Single low-density polyethylene microplastics stress and drought co-exposure effects on lettuce (Lactuca sativa) physiology, growth, and root development

Microplastics (MPs), plastic particles smaller than 5 mm, can be found agricultural soils and interact with other sources of stress, such as drought. In this study, low-density polyethylene (LDPE) MPs were tested at two contamination levels (0.75 % and 1.5 % w/w), alone or in combination with drought stress (20 % plant available water - PAW), on lettuce (Lactuca sativa) in a pot experiment. The effects of LDPE MPs only or in the co-occurrence with drought stress on lettuce physiology and growth parameters (leaf chlorophyll content - LCC, photosystem II efficiency - Fv/Fm and PIabs parameters, leaf spectral reflectance data, growth parameters, and root growth) were investigated. LDPE MPs proved to interfere with the measured plant physiology and growth parameters, even exacerbating the effects of drought stress. LCC and PIabs parameters were affected by the single presence of 1.5 % LDPE MPs, while the interaction between 1.5 % LDPE MPs and drought stress impacted LCC and Fv/Fm parameters. Most growth parameters were affected by 1.5 % LDPE MPs contamination only, while aboveground biomass moisture content and leaf mass area were affected by the interaction between both 0.75 % and 1.5 % LDPE MPs and drought stress. Root growth parameters were mostly affected by 1.5 % LDPE MPs only and root tissue density, was impacted from the interaction between 1.5 % LDPE MPs and drought stress. This work contributes to expand the knowledge on the behaviour of LDPE MPs in agricultural soils and their interaction with drought stress, in relation to plant physiology and growth.