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Cowpea (Vigna unguiculata (L.) Walp.) yield is a crop strongly influenced by environmental conditions. Average seed yield can decrease to a great extent when drought conditions occur, especially when they prevail during flowering and seed filling periods. Identifying genotypes presenting yield stability is one of the most important breeding goals. Local varieties or crop landraces are genetic resources that, despite exhibiting intermediate yield production capacity, present high yield stability in low-input cropping systems.In the countries around the Mediterranean basin, a remarkable number of cowpea landraces are still cultivated on a small scale by farmers mainly for their own use and consumption. These landraces could serve as important sources of adaptive traits and resistance to drought for the upcoming climatic change. However, the evaluation of cowpea landrace material originated from Southern European countries is considered limited in proportion to the number of local varieties that are available. Efforts are therefore made to evaluate more cowpea landraces originated from countries around Meditteranean sea in order to be later included in breeding programs.
Cowpea (Vigna unguiculata (L.) Walp.) seed and fresh pod yield is usually strongly influenced by prevailing environmental conditions, expressing significant environmental effects (E) and genotype x environment interactions (G x E) [1],[2],[3],[4],[5],[6],[7]. Average seed yield can especially be decreased when drought conditions occur during the flowering and seed filling periods [8],[9],[10]. Identification of genotypes expressing yield stability under different environments and throughout cropping seasons is considered one of the most important breeding goals [5],[11], while yield is also recorded as the most desirable trait for farmers [12],[13],[14]. Currently, we are at a point at which the observed extreme climatic conditions may affect the stability of various crops yield globally [15],[16],[17]. The ability of a crop to produce consistent yields in various environments and under changing weather conditions therefore comes to the fore. Landraces consist of genetic material with intermediate yield, production efficiency, and yield stability when they are cultivated in low-input cropping systems [18],[19] and therefore, they are important in terms of stable yield production [20],[21] and of improving resistance to various abiotic stresses [22],[23],[24].
Cowpea cultivation in Southern Europe, including Mediterranean countries, starts in late spring (late April) and lasts until the beginning of autumn (early October) [1],[25],[26]. During the summer period, cowpea is confronted with water scarcity and high air temperatures, like in many other areas of the world [27],[28], while in many cases, cultivation faces additional limiting soil factors [29]. The evaluation of cowpea landrace material originated from Southern European countries is considered limited in proportion to the number of local varieties that are available. This encyclopedia article therefore aims to provide more information regarding cowpea landraces of Greek origin with interesting morphological traits, adapted to different microclimates with regards to their phenological characteristics and traits related to seed yield.
Statistical significant differences were observed among the five landraces material evaluated for three consecutive experimental years. More specifically, there was a statistically significant interaction among the accessions and the experimental years for all the phenological traits studied, except for days from sowing till the appearance of mature pods, in 50% of the plants. The accessions differed statistically significantly from each other regarding all three phenological characteristics studied, while statistically significant differences were observed among the experimental years (p < 0.001). Furthermore, there was not a statistically significant interaction of accession x experimental year for the number of pods per plant, pod length, seed weight per plant, and number of seeds per plant. Seed weight per plant did not differ statistically significantly among the three experimental years and the accessions, while pod length and hundred-seed weight did not differ statistically significantly among the experimental years.
Seed yield (kg ha-1) varied between 577.78 kg ha-1 (VG2-Atsiki, Lemnos) and 1058.33 kg ha-1 (VG23-Logothetianika, Kythira) in 2015, between 389.49 kg ha-1 (VG4-Marathi, Mykonos) and 690.47 kg ha-1 (VG23-Logothetianika, Kythira) in 2016, and between 683.19 kg ha-1 (VG2-Atsiki, Lemnos) and 1053.15 kg ha-1 (VG3-Alinda, Leros) in 2017 (Figure 1). However, seed yield (kg ha-1) did not differ statistically significantly among the accessions and the experimental years, while there was not a statistically significant interaction between accessions and experimental years.
Figure 1. Seed yield (kg ha−1) for each accession and each experimental year. Seed yield for each accession for the three consecutive experimental years (2015, 2016, 2017) is presented in orange, green, and blue, respectively.
