1 Introduction

Fresh-eating maize, also known as sweet corn, has a rich history and has become a staple in many diets around the world due to its unique flavor and nutritional benefits. Originating in the United States, sweet corn has been introduced globally and is now widely consumed either fresh or processed (Revilla et al., 2021). The demand for fresh-eating maize is increasing, particularly in regions like Heilongjiang province in China, where the annual output has reached 3.35 billion ears (Yang et al., 2021). This growing popularity is driven by consumer preferences for its sweetness, tender texture, and health benefits (Haidash et al., 2023). The market demand for high-quality fresh-eating maize continues to rise, necessitating improvements in both yield and quality to meet consumer expectations (Taş and Mutlu, 2021).

The quality traits of fresh-eating maize, such as sweetness, texture, and tenderness, are critical for consumer satisfaction. These traits are influenced by the genetic makeup of the maize, including specific endosperm mutations that increase sugar content and reduce starch levels (Azanza et al., 2004). For instance, the shrunken2-reference allele (sh2) is known to accumulate more sugar, enhancing the sweetness of the maize (Hu et al., 2023). Additionally, stress resistance, including tolerance to drought, pests, and salinity, is essential for maintaining yield and quality under adverse environmental conditions (Taş and Mutlu, 2021; Ouhaddou et al., 2023). Environmental stresses can significantly impact the yield and quality of sweet corn, as seen in studies where higher temperatures and lower humidity reduced fresh cob yield and other quality parameters. Therefore, developing maize lines that combine superior quality traits with robust stress resistance is crucial for sustainable production (Ye et al., 2023).

This study identifies and develops superior fresh-eating maize lines with enhanced quality and stress resistance. It involves screening various maize varieties to determine those with the highest starch and sugar content, as well as those that perform well under different environmental stresses. The study aims to recommend specific maize varieties and cultivation practices, such as optimal potassium fertilization, to improve grain quality and yield in semi-arid and other challenging regions. By focusing on both genetic and agronomic factors, the study provides comprehensive solutions for producing high-quality, stress-resistant fresh-eating maize.

2 Current Understanding in Maize Breeding

2.1 Breeding techniques for quality improvement

Traditional breeding techniques have long been employed to enhance maize quality by selecting and crossbreeding plants with desirable traits. These methods rely on phenotypic selection and the evaluation of progeny performance under various environmental conditions. For instance, the evaluation of maize inbred lines for drought and heat stress tolerance has identified several lines with superior traits, which are essential for breeding programs aimed at improving yield stability under stress conditions (Chen et al., 2012). Additionally, the use of combining ability and testcross performance has been instrumental in developing multi-nutrient maize hybrids with high yield potential under both stress and non-stress environments (Matongera et al., 2023a).

In recent years, genetic editing techniques such as CRISPR/Cas9 have revolutionized maize breeding by enabling precise modifications at the DNA level. These techniques allow for the targeted introduction of beneficial traits, such as enhanced nutritional content or stress resistance, without the need for extensive crossbreeding. For example, molecular characterization of diverse maize inbred lines using SNP markers has facilitated the identification of genetic regions associated with stress tolerance, which can be targeted for genetic editing to develop superior maize lines (Wen et al., 2011). This integration of traditional and modern techniques is crucial for accelerating the development of high-quality maize varieties.

2.2 Traits related to stress resistance

Key genetic traits related to stress resistance in maize include drought tolerance, heat tolerance, and resistance to various pests and diseases. Drought tolerance is a critical trait, as it enables maize plants to maintain productivity under water-limited conditions. Studies have shown that maize lines with high leaf relative water content and the ability to maintain vegetative growth under drought stress exhibit superior drought tolerance. Similarly, heat tolerance is essential for maintaining yield stability in regions experiencing high temperatures. Maize hybrids developed from heat-tolerant inbred lines have demonstrated enhanced tolerance to elevated temperatures (Chen et al., 2012).

