**2. Speed breeding techniques**

#### **2.1 Crops under controlled environment**

Speed breeding techniques involve deliberate manipulation of environmental conditions for the rapid advancement of crop cycle. The use of controlled growth chambers

#### *Speed Breeding: A Propitious Technique for Accelerated Crop Improvement DOI: http://dx.doi.org/10.5772/intechopen.105533*

equipped with manipulation provisions for light intensity, temperature regime, photoperiod, soil moisture, carbon dioxide level and nutrition supply will influence/alter the plant physiological growth process [17]. Researchers employ these modifications in a crop improvement program to achieve increased generation per year. The early flowering was induced in IR 64 rice variety by altering the light exposure in the growth chamber [18]. Similarly, a photoperiod of 22 hours of light exposure reduced flowering duration in wheat genotypes [19]. The breeding strategy can be efficiently planned with photosensitive crops through the adoption of light-based speed breeding protocols. The quality of light delivered per day highly influences the photosynthesis rate, gas exchange, transpiration rate, stomatal activity, and other plant developmental processes [20]. Adoption of 360–650 μmol/m2 light intensity with 400–700 nm of PAR (photosynthetic active radiation) was found successful in barley, wheat, chickpea, canola, and other major crops for early flowering and seed set [15]. The induction of early flowering was observed in legumes such as chickpea, faba beans, and pea with the use of blue and far-red light spectrums [21]. Early flowering was induced in groundnut by continuous exposure (24 hours) of 450 W lamps 25 days after germination [22].

The temperature variation plays a crucial role in the transition from vegetative to flowering stage in crop plants [23]. It influences the seed germination rate, plant growth, flowering period, seed set per cent, and maturity [24]. A temperature range of 12–30°C for germination and 25–30°C for other developmental processes (growth, flowering, and seed formation) is found suitable for most of the species [25]. Rapid plant development is observed on introducing the crop to altered temperature regime (17°C/32°C) and photoperiod in groundnut [22].

A shift from vegetative to reproductive phase is reported in crop plants at increased CO2 levels [25]. Plants' response to CO2 levels highly varies with the genotype of a species. The experimental genotype has to be evaluated with a critical range of CO2 levels in growth chambers to determine the optimum value for induction of earliness in flowering. The breeding cycle was enhanced up to five generations per year in soybean by manipulating CO2 supply (> 400 ppm) coupled with light exposure of 14 hours cycle in a growth chamber [26].

Most crop species exhibit early flowering and seed set on subjecting to moisture stress [27]. Modulation of soil moisture status in speed breeding protocol helps in rapid generation advancement of crop species. The high induction of grain filling and maturation is observed in barley, wheat, and chickpea on the gradual decrease of moisture status at the end of the flowering stage [15].

High-density planting is a low-cost strategy in speed breeding as it contributes to rapid generation turnover along with the maintenance of large population size. Crops raised at high density tend to compete with each other resulting in early induction of flowering and seed maturity [28]. The earliness in flowering at high density was reported in rice, sorghum, and cotton [25]. On contrary, many researchers found no deviations in flowering initiation at high-density planting [29]. Therefore, the genotypic responses need to be investigated in each species to validate the use of highdensity planting as a component in speed breeding.

Application of plant growth hormones and essential nutrients tend to regulate flowering and seed set under *in vitro* conditions [30]. More breeding cycles per year can be generated through the use of growth regulators with other approaches. Around eight generations per year were obtained in lentil and faba bean with the use of plant growth regulators *viz*., auxin, cytokinin, and flurprimidol under modified temperature (22°C light/18°C dark) and photoperiod (18 hr. light/6 hr. dark) in growth chambers [31].

The immature seeds obtained from plants grown under speed breeding protocols with an extended duration of photoperiod (22 h of light) proved to be viable in wheat and barley [15]. A similar finding on early seed harvest was reported in wheat cultivars [32]. The advancement of subsequent generations can be hastened by the adoption of early harvest with other speed breeding techniques. The immature seeds (37 days after postanthesis) from plants grown under CO2 supplementation exhibited a high germination rate similar to control in soybean [26]. Around 7–8 generations/ year is achieved in lentils by integrating early harvest with the application of plant growth regulators [16].

