Germination and seedling establishment is perhaps the most critical period in the life history of any plant. This study addresses the processes and mechanisms involved in the case of red mangrove (Rhizophora mangle L.) seedlings. This is the major neo-tropical representative of a pan-tropical family, often dominating intertidal communities to near monoculture. The family includes 3 other mangrove, and ca. 16 non-mangrove genera. The mangroves (but not the others) are all viviparous. The propagule is a quiescent hypocotyl topped by a quiescent plumule. “Germination” is defined as the resumption of growth by the propagule, and a seedling is “established” when it can reasonably be expected to survive the range of environmental conditions to which it has adapted over the last 65 Ma. This phase is particularly critical in the restoration of mangrove forests, due to the extreme variabilities that can be expected, especially wave action. Tolerance of waves also underlies the effectiveness of mangroves as storm and surge buffers.

Although propagules may fall and stick in a muddy substrate, the majority drop into water. Rhizophora propagules can float for several months, germinating only after washing up on a shore line or becoming snagged under a root or between rocks, and thus, growth begins from a horizontal position. In this presentation, (1) this process will be illustrated using time-lapse photography, and (2) its consequences will be compared to growth from a vertical position based on rates of leaf initiation and stem growth.
Results/Conclusions

Germination begins with the appearance of 4-6 adventitious roots from the base of the hypocotyl, each on the order of 1.5 mm in diameter. These elongate downward and outward and thicken to >3 mm within a few weeks, forming the strong anchor essential to survival. Thereafter, during seedling establishment, the propagule elevates the shoot apex to a vertical orientation. This involves five stages: a “push-ups” stage in which the hypocotyl elevates ca. 10˚; a stage characterized by strong diageotropic curvature at the midpoint of the hypocotyl; the beginning of elevation due to curvature at the base of the hypocotyl; autotropic straightening of the distal curved zone; and finally, full straightening and leaf opening. All stages are characterized by elevation during the day and relaxation at night although autotropic straightening is largely accomplished at night.

The consequences of elevation-restrained germination and re-orientation after germination will also be presented, and discussed with respect to their real-world context.