|Abstract:||The hydroponic culture system, growing vegetables using nutrient solutions without the presence of soil, has become one of the most popular approaches in today’s agricultural production. Yet it is questionable whether a hydroponic system can grow vegetables with comparable quality to soil-grown ones. Furthermore, when abiotic stresses such as environmental pollutants, microplastics (MPs), are introduced, there may be a unique matrix-dependent response consequently affecting plant performance and ultimately affect human health. This dissertation utilizes lettuce (Lactuca sativa L.) as a model vegetable to evaluate product quality of hydroponic produce and the ability of hydroponic plants to withstand biotic stress, MPs, as compared to conventional soil-grown lettuce.
Giant Caesar lettuce was grown in site-by-site laboratory-constructed hydroponic or soil systems for 35 days. For morphological features, hydroponic lettuce had significantly longer and heavier roots, otherwise, there were no significant differences found in the above-ground plant size, biomass, and leaf features between hydroponic and soil-grown lettuce. Hydroponic lettuce had higher moisture and lower ash content. For functional quality, no significant difference in bioactive compounds (ascorbic acid, chlorophyll a, chlorophyll b, beta-carotenes, and total phenolics) was found in freeze-dried lettuce. However, all compounds analyzed were significantly higher in soil-grown lettuce based on fresh weight. Moreover, antioxidant capacity of hydroponic lettuce was significantly lower on both dry- and wet-basis. For sensory quality, hydroponic lettuce had softer leaves and firmer midribs, which potentially linked to the increased lignin in plant cell walls.
Three different exposure concentrations (0.005%, 0.025%, and 0.1%) of polyethylene microplastics (PE-MPs) on hydroponic and soil-grown lettuce for 30 days led to similar results in inhibition of plant growth (head and root), decreased ascorbic acid production, increased plant firmness and lignin in plant cell walls, as well as shortened shelf life. In addition, after treatment of PE-MPs, the moisture content of hydroponic lettuce decreased, leading to improved ash, bioactive compounds, and antioxidant capacity per unit weight of lettuce, which opposed the discovery of soil-grown lettuce.
To investigate the possible uptake of PE-MPs by lettuce, fluorescent PE-MPs and nonfluorescent PE-MPs were mixed at the ratio of 1:1 (w/w) and added in hydroponic and soil growth systems at concentrations of (0.005%, 0.025%, and 0.1%) for 30 days. Results showed no presence of PE-MPs in the leaves and stems of both types of lettuce. For hydroponic lettuce, the surfaces of leaves, stems and roots were damaged, and PE-MPs were found on the root surface and in the internal root structure (possible entry through damaged root tissue). For soil-grown lettuce, the surfaces of leaves, stems, and roots were dust-laden by large amount of crystal-like particles (perhaps plant wax), and few or almost no PE-MPs were observed on the roots.
Overall, this research demonstrated that lettuce grown hydroponically was not identical to the quality of lettuce grown in soil. PE-MPs affected lettuce growth and quality regardless of growth system, but it was not possible to draw conclusions as to which lettuce plants were more harmed, due to different changes in selected parameters. PE-MPs were more likely to accumulate on lettuce roots in hydroponic conditions. Although no PE-MPs were detected in the edible parts of lettuce, our results indicated that the association of MPs in terrestrial plants is a great concern about food security.