1 Introduction

Wheat is one of the most important staple crops globally, playing a crucial role in food security and human nutrition. It contributes approximately 20% of the total dietary calories and proteins worldwide, making it a fundamental component of the human diet (Shiferaw et al., 2013). Wheat is not only a major source of starch and energy but also provides essential nutrients such as protein, vitamins (notably B vitamins), dietary fiber, and phytochemicals, which are beneficial for health (Shewry and Hey, 2015). The crop's significance is underscored by its ability to support the dietary needs of a growing global population, particularly in developing regions where food demand is increasing annually (Shiferaw et al., 2013).

The effect of plant density on wheat yield and agronomic traits is a critical area of study. Plant density influences various agronomic traits, including grain size, number, and overall yield. For instance, targeted overexpression of certain proteins in wheat can lead to significant increases in grain size without negatively affecting grain number, thereby boosting yield under optimal plant density conditions (Calderini et al., 2020). Additionally, optimized growing conditions, such as those in controlled-environment vertical farms, can result in wheat yields several hundred times higher than traditional field yields, highlighting the potential of plant density optimization in enhancing productivity (Asseng et al., 2020).

Despite its importance, achieving high global wheat yields faces several challenges. Climate change, increasing population, and water shortages are significant hurdles that need to be addressed to ensure sustainable wheat production (Li et al., 2021). Moreover, the productivity gains from past agricultural revolutions are slowing, necessitating new approaches to meet future food demands (Shiferaw et al., 2013). Optimizing planting density is one such approach that can play a pivotal role in overcoming these challenges. By adjusting plant density, it is possible to enhance wheat's resilience to environmental stresses and improve yield potential (Wang and Liu, 2021).

This study aims to explore methods and practices for optimizing wheat planting density to achieve high yields. It seeks to synthesize current research on the impact of planting density on wheat yield and agronomic traits, identify the challenges faced in optimizing planting density, and propose strategies to address these challenges. Ultimately, the study aims to contribute to the development of sustainable wheat production practices that can meet the growing environmental and demographic pressures, thereby supporting global food security.

2 Plant Density and Its Role in Wheat Production

2.1 Understanding plant density

Plant density refers to the number of plants per unit area and is a critical factor in determining the overall productivity of wheat crops. It influences various physiological and ecological processes, including competition for light, water, and nutrients. Optimal plant density can maximize yield by ensuring that plants have adequate space to grow and access resources efficiently. Conversely, suboptimal plant density, whether too high or too low, can lead to reduced yields due to increased competition or underutilization of available resources.

2.2 Plant density's impact on light interception and photosynthesis

The arrangement and density of plants significantly affect light interception and photosynthesis. In dense plantings, the upper leaves may intercept most of the light, shading the lower leaves and reducing their photosynthetic efficiency. This phenomenon, known as self-shading, can limit the overall photosynthetic capacity of the crop. On the other hand, too sparse a planting can result in excessive light reaching the soil surface, leading to increased soil evaporation and reduced water use efficiency. Therefore, achieving an optimal plant density is crucial for maximizing light interception and photosynthesis, which are essential for high grain yield (Renny-Byfield and Wendel, 2014; Tossi et al., 2022).

2.3 Relationship between plant density and resource use efficiency

Resource use efficiency, including water, nutrients, and light, is closely linked to plant density. At optimal densities, plants can effectively utilize available resources, leading to improved growth and yield. For instance, polyploid wheat varieties, which have been shown to exhibit enhanced tolerance to abiotic and biotic stresses, may benefit from specific plant density adjustments to maximize their genetic potential (Matsuoka et al., 2014; Renny-Byfield and Wendel, 2014; Tossi et al., 2022). High plant density can lead to increased competition for resources, potentially reducing individual plant performance. Conversely, low plant density may result in underutilization of resources, leading to suboptimal yields. Therefore, understanding and managing plant density is essential for optimizing resource use efficiency and achieving sustainable wheat production.

3 Optimal Plant Density Across Different Wheat Varieties

3.1 Variety-specific plant density requirements

The optimal plant density for wheat can vary significantly depending on the specific variety being cultivated. Research has shown that different wheat varieties exhibit unique responses to plant density, which can influence their overall yield and agronomic performance. For instance, the genetic diversity within tetraploid wheat subspecies has been extensively studied, revealing that certain landraces and modern germplasm are better adapted to specific environmental conditions, which includes optimal plant density (Ganugi et al., 2021). Additionally, the transcriptional landscape of polyploid wheat indicates that gene expression patterns can vary significantly across different cultivars, suggesting that plant density requirements may also be influenced by these genetic differences (Ramírez-González et al., 2018).

