Introduction

Maize (Zea mays L.) stands as a globally significant cereal crop, underpinning food security, animal feed production, and various industrial applications. Phosphorus, an indispensable macronutrient, plays a pivotal role in numerous physiological processes within plants, including energy transfer, nucleic acid synthesis, and photosynthesis. Phenological traits, such as days to tasseling, silking, and physiological maturity, along with their accumulated Growing Degree Days (GDD), serve as robust indicators of plant development and can unveil crucial adaptive responses to environmental stressors. A comprehensive understanding of the genetic underpinnings and the intricate relationships between these phenological traits and key yield components under phosphorus stress is fundamental for designing effective maize breeding programs. This study was conducted to evaluate genetic diversity using phenological traits as a function of growing degree days (GDD) and to examine the importance of these traits on grain yield under optimal conditions and phosphorus deficiency stress.

Materials and Methods

Diverse maize germplasm including 93 lines was selected and prepared among the breeding lines produced in plant breeding programs of various research institutions in Iran including Razi University of Kermanshah, Khorasan Razavi Agricultural and Natural Resources Research Center and Karaj Seed and Plant Improvement Institute. Seeds of selected maize genotypes were evaluated under two optimal and phosphorus deficiency stress conditions in pots in a completely randomized design with three replications at the Faculty of Agriculture, Urmia University. The growth degree day for each genotype in each phosphorus supply and deficiency stress conditions was calculated using the following equation:

GDD= ∑ [(Tmax+Tmin)/2]-Tb

Analysis of variance under optimal and phosphorus deficiency stress conditions was performed using SAS. Pearson correlation coefficient was calculated using the corrplot package in RStudio software. Cluster analysis was performed using the Ward method in the d3heatmap package in R software.

Results

The comprehensive statistical analyses yielded insights into the performance of the maize inbred lines under varying phosphorus conditions. Analysis of variance (ANOVA) consistently revealed statistically significant differences among the 93 maize inbred lines for all evaluated traits under both optimal and phosphorus deficiency conditions (P < 0.01).

Correlation analysis provided into the interrelationships among traits. Grain yield consistently displayed a weak positive correlation with phenological traits under both optimal and phosphorus deficiency conditions. This finding suggests that while phenological development is a crucial aspect of plant growth, it may not be the primary determinant of grain yield under phosphorus stress, implying that other physiological or morphological mechanisms might play a more dominant role in maintaining yield under nutrient limitations. Factor analysis successfully identified three principal factors in both the optimal and phosphorus deficiency environments. The three factors cumulatively explained a substantial proportion of the total variation, accounting for 79.74% under optimal conditions and an even higher 82.34% under phosphorus deficiency stress. The high percentage of explained variance by a limited number of factors indicates the presence of strong underlying biological relationships among the measured traits, providing a valuable framework for understanding the complex interplay of traits under different phosphorus availabilities and guiding selection strategies in breeding programs.

Cluster analysis effectively grouped the maize inbred lines based on their phenotypic responses, revealing distinct patterns of adaptation. Under optimal phosphorus conditions, the inbred lines were robustly classified into four main groups, each comprising genotypes with similar performance characteristics. In contrast, under phosphorus deficiency stress, the inbred lines were categorized into five distinct clusters, indicating a more diversified range of responses to the nutrient stress. This differential clustering pattern underscores the varying levels of tolerance and susceptibility among the genotypes and is crucial for identifying promising candidates for targeted breeding.

Conclusion

Based on the results, a specific set of inbred lines demonstrated consistent and superior performance under both optimal and phosphorus deficiency conditions. These lines were identified as Ma001, Ma002, Ma003, Ma004, Ma005, Ma006, Ma007, Ma008, Ma009, Ma010, Ma011, Ma012, Ma013, Ma015, Ma018, Ma020, Ma021, Ma022, Ma023, Ma025, Ma026, Ma027, Ma028, Ma053, Ma064, Ma066, Ma073, Ma104, Ma105, Ma118, Ma122, and Ma123. These genotypes consistently exhibited greater tolerance to phosphorus deficiency, characterized by either better maintenance of their yield components or a less severe negative impact on their phenological development when compared to other lines under stress. These lines represent invaluable genetic resources that can be directly integrated into maize breeding programs.