Improving pea production – Yield and nitrogen content of pea cultivars with different leaf types

Pea (Pisum sativum L., 2n = 14) is the most important cool season grain legume in Europe. Peas are mainly used as protein-rich feed for animals as an alternative to soybeans and faba beans. There is a special interest in peas in organic agriculture also because of their high value as a pre-crop due to the nitrogen fixation by symbiotic rhizobia and the high nitrogen content in the residues. For a long time, the main limitation for pea production was the high susceptibility to lodging of the normal-leafed type and consequently severe problems with harvesting. A semi-leafless mutant reduced this problem, but the lack of leaflets may have impaired productivity. The aim of this study was to compare the two leaf types with respect to the differences in yield and nitrogen content, and to identify genotypes that could be a resource for combining a high protein content in seed with a high nitrogen yield in the straw. The benefit of this study is to help increasing the pre-crop value of pea and to broaden its genetic diversity for breeding programs. The thesis consists of three different experiments. In the first experiment, 24 normal-leafed and 30 semi-leafless pea genotypes were evaluated in randomized complete block design with two replicates in three environments in 2017 and 2018 near Göttingen, Germany. The materials were recently released to very old cultivars, as well as recent breeding lines and some genetic resources, collected from gene banks, seed companies and breeders in Europe. Fourteen important morphological and agronomic traits were measured: light interception, lodging, plant length, plot height, seed dry yield, nitrogen content in seed, nitrogen yield in seed, straw dry yield, nitrogen content in straw, nitrogen yield in straw, biomass yield, total nitrogen yield, harvest index, and nitrogen harvest index. For the comparison of semi-leafless and normal-leafed peas under comparable genetic backgrounds, in the second experiment near-isogenic bulks were established for four crosses. The crosses were performed in 2013 -2014 with parents that are different in leaf type (normal-leafed and semi-leafless) and plant length (long and short), and lines in F4, F6 were developed, respectively. In each cross, four bulks of 10 segregating lines each were bulked which are the combination of different leaf type and plant length: semi-leafless, short; semi-leafless, long; normal-leafed, short; normal-leafed, long. These four bulks of each of the four crosses were tested in four environments in 2018 and 2019 for the traits: light interception, plant length, plot heigh, lodging, seed dry yield, straw dry yield, nitrogen content in seeds, nitrogen content in straw, nitrogen yield in seed, nitrogen yield in straw, biomass yield, total nitrogen yield, harvest index and nitrogen harvest index. In the third experiment, a collection of 46 pea genotypes from the initial diversity trial were analysed for genetic diversity using the GenoPea 13.2K SNP Array chip by the company Trait Genetics. In the first experiment, seed yield and straw yield were higher for semi-leafless genotypes (51% and 40%, respectively, compared to normal-leafed genotypes). However, nitrogen content in seed and nitrogen content in straw were lower for these genotypes (10% and 30%, respectively). A negative correlation of seed yield with nitrogen content in seed was observed in both normal-leafed and semi-leafless peas with correlation coefficients (r) of -0.67 and -0.36, respectively. A similar relationship was also recorded between straw yield and nitrogen content in straw (r = -0.48 for normal-leafed, and r = -0.30 for semi-leafless peas). However, there was no correlation between nitrogen yield in seed and nitrogen yield in straw. In addition, semi-leafless peas were significantly higher than normal-leafed genotypes in total nitrogen yield, biomass yield, plot high, and nitrogen harvest index. However, most normal-leafed genotypes in this experiment were released earlier than semi-leafless genotypes, and some of them even were genetic resources not adapted to European climate. Some recent normal-leafed cultivars reached the yield level of semi-leafless genotypes. The lodging scores of normal-leafed genotypes were on average 7 to 9 (1 = no lodging, 9 = total lodging), whereas this value of semi-leafless accessions were just between 2 and 3. The first component of a principal components analysis (PCA) separated the two leaf types with some exceptions. It was positively influenced by the traits seed dry yield, nitrogen yield in seed, harvest index, nitrogen harvest index, total nitrogen yield, biomass yield, with higher values for the semi-leafless group, while the traits lodging, nitrogen content in seed and nitrogen content in straw had a negative impact, with higher values for the normal-leafed group. The results of the second experiment illustrate that normal-leafed peas were about 25% higher in seed dry yield although seriously lodging, while the semi-leafless type was almost 5% higher in straw dry yield, however about 6% higher biomass yield was recorded for normal-leafed peas. The average difference of nitrogen content in seed and in straw between the two leaf types was small but significant. Normal-leafed peas were about 26% higher in nitrogen yield in seed, but approximately 11% lower in nitrogen yield in straw compared to the semi-leafless group. In the principal components analysis, the first component explained about 48% of the total variance, and also distinguished between the two leaf types. Normal-leafed peas had higher values for harvest index, nitrogen harvest index, seed dry yield, nitrogen yield in seed, light interception, total nitrogen yield, and nitrogen content in seed. In all field experiments, the light interception of normal-leafed peas is always significantly higher than that of semi-leafless peas, 83.5% vs 81.1% in the first experiment, and 84.2% vs 78.4% in the trial of bulks of leaf types, respectively. The genotyping by SNPs markers of 46 accession resulted in a principal component analysis where the first two principal components explained about 20.5% of the total variation. The first component explaining 14.2% and separated the research materials in the two groups semi-leafless type and normal-leafed type, whereas the second component, which explained 6.3% of variance reveal a small outlier group which consists of the winter peas from both leaf types, and two genotypes from Afghanistan at similar position. A dendrogram with bootstrap values using the genetic distance between the 46 pea genotypes illustrated also two groups of genotypes mainly corresponding to the two leaf types. There are a few interesting exceptions from the clear clustering into semi-leafless and normal-leafed cultivars. The cluster of semi-leafless cultivars contains four normal-leafed cultivars. In the normal-leafed cluster, there is a group of winter cultivars (GR409, GR293EW, Pandora) also containing the semi-leafless cultivar Specter, because in breeding of winter pea there was obviously not such a strong distinction between the two leave types. Some new semi-leafless cultivars from Europe such as Salamanca, Cheyenne, Astronaute, Santana, Casablanca, Alvesta, Navarro, Cleopatra and Camilla were assigned together in a sub-group. In general, semi-leafless peas have a lower genetic distance to each other than the normal-leafed peas. The results of the present study demonstrate the role of normal-leafed pea cultivars in breeding, especially for low-input production systems. The absence of a negative correlation between nitrogen yield in seed and in straw indicates that a high protein yield can be combined with a high pre-crop value. The value of pea in crop rotations can be increased by using superior cultivars and by breeding for both traits. Although seriously lodging, normal- leafed genotypes still have a potential of high yield and protein content especially for intercropping and organic agriculture. Due to the large distance between the two leaf types, normal-leafed peas are an interesting resource to increase the genetic diversity even when breeding semi-leafless peas.

2022-11-04