Speed breeding: An innovative approach in crop breeding

Introduction

Crop improvement has been central to ensuring global food security, particularly in the face of rising population pressure, climate variability and diminishing natural resources. Traditional plant breeding, though highly successful in improving yield and quality, is inherently slow due to long crop life cycles, seasonal dependence and limited generations per year. In most fieldbased breeding programmes, only one or two generations can be advanced annually, resulting in prolonged timelines of 8-12 years for the release of a new variety (Tester & Langridge, 2010) [13] . The urgency to accelerate genetic gain has intensified in recent decades owing to climateinduced stresses such as heat, drought, salinity and emerging pest-disease complexes. Conventional breeding approaches alone are insufficient to cope with the speed at which these challenges are evolving. As a result, innovative breeding strategies that reduce generation time while maintaining genetic integrity have gained prominence (Hickey et al., 2019) [6] . Speed breeding is one such revolutionary approach that enables rapid generation advancement by manipulating environmental parameters under controlled conditions. By extending photoperiods up to 20-22 hours per day and maintaining optimal temperature and light intensity, plants complete their life cycle much faster than under natural conditions. This approach was first demonstrated effectively in wheat and barley, where up to six generations per year were achieved without compromising seed viability or phenotypic expression (Watson et al., 2018) [16] . The significance of speed breeding extends beyond simple time reduction. It enhances the efficiency of selection, accelerates fixation of desirable alleles and complements modern molecular breeding tools such as marker-assisted selection, genomic selection and genome editing. Consequently, speed breeding has emerged as a key component of next-generation crop improvement programmes aimed at developing high-yielding, stress-tolerant and climateresilient cultivars (Ghosh et al., 2018; Farooq et al., 2021) [5,4] .

Concept and Principles of Speed Breeding

Speed breeding is a crop improvement strategy designed to minimize generation time by accelerating plant growth and reproductive development through precise manipulation of environmental factors under controlled conditions. Unlike conventional breeding, which relies heavily on natural seasonal cycles, speed breeding creates an artificial but optimized growth environment that allows crops to complete their life cycle in a significantly shorter duration. The fundamental concept of speed breeding lies in rapid generation advancement (RGA). By shortening the time from seed to seed, breeders can advance multiple generations per year, thereby increasing genetic gain per unit time. This approach does not involve genetic modification; rather, it exploits the inherent plasticity of plant developmental processes in response to environmental cues (Watson et al., 2018) [16] .

Core Principles of Speed Breeding 2.1.1 Extended Photoperiod

Photoperiod manipulation is the most critical principle of speed breeding. Plants are exposed to prolonged light periods of 20-22 hours per day, which enhances photosynthetic activity and accelerates vegetative growth. Extended photoperiods also promote early floral induction, especially in long-day and dayneutral crops such as wheat and barley (Ghosh et al., 2018) [5] . Continuous or near-continuous light increases assimilate production, enabling plants to reach reproductive maturity faster.

Optimized Temperature Regimes

Temperature plays a crucial role in regulating plant metabolic activity and developmental rate. Speed breeding protocols typically maintain day temperatures of 22-25 °C and night temperatures of 17-20 °C, which are optimal for most temperate crops. Such conditions accelerate enzymatic reactions, cell division and floral development without inducing heat stress (Farooq et al., 2021) [4] .

High Light Intensity and Quality

Adequate light intensity is essential to support extended photoperiods. Artificial lighting systems, particularly LED lamps, are commonly used to deliver high photosynthetically active radiation (PAR). LEDs also allow manipulation of light spectra, particularly red and blue wavelengths, which influence stem elongation, leaf expansion and flowering responses (Cazzola et al., 2020) [1] .

Controlled Growth Environment

Speed breeding relies on growth chambers or glasshouses where humidity, CO₂ concentration, irrigation and nutrient supply can be precisely regulated. Controlled environments reduce biotic and abiotic stresses, ensuring uniform growth and minimizing environmental noise during selection (Hickey et al., 2017) [7]

Early Seed Harvest and Rapid Drying

Another important principle is harvesting seeds before full physiological maturity, followed by rapid drying. Studies have shown that seeds harvested at the soft-dough stage retain high viability and germination capacity, allowing immediate advancement to the next generation (Watson et al., 2018) [16] .

