Chewing and other cues induce grass spines that protect meristems

Abstract

Grasses frequently have unidirectional hairs, prickles, and spines; these leaf features have been hypothesized to move herbivores and their chewing damage away from grass meristems, which are located basally. Observations of chewing damage to two grasses, Andropogon virginicus and Phragmites australis, were consistent with this hypothesis as leaf tips received 10 × and 2 × more damage than bases. Grasshoppers were no more likely to land on leaf tips than bases although they oriented towards the tips after landing. Leaves of A. virginicus that were damaged by chewing herbivores had fewer spines than leaves on the same or neighboring plants that lacked damage. This suggests that herbivores chose less spiny leaves. At a larger spatial scale, plants in neighborhoods favorable for grasshoppers had more spines than plants in less favorable neighborhoods. We found no evidence that marginal spines allowed leaves to shed water more rapidly, a potential alternative benefit. The density of spines on new leaves increased following cues of damage. A. virginicus leaves produced after an adjacent leaf had been clipped with scissors had 13% more spines than new leaves on unclipped plants. Clipping with scissors failed to increase spine density for new P. australis leaves although experimental chewing by caterpillars led to the production of new leaves with 24% more spines than controls. Unchewed new leaves within 20 cm of a chewed neighbor had 13% more spines than controls. Grasses are capable of responding to cues of tissue damage to their own and neighboring leaves, potentially reducing herbivory to meristems.