3.2 Hybrid wheat and density optimization

Hybrid wheat varieties, which result from the crossing of different wheat species, often exhibit unique characteristics that can influence their optimal plant density. The process of hybridization and subsequent polyploidization can lead to significant genomic and phenotypic changes, which may affect how these hybrids respond to different planting densities (Wang et al., 2014; Blasio et al., 2022). For example, the famous chromosomal translocation 1BL/1RS in hybrid wheat has been shown to contribute to improved agronomic traits, including better adaptation to varying plant densities (Wang et al., 2014). Furthermore, genome-wide association studies have identified specific genetic loci associated with adaptation to plant density, highlighting the potential for optimizing planting density in hybrid wheat through targeted breeding programs (Sukumaran et al., 2015).

3.3 Case study: comparative analysis of different varieties

A comparative analysis of different wheat varieties can provide valuable insights into their specific plant density requirements. For instance, a study on the genetic mechanisms of allopolyploid speciation in wheat has shown that hybrid genome doubling can lead to the emergence of new wheat varieties with distinct agronomic traits, including optimal plant density (Matsuoka et al., 2014). Additionally, research on the evolution of polyploid Triticum wheats under cultivation has highlighted the role of natural hybridization and allopolyploid speciation in the diversification of wheat varieties, which can influence their response to plant density (Matsuoka, 2011). By comparing the performance of various wheat varieties under different planting densities, researchers can identify the specific requirements for each variety and develop strategies to optimize their cultivation.

4 Environmental Factors Affecting the Optimal Planting Density of Wheat

4.1 Climate conditions

Climate conditions play a crucial role in determining the optimal planting density of wheat. Variations in temperature, precipitation, and humidity can significantly impact wheat growth and yield. Polyploid wheat, such as bread wheat (Triticum aestivum), has shown adaptive plasticity, allowing it to thrive in diverse climatic conditions (Liu et al., 2021). This adaptability is partly due to the genetic diversity and resilience conferred by polyploidy, which enhances tolerance to abiotic stresses such as salinity and drought (Figure 1) (Tossi et al., 2022; Wang et al., 2022). The ability of polyploid wheat to withstand these stresses is linked to genetic mechanisms like homoeologous exchange, which facilitates rapid evolution and adaptation (Wang et al., 2022). Therefore, understanding local climate conditions is essential for optimizing planting density to ensure maximum yield and crop health.

Figure 1 Quantitative Analysis of Morphological and Physiological Traits in Synthetic Allotetraploid Wheat under Salinity and High Osmotic Stress Conditions (Adapted from Wang et al., 2022) Image caption: A: Morphological performance (including shoot length and condition) of synthetic allotetraploid wheat (AT3-F2-HOT-1-Line), its diploid progenitor (AT3-Eup), and parental lines TMU38 and TQ27 under control, salinity, and high osmotic stress conditions. Scale bar = 5 cm; B~Q: Quantification of morphological and physiological traits across these materials under different conditions, including shoot length, fresh shoot weight, chlorophyll content, photosynthetic rate, Na content, cell membrane permeability, transpiration rate, and relative water content, as well as the percentage changes of these traits under stress conditions (Adapted from Wang et al., 2022)

4.2 Soil type and fertility

Soil type and fertility are fundamental factors influencing the optimal planting density of wheat. The physical and chemical properties of the soil, including texture, structure, pH, and nutrient content, affect root development and nutrient uptake. Polyploid wheat varieties have been shown to possess enhanced tolerance to various soil conditions, which can be attributed to their complex genetic makeup (Matsuoka, 2011; Liu et al., 2021). For instance, synthetic polyploid wheat has demonstrated improved adaptation to different soil types, which can be leveraged to optimize planting density in varying soil conditions (Ruiz et al., 2020). Additionally, the genetic diversity in polyploid wheat allows for better nutrient utilization, which is crucial for maintaining soil fertility and achieving optimal plant growth (Heslop-Harrison et al., 2022).

4.3 Irrigation practices

Irrigation practices are critical in managing the water needs of wheat crops, especially in regions with limited rainfall. Polyploid wheat varieties have shown improved water use efficiency and tolerance to water stress, which can be beneficial in optimizing planting density under different irrigation regimes (Tossi et al., 2022; Wang et al., 2022). The genetic mechanisms underlying this tolerance include homoeologous exchanges and transcriptomic changes that enhance the plant's ability to cope with water scarcity (Blasio et al., 2022; Wang et al., 2022). Effective irrigation practices, tailored to the specific needs of polyploid wheat, can help maintain optimal soil moisture levels, reduce water wastage, and ensure consistent crop performance. Understanding the interaction between irrigation practices and planting density is essential for maximizing yield and resource efficiency in wheat cultivation.

