The aim of this experiment was to determine plant responses to Coccus hesperidum L. infestation on two host plants. Groups of five lemon and five fern plants were colonised by various numbers of mobile C. hesperidum instar nymphs. After 6 months, all scale insect individuals were counted on each plant. According to the insect density, the plants were divided into a five-degree series. In all density classes of host plants tested, the infestation of scale insects decreased the chlorophyll and carotenoid content as well as the value of three indicators of photosynthetic activity. The strongest decrease in the analysed pigments was observed for the smallest abundance of insects (firstclass density) in lemon leaves and in second-class density in fern leaves. The strongest reactions of the chlorophyll fluorescence indicators were observed in density classes III and IV in the fern leaves and density classes IV or V in the lemon leaves. The reactions depended on the specific properties of plants and abundance of insects feeding on them.

Gall formation is induced by an insect, which changes normal plant development and results in the formation of a new organ, following distinct stages of metabolic and developmental alterations. Research on mechanisms of recognition and responses to biotic stress may help to understand the interactions between galling aphids and their host plants. In this study, Tetraneura ulmi L. (Hemiptera: Eriosomatinae) galls and Ulmus pumila L. (Rosales: Ulmaceae) leaves were used as a model. Concentrations of hydrogen peroxide (H2O2) and thiobarbituric acid reactive substances, electrolyte leakage, as well as the activity of ascorbate peroxidase, guaiacol peroxidase, and catalase (CAT) were determined in galls and two parts of galled leaves (with and without visible damage). Biochemical analyses were performed at three stages of gall development: initial, fully developed, and mature galls. A slight increment in H2O2 content with a strong enhancement of ascorbate peroxidase and CAT activities were observed in galls and galled leaves in the first stage. In subsequent stages of gall development, a progressing increase in H2O2 production and cell membrane damage was associated with declining antioxidant enzyme activities, especially in gall tissues. The stages of gall development are likely to be part of cell death triggered by aphid feeding. It seems that the gall is the result of a biochemical struggle between the host plant and the gall inducer.

Changes of chlorophyll (Chl) a fluorescence and photosynthetic pigment contents were analysed in galled leaves (visibly damaged and undamaged parts) and intact leaves. The values of minimal fluorescence of the dark-adapted state, maximal quantum yield of PSII photochemistry, effective quantum yield of PSII photochemical conversion, and photochemical quenching coefficient decreased in Ulmus pumila L. leaves galled by Tetraneura ulmi (L.) and in U. glabra Huds. galled by Eriosoma ulmi (L.). Colopha compressa (Koch.) feeding affected these parameters only in damaged parts of U. laevis Pall. galled leaves. The increasing number of T. ulmi galls progressively decreased photosynthetic performance. In gall tissues of all analysed aphid species, the lowest photosynthetic pigment content was found, indicating that the photosynthetic capacity must have been low in galls. Significant reduction of Chl and carotenoid contents were observed in damaged and undamaged portions of galled leaves only in the case of T. ulmi feeding.Additional key words: biotic stress; Eriosomatinae, fluorescence parameters; host plant response. Fm -minimal and maximal fluorescence yield of the dark-adapted state, respectively; F0', Fm' -minimal and maximal fluorescence yield of the light-adapted state, respectively; Fs -steady-state fluorescence yield; Fv -variable fluorescence in the dark-adapted state; Fv/Fm -maximal quantum yield of PSII photochemistry; qN -nonphotochemical quenching coefficient; qP -photochemical quenching coefficient; Y -effective quantum yield of PSII photochemical conversion; ΦPSII -quantum efficiency of PSII electron transport. Acknowledgments: This work was supported by University of Life Sciences in Lublin (Project no. OKE/DS/2 in 2013-2017). Authors thank to three anonymous reviewers for their valuable comments, which helped improve this manuscript.

The study aimed at comparing the changes in the content of free amino acids, phenolic compounds and the activity of PAL and TAL caused by two piercingsucking arthropods: the grape mealybug (Pseudococcus maritimus Ehrh.) and the two-spotted spider mite (Tetranychus urticae Koch) in the leaves of orchid and strawberry, respectively. The obtained results show that the amino acid content and the ratio of amino acids to phenolic compounds increased in both plant species infested by the mealybug and the mite. However, such response was weakly dependent on changes in activity of the analysed enzymes. The pest feeding affected accumulation of the phenolic compounds, since the induction of the PAL activity in mealybug-infested orchid leaves during the first 5 h of the experiment preceded the increase in phenolic compounds during the first week of insect feeding. Instead, the increased activity of TAL was accompanied by elevated levels of phenolic compounds in the leaves of strawberry infested by mites. Mechanisms of biochemical plant responses induced by infestation of the studied herbivorous arthropods are discussed.

Pemphigus spyrothecae Pass. which is a member of the subfamily Pemphiginae is one of the gall-inducing aphids that occurs on poplar trees. Phloem feeding of a founding mother on leaf petiole results in the formation of a new organ, i.e., the spiral gall. This study documents aphid development inside the galls during the growing season and the effect of their feeding on leaf architecture and physiology of the host plant. In particular, leaf length, width, and area were measured, as well as hydrogen peroxide (H2O2) content, electrolyte leakage (EL), malondialdehyde (MDA) concentration, and the activity of ascorbate (APX) and guaiacol peroxidase (GPX) were determined in galls and galled leaves. The presence of petiole galls significantly decreased the length, width, and leaf area. Aphid activity increased H2O2 concentration in galls and EL from galls and leaf tissues, which was accompanied by a strong decrease in MDA content and both peroxidase activities, especially in gall tissues. It can be suggested that P. spyrothecae can manipulate physiological machinery of the host plant for its own benefit.