Water household of viviparous mangrove seedlings (Bruguiera gymnorrhiza, Rhizophoraceae) during their on-tree development and post-abscission establishment

Mangrove trees are remarkably well adapted to the tropical intertidal environment. Aerial roots and salt excluding mechanisms at the root and leaf level are some of the important mechanisms that help them surviving the harsh mangrove environment. The formation of viviparous water-dispersed seedlings is another adaptation. All mangrove species belonging to the family Rhizophoraceae are the only true viviparous eudicot seed plants. Vivipary is a phenomenon in which the seed germinates while it is still attached to the parent tree. The seedling or propagule is thus the dispersal unit.
During its on-tree development, the seedling receives water and nutrients from the parent tree. Yet, whether the seedling exhibits (daily) diameter variations that indicate dynamics in internal water storage as is common in trees, remains a question.
In our study we used dendrometers to monitor the dynamics in the diameter of two ex situ developing mangrove seedlings (Bruguiera gymnorrhiza) until their abscission, along with the diameter changes of the respective branches of the parent tree they were attached to. Environmental variables were monitored as well. After abscission, the seedlings were planted and monitored anew with dendrometers to reveal the evolution in their diameter patterns during establishment.
Our results indicate that the seedlings, from the start of data-logging until abscission, decreased in diameter in a stair like pattern while they gained in length. In contrast, the branches showed a pronounced diurnal pattern of diameter shrinking and swelling. The seedling contractions were delayed by a few hours compared to those of the branches, and were followed by periods overnight during which the diameters remained stable. Seedling expansions, when they occurred, coincided with smaller branch contractions, and seemed to be associated with lower average daytime air temperatures and vapour pressure deficit values.
After planting, the seedlings exhibited a general diameter pattern that could be divided into four main parts: a diameter decrease followed by a short stable phase, then a long diameter increase until finally the seedlings started a diurnal diameter pattern similar to those observed in the parent tree branches. This study allows a more complete understanding of mangrove seedling development.