Stem
Elongation Response to Neighbour Shade in Sprawling and UprightPolygonum Species
BEVERLY
COLLINS and GARY WEIN
Savannah
River Ecology Laboratory, P.O. Drawer E, Aiken, South Carolina
29802, USA
Received:
27 March 2000 Returned for revision: 2 May 2000 Accepted: 13 June 2000 Published
electronically: 10 August 2000
Shading
by neighbouring plants, which reduces energy for photosynthesis and lowers the
ratio of red: far red light,
can trigger
a stem elongation or 'overtopping' response in herbaceous plants.We compared the stem elongation
response of two Polygonum species in a greenhouse experiment.P. sagittatum, a sprawling, vine-like annual, and P.
hydropiperoides, an upright perennial, were grown from seeds at three levels
of neighbour shade produced by
crowding a
cohort of real neighbour plants or adult-sized fake neighbour plants that
provided shade and reduced the red: far-red ratio.
We hypothesized that the annual would show a more pronounced elongation response
to developing or adult neighbour shade because vine-like plants are less
mechanically constrained to remain upright and self-supporting. Internodes
on stems of both P. sagittatum and P. hydropiperoides increased in
length as the amount of shading by real or fake neighbours increased. P.
sagittatum climbed on adjacent plants, and had longer stems with more nodes
than those of P. hydropiperoides. Although P. sagittatum and
P. hydropiperoides tended to elongate with crowding, the greater
elongation response to both real and fake neighbours shown by the sprawling
annual reflects its ability to extend upward into a canopy beyond
self-supporting height. Self-supporting P. hydropiperoides can
extend upward with, or overtop, cohort neighbours, but might less readily
elongate into an extant canopy. In dense stands, P. sagittatum can
become structurally dependent on close P. hydropiperoides
neighbours. Our results suggest that the elongation response of P.
sagittatum to neighbours can contribute to structural dependence and could
facilitate coexistence of these species.
