Variability and relationships between characters of physic nut (Jatropha curcas L.) in Burkina Faso

Jatropha curcas is a highly promising species for biodiesel production in Burkina Faso and other countries in the tropics. It is rustic, grows in warm regions and is easily cultivated. These characteristics and high-quality oil yields from the seeds have made it a priority for biodiesel programs. Consequently, this plant merits genetic investigations aimed at improving yields. The present study was conducted to determine the extent of genetic variability and relationships among 30 accessions from Burkina Faso using 7 qualitative characters and 20 quantitative characters. For most of the traits, there were highly significant differences among the accessions. A positive and significant correlation between the 100-seed weight trait and the oil content trait was observed. The oil content trait is significantly and positively influenced by growth traits such as plant height and crown diameter. Based on the petiole base pigmentation, three morphotypes were identified: green morphotype, purple morphotype and brown morphotype. The green morphotype was characterized by very high oil content and high 100-seed weight while the brown morphotype presents low oil content and low 100-seed weight. The purple morphotype registered a high oil content and medium 100-seed weight. These results are important for the continuity of breeding programs, aimed at obtaining cultivars with high grain yield and high oil content in seeds.


Introduction
Physic nut (Jaropha curcas L.) is a perennial oil plant commonly used in tropical areas as a medicinal plant, in the construction of defensive hedges against animals and in the fight against water erosion [1]. It is rustic, grows in warm regions and is easily cultivated. These characteristics and high-quality oil yields from the seeds have made this plant a priority and a highly promising species for biodiesel programs in countries in the tropics [2]. It produces seeds rich in oil which can be used pure after filtration as fuel in diesel engines with indirect injection or serve as raw material for the production of biodiesel by transesterification [3,4]. [5] highlighted the economic interest of the use of oil of this species for the poor countries, in particular tropical Africa and Asia. Indeed, J.
curcas provides various products that contribute to poverty reduction, in particular the promotion of income-generating activities mainly for women such as sale of seeds and soap and the valuation of oil cakes as organic fertilizers. It is an opportunity for developing countries to improve farmers' incomes and even stimulate the rural economy [6]. However, in most countries of sub-Saharan Africa, the establishment of plantations preceded the conduct of agronomic research essential for the sustainable exploitation of the plant. As a result, plant development has been poor and yields obtained during the first years of cultivation have generally been disappointing [7]. Consequently, this species merits genetic investigations aimed at improving yields [2]. So, the viability of the J. curcas-based biodiesel sector is essentially based on highly productive and oil-rich J. curcas genotypes [6]. Tiendrébéogo et al., 2021 Variability and relationships between characters of physic nut (Jatropha curcas L.) Despite strong recommendations for improvement of J. curcas for intensive production, a very little references exist on the genetic characteristics of the plant, the level of productivity of J. curcas in relation to its genetic potential and the cultivation techniques [6]. Indeed, J. curcas is still essentially a wild plant which must be the object of genetic improvement and domestication for its popularization and vaolrization [8,9]. In Burkina Faso, the genetic diversity of the species remains still poorly known [10][11][12][13]. Thus, within the framework of the promotion of the neglected or under-exploited local species, this study was initiated in order to contribute to a better knowledge the genetic diversity and the relationships between agro-morphological traits and seeds oil content of J. curcas.

Plant material
The plant material consisted of 30 elite accessions selected by [11] based on the morpho-metric characteristics of the fruits as well as the oil content of the seeds. In this study, an accession is a set of seeds, from natural pollination, harvested from the same tree during prospecting. The seeds were stored at laboratory temperature without any prior chemical treatment.