5 Physiological and Agronomic Responses to Different Planting Densities of Wheat

5.1 Tillering dynamics in response to plant density

Tillering, the process by which wheat plants produce side shoots, is a critical factor in determining the final grain yield. The dynamics of tillering are significantly influenced by planting density. High planting densities generally lead to reduced tillering due to increased competition for resources such as light, water, and nutrients. This reduction in tillering is primarily a result of shading, which limits the photosynthetic capacity of lower leaves and thus reduces the energy available for the production of additional tillers (Postma et al., 2020). Conversely, lower planting densities allow for more light penetration and resource availability, promoting greater tiller production and potentially higher yields per plant.

Research has shown that the tillering potential (TP) of wheat varieties can significantly affect the agronomic optimum plant density (AOPD). High TP varieties tend to require lower AOPD, especially in high-yield environments, as they can compensate for lower plant numbers by producing more tillers per plant. This compensation mechanism is crucial for optimizing yield in varying environmental conditions (Bastos et al., 2020). Therefore, understanding the tillering dynamics in response to plant density is essential for developing planting strategies that maximize wheat yield.

The study by Wheeldon et al. (2020) demonstrates that wheat yield and its components vary significantly across different yield environments and tillering potentials. In high-yield environments, yield increases notably, mainly benefiting from an increase in spike number per unit area and thousand-kernel weight. Plants with high tillering potential exhibit higher spike and grain numbers across various yield environments, which is particularly important for increasing yield in low-yield environments (Figure 2). This indicates that optimizing tillering potential and managing planting density in different yield environments can help stabilize and improve wheat yield.

Figure 2 Performance of Winter Wheat Yield and Its Components Across Different Yield Environments and Tillering Potentials (Adapted from Wheeldon et al., 2020) Image caption: This figure illustrates the yield and yield components of winter wheat under agronomically optimal planting density (AOPD) conditions, specifically: (A) grain yield, (B) grain number per spike, (C) spike number per plant, (D) spike number per unit area, (E) grain number per unit area, and (F) thousand-kernel weight. These traits are compared across high, medium, and low yield environments and between high and low tillering potentials. In the box plots, the same letters indicate no significant difference (α = 0.05) (Adapted from Wheeldon et al., 2020)

5.2 Plant height and canopy architecture

Plant height and canopy architecture are also influenced by planting density. At higher densities, wheat plants tend to grow taller with a more elongated stem structure. This is a response to competition for light, as plants strive to outgrow their neighbors to access sunlight. However, this increased height can lead to a higher risk of lodging, where plants fall over, which can negatively impact yield and complicate harvesting (Postma et al., 2020). On the other hand, lower planting densities generally result in shorter plants with a more robust stem structure, reducing the risk of lodging.

Canopy architecture, including leaf orientation and distribution, is another critical factor affected by planting density. Dense planting can lead to a more closed canopy, which can reduce light penetration to the lower leaves and affect photosynthesis. This can result in a lower overall biomass and grain yield per plant. In contrast, a more open canopy at lower planting densities can enhance light distribution throughout the plant, improving photosynthetic efficiency and potentially increasing yield (Postma et al., 2020). Therefore, optimizing planting density is crucial for balancing plant height and canopy architecture to maximize wheat productivity.

5.3 Root development and competition

Root development is a vital aspect of plant growth that is significantly influenced by planting density. At higher densities, root competition becomes more intense, leading to a reduction in the overall root biomass per plant. This competition can limit the plant's ability to uptake water and nutrients, which are essential for growth and development. Studies have shown that plants at higher densities generally have fewer shoot-born roots and a reduced rooting depth, which can negatively impact their ability to access deep soil resources (Postma et al., 2020). However, the overall rooting depth seems to be less affected by planting density, suggesting that plants may adapt by altering root architecture rather than root depth.

The competition for resources at higher planting densities can also affect the physiological responses of wheat plants. For instance, the reduced root biomass can lead to lower water and nutrient uptake, which can limit photosynthesis and growth. This competition can be particularly detrimental in environments with limited resources, where the ability to efficiently utilize available resources is crucial for survival and productivity (Postma et al., 2020). Therefore, understanding the impact of planting density on root development and competition is essential for developing strategies to optimize wheat yield under different environmental conditions.

