Reducing the release time of new peanut varieties is a key objective of peanut breeding programs. In the Australian Peanut Genetic Improvement Program release time has traditionally taken 10 to 15 years; however, the relatively recent use of winter breeding and seed increase nurseries has significantly reduced release times. Despite these improvements, full-season maturity cultivars are still limited to two generations in a calendar year, when grown under optimal environmental conditions. This paper describes a new speed breeding technique, which combines controlled environment conditions, continuous light in conjunction with optimal temperature, and a single seed descent breeding strategy in a greenhouse environment. Speed breeding was successful in reducing generation time of full-season maturity cultivars from 145 to 89 days. Speed breeding can rapidly progress the inbreeding of F2, F3 and F4 generations in less than 12 months, and potentially accelerate the development of first cross to commercial release in around six to seven years.
Reducing the period of time required to develop new commercial cultivars is of considerable interest to plant breeders across peanut breeding programs. Accelerated development enables plant breeders to increase the number of breeding generations per calendar year which can considerably improve the efficiency of breeding programs (
Single seed decent (SSD) has been successfully used in international peanut breeding programs, where multiple generations per year have accelerated the inbreeding process to progress fixed lines to multi-site evaluation trials (
The objective of this study was to assess the potential use of speed breeding techniques in a peanut breeding system. This paper describes a speed breeding system for the rapid development of a population (named cv. P27). This population was inbred from the F2 to F5 generation, with the greenhouse system used for the F2 and F3 generations, and field environments used for F4 and F5 generations (
All experiments described in this paper were carried out in the greenhouse at the Queensland Dep. of Agriculture, Fisheries and Forestry (DAAF) J. Bjelke Petersen Res. Stn. at Kingaroy in south-eastern Queensland (150°50′ E, 26° 33′S).
Two preliminary trials were conducted to determine the optimal pot culture system to be used in the main trials with the dual objectives of (1) assessing the ideal plant population to be used in large pots and (2) assessing the potential for the 24 hour light system to discriminate genotypes for photoperiod sensitivity. For the preliminary plant population trial, four different grades of seeds were selected from a breeding line (cv. D136-p7-5); with grade 1 (sieve 23), grade 2 (sieve 22), manufacturing grade (sieve 21) and through-sieve oil grade (sieve 20) all sown in 12 × 30 cm pots. Twenty-five seeds of each grade were planted in each pot with three replications. The potting media consisted of two layers, a lower section of two parts krasnozem (sourced from the A horizon of a field trial plot) to one part alluvial sand; and an upper section, 50 cm in depth, of pasteurised peat mix consisting of nine parts alluvial sand, six parts peat moss and one part krasnozem. A granular nitrogen, phosphorus and potassium basal fertilizer was applied to the peat mix prior to sowing, at a rate of 14, three and 10 kg/ha, respectively. Water for the pots was provided manually to the base of each pot as required to maintain pots close to field capacity. Ten days after sowing an emergence count was performed and the pots were then thinned to three different plant populations; 15, 10 and five plants per pot. Each seed grade had one replicate of each plant population. The experiment was carried out in a greenhouse environment under continuous (24 hr) light conditions, with light provided by 450 watt photosynthetically active radiation (PAR) lamps. The PAR lamps were positioned 1 m above the pots with each lamp providing continuous light for six pots. This study was performed in mid to late-winter (sown 26 Jul. 2009), therefore gas heating was provided in the greenhouse and warm water was used for manual irrigation during early stages of growth. The heating regime provided daily maximum temperatures of 28 ± 3C and daily minimum temperatures of 17 ± 3C. Due to the vigorous vegetative growth habit of the plants, pots were rotated twice weekly to ensure any shading effects were minimised. The plants were harvested at 91 days after sowing (DAS) and pod and kernel traits were measured and recorded.
