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We have tested the hypothesis that a plant may detect the ptesence of a neighbour, befote being shaded by it, thtough the petception of the spectt-al composition of reflected sunlight. Within seedlitig canopies the red : far-red t-atio (R : FR) of the light received by a sensor with a geotnett-y appt-oxitnating that of a stetn was significantly teduced by selective reflection. This effect was observed befote any reduction in the atnount of photosytithetic light energy received by an individual seedling could be detected. Stnall gt-een fences of grass, east-west orientated, alteted the specttal distribution of the light on the north (sunlit) side of thetn. Fully illuminated seedlings of Sinapis alba giown on the north side of these gt-een fences produced longer internodes and had a lower leaf: stetn dry weight ratio than those grown in ftont of fences of bleached grasses. A sitnilar tedistribution of gtowth was elicited in seedlings of Chettopodinm album. Datura ferox and S. alba growing iti full sunlight by exposing plants to additional small quantities of fat-red reliected by seleetive tnirtot-s. These t-esults suggest that the change in the R: FR ratio serves as an early warning signal of oncotning cotnpetition.

Symbiotic microorganisms that live intimately associated with terrestrial plants affect both the quantity and quality of resources, and thus the energy supply to consumer populations at higher levels in the food chain. Empirical evidence on resource limitation of food webs points to primary productivity as a major determinant of consumer abundance and trophic structure. Prey quality plays a critical role in community regulation. Plants infected by endophytic fungi are known to be chemically protected against herbivore consumption. However, the influence of this microbe-plant association on multi-trophic interactions remains largely unexplored. Here we present the effects of fungal endophytes on insect food webs that reflect limited energy transfer to consumers as a result of low plant quality, rather than low productivity. Herbivore-parasite webs on endophyte-free grasses show enhanced insect abundance at alternate trophic levels, higher rates of parasitism, and increased dominance by a few trophic links. These results mirror predicted effects of increased productivity on food-web dynamics. Thus 'hidden' microbial symbionts can have community-wide impacts on the pattern and strength of resource-consumer interactions.