Cell Wall Solubilization in Pedicel Abscission of Begonia Flower Buds

Effects of metabolic inhibitors and growth regulators on the course of abscission and on the activities of cell wall solubilizing enzymes were studied in pedicel explants of Begonia flower buds. Actinomycin D, chloramphenicol and 2,4-dinitrophenol slightly retarded abscission, whereas cycloheximide exerted a strong inhibition if applied until 10.5 h after explant excision. Indoleacetic acid retarded and ethylene promoted abscission and cell wall solubilization. However, the activities of cell wall solubilizing enzymes did not correspond with the course of abscission. No polygalacturonase and pectic acid and pectin transeliminases could be detected in the abscission zone during abscission, whereas a low pectin methylesterase activity did not change. Endo- and exocellulase activities did not increase until about 10 h after the onset of abscission, indicating that they are the result rather than the cause of abscission.     

Differential response to climatic variation of free‐floating and submerged macrophytes in ditches

1. Experimental studies have indicated in freshwater ecosystems that a shift in dominance from submerged to free‐floating macrophytes may occur with climate change because of increased water surface temperatures and eutrophication. Field evidence is, however, rare. 2. Here, we analysed long‐term (26 years) dynamics of macrophyte cover in Dutch ditches in relation to Dutch weather variables and the North Atlantic Oscillation (NAO) winter index. The latter appears to be a good proxy for Dutch weather conditions. 3. Cover of both free‐floating macrophytes and evergreen overwintering submerged macrophytes was positively related to mild winters (positive NAO winter index). On the other hand, high cover of submerged macrophytes that die back in winter coincided with cold winters (negative NAO winter index). Our results therefore suggest that the effect of weather on macrophyte species depends strongly on their overwintering strategy. 4. The positive relation of free‐floating macrophytes with the NAO winter index was significantly stronger in ditches in organic soil than in those in inorganic soil. This may be because of increased nutrient loading associated with increased decomposition of organic matter and increased run‐off to these ditches during mild wet winters. 5. Our results suggest that mild winters in a changing climate may cause submerged macrophytes with an evergreen overwintering strategy and free‐floating macrophytes to outcompete submerged macrophytes that die back in winter.