Abstract
Pigeonpea [Cajanus cajan (L.) Mill.] may have cross- or self-pollination that allows the use of hybrids or improved genotypes by conventional breeding approaches in pigeonpea production. The objectives of this research were to: 1) compare the agronomic performance of a hybrid vs. breeding lines (BLs) developed by pedigree and bulk methods, and 2) determine if there were average and useful heterosis and heterobeltiosis for quantitative traits. The hybrid 13KLAF1 and UPE-1, UPE-2, UPE-3, UPE-4, UPE-5 BLs, their parents (13KPP-264-05 and ‘Lázaro’), and one early maturity check ‘ICPL 86012’ were evaluated in field trials in Isabela and Lajas, PR. Data of plant height, days to flowering and harvest, seed yield, and weight of 100 dry-seeds were noted for all genotypes. There were no significant differences between 13KLAF1 and UPE BLs for plant height in both locations. ‘Lázaro’ was late maturity in Isabela (157 days) and Lajas (124 days), as expected. The weight of 100 seeds for all genotypes did not reach values more than 18.5 and 16.3 g such as those observed for 13KPP-264-05 in Isabela and Lajas, respectively. The higher seed yields were observed for 13KLAF1 (3112 kg·ha−1), UPE-4 (2970 kg·ha−1), and ‘ICPL 86012’ (2739 kg·ha−1) in Isabela, whereas 13KLAF1 (1599 kg·ha−1) and ‘ICPL 86012’ (1450 kg·ha−1) produced the higher yields in Lajas. The 13KLAF1 showed an average heterosis and heterobeltiosis of 87% and 60%, and 78% and 30% for seed yield in Isabela and Lajas, respectively. The useful heterosis was >125% in both locations.
Pigeonpea [Cajanus cajan (L.) Mill.] is an important source of protein (20% to 22%) for human consumption (Saxena et al. 2015, 2021a). This legume is planted on more than 6.0 million ha worldwide in various countries in Africa, Asia, and the Americas (Sinha et al. 2020). Pigeonpea may have self- or cross-pollination (ranges varied between 3% and 40%) (Abrams 1967; Sameer et al. 2017). Both characteristics have been used to develop new cultivars or hybrids with higher productivity, early maturity, insensitivity to the photoperiod, and resistance to pests and diseases, among other desirable characteristics (Saxena and Sawargaonkar 2014; Saxena et al. 2021a; Vales et al. 2012; Viteri et al. 2020; Viteri and Linares-Ramírez 2023, 2024).
Saxena et al. (2021a) reported that more than 80 cultivars have been released since 1960 though hybridization and pedigree selection. For instance, ‘CRG 2012-25’ (Kannan Bapu et al. 2021), ‘ICPL 88039’ (Pande et al. 2006), and ‘IPA 203’ (Choudhary 2016) with yield productions from 1500 to >2000 kg·ha−1 were released in India. Also, ‘Cortada’ (Bosques-Vega et al. 2000a), ‘Guerrero’ (Bosques-Vega et al. 2000b), and ‘Lázaro’ (Viteri et al. 2020) were developed though hybridization and combining pedigree and bulk breeding methods in Puerto Rico. Dry-seed yields of these cultivars do not exceed 2000 kg·ha−1 (Sarmiento et al. 2021; Viteri et al. 2020; Viteri and Linares-Ramírez 2024); however, only ‘Lázaro’ can be planted year-round because its insensitivity to the photoperiod (Viteri et al. 2020).
Hybrid vigor (referred to average and useful heterosis and heterobeltiosis) is the superior agronomic performance of the F1 hybrids compared with the average performance of the two parents, a commercial cultivar, or the better parent (Hayes et al. 1955; Sinha et al. 2020). This has been used in combining ability studies to identify additive and nonadditive gene actions (Srinivas et al. 1998) that can be used in pigeonpea breeding programs. In fact, this information allowed for development of heterotic groups to increase pigeonpea productivity (Saxena and Sawargaonkar 2014; Saxena et al. 2021b). Ashutosh et al. (2017) reported heterosis values between 17% and 49% in Bahar × BSMR 846, MA6 × BDN 2029, and MAL13 × BSMR 846 F1 populations for the number of pods per plant and seed yield. Likewise, 14% to 35% of heterosis was reported for days to maturity, plant height, number of primary branches/plant, number of secondary branches/plant, number of pods/plant, 100-seed weight, and seed yield for crosses between BDN I, BDN 2, BSMR 175, BSMR 736, and Daithana with 12 different male parents (Phad et al. 2009). In Puerto Rico, there is no information related to heterosis effects between crosses of promising pigeonpea breeding lines or cultivars. Thus, the objectives of this research were to: 1) compare the agronomic performance of a hybrid vs. BLs developed by pedigree and bulk methods, and 2) determine if there were average and useful heterosis and heterobeltiosis for quantitative traits.
