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The structural diversity and high reactivity of phenolic compounds synthesized by higher plants and the toxicity of many intermediate products of their hydrolysis or oxidation causes that they may play an important role in the resistance of plants to different stresses, including feeding by spider mites (Acarina: Tetranychidae). The results of our studies as well as literature data indicate that the feeding of these pests brings about the induction of a defensive response. The active defensive mechanism of a plant is determined by different phenolic compounds and enzymes involved in their synthesis (PAL) and oxidation (PPO, POX). The induced defensive mechanism is usually associated with some degree of specificity. In consequence, the induced resistance reactions, by limiting the further development of the mite pests, protect the plant from further damage.
The influence of PGPR on the susceptibility of glasshouse cucumber and tomato to spider mites was studied. lt was found that plants treated with PGPR suppressed the development of mile populations on both studied crops. The effect was more evident for cucumber, as compared to tomato plants. Mite population was smaller by more than 40% on the leaves of the susceptible cucumber cultivar, Corona growing in the presence of rhizobacteria, as compared to that without bacteria. The development of spider mile population on the bacterized susceptible tomato cultivar, Romatos was more inhibited on young leaves (43%) than on older ones (34%). As the injury of plants progressed the level of inhibition was decreased.
Experimental cucumber and tomato plants were cultivated under glasshouse conditions with or without PGPR. Young plants were infested with spider mites (T. urticae - cucumber and T. cinnabarinus - tomato). Leaves were analysed for protein and amino acid concentration. In infested leaves of the cucumber cultivar, susceptible to spider mites (Corona), an important decrease in protein content occurred in both bacterized and nonbacterized plants. Amino acid content was higher in mile infested leaves and the presence of PGPR did not influence this phenomenon. The content of proteins was also studied in the leaves of the less susceptible cucumber cultivar (Aramis). Spider mile feeding caused a small decrease in their concentration. The presence of bacteria in the root system caused an increase of soluble proteins in the leaves. In infested leaves of the highly susceptible tomato cultivar (Romatos), cultivated without bacteria, an evident increase of amino acid content was found opposite to plants with PGPR. The protein concentration was also increased in injured leaves of this cultivar but the presence of bacteria in the root system of mite infested plants caused a decrease of these compounds in their leaves. lnjured foliage of the less susceptible tomato cultivar (Sionka) had a lower protein concentration in bacterised plants as compared to those nonbacterised and both controls.
Luring natural enemies to the plants colonized by pests is a complex process, which is still not fully recognized, and the efficiency of which is influenced by many factors. A profile of Herbivore‐Induced Plant Volatiles (HIPVs) depends on species/cultivars of the hostplant and its developmental stage, as well as species of the pest, its developmental stage and density. The results of recent studies show that emission of HIPVs significantly varies with abiotic environmental factors (soil drought, salinity, temperatures, light intensity, etc.). Furthermore, it was shown that the presence of endosymbiotic organisms may change the composition of volatile compounds emitted from the plants colonized by pests, resulting in modification of the behaviour of natural enemies. Detailed knowledge concerning mechanisms of indirect defense should contribute to employment of volatile compounds in agricultural practice as a method compatible with other methods used in integrated plant protection.
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