For the preliminary photoperiod trial, three cultivars with suspected differing photoperiod insensitivities were used, Sutherland, a runner type (
The preliminary plant population experiment indicated that 10 plants per pot would be the ideal plant population for a SSD breeding strategy in the 30 cm pots used in the main speed breeding study. The ‘Anova’ pots have previously been used for growing wheat plants under CEnvC and were developed by Ian DeLacy and Mal Hunter at the Univ. of Queensland, School of Land, Crop and Food Sciences (
Warm greenhouse temperatures during the initial phase of growth necessitated the operation of two evaporative coolers during this early period. Greenhouse daily maximum temperature was maintained at approximately 32 C, with a minimum daily temperature of approximately 22 C. Relative humidity was maintained at approximately 65%, however variability around this average was observed. Due to reduced greenhouse temperature during the first week of April 2010, the gas-fired heating units were operated to maintain an optimum diurnal temperature of 18 ± 3C. The heating units remained in operation for the remainder of the growing period of this F2 generation.
Light was provided by four 450 watt PAR lamps that were suspended one m above the pots (
F2 plants growing in the greenhouse under controlled environment conditions at 15 days after sowing.
The F3 generation was planted on 7 Aug. 2010. Due to a number of plants not producing viable F3 seed, only 270 F3 seeds were sown, in addition to 10 parental checks. A plant population of 10 plants per pot was again used with additional seed planted, where available, to account for emergence losses. Seeds were assigned randomly throughout the pots with all seeds labeled to ensure subsequent generations could be traced back to the individual F2 line. Twenty-eight 30 cm ‘Anova’ pots were used with the same potting media, nutrition, watering regime and continuous light set-up as used for the F2 generation. Gas fired heaters were in use at planting time until the end of September due to cool temperatures experienced during this period. Seedlings were thinned to the required population at 20 DAS. All pots were rotated on a weekly schedule to minimise the effects of shading during the growth period. The harvesting of the F3 generation occurred on schedule at 89 DAS. All 28 pots were harvested and plant, pod and kernel characteristics were measured as for the F2 generation experiment.
The preliminary plant population trial was characterised by extreme vegetative growth up to harvest at 91 DAS. Despite the vigorous vegetative growth which resulted in intense intra-plant competition within each pot, along with a very early harvest date, most plants produced mature pods and viable kernels. Of the 120 plants harvested, only six plants did not produce a single pod. There were considerable differences in the mean number of pods per plant among the different plant population treatments. The treatment with a population of five plants per pot produced a mean number of 15.8 pods per plant. The ten and fifteen plants per pot populations produced 8.9 and 4.7 pods per plant, respectively (
Conversion rate of flower to peg and peg to pod in preliminary photoperiod trial.
Mean number of pods per plant for varying populations and grades.
Within the preliminary photoperiod study, the time to first flower was consistent across the three cultivars, with Sutherland, Wheeler and TAG-24 producing their first flowers at 25, 27 and 25 DAS, respectively. There were however considerable differences between the three cultivars with respect to the success rate of conversion of flowers to pegs. TAG-24 had the highest flower to peg conversion (71%), followed by Wheeler with 58% and Sutherland with 21%, respectively. Sutherland also had fewer flowers during the tagging period with only 42 flowers identified compared to 50 for Wheeler and 52 for TAG-24. There was also a distinct cultivar effect on the conversion of pegs to pods with Sutherland converting 56% of its pegs to pods, compared to 90% for Wheeler and 84% for TAG-24 (
The use of continuous light in combination with optimum temperature and humidity in the greenhouse facility considerably increased the rate of plant development compared to field conditions. The parental lines, Farnsfield and D147-p3-115, are both full season maturity varieties which require 140 and 145 days in the field to reach full maturity. Using our speed breeding techniques enabled generation time to be reduced to 113 days for the F2 generation and 89 days for the F3 generation. The three week extension required for the F2 growing period was traced back to a decrease in mean temperatures experienced for a 14 day period during May. A mechanical fault in the heating system caused this problem which was rectified immediately after being discovered. During this period there was however a significant decrease in greenhouse temperature, with minimum temperatures decreasing from around 15 C to less than 10 C, as well as a reduction in maximum temperatures form 25 C to 20 C. Temperature was more closely monitored during the F3 generation and no subsequent problems were experienced. There were considerable plant losses observed during both F2 and F3 generations, which resulted from lack of effective emergence and subsequent plant competitiveness in not being able to produce viable pods and/or kernels. From the initial 400 seeds planted in the F2 generation, there was a 68% seed recovery with 270 plants producing viable F3 seeds available for planting. From the F3 seed source, there was a 74% recovery rate, with a total of 201 viable F4 seeds harvested (
Reproductive success rate of F2 and F3 generations.