Materials and Methods
Germplasm and hybrid development.
The 13KLAF1 hybrid and UPE-1, UPE-2, UPE-3, UPE-4, and UPE-5 BLs were developed from the biparental cross 13KPP-264-05 × ‘Lázaro’. 13KPP-264-05 is a promising pigeonpea breeding line (BL) developed by the University of California, Riverside (Huynh and Roberts, personal communication). This genotype has flowers with petals of yellow color and reached harvest maturity <132 d after planting in Puerto Rico (Viteri and Linares-Ramírez 2023). Lázaro is a cultivar developed by the University of Puerto Rico that has flowers with red-colored petals on the outside and yellow inside and reaches harvest maturity between 130 and 149 d after planting (Sarmiento et al. 2021; Viteri et al. 2020). Both genotypes have determinate flowering habit and are insensitive to the photoperiod. Seed yields of 13KPP-264-05 and ‘Lázaro’ varied from 300 to 2500 kg·ha−1 (Sarmiento et al. 2021; Viteri et al. 2020; Viteri and Linares-Ramírez 2023). UPE determinate BLs were developed by the pedigree selection of early maturity (<135 d) and higher yield (i.e., >80 g per plant) from F1 to F6. The F7:8 seed was harvested in bulk and genotypes with seed yields of more than 2000 kg·ha−1 were selected in Isabela (Viteri and Linares-Ramírez 2023). As indicated previously, the hybrid seed (i.e., 13KLAF1) was developed by crosses between 13KPP-264-05 (female) and ‘Lázaro’ (male) through emasculation at the Isabela Research Substation, University of Puerto Rico in Jan 2022.
Pre-germination, transplanting, and field trials.
A total of 144 F8 seeds of UPE-1, UPE-2, UPE-3, UPE-4, and UPE-5 BLs, the hybrid 13KLAF1, their parents (13KPP-264-05 and ‘Lázaro’), and ‘ICPL 86012’ [an early mature check from India (Chauhan et al. 1999)] were used for the experimental trials in Isabela and Lajas Research Substations at the University of Puerto Rico. One seed of each genotype was planted in a biodegradable square (4.5 × 4.5 cm) Jiffy-Strips® containing Pro-Mix BX at pH 5.9 in the greenhouse. The plants were transplanted to the field 10 d after sowing. The genotypes were arranged in a randomized complete block design with three replications in both locations. The experimental plot consisted of 24 plants of each genotype planted in three rows by 4.3 m long, with a spacing of 0.9 m between rows and 0.6 m between plants per replication and location (n = 72 plants). The two plantings were on Mar 2022 where long-day conditions (>12 h) occurred in Isabela and Lajas (Sarmiento et al. 2021; Viteri et al. 2020; Viteri and Linares-Ramírez 2024).
Overhead and drip irrigation were used in Isabela and Lajas, respectively. The plants were grown under Oxisol soils (Coto series, very-fine, kaolinitic, isohyperthermic Typic Eutrustox) (Muñoz et al. 2018), at temperatures from 20 to 31 °C, mean annual precipitation of 1592 mm, and an average relative humidity of 70% in Isabela (Sarmiento et al. 2021; Viteri et al. 2020). In contrast, the plants were grown in soils belonging to the Vertisol order, San Anton (fine, smectitic, isohyperthermic Typic Haplusterts) series (Muñoz et al. 2018), under temperatures between 19 and 33 °C, an annual precipitation of 1143 mm, and an average relative humidity of 80% in Lajas (Sarmiento et al. 2021; Viteri et al. 2020).
Hybrid identification and evaluation of quantitative traits.