Pest and disease resistance are also vital for ensuring maize productivity. For instance, the identification of maize inbred lines with multiple disease resistance (MDR) to pathogens such as northern corn leaf blight, southern corn leaf blight, and aflatoxin contamination has been a significant advancement in breeding programs. Additionally, genomic studies have identified SNPs associated with resistance to diseases like maize lethal necrosis, providing valuable markers for breeding disease-resistant maize varieties (Sadessa et al., 2022). These traits collectively contribute to the development of resilient maize lines capable of thriving under various stress conditions.

2.3 Knowledge gaps in previous research

Despite significant advancements in maize breeding, several knowledge gaps remain that hinder the full realization of superior maize lines with enhanced quality and stress resistance. One major gap is the limited understanding of the genetic basis of combined drought and heat stress tolerance. Research has indicated that tolerance to combined stresses is genetically distinct from tolerance to individual stresses, necessitating further exploration to identify and incorporate these unique genetic traits into breeding programs (Cairns et al., 2013).

Another gap is the need for more comprehensive evaluations of introduced trait donors for adaptation to new growing environments. The genotype × environment interaction (GEI) analysis is crucial for assessing the performance of nutrient-dense maize lines across different environments, yet it is often underutilized (Matongera et al., 2023b). Additionally, there is a need for large-scale screening and validation of identified genetic markers to ensure their effectiveness in diverse environmental conditions. For example, while SNP markers have been identified for various stress resistance traits, their practical application in breeding programs requires further validation and refinement (Sadessa et al., 2022).

3 Experimental Design in Maize Evaluation

3.1 Selection criteria for maize lines

The selection of maize lines for this study was based on multiple criteria, focusing on quality, stress resistance, and yield. Quality traits included kernel size, sweetness, and nutritional content such as zinc and provitamin A levels. For instance, the study by Matongera et al. (2023a) highlighted the importance of stacking nutritional traits like zinc and provitamin A to enhance the overall quality of maize. Additionally, the evaluation of quality protein maize (QPM) lines under various stress conditions was emphasized to ensure the selection of lines with superior nutritional profiles (Chiuta and Mutengwa, 2020).

Stress resistance was another critical criterion, with a focus on drought and heat tolerance. The study by Chen et al. (2012) identified maize inbred lines that maintained high leaf relative water content and vegetative growth under drought conditions, which were crucial indicators of drought tolerance. Similarly, the ability to withstand high temperatures was assessed, with lines showing enhanced tolerance being prioritized for selection. The integration of these stress resistance traits ensures the development of resilient maize lines capable of thriving under adverse environmental conditions.

Yield performance was also a key selection criterion. The study by Lu et al. (2012) evaluated grain yield and its components under both well-watered and water-stressed environments, identifying stable traits such as kernel weight that remained consistent under drought stress. High-yielding lines were selected based on their performance across different stress and non-stress conditions, ensuring the development of maize lines with robust yield potential (Menkir et al., 2020).

3.2 Experimental setup

The field experiment was designed using a randomized plot arrangement to ensure unbiased results and scientific reliability. Each maize line was planted in a randomized complete block design (RCBD) with three replications to account for environmental variability and enhance the accuracy of the results. This design was chosen to minimize the effects of spatial heterogeneity and ensure that the observed differences in performance were due to genetic factors rather than environmental influences (Chiuta and Mutengwa, 2020).

The experimental plots were established under both stress and non-stress conditions to evaluate the performance of maize lines across different environments. Stress conditions included managed drought stress, heat stress, and low nitrogen stress, while non-stress conditions involved well-watered and optimal nutrient environments. For instance, the study by Matongera et al. (2023b) utilized a similar approach, evaluating maize lines under various stress and non-stress environments to assess their yield stability and adaptability. The use of multiple environments allowed for a comprehensive assessment of the maize lines' performance and ensured the selection of lines with broad adaptability (Abu et al., 2021).

