ABSTRACT
Shea trees identified for agronomical performances were compared through leaf and fruit characteristics. Seventy adult trees were selected using purposive sampling from seven sites along a south-north climatic gradient covering four agro-climatic zones. The effects of two factors (type of performance and site) on leaf and fruit parameters were investigated. Results showed significant effect of the type of the performance as shea trees performant for pulp production had longer petiole and laminar, wider laminar basis and top and had longer, wider and heavier fruits with more abundant pulp as compared to those performant for butter production. The factor site was found significant for all leaf and fruits parameters but the effect of climatic gradient was rare. The leaves in Siby and Kaniko had the longest laminar and the widest laminar basis, but those in Kaniko and Noumoudama had the widest laminar as compared to all other sites. The leaves in Noumoudama had the shortest petiole as compared to the rest. The leaves in Nampossela, Zanzoni, Solosso and Noumoudama had the widest laminar top as compared to those of Kaniko, Siby and Fougatiè. The longest fruits were observed at Nampossela, while the widest and heaviest fruits as well as the most abundant pulp were observed at Kaniko and the heaviest nuts were observed at Siby. The smallest and slightest fruits were observed at Noumoudama and Solosso in more arid zones, but often, not significantly different from some sites in wettest zones regarding certain fruit parameters. This study highlighted phenotypic descriptors of agronomical performance through the leaves and the fruits of this tree species.
Key words: Agronomical performance, climatic gradient, leaf and fruit characteristics, Mali, Vitellaria paradoxa.
Morphological traits of plants are usually influenced by genetic and environmental factors. The variation of local environmental conditions such as light, water, temperature, wind, etc., causes modifications in the development and the growth of plants. Hence, according to their environment, individuals of the same species may display different morphological traits as well as a similar response could be displayed by different plants species growing in the same environment. Anthropogenic activities could also influence plants’ morphological traits. For instance, human horticultural practices (crossing, selection, grafting, etc.) may affect directly the phenotypic traits of domesticated plants species like Vittelaria paradoxa (V. paradoxa).
V. paradoxa (Shea butter tree, karité in french), is a semi-domesticated forest tree species covering a wide geographical area in Mali. This economically valuable species faces environmental condition effects and human practices, which certainly, have an impact on its development, particularly on the morphological traits of organs like leaves, flowers, fruits and nuts. Thus, an important variation of trunk, leaves, fruits and nuts of this species and strong correlation between several morphological traits were observed in several countries like Mali, Burkina Faso, Cameroon, Ghana and also Nigeria (Enaberue et al., 2012).
Most of studies concerning shea trees species were focused on populations located in various agro ecological zones because of the economical and socio-cultural value of the species due to the several uses of shea butter nowadays becoming very important in the international market (Divine et al., 2014).
Another shea products becoming internationally important is shea pulp which is very nutritious and plays an important alimentary role for rural populations (Aguzue et al., 2013; Divine et al., 2014; Fernande et al., 2014). These two products could be considered as the most important agronomical products of shea tree.
Unfortunately, little is known about shea trees performant for pulp production. This is why the project “Projet d’Appui aux Filières Agricoles (PAFA)” aiming to improve shea parklands and shea products by identifying and promoting performant vegetal material, was interested in these shea trees and aimed to base investigation on local populations knowledge. It is important also to investigate if shea trees performant for pulp production were different from those performant for butter production, regarding leaf and fruit characteristics but also if this difference varies according to environmental conditions.
The characterisation of a forest tree species based on its agronomical performance is useful in the objective of genetic material improvement and in improved material dissemination. Also, taking into account local knowledge of rural populations (men and women), which are the main and direct users of the resource, is one of the first steps in identifying varieties or individuals of a plant species.
The study was conducted by investigating the variation of morphological traits of leaves and quantitative traits of fruits of individual trees performing well in the production of abundant fruits with high fat content and individual trees performing well in the production of fruits with abundant and succulent pulp. The objective of the study was to determine the variation of morphological traits of leaves and that of quantitative traits of fruits for these trees with the aim of finding marker variables for these two types of agronomical performance of shea trees, to contribute to facilitate identification and choice of plus trees for multiplication and promotion of performant vegetal material.
Study sites
The study was conducted in seven sites, selected along the south-north climatic gradient (Map 1) and belonging to four agro climatic zones (Table 1). Sites were identified in concert with local forest office technicians and local community authorities, based on the abundance of the resource in villages’ territories and the presence of known associations or groups of people collecting, processing and commercialising shea products.
Experimental procedures
Once the sites were identified, local populations were met to collect their knowledge of shea trees performant for butter and pulp production through a survey. In each site, three to four women and three men were designated to respond to the questions and the interview allowed each person interviewed to freely give his point of view. Persons interviewed were suggested on consensus by the villagers based on their experience and knowledge of addressed issue. After interviews, guides (men and women) were designated for the localization of shea trees performant for the two products. The choice of guide people was done on consensus by the villagers in the same way as the people chosen for the interview. The number of guides varied from two to six persons according to site.
