Laboratory of Plant Systematics
David Rabaey
Micromorphology of pit membranes in tracheary elements in Asterids
Introduction
Water transport in green plants is one of the most important regulatory systems required for photosynthesis, which has crucial implications for all life on earth. For this purpose plants need a hydraulic system and this involves water-conducting elements that are both efficient as well as safe. Since xylem cells have a limited length, water must pass from one element to another via so called pits. These are minute openings in the secondary cell wall of vessels and tracheids. This opening, however, is not complete due to the presence of a pit membrane. Interestingly, there is a large anatomical variation in the structure of water conducting elements as well as in the micromorphology of pits. The anatomical variation of pits can partly be explained in terms of systematic relationships and can partly be caused by environmental and functional aspects. Recent physiological findings highlight the importance of pits in water transport, suggesting that the micromorphology of pit membranes may affect flow resistance and vulnerability to air entry. Nevertheless, many secrets about the structure of pits and pit membranes remain hidden in water transporting elements of plants.
Objectives
By studying various pit characters selected woody plants, we would like to obtain detailed information about these structures and especially their systematic and ecological significance. These observations will also help us to understand the possible function. The species selected will include samples with genuine tori, plasmodesmata associated thickenings, vestures,.... This research takes place in close collaboration with Dr. Steven Jansen (Royal Botanic Gardens, Kew) and Dr. Frederic Lens.
So far, plasmodesmata associated thickenings (pseudo-tori) have been described in a selected number of genera. Till now, we were able to describe this feature in various genera belonging to the families Elaeagnaceae, Ericaceae, Rhamnaceae, Rosaceae, Oleaceae, and Pittosporaceae. Based on our preliminary observations, we hypothesize that plasmodesmata associated thickenings are much more abundant than previously thought and especially quite common in species from cold to cold-temperate regions. Ontogenetic work suggests that all pseudo-tori observed in various angiosperm orders are homologous. New records of tori were reported in Chionanthus and Picconia (Oleaceae) and Cercocarpus (Rosaceae).