#### **2.2 Accelerated crop improvement through integration of novel approaches**

Speed breeding is a feasible platform that allows the integration of modern approaches along with generation advancement techniques. The conventional breeding techniques (pedigree selection, mass selection, pure line selection, bulk selection, and recurrent selection) of line development require more number of inbreeding and selection processes. These methods were not found amenable for inclusion in speed breeding protocols [25]. The use of modern techniques coupled with highthroughput phenotyping platforms in speed breeding would highly augment the crop improvement program. The target-specific traits involved in biotic and abiotic stress can be improved at a faster rate by creating artificial environments with accurate phenotyping.

Few modifications in conventional selection methods proved efficient for inclusion in speed breeding protocol. The single plant selection method was employed in the handling of backcross progeny at earlier generations (F2 and F3). A rigid selection for the trait under transfer and characteristics of the recurrent parent was made in segregating generations (F2 and F3) after the first and third backcross. Each F2 selected plant was harvested separately for the advancement of generation (F3) following the progeny-row method. The inclusion of selection in the early generation reduced the number of backcrosses and thereby saves labor, time, and other resources. The modified backcross method was employed in barley for the rapid development of introgression lines [33]. The European barley cultivar (Scarlett) was crossed with other donor parents to evolve lines exhibiting resistance to blotch and leaf rust. The lines under evaluation were raised under growth chambers with continuous light exposure at 22°C. Similarly, the single plant selection in combination with the speed breeding protocol was followed in wheat for multiple trait improvement [34].

Single-seed descent serves as a promising selection approach for inclusion in speed breeding techniques in field and controlled environments. The attainment of homozygosity is accelerated through constant inbreeding of segregating population by forwarding a single seed of each individual to the next generation. It allows for the advancement of generations in growth chambers and small nursery fields [35]. The single-seed descent method provides the opportunity for high-density planting and proves to be a very effective strategy for resource-limited environments [36]. The popular rice cv. Nipponbare was developed by adopting a single-seed descent method with rapid generation techniques at growth chambers [37]. Around 450 inbred lines evolved rapidly under field conditions following the single-seed descent method in wheat [38]. No selection is imposed in any successive generation which may carry more inferior progenies in a population compared to other selection methods.

#### *Speed Breeding: A Propitious Technique for Accelerated Crop Improvement DOI: http://dx.doi.org/10.5772/intechopen.105533*

A slight deviation from the single-seed descent method was found successful in legume species. The selection of one pod per plant was followed from F2 to F4 generation instead of a single seed. Single-pod descent selection provides scope for maintaining each F2 line in advanced generations compared to the single-seed descent method. It also possesses the advantage of early selection of pods, which is not feasible in the single-seed descent method. The mean yield of progenies developed from single-pod descent (7.96 g / plant) was higher compared to the single-seed descent (6.42 g/plant) selection method in soybean [39]. However, the conduct of preliminary test trials under controlled environments is required to validate the selection efficiency of the single-pod descent method in legume crops [25].

The precise identification of candidate genes has become feasible due to recent advancements in genotypic platforms and high-throughput phenotyping techniques. The development of mapping population (F2, recombinant inbred line (RIL), and backcross) requires a longer generation time on conventional approaches. The inclusion of the speed breeding technique promotes rapid identification and validation of QTL (quantitative trait loci) [21]. It facilitates minimal backcross (1–2) to introgress the target gene in a superior genotype (over 99% of the recurrent genome). The use of marker-assisted selection (MAS) in speed breeding protocol facilitates gene discovery at a faster rate and thereby meets the challenges associated with food production. The SNP marker-assisted selection is combined with speed breeding protocols for rapid development of mapping population (BC3F3) associated with salinity tolerance in rice [40].

The marker-assisted selection is efficient only with a few QTLs exhibiting a major effect on the trait of interest. At present, researchers employ a genomic selection approach in the breeding strategies, which is effective for complex trait improvement. It paves way for the identification of several minor QTLs, which is involved in the governance of biotic and abiotic stress resistance. With the development of nextgeneration sequencing (NGS) technologies, the cost and time involved in genomic selection are drastically reduced [41]. The genomic-estimated breeding values (GEBVs) of individuals are estimated based on genotype and phenotype datasets of a training population. It results in high accuracy of measuring the genetic worth of an individual compared to other selection methods [42]. The rapid genetic gain was realized in wheat through the implementation of genomic selection with other speed breeding protocols [43]. Several haplotypes related to yield improvement have been identified in rice and many other species. Introgression of haplotype into superior cultivars requires more breeding cycles and is highly time-consuming. The haplotype breeding can be accelerated by the integration of speed breeding protocols with the genomic selection approach [9]. Speed breeding also serves as a promising strategy for the rapid advancement of generations in transgenic crops [44].