Physiological Basis of Speed Breeding

At the physiological level, speed breeding accelerates plant development by enhancing carbon assimilation, hormonal signalling and flowering gene expression. Prolonged light exposure influences the circadian clock and upregulates flowering-related genes such as CONSTANS and FLOWERING LOCUS T, leading to early transition from vegetative to reproductive stages (Farooq et al., 2021) [4] . Importantly, these changes are phenotypic and reversible, ensuring that genetic stability is maintained across generations. The concept of speed breeding is grounded in well-established plant physiological principles and represents a practical, nontransgenic approach to accelerating crop improvement. Its effectiveness across multiple crop species highlights its potential as a core component of modern breeding programmes.

Methodology and Protocols of Speed Breeding

The methodology of speed breeding is centred on precise environmental control to accelerate plant growth, flowering and seed set. Protocols are adaptable to crop species, breeding stage and available infrastructure, but they share common operational elements that ensure rapid and reproducible generation advancement (Watson et al., 2018; Ghosh et al., 2018) [16,5] .

Infrastructure and Growth Facilities

Speed breeding can be implemented in growth chambers, glasshouses or controlled-environment rooms. Growth chambers offer the highest level of control over temperature, light and humidity, whereas glasshouses provide a cost-effective alternative when supplemented with artificial lighting (Hickey et al., 2017) [7] . Light is supplied using LEDs or metal halide lamps, positioned to ensure uniform illumination. LEDs are increasingly preferred due to their energy efficiency, spectral flexibility and reduced heat emission (Cazzola et al., 2020) [1] .

Environmental Conditions for Speed Breeding

The success of speed breeding depends on maintaining optimal and consistent environmental parameters. Standardized conditions used across many studies are summarized in Table 1. [5] Light intensity 400-600 µmol m⁻² s⁻¹ Sustains high photosynthesis Cazzola et al., 2020 [1] Relative humidity 60-70% Reduces moisture stress Hickey et al., 2017 [7]

Stepwise Speed Breeding Protocol

A generalized speed breeding protocol followed across crops includes the following steps:

Crop-Specific Protocol Adjustments

While the basic methodology remains consistent, crop-specific modifications are often required. For example, legumes may require adjusted nutrient regimes, whereas cereals respond strongly to photoperiod extension. Rice and other short-day crops may require modified light schedules to avoid delayed flowering (Yadav et al., 2021) [17] .

Reproducibility and Selection Efficiency

One of the strengths of speed breeding methodology is its high reproducibility. Uniform environmental conditions reduce phenotypic variability caused by external factors, allowing breeders to make more accurate selections at early generations. This is particularly advantageous when integrating speed breeding with molecular selection tools (Voss-Fels et al., 2019) [15] .

Crop-wise Applications of Speed Breeding (Elaborated)

Speed breeding has been successfully applied across a wide range of crop species, particularly cereals, pulses and oilseeds. Its effectiveness varies among crops depending on their photoperiod sensitivity, growth habit and reproductive biology. Nevertheless, numerous studies have demonstrated that speed breeding can significantly reduce generation time and accelerate genetic improvement across diverse cropping systems (Hickey et al., 2017; Watson et al., 2018) [7,16] .

Cereals 4.1.1 Wheat

Wheat was among the first crops in which speed breeding protocols were standardized. Under extended photoperiods of 22 hours, wheat plants can flower within 35-40 days, allowing up to six generations per year. Speed breeding has been extensively used to accelerate the development of disease-resistant and stress-tolerant wheat lines without compromising grain quality or seed viability (Watson et al., 2018) [16] .

Barley

Barley responds strongly to photoperiod extension. Studies report four to six generations per year using speed breeding, enabling rapid fixation of desirable traits such as malting quality and disease resistance (Hickey et al., 2017) [7] .

Rice

Rice, being a short-day crop, requires modified speed breeding protocols. Nevertheless, successful acceleration of generation cycles (three to four generations per year) has been achieved through careful manipulation of photoperiod and temperature, facilitating faster development of early-maturing and stresstolerant rice genotypes (Yadav et al., 2021) [17] .

Pulses 4.2.1 Chickpea

Speed breeding has shown considerable promise in chickpea, a crop traditionally limited to one generation per year under field conditions. Using controlled environments, researchers have achieved up to four generations annually, enabling rapid advancement of segregating populations and early identification of superior lines (Samineni et al., 2020) [12] .