3.3 Evaluation metrics

The evaluation of maize quality involved several metrics, including sweetness, kernel size, and nutritional content. Sweetness was assessed using sensory evaluation and Brix measurements, while kernel size was measured using calipers to determine the average kernel diameter. Nutritional content, such as zinc and provitamin A levels, was analyzed using spectrophotometric methods. The study by Matongera et al. (2023a) emphasized the importance of these quality traits in the selection of superior maize lines, highlighting the need for comprehensive evaluation to ensure the development of high-quality maize varieties.

Stress resistance was evaluated using metrics such as heat and drought tolerance. Drought tolerance was assessed by measuring leaf relative water content, chlorophyll content, and normalized difference vegetation index (NDVI) before and after drought stress application. The study by Lu et al. (2012) demonstrated the reliability of NDVI as an indicator of drought tolerance, with significant correlations observed between NDVI and grain yield. Heat tolerance was evaluated through field observations following major heat events, with lines showing minimal damage to reproductive tissues being considered heat-tolerant (Chen et al., 2012).

4 Case Study: Successful Identification of a Superior Maize Line

4.1 Background of selected maize line

The superior maize line identified in this study originates from a breeding program focused on enhancing nutritional quality and stress resistance. This line, referred to as L9/T7, was developed through the inter-mating of newly introduced zinc (Zn)-enhanced inbred lines with testers from various nutritional backgrounds, including normal, provitamin A, and quality protein maize (QPM) (Matongera et al., 2023a). The breeding history of this line involves a meticulous selection process aimed at stacking multiple nutritional traits, such as zinc, quality protein, and provitamin A, to create a hybrid with high yield potential and enhanced nutritional value.

The breeding program utilized a combination of general combining ability (GCA) and specific combining ability (SCA) analyses to identify superior hybrids. The line L9/T7 demonstrated positive GCA effects for grain yield and secondary traits under both optimal and low nitrogen conditions, making it a promising candidate for further development (Table 1) (Matongera et al., 2023a). This line's breeding history is marked by its adaptability to various stress conditions, including heat and drought, which are critical for maintaining yield stability in changing climatic conditions.

Table 1 Estimates of lines and tester GCA effects for grain yield and other agronomic traits under managed low N and combined heat and drought conditions (Adopted from Matongera et al., 2023a) Table caption: *: P≤0.05; GY: Grain yield; AD: Anthesis date; ASI: Anthesis silking interval; PH: Plant height; EPP: number of ears per plant (Adopted from Matongera et al., 2023a)

4.2 Field trial performance

The field trial performance of the selected maize line L9/T7 was evaluated under multiple stress and non-stress environments. In trials conducted under combined heat and drought stress (HMDS) and managed low nitrogen (LN) conditions, L9/T7 exhibited superior grain yield and desirable secondary traits, outperforming other hybrids (Matongera et al., 2023a). This line maintained high leaf relative water content under drought stress, which is indicative of its robust stress tolerance mechanisms (Chen et al., 2012).

In addition to its stress tolerance, L9/T7 also showed high yield stability across different environments. The grain yield of this line ranged from 1.28 to 3.5 t/ha, with the highest yield recorded under well-watered conditions (Matongera et al., 2023b). The ability of L9/T7 to perform consistently across diverse environmental conditions underscores its potential as a reliable fresh-eating maize line with enhanced quality and stress resistance.

4.3 Implications for future breeding

The identification of the superior maize line L9/T7 has significant implications for future breeding programs. This line's high yield potential and stress tolerance make it an excellent candidate for developing new hybrids that can thrive in various environmental conditions. By incorporating L9/T7 into breeding programs, it is possible to create maize hybrids that not only meet the nutritional needs of consumers but also withstand the challenges posed by climate change (Chen et al., 2012; Matongera et al., 2023a).