During the second experimental year, accessions presented shorter flowering duration, earlier pod maturity, lower number of pods per plant, lower number of seeds per plant, and lower seed yield (kg ha-1) than in the other two experimental years. Therefore, the accessions in the second experimental year (presented in green color) were grouped separately (second and third quadrant) from the two other experimental years through principal component analysis (PCA) (Figure 2). Accessions in the experimental years 2015 and 2017 were grouped in the first, third, and fourth quadrants, with the highest yield accessions to be depicted in the first and fourth quadrants. Landraces VG3 and VG23 were presented as the highest yielding accessions during 2017 and 2015, respectively, while VG4 was the lowest yielding accession in 2016 (Figure 2). Most accessions expressed similar values for every studied trait for each accession under the 2015 and 2017 experimental years and therefore were depicted at close distances, with the exception of local population VG3 (Alinda, Leros) and local population VG23 (Logothetianika, Kythira). Landrace VG2 (Atsiki, Lemnos) was among the least productive accessions in all three experimental years (Figure 2).
Figure 2. Classification of accessions through principal component analysis (PCA) and diagrammatic representation of the eigenvectors of studied traits. DFL: days from sowing to 50% flowering, FDUR: flowering duration, DMAT: days from sowing to 50% pod maturity, PH: plant height, NPOD: number of pods per plant, PODL: pod length, SPOD: number of seeds per pod, SEEDW: seed weight, NSEED: number of seeds per plant, 100 SW: hundred-seed weight. Accessions are presented with different symbols, while experimental years are indicated by different colors (2015: red, 2016: green, and 2017: blue).
Cowpea genotypes with short biological cycle and short flowering time can avoid the water scarcity that often prevails in the area during the summer months [30], while genotypes with long biological cycles seem to cope better under high temperatures as they gradually enter flowering and podding stages [31],[32]. Cowpea landraces evaluated were diverse and represent promising material for high air- temperature and drought tolerance or avoidance. In particular, local population VG2 from Atsiki, Lemnos could be suitable material for drought avoidance [33].
The average cowpea seed yield in Greece fluctuates from 1 to 3.5 t ha-1 [34]. However, the average seed yield achieved, ranged from 0.39–1.05 t ha-1 and was therefore lower than the average seed yield reported for Greek conditions. The observed seed yield values were also lower than the average seed yield of cowpea landraces reported in other countries, such as in Ethiopia (2.05 t ha-1) and Brazil (1.05 t ha-1) [35],[36]. In addition, all landraces under study did not differ statistically significantly from each other, nor compared to the improved line, regarding their seed yield (kg ha-1). This fact is probably due to the unfavorable field conditions, which led to the production of low seed yield and did not allow landraces’ potential to be unfolded [37],[38],[39]. Landraces unfold their potential when they are cultivated per se in the regions where they have been adapted, and despite their low productivity in ex situ cultivation, they should be further evaluated on-farm in each one’s region of origin. Therefore, the performance, including both productivity and stability of the examined landraces of the present experiment, should also be assessed in their region of origin as well as in other soil and climatic environments[24],[40],[41],[42],[43], as different cowpea landraces were found to be better adapted to diverse environments [44].
Despite that, there were not statistically significant differences among the accessions regarding seed yield, VG23 (Logothetianika, Kythira Island) was a landrace that exceeded the seed yield production of IT97K-499-35 breeding line in each one of the three experimental years. VG23 also presented a high number of pods and seeds per plant as well as large pod length, which are traits that have been strongly and positively related to seed yield. Therefore, VG23 was the most productive accession in the current soil and climatic conditions. The statistically significantly higher genetic diversity that has been previously recorded for the VG23 landrace in comparison to twenty-two other cowpea landraces of Greek origin [45], could be the reason for its increased seed yield production efficiency during the three experimental years.
Promising variability observed among and within the five landraces studied regarding their phenological and seed yield related traits. However, there were not statistically significant differences in terms of their seed yield (kg ha-1), a fact which is probably recorded due to the unfavorable soil and climatic conditions prevailed, that affected accessions performance and prevent them to unfold their potential. Therefore the importance of evaluating landraces to different environments as well in their regions of origin was highlighted. Among the landraces, VG23 (Logothetianika, Kythira Island) was the most productive under the present soil and climate conditions and therefore could be utilized in a breeding program aiming to increase seed yield production.