Effect of dimethyl phthalate (DMP) on germination, antioxidant system, and chloroplast ultrastructure in Cucumis sativus L.

Abstract

Pollution of agricultural soils caused by widely employed plastic products, such as phthalic acid esters (PAEs), are becoming widespread in China, and they have become a threat to human health and the environment. However, little information is available on the influence of PAEs on vegetable crops. In this study, effects of different dimethyl phthalate (DMP) treatments (0, 30, 50, 100, and 200 mg L⁻¹) on seed germination and growth of cucumber seedlings were investigated. Although germination rate showed no significant difference compared to control, seed germination time was significantly delayed at DMP greater than 50 mg L⁻¹. Concentrations of DMP greater than 30 mg L⁻¹ reduced cucumber lateral root length and number. The measurement of five physiological indexes in cucumber leaves with increasing DMP concentration revealed a decrease in leaf chlorophyll content, while proline and H₂O₂ contents increased. Peroxidase (POD) and catalase (CAT) activities increased in cucumber plants under 30 and 50 mg L⁻¹ DMP treatments compared to control; while after a 7-day treatment, these activities were seriously reduced under 100 and 200 mg L⁻¹ DMP treatments. According to transmission electron microscopy (TEM) micrographic images, the control and 30 mg L⁻¹ DMP treatments caused no change to leaf chloroplast shape with well-structured thylakoid membrane and parallel pattern of lamellae. At concentrations higher than 30 mg L⁻¹, DMP altered the ultrastructure of chloroplast, damaged membrane structure, disordered the lamellae, and increased the number and volume of starch grains. Moreover, the envelope of starch grains began to degrade under 200 mg L⁻¹ DMP treatment.