Experimental site
The agro-morphological characterization was carried out on a plantation installed in the experimental station of the Institute for Rural Development (IDR) of Gampela at 1°21'0.9'' West longitude, 12°24'10.7'' North latitude and 924 m altitude. The station is characterized by very heterogeneous, deep soils, of low physicochemical fertility and a predominantly sandy-clay texture [14]. The climate of the zone is of the sudano-sahelian type characterized by the alternation of two seasons, namely a rainy season which extends from june to october and a dry season from november to may [15]. The annual rainfall recorded in the station during the experimentation varied between 728 and 984.8 mm. Average air temperatures during the rainy months range between 35°C and 40°C for the maximums and between 18°C and 19°C for the minimums [15].

Experimental design
The experimental design used is a completely random experiment plan consisting of 30 elementary plots of 8 m x 8 m. Each elementary plot contains 9 plants arranged in three lines of 8 m in length each. The spacing between the lines was 4 m. Each line includes three plants with a spacing of 4 m. The aisles between the elementary plots were also 4 m.

Qualitative characters
Based on the work of [16], seven qualitative variables were chosen to characterize the phenotypic variability of the accessions studied. The Table 1 presents the different characters with their modalities.

Quantitative characters
Based on the work of [17,2,18,16,6], 20 quantitative variables were chosen to characterize the genetic diversity of J. curcas five years after planting. These parameters were measured on three plants sampled by accession. These are first of all the parameters linked to the vegetative development of the trees, measured three months after the appearance of the first leaves. Those are: • The dendrometric characters of trees such as plant height (PH) measured from the base to the apex of the main stem, crown diameter (CD) measured between both ends of the plant, stem diameter (SD) measured at the collar using a tape measure (SD = collar circumference / π) and number of main branches (NB) evaluated by counting the branches coming out from the trunk .
• The characters linked to the dimensions of the leaves measured on three fresh leaves, fully developed and not parasitized, per individual: these were the width of the leaf (WL) measured between the two ends of the leaf, the length of the leaf (LL) measured from the petiole to the tip of the leaf and the length of the petiole (LP) measured from the insertion of the stem to the insertion of the leaf.
The traits linked to the productivity of the accessions such as fruits weight (FRW), seeds weight (SEW) and pulps weight (PUW) per accession were also evaluated at the ripe and dry fruit stage using an electric scale. Moreover, others traits relative to the fruits were measured. They are the average length of the fruit (ALF), the average diameter of the fruit (ADF) evaluated using a digital caliper, the average weight of the fruit (AWF) and the average weight of the pulp (AWP) determined using an electric Tiendrébéogo et al., 2021 Variability and relationships between characters of physic nut (Jatropha curcas L.) scale on ten fruits. The average number of seeds per fruit (ANS) was determined by counting.
As concerning the seeds, the characters measured were the 100-seed weight (SW100) determined by weighing a random sample of 100 seeds of each mother plant using an electric scale, the average length of the seed (ALS), the average width of the seed (AWS) and the average thickness of the seed (ATS) measured with a digital caliper on ten seeds. The seeds oil content (OIL) was determined by the soxhlet extraction method using hexane as the solvent [19]. [16] proposed a descriptor with four classes based on the seeds oil content character in relation with the weight of the seed: low (0-20%), medium (21-30%), High (31-40%) and very high (> 40%).

Data analysis
The data collected were analyzed with XLSTAT 2020.3.1.11 software. Analysis of variance (ANOVA) and of Student Newman Keuls means separation test at α=5% were carried out in order to compare accessions. Pearson's R coefficient was used to measure correlations between quantitative traits while the relationships between discriminate qualitative and quantitative traits were determined through multiple correspondence analysis (MCA). Indeed, the quantitative traits have been previously transformed into qualitative traits according to the classes obtained in the Student Newman Keuls means separation test.

Results
Analysis of the phenotypic variability of J. curcas using qualitative variables The results recorded in Table 2 showed that all the accessions studied had green stems and green leaves, a cream-colored latex and alternate leaves. However, two characters presented variability. Thus, the majority of plants (70%) had a basal branching pattern while a minority (30%) revealed an intermediate branching pattern (Figure 1). The petiole base expressed several pigmentation color ( Figure 2): green (33.33%), brown (13.33%) and mostly purple (53.34%).

