Contact:
Dr Nikolaus Wellner
T: +44 (0)1603 255012
E: wellner@ifr.ac.uk

Abscission zones in Poinsettia

Physiological changes associated with programmed tissue death

Poinsettia (Euphorbia pulcherrima), known as Christmas star, is an economically valuable ornamental plant worldwide, especially during the Christmas season. However, these plants are easily affected by premature flower abscission, which results in a severe loss of postharvest quality.

To study this process, scientists at the Norwegian University of Life Sciences have developed an abscission induction system by decapitation of the flower bud. Normal abscission takes 3-4 weeks, depending upon environmental conditions. However, decapitation of the flower pedicels at specific sites leads to a rapid abscission process and full abscission including cell separation occurs in 7 days.

Poinsettia flower abscission led to various chemical changes in cell wall components. The most prominent changes were related to pectin and phenolic compounds. Immunolabeling analysis with pectic monoclonal antibodies showed distinct differences between abscising and non-abscising cells. The (1-›4)-b-D-galactan epitope was absent in the abscission zone at day 6. (1-›5)-a-L-arabinan and JIM7 homogalacturonan epitopes considerably decreased in their numbers in the abscission zone. In contrast, arabinogalactanprotein did not show drastic changes. For a more detailed differentiation in cell wall component changes we used FT-IR and FT-Raman analysis. FT-IR bands in the 1200-900 cm-1 region, assigned to polysaccharides, showed that the relative amounts of pectic polysaccharides decreased in the abscission zone. Moreover, the decrease of ester carbonyl bands in theabscission zone showed some degree of de-esterification. Enzymatic action may be involved in the degradation of pectic material. FT-IR and FT-Raman spectra showed an increased protein levels during the abscission process, which may indicate higher levels of cell wall degrading enzymes in the abscission zone. FT-Raman analysis emphasized the accumulation of phenolic ring materials in the abscission zone as well as the loss of carotenoid pigments. Increasing lignin accumulation in the abscission zone was also detected by lignin specific staining method.

Structural differentiation was found with classified FT-IR images. The vascular bundle area in unabscised control plant had different cell wall composition than parenchyma cells. During the abscission process, the reduction of the pectin content was more apparent in the parenchyma cells, indicating that there are tissue-specific variations in the molecular changes during abscission. Conclusively, these non-destructive analyses suggested that carbohydrates were reduced, especially pectin, in the abscission zone cell walls.  In addition, these FT-IR and FT-Raman analyses provided strong evidence for the incorporation of lignin in the cell walls and increased protein levels in the abscission zone.