Structure of sieve elements exposed to different degrees and forms
of injury.
The direction of translocation before injury was from
left to right. The nature of the structural responses depends on the degree
of injury and the time frame of observation. The upper sieve element presents
a strong response, mostly arising in the regions near the transverse cutting
surface in tissue slices. A composite double-layered plug topped off by starch
grains has been formed in front of the sieve plate. The location of the plug
is dependent of the degree of injury. The plug is mostly formed at the sieve
plate side facing the translocation stream. In case of extreme damage (at
the very cutting edges), however, plugs are formed at the sieve plate side
distal to the site of injury due to surging as result of pressure loss. At
the other side of the sieve plate, proteinaceous strands may or may not extend
from one to the other end of the sieve element. The strands often branch
to the side walls. In case of severe injury, the strands are absent; they
have probably been torn away by the force of the pressure release and have
sedimented onto the plug. Vesicles building up from about 30 min after injury
are attached to the strands, often in the vicinity of the sieve plate. P
plastids as well as parietal proteins are absent. The lower sieve tube shows
the response to high intensities of laser light. The parietal proteins produce
a network of strands which deposit to variable degrees on the side of the
sieve plate facing the mass flow stream. The P plastids remain intact and
are located parietally. The emergence of vesicles has never been observed.
C = callose; CC = companion cell; N = nucleus; Pg = plug; Pl = P plastid;
PP = parietal protein; SE = sieve element; SG = starch grain; St = strand;
V = vacuole; Ve = vesicle.
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The direction of translocation before injury was from
left to right. The nature of the structural responses depends on the degree
of injury and the time frame of observation. The upper sieve element presents
a strong response, mostly arising in the regions near the transverse cutting
surface in tissue slices. A composite double-layered plug topped off by starch
grains has been formed in front of the sieve plate. The location of the plug
is dependent of the degree of injury. The plug is mostly formed at the sieve
plate side facing the translocation stream. In case of extreme damage (at
the very cutting edges), however, plugs are formed at the sieve plate side
distal to the site of injury due to surging as result of pressure loss. At
the other side of the sieve plate, proteinaceous strands may or may not extend
from one to the other end of the sieve element. The strands often branch
to the side walls. In case of severe injury, the strands are absent; they
have probably been torn away by the force of the pressure release and have
sedimented onto the plug. Vesicles building up from about 30 min after injury
are attached to the strands, often in the vicinity of the sieve plate. P
plastids as well as parietal proteins are absent. The lower sieve tube shows
the response to high intensities of laser light. The parietal proteins produce
a network of strands which deposit to variable degrees on the side of the
sieve plate facing the mass flow stream. The P plastids remain intact and
are located parietally. The emergence of vesicles has never been observed.