Induced Volatile Emissions, Photosynthetic Characteristics, and Pigment Content in <i>Juglans regia</i> Leaves Infected with the Erineum-Forming Mite <i>Aceria erinea</i>

Persian walnut (Juglans regia L., Juglandaceae), one of the essential nut crops, is affected by different diseases, including mite attacks which result in gall and erineum formation. As the proportion of leaf area covered by mite galls or erineum is typically relatively low, the impact on tree photosynthetic productivity is often considered minor, and no pest control management is usually suggested. However, the effect of erineum-forming mites on walnut photosynthesis might be disproportionately larger than can be predicted from the leaf area impacted. In the present study, we studied how the foliage photosynthetic characteristics, pigment contents, and stress-induced volatile organic compounds scaled with the severity of infection varied from 0% (control trees) to 9.9%, by erineum-forming mite Aceria erinea in J. regia. Both leaf net assimilation rate (up to 75% reduction) and stomatal conductance (up to 82%) decreased disproportionately, increasing infection severity. Leaf total chlorophyll and β-carotene contents also decreased with infection severity, although the reduction was less than for photosynthetic characteristics (28% for chlorophyll and 25% for β-carotene). The infection induced significant emissions of green leaves volatiles ((Z)-3-hexenol, (E)-2-hexenal, (Z)-3-hexenyl acetate and 1-hexanol), monoterpenes and the homoterpene 3-(E)-4,8-dimethyl-1,3,7-nonatriene, and these emissions scaled positively with the percentage of leaf area infected. These results collectively indicate that erineum-forming mite infection of walnut leaves results in profound modifications in foliage physiological characteristics that can significantly impact tree photosynthetic productivity