Computed Tomography and light microscopy: combining visualisation techniques in the study of mangrove seedling development

Abstract

When seedlings grow into young plants their tissue proportions change over time. Viviparous mangrove seedlings of the Rhizophoraceae are different from other young trees. They consist of a thickened cylinder-shaped hypocotyl that allows the seedlings to float and disperse before establishment. Despite the crucial role in the ecological and biogeographical success of mangroves, not much has been published about the internal development of mangrove seedlings in their early life stages. We used X-ray CT-scanning and light microscopy to investigate the internal development (i) over time and (ii) with hypocotyl height in seedlings of the mangrove species Bruguiera gymnorrhiza and Ceriops tagal. While light microscopy offered cell- and tissue identification in destructive transverse sections, X-ray CT-scanning allowed investigating the internal tissue development of living plants over time in a non-destructive way. Our results indicated that the vascular tissue proportionally increased over time and with hypocotyl height in both species in accordance with the growing importance of this tissue in the developing seedlings. As a result, the cortex, composed of an inner and outer zone, proportionally decreased over time and with height in both species. No clear trends over time and with height could be observed regarding the proportion of the pith tissue. A decrease in average density of all tissues together with height was discerned in both species indicating the seedlings were heavier at their base. The latter suggests a supporting role of the seedling base in tidal and wind action. The combination of CT-scanning and light microscopy offered the advantages of both methods in the developmental study of young mangrove plants, and opens perspectives in the study of internal development of young plants in general.