Analysis of the phenotypic variability of J. curcas using quantitative variables
The results of the significance of the means separation test of Newman Keuls were performed with vegetative development parameters (Table 3), fruit parameters (Table 4) and seeds parameters ( Table 5). Most of the vegetative development parameters and seeds traits significantly discriminate the accessions studied except the length of the petiole (LP), the average length of the seed (ALS) and seeds weight per accession (SEW). For the fruit traits, only three of the seven measured characters, precisely the average length of the fruit (ALF), the average weight of the fruit (AWF) and the average weight of the pulp (AWP) showed variability. Based on the each of three characters of interest namely oil content (OIL), 100-seed weight (SW100) and crown diameter (CD), three classes were obtained within accessions. Indeed, according to the value of the character, three performance classes Tree (> 5m) 0 (low, medium, high) were registered with the 100-seed weight trait and crown diameter, respectively. The seeds oil content character showed also three performance classes which ranged medium, high and very high according to the descriptor classes.

Correlation between characters
The correlation of Pearson (Table 6) showed positive and significant correlations at the 5% and 1% threshold between the characters studied. Thus, the character oil content was positively correlated with the characters 100-seed weight (r = 0.253), plant height (r = 0.344) and crown diameter (r = 0.260). The 100seed weight character was positively correlated with the characters linked to vegetative development such as plant height (r = 0.522), crown diameter (r = 0.443) and length of the petiole (r = 0.272). Furthermore, the crown diameter character is positively correlated with all the other characters studied. These different correlations indicate that plants with significant vegetative development produce seeds of high weight and high oil content. The characters linked to the yield (FRW, SEW and PUW) are positively correlated with the characters relating to the scale of the plant (PH, SD and CD). So, large-scale plants have also high productive potential. Furthermore, the characters of the seeds (ALS, AWS and ATS) are positively influenced each other.

Association between characters
The results of multiple correspondence analysis (MCA) recorded in Figure 3 showed three associations of the characters. On the plan formed by axes 1 and 2 with 45.41% of the total inertia, the F1 axis (23.14% of total inertia) opposed two groups of variables precisely group 1 and group 3. Group 1 combined the characters very high seed oil content (> 40%), intermediate

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Page 4 of 11 Tiendrébéogo et al., 2021 Variability and relationships between characters of physic nut (Jatropha curcas L.) The values for each class followed by the same letters are not significantly different at the 5% level.
branching of the stem, green pigmentation of the petiole base, high 100-seed weight (≥ 62.85 g) and large crown diameter (≥ 4 m) while the group 3 is formed by association between the medium oil content (21-30%), the brown pigmentation of the petiole base and the low 100-seed weight (≤ 36.9 g). Group 2, correlated to axis F2 with 22.127% of total inertia, combined the high oil content (31-40%), the basal branching of the stem, the purple pigmentation of the petiole base, the medium 100-seed weight (43.30-43.85 g), the medium crown diameter (2.4-3.467 m) and the small crown diameter (≤ 2.1 m). The values for each class followed by the same letters are not significantly different at the 5% level. Tiendrébéogo et al., 2021 Variability and relationships between characters of physic nut (Jatropha curcas L.) The values for each class followed by the same letters are not significantly different at the 5% level. Table 6.
Matrix of correlations between the studied characters.

