Action of papain and chitosan on phytotoxicity and antioxidant system of Phaseolus vulgaris L.

Abstract

The transition toward sustainable agriculture has driven the adoption of bio-inputs derived from secondary metabolites and biopolymers as alternatives to synthetic agrochemicals. However, the application of these molecules requires a thorough understanding of the boundaries between biostimulation and phytotoxicity, as well as the energetic costs associated with the activation of plant defense mechanisms. This study evaluated the morpho-agronomic responses and antioxidant biochemical profile of Phaseolus vulgaris L. subjected to papain and chitosan treatments. Initially, a critical review was conducted on the thresholds between eustress and distress in plant physiology under the use of elicitors. Subsequently, the non-target effects of increasing papain concentrations (0 to 16%) applied via soil drench and foliar spray were investigated. The results revealed a delivery route-dependent paradox: foliar application induced acute oxidative stress, with a dose-dependent increase in malondialdehyde (MDA), accompanied by marked activation of guaiacol peroxidase (GuPOX) and phenolic compounds, whereas soil application maintained plant homeostasis due to rhizosphere buffering. Composite Z-Score modeling identified the 2% foliar treatment as the optimal operational dose. Finally, rhizosphere applications of papain (4%), chitosan (2%), and their combination (P+Q) were evaluated. Papain applied alone acted as a phytotoxic stressor, whereas chitosan reduced pod production due to energetic trade-offs. However, the combined application (P+Q) produced a synergistic hormetic effect, in which chitosan mitigated papain-induced toxicity and restored agronomic performance. These findings demonstrate that the biotechnological success of these bio-inputs depends on the precise optimization of dose, molecular combinations, and delivery route.