6 Wheat Planting Density and Yield Composition

6.1 Relationship between plant density and grain number

The relationship between plant density and grain number in wheat is a critical factor influencing overall yield. Research has shown that genotypes performing well under high plant density conditions tend to produce a higher grain number compared to those under low-density conditions. For instance, a study involving a wheat association mapping initiative (WAMI) panel of 287 elite lines demonstrated that grain number (GNO) was significantly influenced by plant density. The inner rows, representing high plant density, consistently showed higher grain numbers compared to the outer rows, which had lower plant density. This pattern suggests that genotypes adapted to high-density planting can maximize grain number, contributing to higher yield potential (Sukumaran et al., 2015).

Moreover, the genetic basis of this adaptation has been explored, revealing that specific loci are associated with the ability to thrive under different planting densities. For example, a major locus on chromosome 3B was identified, explaining a significant portion of the variation in grain number under high-density conditions. This genetic insight is crucial for breeding programs aiming to develop wheat varieties that can maintain high grain numbers even when planted densely, thereby optimizing yield (Sukumaran et al., 2015).

6.2 Grain weight and plant density

Grain weight, another vital component of yield, exhibits a complex relationship with plant density. Unlike grain number, grain weight tends to be less affected by variations in planting density. In the same WAMI panel study, it was observed that the 1000-kernel weight (TKW) remained relatively stable across different planting densities. This stability suggests that while plant density significantly impacts grain number, it does not necessarily translate to changes in grain weight (Sukumaran et al., 2015).

However, the genetic mechanisms underlying grain weight are intricate and involve multiple genes. For instance, genes such as TaGNI, TaGW2, and TaCKX6 have been identified to influence grain weight. These genes are part of a broader genetic network that regulates the trade-off between grain weight and grain number. Understanding these genetic interactions is essential for breeding strategies that aim to balance these two traits to achieve optimal yield (Tillett et al., 2022). By stacking beneficial alleles of these genes, it is possible to enhance grain weight without compromising grain number, even under varying planting densities.

6.3 Harvest index optimization

Optimizing the harvest index (HI), which is the ratio of grain yield to total biomass, is crucial for improving wheat yield. The harvest index is influenced by both plant density and the genetic makeup of the wheat varieties. Studies have shown that certain genetic loci are associated with higher harvest indices, which can be targeted in breeding programs to develop high-yielding wheat varieties (Quarrie et al., 2005).

For instance, quantitative trait loci (QTL) analysis has identified several genomic regions that harbor stable loci for yield-related traits, including the harvest index. These loci are distributed across various chromosomes, such as 4A and 4B, and are associated with traits like spike yield and plant height, which indirectly affect the harvest index. By selecting for these QTLs, breeders can develop wheat varieties that optimize the harvest index, thereby enhancing overall yield (Guan et al., 2018). Additionally, understanding the environmental interactions with these genetic factors can further refine planting strategies to maximize the harvest index under different growing conditions.

7 Management Strategies for Optimizing Wheat Planting Density

7.1 Seeding rate adjustment

Adjusting the seeding rate is a fundamental strategy for optimizing wheat planting density. The seeding rate directly influences plant population, which in turn affects competition for resources such as light, water, and nutrients. Research has shown that different wheat genotypes respond variably to changes in plant density, with some genotypes performing better under high-density conditions while others thrive in low-density settings. For instance, a study on a wheat association mapping initiative (WAMI) panel revealed that genotypes that excelled under high-density conditions (inner rows) were generally the best performers overall, despite showing less response to reduced competition in low-density conditions (outer rows) (Sukumaran et al., 2015). This suggests that selecting the appropriate seeding rate based on genotype can significantly impact yield outcomes.

Moreover, the genetic basis of adaptation to planting density has been explored, identifying specific loci associated with yield performance under varying densities. For example, a major locus on chromosome 3B was found to explain a significant portion of the variation in yield and grain number under different planting densities (Sukumaran et al., 2015). This genetic insight can be leveraged to develop functional markers for early selection of high-performing genotypes, thereby optimizing seeding rates to match the genetic potential of the wheat varieties being cultivated.

7.2 Row spacing optimization

Row spacing is another critical factor in managing wheat planting density. Optimal row spacing can enhance light interception, reduce disease incidence, and improve resource use efficiency. Studies have indicated that the spatial arrangement of plants within a field can significantly influence yield components such as grain number and 1000-kernel weight. For instance, research involving the WAMI panel demonstrated that inner rows (high plant density) consistently outperformed outer rows (low plant density) in terms of grain yield and number, although kernel weight remained unaffected by plant density (Sukumaran et al., 2015). This highlights the importance of considering row spacing in conjunction with seeding rate to achieve the best agronomic outcomes.

