Stem anatomical variations in seedlings of the mangrove Bruguiera gymnorrhiza grown under periodical waterlogging

The purpose of this study was to investigate the stem anatomical adaptations of the mangrove Bruguiera gymnorrhiza seedlings under experimental conditions that simulated semidiurnal tides. The inundation periods in these treatments were 0, 2, 4, 6, 8, 10 and 12 h. In the 0–6 h treatments, cortex thickness, pith radius and stem radius increased, but then declined when inundation period exceeded 6 h. Significant negative linear correlations were found between waterlogging duration and vessel element length, bar number per perforation plate and gelatinous fibre ratio. Whereas vessel density and fibre diameter showed only slight fluctuations, fibre cell wall thickness declined dramatically from 2 h treatments onwards, in contrast, vessel wall thickness increased under long-term waterlogging conditions (>6 h). Tangential vessel diameter and fibre length exhibited similar tendencies, first decreasing significantly with prolonged inundation, and then increasing under long-term waterlogging duration up to 6 h. These results indicated that the internal structure of B. gymnorrhiza seedlings was affected by periodical waterlogging. The distinct structural changes with short (<6 h) or long (> 6 h) term inundation suggest distinct strategies for maintaining a balance between growth, conductive capacity, conductive safety and mechanical strength.