Increased photosynthesis and water potential in Silphium integrifolium galled by cynipid wasps. Oecologia 93

Abstract. Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend agiinst her-bivory, yet the effects of insect herbivores on photosyn-thesis and water potential are seldom assessed. We presbnt evidence qhat cynipid wasp galls formed by Antisirophus silphii on Sil phium integrifolium increase photosynthesis(A), stomatal conductance (g), and xylem water potential(Y). Preliminary data showed that in drought-stressed plants galled shoots had36 % greater A, and l0o/o greater stem Y than ungalled shoots, while in well-watered plants leaf gas exchange was not aflected by galls. We hypothe-sized that l) galled shoots have higher y, g, and-A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decriases in A and g if water availability decreases. A field experi-ment.testing the first hypothesis found that galls increase