SIEVE TUBES IN ACTION

A method was designed for in vivo observation of sieve element/compani on complexes by using confocal laser scanning microscopy. A leaf attac hed to an intact fava bean plant was mounted upside down on the stage of a confocal microscope. Two shallow paradermal cortical cuts were ma de in the major vein. The basal cortical window allowed us to observe the phloem intact. The apical window at 3 cm from the site of observat ion was used to apply phloem-mobile fluorochromes, which identified li ving sieve elements at the observation site. In intact sieve tubes, th e sieve plates did not present a barrier to mass flow, because the tra nslocation of fluorochromes appeared to be unhindered. Two major occlu sion mechanisms were distinguished. In response to intense laser light , the parietal proteins detached from the plasma membrane and formed a network of minute strands and clustered material that aggregated and pressed against the sieve plate. In response to mechanical damage, the evenly distributed P plastids exploded, giving rise to the formation of a massive plug against the sieve plate. In case of mechanical damag e, the parietal proteins transformed into elastic threads (strands) th at extended throughout the sieve element lumen. Our observations cover the phenomena encountered in previous microscopic and electron micros copic studies and provide a temporal disentanglement of the events giv ing rise to the confusing mass of structures observed thus far.