The pattern of acropetal and basipetal cytoplasmic streaming velocities inChara rhizoids and protonemata, and gravity effect on the pattern as measured by laser-Doppler-velocimetry
D. Ackers et al., The pattern of acropetal and basipetal cytoplasmic streaming velocities inChara rhizoids and protonemata, and gravity effect on the pattern as measured by laser-Doppler-velocimetry, PLANTA, 211(1), 2000, pp. 133-143
The spatial pattern of acropetal and basipetal cytoplasmic streaming veloci
ties has been studied by laser-Doppler-velocimetry (LDV) in the positively
gravitropic (downward growing) rhizoids of Chara globularis Thuill. and for
the first time in the negatively gravitropic (upward growing) protonemata.
The LDV method proved to be precise and yielded reproducible results even
when tiny differences in velocities were measured. In the apical parts of t
he streaming regions of both cell types, acropetal streaming was faster tha
n basipetal streaming. Starting at the apical reversal point of streaming,
the velocity increased basipetally with the distance from that point and be
came fairly constant close to the basal reversal point; subsequently, the v
elocity decreased slightly acropetally as the apical reversal point was aga
in approached. There was no change in velocity at the basal reversal point.
However, at the apical reversal point there was an abrupt decrease in velo
city. The pattern of the ratio of acropetal to basipetal streaming velocity
(VR) was a function of the relative distance of the site of measurement fr
om the apical reversal point rather than a function of the absolute distanc
e. Upon inversion of the rhizoids, the VR decreased on average by 3.8% (+/-
0.4%), indicating that the effect of gravity on the streaming velocity was
merely physical and without a physiological amplification. Rhizoids that h
ad developed on the slowly rotating horizontal axis of a clinostat, and had
never experienced a constant gravity vector, were similar to normally grow
n rhizoids with respect to VR pattern. In protonemata, the VR pattern was n
ot significantly different from that in rhizoids although the direction of
growth was inverse. In rhizoids, oryzalin caused the polar organization of
the cell to disappear and nullified the differences in streaming velocities
, and cytochalasin D decreased the velocity of basipetal streaming slightly
more than that of acropetal streaming. Cyclopiazonic acid, known as an inh
ibitor of the Ca2+-ATPase of the endoplasmic reticulum, also reduced the st
reaming velocities in rhizoids, but had slightly more effect on the acropet
al stream. It is possible that the endogenous difference in streaming veloc
ities in both rhizoids and protonemata is caused by differences in the cyto
skeletal organization of the opposing screams and/or loading of inhibitors
(like Ca2+) from the apical/subapical zone into the basipetally streaming e
ndoplasm.