Maize Genomics and Genetics 2025, Vol.16, No.1, 45-59 http://cropscipublisher.com/index.php/mgg 47 When it comes to breeding, both old and new methods are used. For hundreds of years, farmers have known how to select good varieties for hybridization, and slowly cultivated high-yield and disease-resistant corn (Flint-Garcia et al., 2009; Nelimor et al., 2020). But now the situation is different, and scientists have begun to use some high-tech methods. In recent years, genome analysis technologies such as GWAS have been particularly popular (Ruanjaichon et al., 2022; Dang et al., 2023). Although traditional breeding has also achieved good results, the new method is indeed faster and more accurate. It can directly find genetic markers that control excellent traits, allowing breeding work to avoid many detours (Ruanjaichon et al., 2022; Dang et al., 2023). Interestingly, many breeding projects now use both old and new methods to complement each other. 3 Phenotypic Diversity of Maize 3.1 Phenotypic traits of maize and their importance in breeding When breeders look at corn, they first focus on a few key indicators. For example, grain yield and plant height are commonplace but particularly important traits. But what's interesting is that details such as 100-grain weight and the number of cobs per plant can more accurately predict the yield (Al-Naggar et al., 2020; Naggar et al., 2020). When it comes to yield prediction, you may not think that even root characteristics are critical. Although the underground part is usually invisible, the degree of root development is closely related to the growth of corn (Tucker et al., 2019). Of course, different places may value different traits - arid areas may pay more attention to the root system, while fertile areas may pay more attention to ear characteristics. But in any case, these traits are important bases for evaluating the quality of varieties. Breeding experts have found that some characteristics of corn are particularly "stubborn" - indicators such as ear height and thousand-grain weight are basically stably inherited by offspring (Ababulgu et al., 2016). This is very interesting, because as long as these traits are selected, the success rate of breeding new varieties can be greatly improved. But then again, just focusing on yield is not enough. Now people pay more attention to the comprehensive evaluation of various phenotypic characteristics, so as to find truly high-quality genotypes (Mhoswa et al., 2016). For example, some corn may not have the highest yield, but it is particularly resistant to drought. Combining these characteristics often breeds hybrid varieties that are both high-yielding and resistant to stress. 3.2 Methods for evaluating major phenotypic traits To study whether corn grows well, we need to rely on field investigation and data analysis. Farmers and scientists often conduct experiments in drought or fertilizer-deficient plots, mainly looking at intuitive indicators such as corn cob size and plant height. Interestingly, the same variety may perform very differently in different plots. There are many ways to analyze data now, such as principal component analysis (PCA), which can clarify complex trait relationships (Al-Naggar et al., 2020). There is also a GT biplot analysis that is quite practical, which can simultaneously show the interaction between variety characteristics and environmental factors (Naggar et al., 2020). Although it sounds professional, it is actually to have a more comprehensive understanding of the growth characteristics of corn. When it comes to predicting corn yields, even machine learning is now used. These algorithms are smart and can find details that we might overlook - such as the shape of the corn cob or the development of the root system, which have a significant impact on the final yield (Tucker et al., 2019). However, machine learning is not a panacea. Researchers have found that it is more effective when used in conjunction with path coefficient analysis (Mhoswa et al., 2016). This method can distinguish which traits directly affect yield and which ones work indirectly through other factors. For example, some seemingly important characteristics may just be "hitchhiking". By clarifying these relationships, you can more accurately grasp the key points when breeding. 3.3 Influence of cultivation environments on maize phenotypic diversity How corn grows actually depends a lot on its "face" - here we are talking about the growing environment. How much water is applied, what fertilizer is applied, and the temperature will make the corn grow differently. Take the 100-grain weight as an example. The same variety of corn may differ a lot in dry land and fertile land (Al-Naggar et al., 2020). Interestingly, some characteristics are particularly "delicate". For example, the number of cobs per
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