Does resource availability modulate shade avoidance responses to the ratio of red to far-red irradiation? An assessment of radiation quantity and soil volume

Abstract

We conducted two experiments to investigate the expression of shade avoidance in response to low ratios of red to far-red irradiation (R[ratio ]FR) in the C₄ perennial grass Schizachyrium scoparium at two levels of above-ground (photosynthetic photon flux density (PFD) 400–700 nm) and below-ground (soil volume) resource availability and in plants of two ages. Young plants showed greater sheath and ramet height in response to low R[ratio ]FR and old plants showed reduced ramet initiation in experiments one and two, respectively, but both responses were not expressed simultaneously in either group. Growth of all ramet variables, including ramet initiation, was suppressed in small as opposed to large soil volumes. By contrast, architectural variables, but not ramet initiation, were greater for plants grown in low as opposed to ambient PFD. However, expression of shade avoidance to low R[ratio ]FR was not significantly affected by either level of PFD or soil volume. We must conclude that the levels of resource availability provided do not modulate shade avoidance in this perennial grass. These results demonstrate that S. scoparium is capable of expressing shade avoidance in response to low R[ratio ]FR, but inconsistent juvenile ramet initiation and architectural responses in plants of different ages and phenological stages of development indicate that the shade avoidance response might not be expressed consistently throughout the life of plants.