Resupinate floral dimorphy in Chamaecrista nictitans (L.) Moench (Fabaceae-Caesalpinioideae)

Context Resupination is the movement of developing buds that leads to a vertical inversion of the flowers at an angle of 180 degrees. Chamaecrista nictitans exhibits two anther sizes, and nonresupinate and resupinate flowers in the same individual, which is known as monomorphic resupinate dimorphy. Aims We aimed to investigate the influence of monomorphic resupinate dimorphy upon pollen deposition on pollinators, capture by the stigma and on plant reproduction, using three populations of C. nictitans from NE Brazil as a model. Methods We assessed the floral biology, the proportion of nonresupinate and resupinate flowers in the populations, dynamics of pollen transfer between floral morphs, and the plant's reproductive system. Key results All flowers have two anther sizes with similar pollen viability. The proportion of nonresupinate:resupinate flowers was 3:1 in all populations. The bee Florilegus (Euflorilegus) sp. was the only pollinator observed and had pollen of both flower morphs deposited on the underside part of the thorax and abdomen. The stigma of nonresupinate flowers received pollen from the pollinators' underside of the abdomen, whereas the stigma of resupinate flowers received pollen from the pollinators' underside of the thorax. The species is self-compatible but does not set fruits by spontaneous self-pollination. Therefore, natural fruit set had resulted from both intramorph- and intermorph-pollination in the same or in different individuals. Conclusions Both flower types have a similar pattern of pollen deposition on the pollinator's body (underside of the abdomen and thorax) and only differ in relation to areas of pollen capture by the stigma (underside of the abdomen or underside of thorax). Implications The monomorphic resupinate dimorphy observed here improves the area of pollen deposition by the anthers on pollinator's body and makes the capture of pollen by stigma sectored in the different floral types, similar to what is observed in other species presenting floral polymorphisms.