A Plant Parasite uses Light Cues to Detect Differences in Host-Plant Proximity and Architecture.

Sunlight filtered by green plant tissue becomes diminished in its ratio of red to far-red wavelengths (R:FR). Some parasitic plants exploit this change by growing toward regions of low R:FR to locate potential hosts. In principle, variation in R:FR can also convey vital information about the proximity or shape of potential hosts. Here we demonstrate that the parasitic vine Cuscuta epilinum Weihe (Convolvulaceae) can distinguish fine-scale differences in R:FR associated with the proximity and architecture of potential hosts. We conducted dual-choice experiments by placing parasite seedlings between targets and observing their growth relative to the targets. Targets consisting of FR lights or model host plants (with realistic R and FR reflectance) probed the effects of R:FR, host shape and proximity on parasite choice. Seedlings distinguished between low-R:FR fields of differing R:FR ratios and exhibited preferences for nearer plant models versus identical models placed 4 cm further away; they also distinguished between same-sized models exhibiting shape differences. Our results indicate that parasites can discriminate minute differences in R:FR signatures corresponding to host factors (proximity and shape) that impact seedling survival. This keen sensory ability underpins the parasite's sophisticated foraging behaviour and highlights the broader importance of light cues in plant ecology. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.