1. Introduction
Peas (
Pisum sativum L.) are cultivated worldwide in temperate climates and are the fourth most important legume crop. Meanwhile, garden peas are also one of the main edible bean crops in China, with a perennial planting area of about 1.4 million hectares, accounting for 55.3% of the world’s total area [
1]. China is the world’s largest producer of garden peas and plays an important role in the world’s pea production. China has a variety of agricultural ecological types. Garden pea is often cultivated during winter in subtropical and early spring in temperate zones by smallholder farmers. Winter sowing areas are mainly in Yunnan, Jiangsu, Sichuan, Guangdong, Fujian, Zhejiang, and Shandong; spring sowing areas are mainly in Gansu, Ningxia, Inner Mongolia, Hebei, and other places [
2]. In recent years, there has been an unprecedented demand for new varieties of garden peas from farmers due to the improvement in people’s diets and the fact that peas have a short growing period and are easy to manage, which can enrich the soil [
3]. However, for a long time, when introducing and breeding pea varieties in China, the cultivar improvement of dry pea is concerned. The breeding of pea plant type was mainly vine type, and the improvement of modern pea cultivar resistant to biotic and abiotic stress was mainly focused on resistance to drought in the Northern Arid and Semi-arid region [
4], and resistance to frost for winter-hardy pea cultivar [
5], and resistance to root rot diseases (
Aphanomyes euteiches) [
6]. Unfortunately, the program of garden pea cultivar with high resistance to lodging and powdery mildew (
Erysiphe pisi DC.) and wide suitability for irrigated cropland were largely neglected. The improvement of semi-leafless peas with resistance to lodging, resistance to powdery mildew, and better suitability for irrigated conditions or rain-fed agricultural areas with annual precipitation of 450–650 mm may be the most effective measure to increase pea production potential and the relative profitability of grain legumes.
Semi-leafless pea is a leaf mutant caused by the
afila gene (
af) [
7], with normal stipules and all leaflets are replaced by tendril complexes which improve canopy structure and ventilation and light transmission conditions of the colony [
8]. Moreover, developed tendrils can intertwine each other to form scaffolding structures, thus significantly improving the lodging resistance of pea. Lodging is a major constraint to the production of many crops including pea [
9]. The lodging resistance breeding of pea has important application value in agricultural production [
10].
Modern field pea cultivars with better lodging resistance are very dependent on the
afila gene, as they tend to be bush-type tendrilled architecture with low-leaf biomasses [
11]. Over more than the past 20 years, steady progress has been made in improving the agronomic and quality characteristics of semi-leafless field pea as evidenced in the cultivars released in Canada, USA, France and other main pea-producing countries [
12]. However, breeding and utilization of semi-leafless pea varieties in China lag far behind outside countries. Over the past decade, few semi-leafless dry pea varieties such as Yunwan 35 [
13], Longwan 6 [
14], and Longwan 10 [
15] were registered and released; but semi-leafless garden pea varieties are even few. Semi-leafless varieties of pea have considerable agronomic importance and they have been suggested that they may have a better standing ability [
16], and higher resistance to powdery mildew than conventional varieties [
17].
Developing high-yielding, strong lodging resistance and powdery mildew resistant cultivars are important in pea breeding strategy [
18], and evaluation across locations and years would form a basis for breeding [
19]. In China, dry pea breeding programs has been strengthened with the launch of the China Agriculture Research System of MOF and MARA-Food Legumes (CARS-08) in 2008, but little attention has been paid to the improvement of garden pea varieties. Current commercial garden pea varieties, Qizhen 76, Tiancui 761, Taizhong 11, introduced from the World Vegetable Centre (AVRDC), Tainan, Taiwan [
2], which are susceptible to powdery mildew and a long maturity period in spring sowing areas. Moreover, these climbing garden varieties are expensive to trellis with wooden posts and plastic strings used for vine staking, while semi-leafless garden pea varieties could reduce these costs [
11]. Therefore, developing new powdery mildew resistant and early maturing cultivars is a paramount solution for the resource-poor farmers growing pea. This work presents the overall performances of the recently developed and released garden cultivar Longwan 5, an edible-podded sweet pea cultivar with high resistance to powdery mildew.
