Effects of lead stress on the growth, physiology, and cellular structure of privet seedlings

In this study, we investigated the effects of different lead (Pb) concentrations (0, 200, 600, 1000, 1400 mg kg-1 soil) on the growth, ion enrichment in the tissues, photosynthetic and physiological characteristics, and cellular structures of privet seedlings. We observed that with the increase in the concentrations of Pb, the growth of privet seedlings was restricted, and the level of Pb ion increased in the roots, stem, and leaves of the seedlings; however, most of the ions were concentrated in the roots. Moreover, a decreasing trend was observed for chlorophyll a, chlorophyll b, total chlorophyll, net photosynthesis (Pn), transpiration rate (Tr), stomatal conductance (Gs), sub-stomatal CO2 concentration (Ci), maximal photochemical efficiency (Fv/Fm), photochemical quenching (qP), and quantum efficiency of photosystem II (ΦPSII). In contrast, the carotene levels, minimum fluorescence (F0), and non-photochemical quenching (qN) showed an increasing trend. Under Pb stress, the chloroplasts were swollen and deformed, and the thylakoid lamellae were gradually expanded, resulting in separation from the cell wall and eventual shrinkage of the nucleus. Using multiple linear regression analysis, we found that the content of Pb in the leaves exerted the maximum effect on the seedling growth. We observed that the decrease in photosynthetic activation energy, increase in pressure because of the excess activation energy, and decrease in the transpiration rate could result in maximum effect on the photosynthetic abilities of the seedlings under Pb stress. Our results should help in better understanding of the effects of heavy metals on plants and in assessing their potential for use in bioremediation.