Morpho-physiological and biochemical responses of Brassica species toward lead (Pb) stress

Brassica species, capable of heavy metals (HMs) hyperaccumulation, differ in their ability to accumulate and tolerate metals present in their environment. In this comparative study, the accumulation, morphological, and physiological responses of three Brassica species i.e., Brassica juncea, B. napus, and B. campestris, against lead (Pb) were examined. Plants were grown in pots under greenhouse conditions and subjected to 0, 50, 100, 150 mM concentrations of Pb for 14 days. The study revealed that 150 mM Pb concentration reduced the plant length and biomass in all the species and this decline was more obvious in B. napus. At 100 mM Pb concentration, plant length increased 3.5% in B. juncea, while decreased by 8 and 36% in B. campestris and B. napus, respectively. B. campestris and B. napus suffered from more pronounced Pb-accumulation in the root followed by shoot as compared to B. juncea. Pb-accumulation in 100 mM treated root of B. campestris and B. napus increased 29 and 80%, respectively as compared to B. juncea Pb treated root. Antioxidant enzyme catalase (CAT) activity was increased in B. juncea and B. campestris up to 150 mM concentration, while in B. napus activity of enzyme decreased at 100 and 150 mM Pb concentration. Phenylalanine ammonia-lyase (PAL) and nitrate reductase activity increased at 50 mM, while the polyphenol oxidase (PPO) and nitrite reductase significantly increased at 150 mM. Brassica species also showed more significant accumulation of amino acid, inhibition of proteins and total sugar content at 100 and 150 mM concentrations. Although all species exhibited enhanced antioxidant activity, activation in B. juncea was relatively higher. These results suggest that B. juncea is relatively more tolerant towards Pb stress as compared to B. campestris and B. napus due to reduced metal uptake and enhanced antioxidant enzyme activities.