Pea

In pea, speed breeding has reduced generation time from approximately 120 days to nearly 70 days. This has proven particularly useful in breeding programmes targeting yield improvement and resistance to biotic stresses (Mobini et al., 2020) [9] .

Oilseeds 4.3.1 Canola (Rapeseed)

Canola responds well to extended photoperiod and optimized temperature regimes. Speed breeding has enabled four to five generations per year, significantly accelerating trait pyramiding for oil content and disease resistance (Ghosh et al., 2018) [5] .

Soybean

Although soybean is photoperiod sensitive, controlled environment techniques have successfully shortened its life cycle, allowing rapid generation advancement and faster development of improved cultivars (Nagatoshi & Fujita, 2019) [10] .

Comparative Performance across Crops

Significance for Breeding Programmes

The successful application of speed breeding across multiple crops demonstrates its versatility and scalability. By enabling rapid cycling of generations, breeders can shorten the time required to release improved cultivars and respond quickly to emerging production challenges. Moreover, speed breeding facilitates early generation selection, thus improving overall breeding efficiency and resource utilization (Hickey et al., 2019) [6] .

Integration of Speed Breeding with Modern Breeding Tools (Elaborated)

Speed breeding alone accelerates generation turnover; however, its true potential is realized when it is integrated with modern molecular, genomic and biotechnological tools. This integration enables breeders to not only advance generations rapidly but also identify, select and fix desirable traits with greater precision, thereby substantially increasing genetic gain per unit time (Hickey et al., 2019; Varshney et al., 2021) [6,14] .

Integration with Marker-Assisted Selection (MAS)

Marker-assisted selection allows early identification of plants carrying desirable alleles using DNA markers linked to target traits. When combined with speed breeding, MAS enables selection at seedling or early vegetative stages, eliminating the need to wait for full phenotypic expression (Voss-Fels et al., 2019) [15] . For example, resistance genes for rusts in wheat or Fusarium wilt in chickpea can be rapidly fixed across successive generations using speed breeding combined with marker screening. This integration significantly shortens the breeding cycle while maintaining selection accuracy (Samineni et al., 2020) [12] .

Integration with Genomic Selection

Genomic selection (GS) uses genome-wide marker information to predict breeding values of individuals. Speed breeding complements GS by enabling rapid cycling between selection and recombination phases. This accelerates the realization of predicted genetic gains in actual breeding populations (Crossa et al., 2017) [3] . Studies indicate that combining speed breeding with genomic selection can double or even triple the rate of genetic gain compared to conventional breeding methods, particularly in complex traits such as yield and drought tolerance (Voss-Fels et al., 2019) [15] .

Integration with Genome Editing Technologies

Recent advancements in genome editing, particularly CRISPR/Cas9 technology, have opened new avenues for precise modification of target genes. Speed breeding facilitates rapid advancement and fixation of edited lines by quickly progressing them through successive generations (Varshney et al., 2021) [14] . This approach is especially valuable for validating gene function and developing elite lines with improved stress tolerance, disease resistance or nutritional quality within a short timeframe.

Integration with Doubled Haploid (DH) Technology

Doubled haploid technology produces completely homozygous lines in a single generation. When combined with speed breeding, DH lines can be rapidly evaluated and advanced, further reducing the time required to develop pure lines (Hickey et al., 2017) [7] .

High-Throughput Phenotyping and Digital Tools

Controlled environments used in speed breeding are ideal for deploying high-throughput phenotyping platforms, including imaging systems, spectral sensors and automated data collection tools. These technologies enable precise, repeatable phenotyping, improving selection efficiency and data quality (Reynolds et al., 2017) [11] .

Comparative Advantage of Integrated Approaches

Implications for Future Breeding Programmes

The integration of speed breeding with modern breeding tools transforms traditional breeding pipelines into high-efficiency, data-driven systems. This integrated approach is particularly critical for addressing complex challenges such as climate resilience, nutritional security and sustainable productivity under resource-limited conditions (Hickey et al., 2019) [6] .

Advantages of Speed Breeding (Elaborated)

Speed breeding offers multiple advantages over conventional crop breeding approaches by significantly enhancing efficiency, precision and responsiveness of breeding programmes. These advantages have positioned speed breeding as a core component of modern and future-oriented crop improvement strategies (Watson et al., 2018; Farooq et al., 2021) [16,4] .