The hybrids were identified because the plants’ flowers had red-colored petals. This is a dominant trait derived from ‘Lázaro’ that was the morphological marker used when the UPE BLs were developed (Viteri and Linares-Ramírez 2023). Data for plant height, the number of days to the initiation of flowering, and number of days when at least 50% of the plants of each genotype reached harvest maturity were collected at reproductive stages in both plantings. The plant height was measured from the soil level to the top of the primary inflorescence when pod and seed set had been completed. Also, the seed yield and weight of 100 dry-seeds (∼16% moisture content) were collected for each genotype. All these traits were recorded for each hybrid plant. However, for the statistical analysis, 10 and five hybrids were chosen randomly to estimate the seed yield and weight of 100 seeds, respectively. Likewise, the same number of plants for 13KPP-264-05, ‘ICPL 86012’, ‘Lázaro’, and the UPE BLs were used to estimate these quantitative traits.
Statistical analysis and hybrid vigor estimation.
A combined analysis of variance was performed for the days to flowering, days to maturity, seed yield, and weight of 100 dry-seeds to determine the effect of the genotype, location, and their interactions. A mixed linear model was used for data analysis where the pigeonpea genotypes and locations were treated as fixed effects while replications were considered as random effects (McIntosh 1983). Also, Bartlett’s test for homogeneity of variances (Cordeiro 1983) was performed for each variable in each location and a Fisher’s least significant difference at P ≤ 0.05 was calculated to discriminate differences among genotypes. Data were analyzed using SAS 9.4 PROC GLM (SAS Institute 2021).
Results
Mean squares for locations, genotypes, and their interactions were significant (P ≤ 0.001) for days to harvest, seed yield, and weight for 100 seeds (Table 1). Likewise, the mean square for the genotype × location interaction was significant only for days to flowering (Table 1). In contrast, the mean square for location was not significant (P > 0.05) for days to flowering and mean squares for the genotype × location were not significant for plant height and days to harvest (Table 1). Variances for the data analyzed from the two locations were not homogeneous for all the quantitative traits with the exception of plant height (Table 2). Thus, the data for plant height, days to flowering, days to harvest, and weight of 100 seeds of the 13KLAF1 hybrid and the eight pigeonpea genotypes were presented separately for Isabela and Lajas.
Combined analysis of variance for quantitative traits for 13KLAF1 hybrid, their parents, ‘ICPL 86012’, and UPE pigeonpea genotypes [Cajanus cajan (L.) Mill.] evaluated at Isabela and Lajas, PR in 2022.
Bartlett’s test for homogeneity of variances for quantitative traits at Isabela and Lajas, PR 2022.
In general, mean plant heigh was not significant in both locations (Table 3). Values between 1.5 and 1.6 m were observed in Isabela and plant heights of 1.2 to 1.4 m were noted in Lajas. Mean days to initiation of flowering varied from 61 to 78 in both locations. All UPE breeding lines, 13KPP-264-05, and ‘ICPL 86012’ genotypes were early flowering compared with 13KLAF1 and ‘Lázaro’ in Isabela and Lajas (Table 3). However, an earlier harvesting time was noted in Lajas (mean of 115 d) compared with Isabela (147 d). ‘Lázaro’ was late maturing in both locations, as expected (Table 3).
Mean of plant height, flowering, and harvest time for 13KLAF1 hybrid, their parents, ‘ICPL 86012’, and UPE pigeonpea genotypes [Cajanus cajan (L.) Mill.] evaluated at Isabela and Lajas, PR in 2022.
The higher mean seed yields were observed for 13KLAF1 (3112 kg·ha−1), UPE-4 (2970 kg·ha−1), and ‘ICPL 86012’ (2739 kg·ha−1) in Isabela, whereas 13KLAF1 (1599 kg·ha−1) and ‘ICPL 86012’ (1450 kg·ha−1) produced the higher yields in Lajas. In fact, 13KLAF1 showed an average heterosis and heterobeltiosis of 87% and 60%, respectively, in Isabela for seed yield. Likewise, 13KLAF1 reached values of 78% and 30% for both types of hybrid vigor in Lajas. The useful heterosis was 126% in Isabela and 184% in Lajas. UPE-1 had significantly lower yields than UPE-3, UPE-4, and UPE-5 in both locations (Table 4). Moreover, UPE-1 had a similar yield (1657 kg·ha−1) in comparison with 13KPP-264-05 (1950 kg·ha−1) and ‘Lázaro’ (1380 kg·ha−1) in Isabela. Also, lower seed yields were observed for UPE-1 (794 kg·ha−1) and ‘Lázaro’ (563 kg·ha−1) in Lajas. The weight of 100 seeds for all genotypes did not reach values more than 18.5 and 16.3 g such as those observed for 13KPP-264-05 in Isabela and Lajas, respectively (Table 4).