2. Materials and Methods
2.1. Cross Information and Breeding Method
Longwan 5 is a garden pea cultivar developed by hybridization between female parent Shuanghua 101 and male parent Baofeng 3. Medium maturity, resistance to powdery mildew, attractive dark green pod with sickle shape, high yielding and lodging resistance were major breeding objectives for the development of the cultivar. The cross between Shuanghua 101 × Baofeng 3 was made in the summer of 2005 at the Qinwangchuan Experimental Station (QES) of Academy of Gansu Agriculture Sciences (AGAS), Lanzhou, China (latitude: 36°43′ N; longitude: 103°38′ E; altitude: 1950 m mean sea level; Annual precipitation: 319 mm; Annual average temperature: 5.9 °C). Shuanghua 101 is a leafy, long vine length and powdery mildew susceptible pure line selected from Qizhen 76. Baofeng 3 is a semi-leafless pea cultivar resistant to powdery mildew, derived from the cross Zhongwan 5 × 90-EP-10, where Zhongwan 5 was developed at Institute of Animal Science, Chinese Academy of Agricultural Sciences (IAS—CAAS), Beijing, China, and 90-EP-10 was obtained from the Institute of Crop Breeding and Cultivation, Qinghai Academy of Agriculture and Forestry Sciences (ICBC—QAAFS), Xining, China.
2.2. Selection Process
The breeding method used for Longwan 5 was a pedigree selection in combination with single seed descent. All breeding activities of the F
1–F
5 generations were conducted in Qinwangchuan Research Station (QRS). In 2006, 20 F
1 were planted. A total of 725 F
2 seeds were bulked and in 2007, F
2 seeds were planted and 120 healthy superior plants were selected (
Table 1). All F
3 to F
5 plant progenies were grown and single plant selection was based on medium maturity, resistance to powdery mildew, lodging resistance, attractive dark green curved pod, and high green pod yield and seed were collected separately. The selected plants from the F
5 were grown as F
6 progeny rows and bulk-harvested to form breeding lines. Plant progenies at the F
7 generation were evaluated in a preliminary screening trial for green pod yield, lodging resistance, resistance to powdery mildew and main characteristics in Yongdeng, Gansu, in 2010. One of the selected lines from the screening trial based on the performance for these traits was X9002 and evaluated in a replicated preliminary yield test at Yongdeng, Gansu province, in 2011. From 2012 to 2014, It was entered into Gansu Pea Coordinated Trials (GPCT) and evaluated in replicated trials at five locations. X9002, named Longwan 5, was registered in the Gansu Crop Cultivar Catalogue (GCCC) in 2015, and subsequently registered in National Pea Varieties Catalogue in China (NPVCC, Registration code: GPD Wandou (2018) 620008) in 2018.
2.3. Testing Sites and Field Evaluation
All F
1 to F
6 plant progenies selection and preliminary screening trial were performed in irrigated field condition at Qinwangchuan Experimental Station (QES) of Academy of Gansu Agriculture Sciences (AGAS), Lanzhou, China. Then multi-sites advanced yield trials were conducted in different agroecological zones for yield traits performance. From 2012 to 2014, Longwan 5 was evaluated in replicated trials at five sites in northwest China with two sites (Yongdeng and Minle) in Hexi Corridor Irrigated Zone (HCIZ), two sites (Tianzhu and Gannan) in High-Latitude Climates (HLC), and one site (Dingxi) in Northern Arid and Semi-arid region (NASR). For the five sites, long-term average total precipitation varied from 319 mm to 621 mm per year, with 53.2–62.2% of the yearly precipitation occurring during the pea growing season. The long-term average annual temperature of the sites was 5.0 °C, with yearly average temperatures ranging from 4.1 °C to 7.1 °C. The daily minimum temperatures in spring varied from −4 °C to −7 °C, and the daily maximum temperatures in summer varied from 19 °C to 26 °C between sites. Additional information on soil types, biotic or crop management factors and climatic characteristics of experimental sites are presented (
Table 2).
2.4. Field Evaluation
Garden pea lines together with check cultivars were evaluated for pod yield traits performance during the main cropping season and location-year was treated as a single environment. During each year, the experiment with genotypes was carried out in a randomized complete block design (RCBD) with three replications. The plot size was 2 × 5 m (10 m
2) having six rows and spacing of 40 cm between rows and 3–5 cm between plants was maintained. Diammonium phosphate (46% P, 18% N) fertilizer with 150 kg/ha was pre-plant incorporated and appropriate cultural practices (weed removal by hand, disease and pest control) were followed. Appropriate irrigation was performed in Minle and Yongdeng sites at the seedling and flowering stages. Peas are usually sown between mid-March and early April. Green pods are picked from late June to mid-July. The lodging resistance of the pea cultivar was identified according to the method described by Bilgili and Uzun et al. [
20]. At the full flowering and pod filling stages, lodging tolerance was rated on a 1–5 scale, where 5 = no lodging and 1 = entire plot lodged.