Reduction in Breeding Cycle Duration

The most significant advantage of speed breeding is the drastic reduction in generation time. Conventional field-based breeding typically allows only one or two generations per year, whereas speed breeding enables three to six generations annually depending on the crop species. This results in a 50-70% reduction in the time required to develop stable breeding lines and release new cultivars (Watson et al., 2018) [16] .

Increased Genetic Gain per Unit Time

By allowing rapid cycling of generations, speed breeding increases the number of selection cycles that can be completed within a given time frame. This accelerates genetic gain, particularly when combined with selection strategies such as genomic selection and marker-assisted selection (Voss-Fels et al., 2019) [15] .

Year-Round Breeding Independent of Seasons

Speed breeding eliminates dependence on seasonal field conditions. Breeding activities can be conducted throughout the year in controlled environments, ensuring continuity of research and efficient utilization of resources. This is especially beneficial in regions with harsh climates or limited growing seasons (Hickey et al., 2017) [7] .

Enhanced Selection Efficiency

Controlled environmental conditions reduce phenotypic variation caused by external factors such as rainfall, temperature fluctuations and pest pressure. This uniformity enhances selection accuracy, particularly for traits with moderate to high heritability (Farooq et al., 2021) [4] .

Compatibility with Modern Breeding Technologies

Speed breeding integrates seamlessly with modern breeding tools including marker-assisted selection, genomic selection, genome editing and doubled haploid technology. This compatibility enables rapid fixation of target traits and validation of gene function (Varshney et al., 2021) [14] .

Rapid Response to Emerging Challenges

Speed breeding enables breeders to respond quickly to emerging threats such as new disease races, pest outbreaks and climateinduced stresses. The ability to rapidly develop and test improved genotypes enhances resilience of agricultural systems (Reynolds et al., 2017) [11] .

Efficient Use of Space and Resources

Due to shortened growth duration and compact plant architecture under controlled environments, speed breeding allows efficient use of space. Multiple generations can be advanced within the same facility, improving cost-effectiveness over time (Cazzola et al., 2020) [1] . [16] Higher genetic gain Improved selection response Voss-Fels et al., 2019 [15] Year-round breeding Continuous breeding cycles Hickey et al., 2017 [7] Improved selection accuracy Reduced environmental noise Farooq et al., 2021 [4] Rapid stress response Climate resilience Reynolds et al., 2017 [11] 7. Limitations and Challenges of Speed Breeding Despite its significant advantages, speed breeding is not without limitations. Understanding these constraints is essential for realistic adoption, optimization and scaling of speed breeding protocols in diverse breeding programmes. Several technical, economic and biological challenges influence its effectiveness and widespread implementation (Watson et al., 2018; Farooq et al., 2021) [16,4] .

High Initial Infrastructure and Operational Costs

One of the primary limitations of speed breeding is the high initial investment required for infrastructure such as growth chambers, controlled-environment rooms and artificial lighting systems. The installation and maintenance of high-intensity lighting, temperature control units and automated systems can be cost-prohibitive, particularly for public-sector breeding programmes in developing countries (Cazzola et al., 2020) [1] .

Energy Consumption and Sustainability Concerns

Extended photoperiods of 20-22 hours per day significantly increase electricity consumption. This raises concerns regarding operational costs and environmental sustainability, especially in regions where energy resources are limited or expensive.

Although LED technology has improved energy efficiency, energy demand remains a critical constraint (Watson et al., 2018) [16] .

Crop and Genotype Specificity

Not all crops or genotypes respond uniformly to speed breeding conditions. Photoperiod-sensitive and perennial crops may exhibit delayed flowering or abnormal growth under extended light regimes. Short-day crops such as rice and soybean require careful protocol modification to avoid flowering delays (Yadav et al., 2021; Nagatoshi & Fujita, 2019) [17,10] .

Physiological Stress and Trait Expression

Prolonged exposure to artificial light and accelerated growth may impose physiological stress on plants. Certain traits, particularly those related to root development, biomass accumulation and stress tolerance, may not be fully expressed under controlled conditions. This can limit the reliability of phenotypic selection for complex traits (Farooq et al., 2021) [4] .

