Lead (Pb)-Induced Oxidative Stress Alters the Morphological and Physio-Biochemical Properties of Rice (<i>Oryza sativa</i> L.)

Lead (Pb) is one of the major environmental heavy metal pollutants, known as being neither essential nor beneficial for any living organisms, and which is detrimental to plant fitness, growth, and productivity, as well as human health. This study investigated the changes in the morphological, physiological, and biochemical properties of rice cultivars exposed to lead (Pb). Therefore, soil was contaminated with a solution containing 0.6 mM or 1.2 mM Pb four weeks prior to transplanting. Then, 4-week-old rice seedlings of Tunnae, Ilmi, Yasmen, Mashkab, and Amber Barka were transplanted into the contaminated soil and grown until maturity. The results showed that a high concentration of lead (1.2 mM) induced significant reduction in the plant height, number of tillers, number of panicles per plant, and the number of spikelets per panicle in Pb-sensitive rice cultivars, while in Pb-tolerant cultivars, a balanced growth of plants and non-significant change in the major yield components were recorded. However, all rice cultivars showed a reduced biomass dry weight. Under the same conditions, we observed a differential enzymatic antioxidant activity, with catalase (CAT) and peroxidase (POD) being the most active. In addition, the proline accumulation and sucrose content increased concomitant with an increase in the Pb concentration, while the total protein and chlorophyll contents significantly decreased. Of all the soluble sugars analyzed, sucrose was the most abundant in response to Pb treatment. Interestingly, the rice cultivars Tunnae and Mashkab exhibited a high degree of tolerance towards Pb stress, with a balanced plant height, number of tillers, number of panicles, and number of spikelets per plant. Therefore, all results collectively suggest that the tolerance to Pb-induced oxidative stress observed in Tunnae and Mashkab could be a result of a synergetic action of both enzymatic and non-enzymatic antioxidant systems, leading to a balanced reduction–oxidation status in rice.