Assessing genetic diversity and reaction of maize genotypes (Zea mays L.) under cadmium stress

Heavy metals especially cadmium (Cd), with potential toxicity and injurious effects for plants and humans, are one of the most important abiotic stresses for crop plants such as maize which lead to a considerable reduction in the crops production in developing countries including Iran. Therefore, assessing genetic diversity for Cd stress tolerance, as one of the basic principles of plant adaptation to abiotic stresses, is very important. In this regard, the present study was carried out to evaluate the genetic diversity of maize pure lines and hybrids based on important agronomic traits associated with grain yield under non-stress and Cd stress environmental conditions. The results of this research can be useful for maize breeders in identifying suitable parental genotypes for maize breeding programs to develop high-yielding and Cd stress-tolerant genotypes in regions contaminated to this heavy metal.

In this study, 95 maize genotypes comprising pure lines and hybrids, were evaluated in a randomized complete block design with three replications under two non-stress and Cd stress conditions. The experiment was performed in a pot experiment in an open area of the Agricultural and Natural Resources Research Station of Jiroft, Kerman province, Iran, in two cropping seasons, 2020-21 and 2021-22. Under Cd stress conditions, cadmium chloride solution (CdCl2.2H2O) with a concentration of 30 mg.L−1 was applied at two important stages of maize plant growth, including six-leaf stage (Code 16 in Zadoks scale) and the appearance of first male flowers (Code 50 in Zadoks scale). The measured traits included 24 different phenological, morphological, and agronomic traits. Grouping of maize genotypes in term of the studied traits was performed using cluster analysis based on Ward’s minimum variance method, and discriminant function analysis was used to confirm and validate the results of cluster analysis.

The results of this study indicated that there was an extensive genetic diversity among the studied maize genotypes for most of the studied traits, especially for grain yield and its components under non-stress and Cd stress conditions. Cluster analysis grouped the maize genotypes into four separate clusters with accuracy probabilities of 91.6% and 97.9% under non-stress and Cd stress conditions, respectively. Comparison of means between these four groups showed that under non-stress conditions, 41 and 24 genotypes in the third and fourth groups were the high yielding genotypes of this experiment, respectively. Under Cd stress conditions, 49 genotypes with the higher grain yield and yield components in the second and third groups (such as Ma002, Ma003, Ma004, Ma005, and Ma007) were identified as Cd-tolerant genotypes. These genotypes mainly showed short to medium phenological periods. Also, 11 genotypes (Ma001, Ma008, Ma030, Ma033, Ma036, Ma039, Ma042, Ma045, Ma072, and Ma089), indicated the lowest means for important agronomic traits including yield and its components under both non-stress and Cd-stress conditions. These genotypes were unable to tolerate Cd stress under the conditions of this experiment and were identified as sensitive genotypes.

The present study led to the identification of 40 tolerant maize genotypes to Cd stress (such as Ma003, Ma005, Ma007, Ma013, Ma014, and Ma015). These genotypes in addition to exhibiting desirable yield and yield components under non-stress environment, also showed suitable yield under Cd stress conditions. Therefore, by carefully selecting the parents among Cd-tolerant genotypes and carrying out targeted crosses between them, it is possible to obtain suitable hybrids for cultivation in Cd-stressed environments by exploiting genetic phenomena such as transgressive segregation and heterosis.