Understanding and Comprehensive Evaluation of Cold Resistance in the Seedlings of Multiple Maize Genotypes

Maize is a cold-sensitive crop, and it exhibits severe retardation of growth and development when exposed to cold snaps during and right after seedling emergence. Although different agronomic, physiological, and molecular approaches have been tried to overcome the problems related to cold stress in recent years, the mechanisms causing cold resistance in maize are still unclear. Screening and breeding of varieties for cold resistance may be a sustainable option to boost maize production under low-temperature environments. Herein, seedlings of 39 different maize genotypes were treated under both 10 °C low temperature and 22 °C normal temperature conditions for 7 days, to assess the changes in seven growth parameters, two membrane characteristics, two reactive oxygen species (ROS) levels, and four antioxidant enzymes activities. The changes in ten photosynthetic performances, one osmotic substance accumulation, and three polyamines (PAs) metabolisms were also measured. Results indicated that significant differences among genotypes, temperature treatments, and their interactions were found in 29 studied traits, and cold-stressed seedlings were capable to enhance their cold resistance by maintaining high levels of membrane stability index (66.07%); antioxidant enzymes activities including the activity of superoxide dismutase (2.44 Unit g^-1 protein), peroxidase (1.65 Unit g^-1 protein), catalase (0.65 μM min^-1 g^-1 protein), and ascorbate peroxidase (5.45 μM min^-1 g^-1 protein); chlorophyll (Chl) content, i.e., Chl a (0.36 mg g^-1 FW) and Chl b (0.40 mg g^-1 FW); photosynthetic capacity such as net photosynthetic rate (5.52 μM m^-2 s^-1) and ribulose 1,5-biphosphate carboxylase activity (6.57 M m^-2 s^-1); PAs concentration, mainly putrescine (274.89 nM g^-1 FW), spermidine (52.69 nM g^-1 FW), and spermine (45.81 nM g^-1 FW), particularly under extended cold stress. Importantly, 16 traits can be good indicators for screening of cold-resistant genotypes of maize. Gene expression analysis showed that GRMZM2G059991, GRMZM2G089982, GRMZM2G088212, GRMZM2G396553, GRMZM2G120578, and GRMZM2G396856 involved in antioxidant enzymes activity and PAs metabolism, and these genes may be used for genetic modification to improve maize cold resistance. Moreover, seven strong cold-resistant genotypes were identified, and they can be used as parents in maize breeding programs to develop new varieties.