How red mangrove seedlings stand up

Abstract

The establishment of Rhizophoracean mangroves usually involves a transition from a horizontal to vertical orientation. Neither how this occurs nor the possible associated ecological benefits and costs have previously been considered. The “righting” of red mangrove (Rhizophora mangle L.) propagules was studied using frequent observation and time lapse photography under growth chamber and greenhouse conditions. Rates of leaf appearance were analyzed as functions of substrate, propagule placement, propagule orientation and salinity. Propagule righting progressed in four phases. First, propagules alternately elevated and relaxed in a diurnal cycle. Next, propagules developed upward curvature, centered approximately two-thirds of the way between the base and tip; curvature developed and relaxed in 24 h cycles. Third, the distal portion straightened and a second center of curvature developed at the base, elevating the whole propagule. Finally, the epicotyl swelled, the stem elongated and leaves unfolded. If bases were in contact with the substrate, initial orientation had no effect on leaf opening. However, propagules without that contact experienced delays throughout the cycle. The delays were longer, initially, at higher salinity. R. mangle propagules are both physiologically and phenotypically highly flexible. This improves their chances of successful establishment in a heterogeneous, unpredictable, and often, high energy environment.