Discussion
The agro-morphological evaluation using quantitative and qualitative characters highlighted the existence of variability for some characters. Similar results for quantitative traits were obtained by [20] and [11,12]. For qualitative characters, this variability relates to branching pattern and petiole base pigmentation. [16] reported similar results on the petiole base pigmentation. However, only two types of stem branching were observed in this study compared to four types obtained by these same authors, thus indicating that the accessions studied are less diversified for this character. No color variation was noted on the stems and leaves of the studied accessions. These results are similar to those of [17], [21] and [22] but differ from those of [16]. Genetic correlations indicate genetic relations between characters. Knowledge of correlations among the characters is useful in designing an effective breeding program for any crops [23]. Indeed, the magnitude and direction of the genetic correlation is important in the choice of breeding methods and the formulation of strategies for simultaneous selection on multiple traits [2]. The correlations noted in this study corroborate those observed by [2] and [23]. In this study, statistically significant correlation of oil content existed with crown diameter and plant height. In addition, 100-seed weight character is significantly correlated with crown diameter, petiole length and plant height. These correlations are interesting for improvement programs because according to [24], the selection of elite accessions to reproduce for oil extraction should be oriented towards those with the best crown and seed weight traits. Furthermore, [25] indicate that the quantity of seeds produced on the one hand is positively correlated with crown, and on the other hand the seed weight and the oil content are positively correlated. Significant and positive correlations between the characters of the seeds would favor the simultaneous improvement of these characters through selection. Thus, in the study, interesting correlations were noted between the characters linked to the weight and dimensions of the seeds. The strong positive correlation between the characters of the seeds indicates that the genes which govern these characters are probably linked or have a pleiotropic effect. Similar results have been reported by [2] and [23] between 100-seed weight and the dimensions of the seed. However, the study found no significant correlation between the oil content and the dimensions of the seeds studied. Similar results have been also reported by [2] and [18]. Furthermore, the study showed a positive and significant correlation between the 100-seed weight trait and the seed oil content trait. [26] and [23] observed a positive and significant correlation between seed weight and oil content while [27] revealed a negative correlation between these two characters. [18] indicated that these contrasting results could be explained by the high weight of the seed coat of some accessions. According to [16], qualitative characters are morphological markers that can be used to identify lines in a relatively short time. These authors reported also that the development of descriptors on perennial plants such as J. curcas can contribute to an effective use of germplasm in the improvement programs. Analysis of the association of characters revealed the existence of three morphotypes differing in the color of the pigmentation of the base of the petiole, the oil content of the seeds and 100-seed weight. [16] identified also three descriptors based on the pigmentation of the petiole base. For perennial plant such as J. curcas, these morphological descriptors could make it possible to select genotypes with high productive potential in the short term. According to [23], complex plant characters such as yield are quantitatively inherited and influenced by genetic effects, as well as by genotype and environment interaction and selection may be difficult and time consuming to improve yield directly especially for perennial crops such as J. curcas. Therefore, identification and use of associations between characters are appropriate. In this study, statistically significant association of 100-seed weight existed with seed oil content. Therefore, seed weight can be considered as important character for early selection of seed sources. Furthermore, according to [11], the characters 100-seed weight and oil content showed the highest values of broad-sense heritability (83.23% and 99.93% respectively) and high expected genetic advance (22.63% and 19.62% respectively). The high broad-sense heritability values associated with high expected genetic advance indicate that effects of the genes are of additive type for these characters and the opportunities to improve seed weight character and oil content character through the selection.

Conclusion
This study highlighted an important agro-morphological diversity within the accessions of J. curcas from Burkina Faso. The variability observed within the accessions studied and the as- Tiendrébéogo et al., 2021 Variability and relationships between characters of physic nut (Jatropha curcas L.) sociations between the characters obtained constitute a database for the genetic improvement programs of J. curcas in Burkina Faso, in particular the development of high-yielding cultivars. A positive and significant correlation between the 100-seed weight trait and the oil content trait was noted. Growth traits such as plant height and crown diameter was also significantly and positively correlated with the oil content trait. Three morphotypes were identified using petiole base pigmentation which are the green morphotype with very high oil content and high 100seed weight, the purple morphotype with medium oil content and medium 100-seed weight and finally the brown morphotype with low oil content and low 100-seed weight. For perennial plant such as J. curcas, these results on relationships between characters could make it possible to select genotypes with high productive potential in the short term.