Stem Elongation Response to Neighbour Shade in Sprawling and Upright Polygonum Species

Abstract

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 bothP. 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 both 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.