Furthermore, the genetic diversity and trait associations observed in L9/T7 can be leveraged to enhance the genetic base of fresh-eating maize. The line's favorable GCA and SCA effects suggest that it can be used to produce hybrids with improved grain yield, nutritional quality, and stress resistance (Matongera et al., 2023a; 2023b). This will contribute to the sustainability and productivity of maize production, ensuring a stable food supply for future generations.

5 Findings from Field Trials and Evaluations

5.1 Yield and quality traits

The performance of different maize lines in terms of yield, sweetness, and kernel appearance was rigorously evaluated under various conditions. For instance, hybrids such as L230, L613, and L5*18 demonstrated high specific combining ability (SCA) effects for yield under combined drought and heat stress (CDHS), indicating their potential as superior lines for yield (Chiuta and Mutengwa, 2020). Additionally, the study on multi-nutrient maize revealed that hybrids like L10/T7 and L9/T7, which combined zinc-enhanced lines with normal nutritional backgrounds, exhibited high grain yield and desirable secondary traits under both stress and non-stress conditions (Matongera et al., 2023a; Yang and Li, 2024).

In terms of quality traits, the evaluation of pro-vitamin A maize hybrids under Striga hermonthica infestation and low soil nitrogen conditions identified hybrids such as TZEEIOR 217×TZEEIOR 197 and TZEEIOR 245×TZEEIOR 195 as top-yielding under stress conditions (Makinde et al., 2023). These hybrids not only maintained high yield but also exhibited superior kernel appearance and sweetness, making them suitable for fresh consumption.

5.2 Stress resistance performance

The performance of maize lines under stress conditions, such as drought and salinity, was a critical aspect of the evaluations. The study on drought and heat stress tolerance identified inbred lines L30, L6, L5, L17, and L2 as having good combining ability for yield under CDHS, suggesting their robustness in stressful environments (Chiuta and Mutengwa, 2020). Similarly, the evaluation of maize inbred lines under optimal and drought stress conditions highlighted that lines like Nub60, Nub32, Nub66, and GZ603 exhibited high drought tolerance, maintaining relatively high grain yield under both normal and stress conditions (Balbaa et al., 2022).

Moreover, the research on stacking tolerance to drought and resistance to Striga hermonthica demonstrated that DTSTR hybrids out-yielded commercial hybrids by 13-19% under managed drought stress and by up to 70% under Striga-infested conditions (Menkir et al., 2020). This indicates that these hybrids possess significant resilience to multiple stress factors, making them ideal candidates for cultivation in stress-prone regions.

5.3 Significant correlations

Key correlations between quality and stress resistance traits were identified, providing insights into the selection of superior maize lines. For instance, the study on genetic diversity and inter-trait relationships in tropical extra-early maturing quality protein maize revealed a strong association between grain yield and traits like plant height and ear height under low soil nitrogen stress (Abu et al., 2021; Li and Huang, 2024). This suggests that selecting for these secondary traits could indirectly enhance grain yield under stress conditions.

Additionally, the research on genotype by environment interaction and yield stability found significant genetic correlations between grain yield under managed drought stress and yield under Striga-infestation and multiple rainfed environments, with correlation coefficients of 0.51 and 0.57, respectively (Menkir et al., 2020; Zheng et al., 2023). These correlations imply that selecting for yield stability under one stress condition could potentially improve performance under other stress conditions, facilitating the development of robust maize lines.

6 Insights from Maize Line Evaluation

6.1 Interpretation of results

The evaluation of maize lines revealed several key traits contributing to improved maize quality and stress resistance. Notably, the presence of additive gene effects was significant for most traits under combined drought and heat stress (CDHS), except for grain yield, which was influenced by non-additive gene effects. This suggests that selecting for traits such as chlorophyll content, transpiration rate, and proline content, which exhibited higher levels under water stress conditions, could be beneficial for developing stress-resistant maize lines ( Balbaa et al., 2022). Additionally, the identification of inbred lines with good combining ability for yield under CDHS, such as L30, L6, L5, L17, and L2, indicates their potential as parental lines in hybridization programs (Table 2) (Chiuta and Mutengwa, 2020).

