Sweet corn (Zea mays var. saccharata) is an important cereal crop worldwide. Its popularity is mainly due to its sweetness, tenderness, and unique flavor, which are all determined by the presence of specific genes and their expression. Understanding the genetic and biochemical basis of these traits helps in the development of improved sweet corn cultivars with higher yield, better nutritional value, and desirable agronomic traits. In this study, we conducted a comprehensive characterization of sweet corn using morphological, molecular, and biochemical approaches to gain insights into the genetic and biochemical basis of its desirable traits, such as sugar and carotenoid content.
Morphological characterization of crop plants is essential for understanding the genetic diversity and variation that exists within a population. In this study, the analysis of variance (ANOVA) inferred significant genetic differences among the 80 sweet corn inbred lines and hence, the possibility of selection of superior genotypes with improved yield potential and high levels of total sugars.
The evaluation of the 80 inbreds for 31 morphological traits showed that all the traits were quite informative with respect to the trait expression cum characterization. Phenotypic characterization of maize inbred lines using DUS traits is well documented previously by many researchers [7, 8]. Similar to our findings, Madhukeshwar and Sajjan [15] also observed enough magnitude of variation for various morphological traits like tassel angle, tassel attitude of lateral branches, tassel density of spikelet’s, plant height, ear shape and 1000 grain weight in a study comprised of two maize hybrids and five parents. In this study, leaf angle between stem and leaf blade for maximum number of inbred lines was small, which is a crucial trait for obtaining higher yield due to its substantial role in light interception by the canopy and hence photosynthetic efficiency, specifically under high density planting stress. However, inbreds with wide leaf angle are thought to be good water harvesters and could be best used under rainfed conditions. The time of anthesis and silk emergence was found to be late in all the inbreds, indicating that more days are available for the lines for photosynthesis, leading to higher biomass production and thus higher crop yield. Most of the inbred lines have dense tassel making them ideal male parent for quality seed production. However, a dense tassel could also cause redundant assimilation, consumption and resource competition with female reproductive organ. Therefore, lines with light tassel with more partitioning of assimilates towards ear could serve as ideal female parents. Furthermore, lines found with medium ear placement are believed to be ideal for making harvesting easy and improving lodging resistance. Plant height, number of rows of grains per ear, ear diameter, ear length and 1000 kernel weight are important secondary traits for the identification of genotypes from this study for higher productivity.
Carotenoids are comparatively found to be abundant in sweet corn; thus, their quantification is vital from the nutritional point of view [14]. The total carotenoids of the 80 early generation sweet corn inbreds (S2 stage) showed a continuous distribution pattern, as expected for a quantitative trait. However, carotenoid content in both parental hybrids was found to be in the intermediate range. The results of the present research are in similar direction with the earlier reports of Song [16] where the total carotenoids were in the range 8.42 to 39.71 µg/g. Similarly, analysis for total sugar content also revealed continuous pattern, however, in case of the parental hybrids from which the 80 inbreds were derived, the mean value for the total sugars was found to be highest. The results of this research are comparable with the previous reports of Ghada and Ibrahim [6]. However, Tosun [27] and Hemavathy and Priyadarshani [9] revealed slightly higher range for total sugars in 49 inbreds (0.66–16.84%) and 26 sweet corn lines (2.05 to 17.14%), respectively.
The scatter plot of total carotenoids and total sugars showed that the non- sugary genotype SMC-3 had low levels of total carotenoids as well as total sugars. The parents viz., Sugar-75 and Mithas showed intermediate levels of total carotenoids with high levels total sugars while as the inbred S9, S23, S27 and S36 showed high levels of total carotenoids and total sugars confirming that these inbred lines could be utilized in sweet corn breeding programmes after further validation for yield attributes, disease resistance and desirable agronomic traits.
In addition to better nutritional profile, the food must have good digestibility in order to have widespread acceptability. High amylose maize is a source of resistant starch which is a type of starch that resists digestion and therefore confers with low glycemic index, thereby can prevent diabetes, obesity and colon cancer [19]. Amylose concentration is found to be high in the su1 kernels [10]. The study has also confirmed that the 80 sweet corn inbreds have increased amounts of amylose in them. The parent sugar 75 was found to contain high content of total carotenoids, total sugars and high amylose. Among the inbreds, the inbreds S5 and S59 were found to have high level of amylose in addition to having high amounts of total carotenoids and total sugars. The results obtained are similar to the results of that of [19, 30].
The results of molecular marker-based validation of sweet kernel trait confirmed that out of the 60inbreds, seven inbreds viz., S21, S28, S47, S48, S49, S53, and S54 carried the alleles specific to sugary trait with respect to the markers umc2061 and bnlg1937. These lines were also found to contain high sugar content, thus confirming the utility of these markers viz, umc2061 and bnlg1937 in marker assisted selection. The other lines which carry the sugar specific alleles for only one of the markers may undergo some recombination between the marker and the gene. Though these lines were also tested for the phenotypic appearance of the grain and with respect to the biochemical traits, the chance of these lines belonging to the non-sugary class is very low. However, these lines are required to be analysed further and validated with more closely linked markers to su-1mutants. Hossain [10] also identified and validated umc 2061 as the most effective and closest marker for su1in sweet corn. Similar studies were carried out by Tosun [27] in 49 sweet corn inbred lines and it was confirmed that the primers which were found to be immensely correlated with the sugar content were phi44 marker for the gene sh1, phi328175 for the ea1 gene, umc1031 for the su gene and umc2276 for the sh2 gene. They also observed that for su1, umc 2061 marker were found useful to determine higher sugar content lines. However, the lines carrying su1 allele with respect to bnlg 1937 had lower sugar content.
The cluster analysis performed on the basis of the phenotypic traits, molecular markers and the biochemical traits revealed that the study have implications in effectively developing improved lines of sweet corn using the information from the clustering pattern which is necessary for successful continuation of any breeding programme. The crosses involving inbred lines from diverse clusters were expected to show maximum heterosis and create wide variability in genetic architecture and could be used as parental source for breeding programmes with selective objective.