Mean seed yield and weight of 100 dry-seeds for 13KLAF1 hybrid, their parents, ‘ICPL 86012’, and UPE pigeonpea genotypes [Cajanus cajan (L.) Mill.] evaluated at Isabela and Lajas, PR in 2022.
Discussion
Significant differences in days to maturity and seed yield were observed in both locations. Higher maximum temperatures (41 °C) at reproductive stages (May and Jun 2022) in Lajas may be associated with the earlier maturity (<125 d) and lower seed yield (1112 kg·ha−1) compared with Isabela (36 °C) where maturity harvesting started at 144 d and mean seed yield was 2300 kg·ha−1. Warm temperatures hasten the time between flowering and maturity in pigeonpea (Silim and Omanga 2001). Furthermore, temperatures >38 °C cause pollen sterility and reduction in anthesis and pod setting leading to significant yield reductions (Choudhary et al. 2018). In addition, further research is necessary to identify if differences in nutrients available for the plants in Oxisol and Vertisol soils are associated with the productivity in Isabela and Lajas, respectively. Lower seed yields were reported in Vertisol soils in other studies (Chauhan et al. 1993; Matsunaga et al. 1994).
UPE-4 BL had a genetic gain for seed yield between 111% and 115% compared with Lázaro, which is the most popular cultivar used in Puerto Rico (Sarmiento et al. 2021; Viteri et al. 2020), in both locations under long-day conditions. Furthermore, UPE-4 had a similar production than the hybrid 13KLAF1 (close to 3000 kg·ha−1) that showed higher values of average and useful heterosis (87% and 126%, respectively) and heterobeltiosis (60%) in Isabela. Thus, conventional breeding is still an important tool to develop genotypes with a similar yield potential than a hybrid in countries where the technology and resources to generate cytoplasmic nuclear male sterility line, its maintainers, and fertility restores lines are restricted (Bohra et al. 2017; Saxena et al. 2016a). However, major disadvantages to developing new genotypes by conventional breeding are: 1) the time to develop a cultivar may take more than 5 years (Saxena et al. 2021a); 2) lower biomass, yield, and resistance to pests and diseases may be observed in cultivars compared with hybrids (Saxena et al. 2006, 2015, 2016a); 3) a recurrent purification of new cultivars is required because the percentages of cross-pollination in pigeonpea may reach up to 40% (Sameer et al. 2017); and 4) limited genetic diversity in the primary gene pool (Kumar et al. 2003) and strong linkage drag that causes barriers for interspecific hybridizations (Saxena et al. 2021a) may limit the development of new cultivars with higher agronomic performance.
This is the first study of hybrid vigor with a F1 population between parents with determinate growth habit conducted in Puerto Rico. It would be important to evaluate the hybrid vigor with indeterminate cultivars that possess taller plants, higher yield, and bigger seed size (>18 g per 100 seeds) (Rangaswamy et al. 2005; Sarmiento et al. 2021; Viteri and Linares-Ramírez 2024). For instance, crosses between indeterminate genotypes ‘Ariel’, ‘Blanco Yauco’, ‘Guerrero’, ‘ICP 7035’, IIPG-7, IIPG-11, ‘Pinto Berrocales’, and ‘Super Pinto’ and even with determinate cultivars Isabella and Lázaro should be recommended. With this, heterosis may be observed with other desirable traits beyond seed yield (e.g., biomass, plant height, early flowering and maturity, and seed weight) (Phad et al. 2009; Sekhar et al. 2004).
Our data indicated that 13KLAF1 hybrid increased the seed yield in Isabela and Lajas. Thus, the use of commercial hybrids may be an important strategy to explore to increase yields for pigeonpea in Puerto Rico. In fact, GTH 1, ICPH 2433, ICPH 2671, ICPH 2740, and ICPH 3762 hybrids reported with >30% of heterosis (Saxena and Sawargaonkar 2014; Saxena and Tikle 2015; Saxena et al. 2013, 2016b, 2021) should be recommended to be tested with local cultivars [e.g., Ariel, Cortada, Guerrero, Lázaro, UPE-4 (synonym ‘Isabella’), and Super Pinto] (Bosques-Vega et al. 2000a, 2000b; Sarmiento et al. 2021; Viteri et al. 2020; Viteri and Linares-Ramírez 2023, 2024) in field trials in contrasting environments.
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