Bactérias promotoras de crescimento e substâncias húmicas como indutoras de defesas diretas e indiretas do cafeeiro à cochonilha Planococcus minor (Maskell) (Hemiptera: Pseudococcidae)

Abstract

Biostimulants have the primary function of providing plant growth, they can also induce direct or indirect resistance to the plant, making them a alternative for sustainable. This thesis aimed to evaluate the effects of humic substances (HS) and the plant growth-promoting bacteria (PGPB), Enterobacter tabaci, combined and isolated, on chemical defenses of the coffee plant Coffea arabica (Rubiaceae) against infestation by the mealybug Planococcus minor (Hemiptera: Pseudococcidae). We verified that PGPB and HS acted as biostimulants in the growth of the root and aerial part of coffee plants, however, their combined use stimulated the growth of more plant characteristics than when used separately. In the host selection, all treatments with biostimulants reduced the colonization of mealybugs in coffee plants, with the HS treatment being the least preferred by them. On the other hand, HS and HS + PGPB treatments were not good hosts for pseudococcids, in addition to stimulating increased activity of the enzymes of oxidative metabolism ascorbate peroxidase (APX) and superoxide dismutase (SOD). Treatment with only HS stimulated catalase activity (CAT). All treatments with biostimulants increased the levels of phenolic compounds in non-infested plants, however, after infestation, only plants treated with PGPB and HS+PGPB showed higher levels of these compounds compared to control plants. We evaluated the attractiveness of the Cryptolaemus montrouzieri (Coleoptera: Coccinellidae). When contrasting P. minor-infested plants against control plants, treated with PGPB or HS+PGPB, C. montrouzieri were attracted to infested plants. Only HS-infested versus HS non-infested was not discriminated. The ladybug was preferentially guided by the volatiles emitted by the mealybugs-infested plants with biostimulants than those emitted by mealybug-infested plants without biostimulants (control). However, when exposed to induced volatiles emitted by HS-treated plants versus HS+PGPB-treated plants, C. montrouzieri did not differentiate between treatments. Furthermore, between PGPB and HS+PGPB, both infested, the natural enemy showed a preference for infested plants treated with both biostimulants. When analyzing induced plant enzymes, we observed that APX was higher in the PGPB treatment than in the HS treatment, but the two treatments did not differ from the control. SOD was higher in HS and in HS+PGPB treatment. On the other hand, CAT was reduced by HS application, since HS and HS+PGPB treatments had less activity compared to control and PGPB. We also quantified the amounts of phytohormones salicylic acid (SA) and jasmonic acid (JA). The infestation by P. minor increased the levels of SA in the coffee plants 2 times compared to the non-infested ones. However, none of the treatments with biostimulants influenced the accumulation of SA in the plant. In contrast, P. minor infestation did not modify JA levels in relation to non-infested plants, however, the HS+PGPB treatment showed higher levels of JA compared to HS or PGPB treatments. Thus, the combined use of biostimulants, HS + PGPB, promoted greater plant growth than the isolated use, however, the combined use did not have a synergistic effect on the direct or indirect defenses of the coffee plant against the mealybug. Furthermore, HS used isolated was the best treatment to increase coffee plant defenses. Although HS and PGPB treatments promoted an increase in phenolic compounds and improved the signaling capacity of the plant's defense pathways, it is not possible to determine the mechanism by which the treatment with HS made the plant more resistant to the white mealybug.