Adjusting row spacing can also mitigate the negative effects of plant competition. By optimizing the distance between rows, farmers can ensure that each plant has adequate access to essential resources, thereby reducing intra-specific competition and promoting uniform growth. This approach is particularly beneficial in high-yielding environments where maximizing resource use efficiency is crucial for achieving optimal yields (Sukumaran et al., 2015). Therefore, careful planning of row spacing, tailored to the specific needs of the wheat genotype and environmental conditions, is essential for optimizing planting density.

7.3 Precision agriculture in plant density management

Precision agriculture technologies offer innovative solutions for managing plant density in wheat cultivation. These technologies enable farmers to monitor and adjust planting density in real-time, based on site-specific conditions and crop requirements. For example, advancements in next-generation sequencing and high-density genetic mapping have provided deeper insights into the genetic factors influencing plant density adaptation (Udall and Wendel, 2006; Renny-Byfield and Wendel, 2014). By integrating these genetic insights with precision agriculture tools, farmers can make data-driven decisions to optimize planting density and improve yield outcomes.

The use of precision agriculture also facilitates the implementation of variable rate seeding (VRS) technology, which allows for the adjustment of seeding rates within a field based on soil fertility, moisture levels, and other environmental factors. This targeted approach ensures that each area of the field receives the optimal seeding rate, thereby enhancing overall productivity and resource use efficiency. Additionally, precision agriculture tools can help monitor plant growth and development, enabling timely interventions to address any issues related to plant density (Udall and Wendel, 2006; Renny-Byfield and Wendel, 2014). By leveraging these advanced technologies, farmers can achieve more precise and effective management of wheat planting density, ultimately leading to improved agronomic performance and sustainability.

8 Challenges and Future Directions for Optimizing Wheat Plant Density

8.1 Balancing plant density and pest management

Optimizing plant density in wheat cultivation is crucial for balancing yield and pest management. High plant density can lead to increased competition among plants, which may reduce the overall health and resilience of the crop, making it more susceptible to pests and diseases. Conversely, lower plant density can reduce competition but may leave more space for weeds and pests to thrive. Advanced breeding techniques, such as genome editing and high-throughput phenotyping, can help develop wheat varieties that are better adapted to varying plant densities and pest pressures (Mondal et al., 2016; Li et al., 2021). Additionally, understanding the genetic basis of plant density adaptation can aid in selecting traits that enhance pest resistance while maintaining high yield potential (Sukumaran et al., 2015).

8.2 Adapting to climate change: adjusting density to enhance resilience

Climate change poses significant challenges to wheat production, including increased temperatures, altered precipitation patterns, and more frequent extreme weather events. Adjusting plant density can be a strategy to enhance the resilience of wheat crops to these changes. For instance, higher plant densities might be beneficial in cooler climates to maximize light interception and photosynthesis, while lower densities could be advantageous in warmer, drier conditions to reduce competition for water and nutrients (Wang et al., 2017; Wang et al., 2022). The expansion of stress response genes, such as small heat shock proteins, in polyploid wheat can provide genetic resources for breeding more resilient varieties (Wang et al., 2017). Moreover, synthetic polyploidy and homoeologous exchanges can enhance tolerance to abiotic stresses, offering new avenues for optimizing plant density under changing climatic conditions (Ruiz et al., 2020; Wang et al., 2022).

8.3 Challenges of adopting optimized density practices

Despite the potential benefits, adopting optimized plant density practices in wheat cultivation faces several challenges. One major challenge is the complexity of the wheat genome, which complicates the identification and manipulation of genes associated with density adaptation (Li et al., 2021; Blasio et al., 2022). Additionally, the integration of new breeding technologies into conventional farming practices requires significant investment in research, infrastructure, and farmer education (Mondal et al., 2016; Liu et al., 2021). There is also a need for comprehensive field trials to validate the effectiveness of optimized density practices across different environments and management systems. Furthermore, balancing the trade-offs between yield, pest management, and environmental sustainability remains a critical challenge that requires a multidisciplinary approach (Sukumaran et al., 2015; Mondal et al., 2016; Li et al., 2021).

Acknowledgments

Thank you to the anonymous peer review for reading the manuscript and providing constructive suggestions for revisions.

Conflict of Interest Disclosure

The authors affirm 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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