Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. The authors present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (psi). Drought-stressed plants galled shoots had 36% greater A, and 10% greater stem psi than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. The authors hypothesize that 1) galled shoots have higher psi, g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and psi, but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO2 concentration relationships were consistent with the explanation that increased psi allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationship or by removing low-psi limitations on non-stomatal components of A. The author's working hypothesis is that galls increase psi and A by reducing the shoot:root ratio so that plant is exploiting a greater soil volume per unit leaf area. They argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased psi and A may protect gall insects from variation in resource availability caused by periodic drought stress.