Everyone with keen eyes that has been walking in wooded areas in the recent years must have noticed small growths on tree leaves called galls. Sometimes the infections can be massive, where it is hard to find one healthy leaf for a whole tree. This prompted the idea to study the physiological effects of galls on trees as such intense infections must have consequences. Furthermore, we were interested whether and how do plants protect themselves once they have been infected.

We started the huge task of collecting data about the penalties of gall infections on tree physiology with oaks. An enormous diversity of gall wasp species can parasitize oak leaves, but the physiological implications of different gall wasp infections are poorly understood. We analysed the effects of infections by four different gall wasp species (Neuroterus anthracinus, N. albipes, Cynips divisa and C. quercusfolii) on foliage photosynthetic characteristics and volatile emission rates in Quercus robur that grow in Tartu Tammik planted by important people that have resided in or visited Tartu. Our work indicated that gall wasp infection resulted in major reductions in foliage photosynthesis rates and elicitation of emissions of green leaf volatiles, mono- and sesquiterpenes and benzenoids in infection severity-dependent manner. Different gall infections resulted in unique emission blends, highlighting a surprisingly selective host volatile response to various gall wasps.

Citation: Jiang, Y., Veromann‐Jürgenson, L. L., Ye, J., & Niinemets, Ü. (2017). Oak gall wasp infections of Quercus robur leaves lead to profound modifications in foliage photosynthetic and volatile emission characteristics. Plant, Cell & Environment, DOI: 10.1111/pce.13050 (link to full text)

Oak galls (pic by Ülo Niinemets)

Abstract

Oak trees (Quercus) are hosts of diverse gall-inducing parasites, but the effects of gall formation on the physiology and biochemistry on host oak leaves is poorly understood. The influence of infection by four species from two widespread gall wasp genera, Neuroterus (N. anthracinus and N. albipes) and Cynips (C. divisa and C. quercusfolii), on foliage morphology, chemistry, photosynthetic characteristics, constitutive isoprene and induced volatile emissions in Q. robur was investigated. Leaf dry mass per unit area (M_A), net assimilation rate per area (A_A), stomatal conductance (g_s), and constitutive isoprene emissions decreased with the severity of infection by all gall wasp species. The reduction in A_A was mainly determined by reduced M_A and to a lower extent by lower content of leaf N and P in gall-infected leaves. The emissions of lipoxygenase pathway (LOX) volatiles increased strongly with increasing infection severity for all four species with the strongest emissions in major vein associated species, N. anthracinus. Mono- and sesquiterpene emissions were strongly elicited in N. albipes and Cynips species, except in N. anthracinus. These results provide valuable information for diagnosing oak infections using ambient air volatile fingerprints and for predicting the impacts of infections on photosynthetic productivity and whole tree performance.