap and the pollinator fauna is shared. Negative sexual interactions between species (reproductive interference) through improper heterospecific pollen transfer have recently been reported between native and invasive species demonstrating pollination-driven competition. We focused on two native Impatiens species (I. noli-tangere and I. textori) found in Japan and examined whether pollinator-mediated plant competition occurs between them. We demonstrate that I. noli-tangere and I. textori share the same pollination niche (i.e., flowering season, pollinator fauna, and position of pollen on the pollinator's body). In addition, heterospecific pollen grains were deposited on most stigmas of both I. noli-tangere and I. textori flowers that were situated within 2 m of flowers of the other species resulting in depressed fruit set. Further, by hand-pollination experiments, we show that when as few as 10% of the pollen grains are heterospecific, fruit set is decreased to less than half in both species. These results show that intensive pollinator-mediated competition occurs between I. noli-tangere and I. textori. This study suggests that intensive pollinator-mediated competition occurs in the wild even when interacting species are both native and not invasive.

Studies of factors influencing spatial variation in flower size offer insights into floral evolution. We investigated altitudinal variations in five flower dimensions of two native Japanese Impatiens species (I. textori and I. noli-tangere) and their interactions with their faunal visitors. These two species have similar floral traits, including flower shape, flowering time, and pollinator species; both species are pollinated mainly by Bombus diversus. In I. textori, all measured flower dimensions were negatively correlated with altitude. In contrast, in I. noli-tangere, no measured flower dimensions correlated with altitude. Thus, the altitudinal pattern of flower size variation differed between these congeneric co-habiting herbaceous plant species. The different patterns suggest that the factors (e.g. altitudinal variations of abiotic factors) that cause variation of flower size differ between these two Impatiens species even though focal species have similar floral traits (e.g. flower shape, flowering time, and pollinator species).ARTICLE HISTORY

Viola (Violaceae) is one of the largest genera in angiosperms. This genus is essentially classified into stemless and stemmed groups based on growth morphology. However, Viola mirabilis var. subglabra is an exception in having intermediate flowering characteristics; cleistogamous (CL) flowers are formed in the axils of stem leaves, whereas chasmogamous (CH) flowers arise from basal rosettes (radical CH (CH(r)) flowers) and also in the axils of the stem (axially CH (CH(a)) flowers). To understand why the pattern of flower production varies in this Viola species, flower production was investigated in 10 Japanese populations from Hokkaido to the western part of Honshu in 2014 and 2015. Furthermore, flower characteristics were also compared between CH(r) and CH(a) flowers in Hokkaido. In this species, the production of CH flowers varied among individuals, and they were categorized into three groups, individuals that produced (i) only CH(r) flowers, (ii) only CH(a) flowers and (iii) both CH flowers. The frequency of these groups differed among populations, but some individuals changed the category between 2014 and 2015. Thus, the production of CH(r) and CH(a) flowers plastically changes depending on individual conditions and/or environmental factors. On the other hand, CH(r) and CH(a) flowers differed in flower size and flowering phenology. These results suggest that two types of CH flowers may play different roles in reproduction in each population, but fruit sets and seed sets did not differ between two types of CH flowers.

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