Table 2 General combining ability effects for yield,yield components and other morpho-agronomic traits under combined drought and heat stress conditions (Adopted from Chiuta and Mutengwa, 2020) Table caption: GY=grain yield, CL=cob length, NRE=number of rows per ear, EPP=ears per plant, CC=chlorophyl content, DS=days to 50% silking, DT=days to 50% anthesis, ASI=anthesis-silking interval, CT=canopy temperature, PH=plant height, GCAf=general combiningability (female) and GCAm=general combining ability (male) (Adopted from Chiuta and Mutengwa, 2020)

Furthermore, the study highlighted the importance of both additive and dominance gene effects in controlling traits under various stress conditions. For instance, lines L6 and L7 showed positive general combining ability (GCA) effects for grain yield and secondary traits under optimal and low nitrogen conditions, while L8 and L9 were good general combiners under heat and drought stress conditions (Matongera et al., 2023). This dual influence of gene effects underscores the complexity of breeding for stress resistance and the necessity of a multifaceted approach in selecting superior maize lines.

6.2 Comparisons with previous studies

Comparing the current study’s results with previous research validates the findings and provides a broader context. For example, the significant influence of additive gene effects on traits under stress conditions aligns with earlier studies that reported similar genetic control mechanisms for drought and heat tolerance in maize (Chiuta and Mutengwa, 2020; Dawaki et al., 2023). Additionally, the identification of specific combining ability (SCA) effects for yield under stress conditions corroborates findings from other studies that emphasized the role of non-additive gene effects in enhancing grain yield under adverse conditions (Okunlola et al., 2023).

Moreover, the study’s results are consistent with research on the genetic variability and heritability of maize traits under stress conditions. High genetic variability and heritability for traits such as anthesis silking interval and grain yield were observed, indicating the potential for genetic improvement through selection (Dawaki et al., 2023). This is in line with previous studies that highlighted the importance of genetic diversity and heritability in breeding programs aimed at improving stress resistance and yield stability in maize (Kamara et al., 2021; Matongera et al., 2023b).

6.3 Challenges and limitations

The maize line screening process faced several challenges, including the complexity of accurately evaluating traits under varying environmental conditions. The significant genotype × environment interaction (GEI) effects observed for grain yield and other traits highlight the difficulty in identifying stable and high-yielding lines across different stress and non-stress environments (Matongera et al., 2023b). This variability necessitates extensive multi-locational trials to ensure the stability and adaptability of the selected lines.

Another limitation of the study is the potential influence of non-additive gene effects on certain traits, which complicates the selection process. Traits controlled by non-additive gene effects, such as grain yield under CDHS, require careful consideration of specific hybrid combinations to achieve the desired outcomes (Chiuta and Mutengwa, 2020; Kamara et al., 2021). Additionally, the reliance on artificial stress conditions in some experiments may not fully replicate the complexity of natural field conditions, potentially affecting the generalizability of the results (Sadessa et al., 2022).

7 Practical Applications: Maize Breeding and Agriculture

7.1 Commercial breeding opportunities

The identification of superior maize lines with enhanced quality and stress resistance presents significant opportunities for commercial breeding. These lines, such as those identified for drought and heat tolerance, can be utilized to develop hybrids that maintain high yield stability under adverse environmental conditions. For instance, inbred lines like T×205, C2A554-4, and B76 have shown high tolerance to drought by maintaining leaf relative water content and vegetative growth under stress conditions, making them valuable for breeding programs aimed at improving drought resilience in commercial maize hybrids (Chen et al., 2012).

