Spray irrigation with microcystins-rich water affects plant performance from the microscopic to the functional level and food safety of spinach (Spinacia oleracea L.).

Irrigation water coming from freshwater bodies that suffer toxic cyanobacterial blooms causes adverse effects on crop productivity and quality and raises concerns regarding food contamination and human exposure to toxins. The common agricultural practice of spray irrigation is an important exposure route to cyanotoxins, yet its impact on crops has received little attention. In the present study we attempted an integrated approach at the macro- and microscopic level to investigate whether spray or drip irrigation with microcystins (MCs)-rich water differently affect spinach performance. Growth and functional features, structural characteristics of stomata, and toxin bioaccumulation were determined. Additionally, the impact of irrigation method and water type on the abundance of leaf-attached microorganisms was assessed. Drip irrigation with MCs-rich water had detrimental effects on growth and photosynthetic characteristics of spinach, while spray irrigation ameliorated to various extents the observed impairments. The stomatal characteristics were differently affected by the irrigation method. Drip-irrigated spinach leaves showed significantly lower stomatal density in the abaxial epidermis and smaller stomatal size in the adaxial side compared to spray-irrigation treatment. Nevertheless, the latter deteriorated traits related to fresh produce quality and safety for human consumption; both the abundance of leaf-attached microorganisms and the MCs bioaccumulation in edible tissues well exceeded the corresponding values of drip-irrigated spinach with MC-rich water. The results highlight the significance of both the use of MCs-contaminated water in vegetable production and the irrigation method in shaping plant responses as well as health risk due to human and livestock exposure to MCs.

Irrigation of radish (Raphanus sativus L.) with microcystin-enriched water holds low risk for plants and their associated rhizopheric and epiphytic microbiome.

Microcystins (MCs) are toxins produced during cyanobacterial blooms. They reach soil and translocated to plants through irrigation of agricultural land with water from MC-impacted freshwater systems. To date we have good understanding of MC effects on plants, but not for their effects on plant-associated microbiota. We tested the hypothesis that MC-LR, either alone or with other stressors present in the water of the Karla reservoir (a low ecological quality and MC-impacted freshwater system), would affect radish plants and their rhizospheric and phyllospheric microbiome. In this context a pot experiment was employed where radish plants were irrigated with tap water without MC-LR (control) or with 2 or 12 µg L-1 of pure MC-LR (MC2 and MC12), or water from the Karla reservoir amended (12 µg L-1) or not with MC-LR. We measured MC levels in plants and rhizospheric soil and we determined effects on (i) plant growth and physiology (ii) the nitrifying microorganisms via q-PCR, (ii) the diversity of bacterial and fungal rhizospheric and epiphytic communities via amplicon sequencing. MC-LR and/or Karla water treatments resulted in the accumulation of MC in taproot at levels (480-700 ng g-1) entailing possible health risks. MC did not affect plant growth or physiology and it did not impose a consistent inhibitory effect on soil nitrifiers. Karla water rather than MC-LR was the stronger determinant of the rhizospheric and epiphytic microbial communities, suggesting the presence of biotic or abiotic stressors, other than MC-LR, in the water of the Karla reservoir which affect microorganisms with a potential role (i.e. pathogens inhibition, methylotrophy) in the homeostasis of the plant-soil system. Overall, our findings suggest that MC-LR, when applied at environmentally relevant concentrations, is not expected to adversely affect the radish-microbiota system but might still pose risk for consumers' health.