Morphological, ultrastructural, biochemical and yield attributes variations in Nigella sativa L. by cadmium (Cd) stress: Implications for human health risk assessment

Abstract

This study was conducted in the pots where each pot was filled with 4 kg soil that was contaminated with varying concentrations of cadmium (Cd) which are 1.5 mM, 2.0 mM, 2.5 mM, 3.0 mM, 3.5 mM, and 4.0 mM equal to 168.61, 224.82, 281, 337.23, 393.44 and 449.64 mg/L respectively), except the control group, each treatment was replicated thrice (n = 3) including control. We evaluated and examined growth, yield, metal stress tolerance, and metal buildup characteristics at the harvesting stage of black cumin (Nigella sativa L.) plants. The findings indicated that all levels of Cd had a substantial (p ≤ 0.05) negative impact on black cumin plants' growth and yield-related characteristics. Low (1.5 mM) to high doses (4.0 mM) of Cd significantly (p ≤ 0.05) reduced chlorophyll a (30%–72%), chlorophyll b (25%–69%) and total chlorophyll (56% to 82%) levels in the fresh leaf tissues, as well as significantly (p ≤ 0.05) decreased the fruit yield (12%–45%) and seed yield (39%–75%) of black cumin plants. Energy dispersive x-ray (EDX) analysis and scanning electron microscopy (SEM) of control and Cd-treated plant leaf samples showed significant Cd accumulation, and the ultrastructure of both revealed that Cd stress reduced the stomatal density and size of the stomatal aperture compared to the control. Bioaccumulation levels of Cd in the seeds of all treated plants are above the threshold limit (0.3 mg/kg) set by the World Health Organization (WHO) for medicinal plants and are unsafe for human consumption. Therefore, farmers must avoid cultivating black cumin crops in Cd polluted soil from economic and health perspectives.