Overall, the greenhouse speed breeding system was a highly effective strategy for reducing generation times in the peanut breeding program. The controlled environment conditions in conjunction with continuous high intensity PAR light, as described by
There were however considerable challenges in maintaining the CEnvC for the speed breeding system, especially during the cool winter months at our elevated location in Kingaroy in S.E. Queensland, where average minimum temperatures during June, July and August are around 5 C (
The SSD breeding strategy worked well in conjunction with the speed breeding technologies employed during the F2 and F3 generations of this study. As described by
Indirect support for the above hypothesis comes from the contrasting results observed in the current study compared to the NASA study reported by
The combination of speed breeding techniques and a single seed decent breeding strategy has the potential to significantly reduce the time in developing new cultivars compared to conventional systems where field based pedigree breeding strategies are commonly employed. Most private and public peanut breeding programs commence preliminary yield (Stage 1) trials by the F5 or F6 generation, by which time the level of heterozygosity has been minimized through the inbreeding process and meaningful selection for more complex quantitative traits made (
This analysis also assumes that full season cultivars are being developed,
Comparison of timeline required to develop F6 fixed lines using Strategy 1, 2 and 3.
The speed breeding system is also less susceptible to adverse biotic and abiotic stresses such as reduced rainfall, low diurnal temperatures and foliar diseases and allows the breeder more flexibility in the generation of new breeding material.
Using speed breeding techniques with continuous light conditions also depends on the cost effectiveness relative to conventional breeding systems (
Costs in $US associated with SSD and pedigree systems for one population developed in the F2 and F3 generations.
Cost analysis results expressed on the basis of developing one population in the F2 and F3 generations, indicated the pedigree system was substantially more cost effective in both F2 and F3 generations. With gas heating included, the SSD F2 system costs US$950 per population (US$718 per population without heating) compared to the pedigree system which cost US$264 per F2 population (
Energy (gas and electricity) is a substantial expense for the speed breeding / SSD system, with heating and lighting accounting for around 60% of total cost for generations grown during the winter months. It may be possible to reduce energy usage by using more energy efficient lights and / or converting to a different heating system, e.g. inverter-based air conditioning. Despite the considerable expense of the speed breeding / SSD system there may still be an important role for it to play in the current AGPIP and indeed other global peanut breeding programs, where the universal aim is always to develop better cultivars more quickly.
The above analysis has clearly shown that generation time can be reduced substantially within a speed breeding / SSD system, and hence new cultivars could be developed up to two years quicker compared to using conventional field based pedigree breeding strategies. The increased cost associated with a speed breeding / SSD system may therefore be a relatively small price to pay where more rapid variety commercialization would be able to recover these relatively small upfront costs. Also, this system may be suitable for smaller peanut breeding programs where land and labor resources are limited. Provided the breeder had access to a relatively small greenhouse capable of being fitted out with high intensity lights and heating, a large number of segregating populations,
The speed breeding / SSD system is ideally suited to a backcrossing breeding strategy, where the major objective is to incorporate a relatively simple inherited trait,
Marker-assisted selection (MAS) is becoming widespread in cereal plant breeding programs across the world and has been used successfully to accelerate selection for specific traits. However, collecting reliable phenotypic data for the mass of genomic data generated through next generation sequencing (NGS) technologies is viewed as a roadblock to efficient implementation of MAS (
The study has demonstrated that speed breeding technologies previously developed for wheat and barley can be successfully transferred to the cultivated peanut and offers peanut breeders a new tool to develop improved cultivars more quickly. The study has clearly shown that generation time can be reduced substantially within a speed breeding / SSD system, and hence new varieties could be developed up to two years quicker compared to using conventional field based pedigree breeding strategies.