Floral anatomy in Serjania clematidifolia (Paullinieae, Sapindaceae): Insights into a monoecious sexual system with multicyclic dichogamy

Abstract

Serjania clematidifolia Cambess. has a monoecious sexual system with multicyclic dichogamy, i.e., staminate and pistillate flowers are produced in more than three phases. The present study evaluated the flowers in the different flowering phases of the species. For morphometric analyses, flowers were measured and the resulting values statistically analysed. For structural analyses, flowers were processed using standard light microscopy techniques. Serjania clematidifolia has white, unisexual, nectariferous, and zygomorphic monosymmetric flowers. In staminate flowers, the anthers are dehiscent, the endothecial cells have bar thickenings, the septum degenerates, and the stomium ruptures, releasing pollen grains in monads at the binucleate microgametophyte stage. In pistillate flowers, the anthers are indehiscent, the endothecial cells near the connective lack parietal thickening, the septum does not degenerate, the stomium does not rupture, and microgametophytes are not released. Male sterility in pistillate flowers is likely associated with structural changes in the endothecium and failures in the process of programmed cell death (PCD) of anther tissues. In pistillate flowers, the stigma is papillose and moist, and the ovary contains campylotropous, bitegmic ovules with seven-celled megagametophytes. In staminate flowers, the stigma has a non-papillose and non-secretory epidermis. The tissues of the style, ovary, and ovules show various levels of degeneration. Ovules may or may not contain megagametophytes. Female sterility in staminate flowers appears to be associated with failures in megagametophyte differentiation or PCD of its cells. We conclude that in S. clematidifolia, (i) the morphometric variation between staminate and pistillate flowers in different flowering phases does not indicate the existence of distinct floral types, (ii) there are no structural differences between staminate or pistillate flowers from different flowering phases, (iii) megagametophytes may or may not differentiate in ovules of staminate flowers, and (iv) microgametophytes differentiate in indehiscent anthers of pistillate flowers.