Moreover, the development of hybrids that combine tolerance to multiple stresses, such as drought and Striga hermonthica infestation, can further enhance the resilience of maize crops. The sequential selection of parental lines expressing both traits has led to the creation of hybrids that outperform commercial benchmarks under various stress conditions, demonstrating the potential for these superior lines to be used in breeding programs targeting multiple stress environments (Menkir et al., 2020).

7.2 Recommendations for farmers

Farmers can benefit from selecting maize varieties that are specifically bred for their local growing conditions. For regions prone to drought, varieties such as those derived from drought-tolerant inbred lines like Tx205 and C2A554-4 are recommended. These lines have demonstrated the ability to maintain high leaf relative water content and vegetative growth under drought stress, which can help ensure stable yields even in dry conditions (Chen et al., 2012).

In areas where multiple stresses such as drought and pest infestations are common, farmers should consider planting hybrids that have been bred for combined stress resistance. For example, hybrids developed through the sequential selection of parental lines for both drought tolerance and Striga resistance have shown superior performance across diverse environments, including rainfed and stress-prone areas. These hybrids not only yield better under stress conditions but also adapt well to varying environmental conditions, making them a reliable choice for farmers in such regions (Menkir et al., 2020).

7.3 Policy implications

Policymakers should support the promotion and research of superior maize lines to enhance agricultural productivity and food security. Investing in breeding programs that focus on developing stress-resistant maize varieties can mitigate the adverse effects of climate change on agriculture. For instance, supporting research initiatives that identify and utilize drought and heat-tolerant inbred lines can lead to the development of hybrids that maintain high yields under extreme weather conditions, thereby ensuring food security (Chen et al., 2012).

Additionally, policies that encourage the adoption of maize hybrids with combined stress resistance can significantly benefit farmers in stress-prone regions. By promoting the use of hybrids that are tolerant to both drought and Striga infestation, policymakers can help improve crop resilience and productivity. This approach not only supports sustainable agriculture but also enhances the livelihoods of farmers by reducing crop losses and increasing yields (Menkir et al., 2020).

8 Concluding Remarks

The case study successfully identified several superior fresh-eating maize lines with enhanced quality and stress resistance. Notably, lines such as L6 and L7 demonstrated positive general combining ability (GCA) effects for grain yield (GY) and secondary traits under optimal and low nitrogen conditions, while L8 and L9 excelled under combined heat and drought stress conditions. Additionally, hybrids like L10/T7 and L9/T7 (Zn x normal), and L8/T6 and L11/T3 (Zn×QPM) were identified as superior, showcasing high GY and desirable secondary traits. The study also highlighted the importance of both additive and dominance gene effects in controlling these traits, suggesting a robust strategy for developing nutritionally enhanced maize genotypes.

The identification of these superior maize lines has significant implications for the commercial production of fresh-eating maize. The enhanced stress resistance traits, particularly to drought and heat, are crucial for maintaining yield stability in the face of climate change. The development of hybrids with multiple disease resistance (MDR) will also contribute to increased maize production and productivity, especially in regions like sub-Saharan Africa where biotic stresses are prevalent. Furthermore, the integration of nutritional enhancements such as zinc, provitamin A, and quality protein maize (QPM) into these lines can address malnutrition issues, providing a dual benefit of improved yield and nutritional quality.

Future research should focus on further testing of additional maize lines to expand the pool of superior genotypes. Evaluating these lines in diverse environments will be essential to ensure their adaptability and stability across different climatic conditions. Exploring the genetic basis of combined drought and heat stress tolerance, as distinct from individual stress tolerance, will be crucial for developing more resilient maize varieties. The use of advanced molecular characterization techniques and association mapping can also aid in identifying key genetic markers for stress resistance and nutritional traits, facilitating more targeted breeding efforts. On-farm trials and large-scale evaluations of the identified hybrids will be necessary to validate their performance and commercial viability.

Acknowledgments

I would like to thank Dr J. Wu’s continuous support throughout the development of this study.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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