Microplastics aggravate zinc deficiency-induced inhibition of physiological-biochemical characteristics in apple rootstock Malus hupehensis (Pamp.) Rehd seedlings

Both microplastic (MP) pollution and zinc (Zn) deficiency have adverse effects on terrestrial plants. However, the combined effect of MPs and Zn deficiency on plant physiology remains unexplored. In this study, a pot-culture experiment and ¹³C stable isotope tracing technology were employed to investigate the combined effects of MPs and Zn deficiency on the growth, photosynthetic physiology and chlorophyll fluorescence characteristics, as well as synthesis and distribution of photosynthetic products in Malus hupehensis (Pamp.) Rehd seedlings. The results revealed significant reductions in biomass, gas exchange parameters, carbohydrate metabolism enzyme activities, and photosynthetic parameters including F_v/F_m, ΦPSII, ETR and q_p in seedlings subjected to both individual and joint treatments of MPs and Zn deficiency compared to the control group. Notably, the combined Zn deficiency and MPs exhibited a more pronounced inhibitory effect on root biomass (RR = −0.42) compared to the single Zn deficiency (RR = −0.37) and MP (RR = −0.26) treatments. Random forest analysis indicated that chlorophyll fluorescence characteristics (37.5 %) had the greatest impact on biomass variation in seedlings, followed by ¹³C accumulation in various organs (26.7 %). MPs exacerbated the inhibition of photosynthesis (Pn and Gs) under Zn deficiency by suppressing chlorophyll fluorescence parameters (F_v/F_m and ΦPSII), further reducing ¹³C accumulation in roots. In conclusion, the addition of MPs intensified the suppression of photosynthetic parameters caused by Zn deficiency, weakened the carbon assimilation capacity of leaves, and hindered the synthesis of photosynthetic products in leaves and their transport to roots, thereby further inhibiting root growth. This study reveals the combined stress of MP pollution and Zn deficiency on terrestrial plants, deepens our understanding of potential ecological risks, and provides scientific basis for the development of effective mitigation measures to protect plant ecosystems.