Limited Field Validation

Genotypes developed or selected under speed breeding conditions require extensive field validation to ensure stable performance under natural environments. Differences between controlled and field conditions may result in genotype × environment interactions that affect yield and stress response (Reynolds et al., 2017) [11] .

Requirement for Skilled Manpower

Successful implementation of speed breeding requires trained personnel with expertise in plant physiology, controlledenvironment management and breeding protocols. Lack of technical expertise can limit adoption and consistency of results (ICAR, 2022) [8] . [1] High energy requirement Increased operational cost Watson et al., 2018 [16] Crop specificity Protocol modification needed Yadav et al., 2021 [17] Physiological stress Altered trait expression Farooq et al., 2021 [4] Field validation need G×E interaction concerns Reynolds et al., 2017 [11]

Skilled manpower

Training and capacity building required ICAR, 2022 [8]

Strategies to Overcome Limitations

Recent research focuses on developing energy-efficient LED systems, optimizing light spectra and integrating speed breeding with field-based selection to overcome these limitations. Publicprivate partnerships and institutional support are also crucial for scaling speed breeding infrastructure (CGIAR, 2021) [2] . [11,6] .

Role of

Accelerated Development of Stress-Tolerant Varieties

Speed breeding enables rapid cycling of generations, allowing breeders to quickly introgress and fix genes associated with tolerance to heat, drought and salinity. Traits that would traditionally require several years to stabilize can be developed within a much shorter timeframe using controlled environment conditions (Watson et al., 2018) [16] .

Rapid Screening under Simulated Stress Conditions

Controlled environments used in speed breeding can simulate climate stress scenarios such as high temperature or limited water availability. This allows early-stage screening of genotypes under stress conditions, improving the efficiency of selection for climate resilience (Farooq et al., 2021) [4] .

Faster Response to Emerging Biotic Stresses

Climate change also alters pest and disease dynamics, leading to the emergence of new pathogen races and pest populations. Speed breeding facilitates rapid development of resistant varieties by enabling faster gene pyramiding and validation of resistance sources (Hickey et al., 2017) [7] .

Support for Climate-Smart Agriculture

By reducing breeding time and enabling rapid varietal turnover, speed breeding supports climate-smart agriculture initiatives. Improved varieties can be released more frequently, ensuring that farmers have access to cultivars better adapted to current climatic conditions (Tester & Langridge, 2010) [13] .

Integration with Predictive Breeding Models

Speed breeding complements climate modeling and genomic prediction tools by allowing quick validation of predicted genotype performance. This integration enhances the reliability of breeding decisions under uncertain future climates (Voss-Fels et al., 2019) [15] . [6,14] .

9.1

Technological Advancements in Controlled Environments Future speed breeding systems are expected to incorporate automation, sensor-based monitoring and artificial intelligence (AI) to optimize environmental parameters in real time. Automated lighting, temperature and irrigation control will enhance precision, reduce human error and improve reproducibility of breeding outcomes (CGIAR, 2021) [2] .

Energy-Efficient and Sustainable Speed Breeding Systems

To address concerns related to energy consumption, research is increasingly focused on developing energy-efficient LED lighting systems and optimized light spectra that maximize photosynthesis while minimizing power usage. Integration of renewable energy sources is also expected to improve the sustainability of speed breeding facilities (Cazzola et al., 2020) [1] .

Expansion to Minor, Orphan and Underutilized Crops

While speed breeding has been extensively applied in major cereals and pulses, its future expansion to minor and orphan crops holds significant promise. Accelerating breeding in these crops can enhance nutritional diversity, regional food security and livelihood resilience in marginal environments (Varshney et al., 2021) [14] .

Integration with Advanced Genomic Technologies

The convergence of speed breeding with advanced genomic tools such as genome-wide association studies, genomic prediction and next-generation sequencing will further accelerate genetic gains. Rapid cycling of generations will allow faster validation of genomic predictions and gene-trait associations (Voss-Fels et al., 2019) [15] .

Role in Global Food and Nutritional Security

Speed breeding is expected to play a pivotal role in achieving global food and nutritional security by enabling faster development of biofortified and climate-resilient crop varieties. Its adoption across national and international breeding programmes will support timely varietal replacement and improved farmer access to superior cultivars (Hickey et al., 2019) [6] .

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