Extended Abstract
1. Introduction: Sweet corn, which is a type of maize harvested at the fresh stage & is consumed as a vegetable. Sweet corn belongs to the family Poaceae & the genus Zea. Although sweet corn is used & accepted as a vegetable, maize, including sweet corn, is the third most important cereal crop after rice & wheat in the world. Smaller plant habitus & chemical composition of the grains are the distinguishing features of sweet corn from other corn types. The sugary1 (su1), shrunken2 (sh2), & sugary enhancer1 (se1 alleles of sweet corn varieties prevent & reduce the conversion of sucrose, which is carried to the endosperm, to starch, & ensure that the grains have a high sugar content. The ratio of fat & protein is higher than in other types of maize due to the larger embryo of sweet corn. The adaptation of varieties to the ecological conditions of a region is important in the selection of sweet corn varieties. Environmental factors that have adverse effects on plant growth & survival are known as abiotic stresses. The abiotic stress factors negatively affect the development of a plant, which adapts to any environment & grows smoothly. However, as a direct consequence of global climate change, increased abiotic & biotic stress events have been reported in different regions, threatening world maize yields. The production of reactive oxygen species is a common phenomenon in plants under drought stress. These reactive oxygen species (ROS) generation led to lipid peroxidation, protein degradation, & nucleic acid damage. To alleviate the adverse effects of reactive oxygen species, plants have evolved an antioxidant defense system that includes enzymes like superoxide dismutase (SOD). In the present research, SOD profiles & plant growth of sweet corn were investigated under drought & non-drought stresses.
2. Materials & Methods: A field experiment using a split-plot design based on a randomized complete block design was performed with two sweet corn cultivars under drought & non-drought stress conditions at the Moghan Agricultural Research Station, Iran. The main plot is the drought condition, one experiment was carried out at normal irrigation conditions, & the other was carried out with interrupted irrigation for 18 days ahead of the flowering stage. The sub-plot was two sweet corn cultivars, KSC403 & Chase. The plant height & grain yield were investigated under drought & non-drought stresses. The crude extracts of fresh leaves were prepared in a tris-HCl extraction buffer [Tris 50 mM, pH 7.5, ascorbic acid (50 mM), sucrose 5%, sodium metabisulfite (20 mM), PEG (2%), 2- mercaptoethanol (0.1%)] before use, with a ratio of 0.5 mg μl^-1 (1W: 2V). The extracts were centrifuged at 10,000 g for 10 min at 4 °C using small Eppendorf tubes. The enzyme extracts were absorbed on filter paper & loaded onto 7.5% horizontal slab polyacrylamide gels (0.6×15×12 cm) with the use of the TBE (Tris-Borate-EDTA) electrode buffer (pH = 8.8). Electrophoresis was performed at 4 °C for 3 h. Two slices prepared from the slab gels were stained after electrophoresis to measure the enzyme activity. The staining SOD isoforms were analyzed based on the sensitivity to 2 mM KCN or 5 mM H₂O₂. The stained gels were then fixed & scanned at an appropriate range of wavelengths. To calculate optical density × area for each isozyme activity, the image analysis software, MCID, was used. The Cu/Zn-SOD gene expression was investigated under drought & non-drought stresses.
3. Results & Discussion: Based on the variance analysis of the data, there were significant differences at the 1% probability level among the different drought stress levels and the maize varieties for plant height and grain yield. The interaction effect of variety × drought stress was also significant only for plant height at the 5% probability level. Drought stress significantly reduced plant height by 19.04% compared to normal irrigation conditions. Additionally, the comparison of the combined means for the hybrid × drought stress interaction for plant height showed that the KSC403 hybrid had the highest plant height under both normal and drought stress conditions. Drought stress significantly decreased the grain yield of the maize hybrids by 20.55%. Comparison of the mean grain yield of sweet maize varieties revealed that the KSC403 variety, with a grain yield of approximately 6.3 tons per hectare, had the highest grain production compared to the Chase variety. Electrophoretic analysis for SOD enzyme using 8% slab polyacrylamide gels showed three isoforms, SOD1 (Fe-SOD), SOD2 (Cu/Zn-SOD), & SOD3 (Mn-SOD), which were detected based on sensitivity to KCN & H₂O₂ The results of the data analysis for the activity of SOD isoforms indicated that the effects of hybrid and drought stress were both significant at the 1% probability level. Additionally, the interaction between hybrid × drought stress was also significant for SOD isoform activity. Drought stress significantly increased the activity of SOD isoforms compared to normal irrigation conditions, with increases of 27.05%, 30.06%, and 25.89% for Fe-SOD, Cu/Zn-SOD, and Mn-SOD, respectively, relative to normal watering conditions. The comparison of the mean activity of SOD isoform interactions between hybrid × drought stress showed that the KSC403 variety exhibited the highest activity of all three SOD isoforms compared to the Chase variety under both normal and drought stress conditions. The Cu/Zn-SOD gene expression using Real-time PCR in sweet corn cultivars indicated that drought stress increased Cu/Zn-SOD gene expression.
4. Conclusion: KSC403 with higher grain yield, plant height, & Cu/Zn-SOD activity ranked as a drought-tolerant hybrid in this study. It can be concluded that the increase in Cu/Zn-SOD activity may reduce damage caused by drought stress in sweet corn.