Trade-offs between the shade-avoidance response and plant resistance to herbivores? Tests with mutant Cucumis sativus

1. Plants exhibit adaptations to many stresses, including light competition and herbivory. The expression of these traits may interact negatively, potentially instigating a trade-off. 2. We employed a combination of genetically altered Cucumis sativus varieties and phenotypic manipulations to test for trade-offs in field experiments. The different genetic lines of C. sativus were altered in their phytochrome-mediated shade responses and the production of terpenoid defence compounds. 3. Cucumber plants constitutively expressing the shade-avoidance response had 93% more herbivory by specialist beetles compared with wild types. The long-hypocotyl mutants also produced leaves with fewer trichomes, greater toughness and a higher carbon to nitrogen ratio (C : N) than wild types. Plants lacking defensive cucurbitacins had 23% longer internodes than the cucurbitacin-producing line. 4. We then manipulated the plant phenotype by artificially imposing neighbours’ shade on plants with and without cucurbitacins. As expected, plants responded to shade by growing longer hypocotyls and first internodes, but few trade-offs were found between plant line and shade treatment and, although herbivory levels were very low, there was a trend towards reduced damage on shaded plants. 5. The use of genetically altered plant lines provided strong evidence for the trade-off hypothesis, while phenotypic manipulations did not support the hypothesis.