How red mangrove seedlings stand up
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Background and aims
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.
KeywordsRhizophora mangle Red mangrove Autotropic straightening Propagule Seedling establishment Geotropism Phototropism Salinity
“push-ups” (6.3 MB)—During the first phase of sprouting, propagules elevate and relax in a diurnal fashion, up during the day and down at night. The movements likely result from alternating hydration and partial desiccation of the propagule, made possible by soil contact at the base. The end of this phase coincides with the penetration of the soil by the adventitious roots. (M4V 6408 kb)
diageotropic curvature (4.7 MB)—During the second phase of sprouting, push-ups are replaced by increasing hypocotyl curvature which does not completely relax at night. The dead propagule does not enter this phase. (M4V 4792 kb)
basal curvature and elevation (3.5 MB)—During the third phase of sprouting, a second center of curvature develops at the base of the propagules, just above the point where roots anchor them. This elevates the whole unit distal to the base. (M4V 3588 kb)
autotropic straightening (and leaf emeergence) (6.3 MB)—During the last phase of sprouting, the elevated propagule straightens as it becomes vertical. This is accomplished by elongation of the inner radius of curvature. At this point, too, the epicotyl bud swells and the shoot emerges… this is NOT developmentally linked to the elevation itself (see main text). (M4V 6444 kb)
composite of all four phases (9.4 MB)—This movie combines all four phases into a single, summarizing movie. (M4V 9599 kb)
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