2.5. Evaluation Resistance to Powdery Mildew
Reactions to powdery mildew (
Erysiphe pisi DC) of Longwan 5 and advanced breeding lines were evaluated during the pod-filling period in field disease nursery on QES in 2014 and 2015 using the method described by Iqbal et al. [
21], and greenhouse evaluations using the method described by Sun and Wang [
17]. Snap pea cultivar Qizhen 76 harbouring the susceptible gene (
Er1), was used as a susceptible check cultivar for evaluating uniformity of inoculation [
22], and Xucai 1 carrying resistance gene (
er1-2) as a resistant check cultivar [
17]. The
E. pisi isolate EPBJ (NCBI accession number KR912079) isolated from greenhouse infected pea, was maintained under greenhouse conditions on the susceptible cultivar Longwan 1 as inoculum for the screening trials. The plants were inoculated with EPBJ using conidia shed by shaking heavily infected plants. Longwan 5 and check cultivars were assessed for resistance 10–14 days after inoculation using a 0–4 scale according to the infected foliage area, macroscopic and microscopic density of mycelia, and sporulation on the lower part of the plant [
23,
24]. Disease severity scales for resistance to powdery mildew are determined according to the infected foliage area in the pea [
25]. (A) 0 = no mycelial growth and sporulation on the plant, (B) 1 = sparse mycelial growth and very little sporulation on less than 5% of leaf, (C) 2 = slight to moderate growth mycelium with conidiophores on 5–25% of leaf, (D) 3 = moderate to heavy growth mycelium with sporulation on 25–75% of leaf, and (E) 4 = abundant growth mycelium with heavy sporulation on greater than 75% of leaf. Plants with scores of 0–2 were classified as resistant (R) and those with scores of 3–4 as susceptible (S) [
23,
24].
2.6. Data Collection and Analysis
Data on green pod yields (GPY) and 11 yield-related traits (YRT) were collected on plot and plant basis from each plot, respectively. Branches per plant (BPP) and date to maturity (DM) were taken when each plot attained 50% flowering and 90% of the pod’s physiological maturity, respectively, and days were calculated beginning from the date of sowing. Data for plant height (PH), pods per plant (PPP), seeds per plant (SPP), hundred green pod weight (HGPW), weight of green pod per plant (WPP), internodes per main stem (IPMS), first blossom node (FBN) and seeds per pod (SPD), double pod rate (DPR) were collected based on five sample plants which were randomly taken from each plot and the average of five sample plants were used for analysis. The measured green pod yield (GPY) value of the plot was converted to kilogram per hectare for analysis. ANOVA was performed using the SPSS statistical program v.26.0 (IBM Corporation, New York, NY, USA) and differences among treatments for all measurements were compared at p = 0.05 and by using Duncan’s multiple-range test.
5. Conclusions
It could be concluded that semi-leafless garden pea cultivar Longwan 5 is superior to existing approved varieties Qizhen 76 and Xucai 1 in terms of green pod yield, medium maturity and double podding. This cultivar is tolerant to lodging, powdery mildew disease, and wide climate resilient for spring cultivation as well as for autumn cultivation under irrigated and rainfed environments in China. Longwan 5 gave a significantly higher average green pod yield (12,376 kg/ha) than check varieties Qizhen 76 (11,132 kg/ha) and Xucai 1 (11,649 kg/ha) across five locations and three years, which was 11.2% and 6.3% higher than control varieties, respectively. Cultivation of this cultivar on large scale will surely increase the production of peas in China and will also prove beneficial for farmers increasing their income. This cultivar may be adapted to other temperate regions of the world. The breeder seeds of Longwan 5, derived from a single line at the F
10 generation, are being maintained at Qinwangchuan Research Station, Institute of Crop Sciences, Academy of Gansu Agriculture Sciences (ICS-AGAS), Lanzhou, China, and conserved in the National Crop Genebank of China (NCGC,
https://www.cgris.net/, (accessed on: 1 May 2022), GenBank accessions: G0007660). Exclusive rights for the sale and production of the pedigreed seed have been awarded to Golden Silk Road Seeds Industry Co., Ltd., Wuwei, China.