Localised shea trees, identified as performant for butter and for pulp production, were marked with yellow paint in all the seven sites in October 2010 when fruiting period was over. GARMIN GPS 72 was used to determine the geographical position of each marked tree which bears also an identity code formed with initials of the site, the type of product (butter, pulp) and tree number. At each site, ten adult shea trees (five trees per type of performance) with diameter at breast height (dbh) varying from 30 to 118 cm giving a total of 70 trees (35 per type of performance) were identified and localised.
Data collection and statistical analysis
Fruits and leaves were harvested in July 2011 from all marked trees in all sites. For leaf characterisation, 50 leaves were sampled per tree and the following variables were measured: laminar and petiole length, laminar width, laminar basis and top width. For fruit quantitative traits, on the same trees (except those of Fougatiè because fruiting has ended when study started), 50 fresh ripe fruits were harvested. Fruits were weighed using electronic balance AND GR 202 and their length and width were measured using Vernier calliper SUNRISE. The pulps of collected fruits were removed on the same day and nuts washed with tap water. Fresh nuts were also weighed using the same balance and the pulp weight was calculated as a difference between the fresh fruit weight and fresh nut weight. Analysis of variance was used to determine the effect of studied factors in comparing LSD means at 5% significance level. The studied factors were the agronomical performance with two levels (shea trees performant for butter production, shea trees performant for pulp production) and site with seven levels as described above (Table 1). The statistical package SYSTAT9 for WINDOWS was used for analyses.
Effect of studied factors on leaf morphological traits
Table 2 shows statistics of leaf variables and the analysis of variance showed significant effects of studied factors. A part from laminar width, the factor type of performance was significant for all leaf variables (Table 2). Shea trees performant for pulp had longer leaf petiole and laminar, wider laminar basis and top as compared to the performant for butter.
The factor site was significant for all leaf variables (Table 2). The leaves in Siby and Kaniko (zone SS) had the longest laminar and the widest laminar basis, but those in Kaniko (zone SS) and Noumoudama (zone Sa) had the widest laminar as compared to all other sites. The leaves in Noumoudama (zone Sa) had the shortest petiole as compared to the rest. The leaves in Nampossela, Zanzoni (zone SS), Solosso (zone SN) and Noumoudama (zone Sa) had the widest laminar top as compared to those of Kaniko, Siby (zone SS) and Fougatiè (zone GN).
Variation of leaf morphological traits for shea trees performant for butter production
Table 3 shows the variation of leaf morphological traits for shea trees performant for butter production according to site. For all leaf variables of shea trees performant for butter production, sites were found very highly significantly different (Table 3).
The variation of leaf laminar length displayed climatic gradient effect as sites of Guinean zone (Fougatiè) and south Sudanian zone (Zanzoni, Kaniko and Siby) had the longest leaf laminar as compared to those of north Sudanian and Sahelian zones. More obvious climatic gradient effect was observed for petiole length and laminar top wide as the site of Sahelian zone (Noumoudama, the most arid site) showed the shortest petiole length and the widest laminar top. For leaf laminar wide, climatic gradient effect was less obvious as sites of south sudanian zone and sahelian zone were found not to be significantly different. The widest laminar basis was found at Kaniko (zone SS), but no climatic gradient effect was displayed for this variable.
Variation of leaf morphological traits for shea trees performant for pulp production
Table 4 shows the variation of leaf morphological traits for shea trees performant for pulp production according to site. Like shea trees performant for butter, sites were found to be very highly significantly different for all leaf variables of shea trees performant for pulp production.
For most variables (laminar length and wide, petiole length, laminar basis wide), results observed for shea trees performant for pulp production (Table 4) were similar to those observed for shea trees performant for butter (Table 3) regarding the trend of the difference between sites relative to agro climatic zones. However, contrary to Shea trees performant for butter, an effect of climatic gradient was not displayed for laminar top wide of shea trees performant for pulp.
From these results, it appears that the trend of the variation according to sites of the leaf morphological traits does not differ very much for the two types of agronomical performance of shea trees. The study showed globally that, independently to the type of performance, shea trees of sites located at the southern part of study area (zones GN and SS) had longer leaf laminar and longer petiole, while those located at the northern part (zone SN and Sa) had wider leaf laminar, wider laminar top and shorter petiole (Table 2). For all performances confounded (Table 2) and per type of performance (Tables 3 and 4), a sort of transitional zone (composed of few sites) was found between the extreme south (Fougatiè, Siby) and the extreme north (Noumoudama, Solosso) regarding some variables, particularly the petiole length.
Effect of studied factors on fruit quantitative traits
Table 5 shows statistics for fruit variables and the analysis of variance showed significant effects of studied factors. A part from nut weight, the type of performance was significant for all other fruit variables. Shea trees performant for pulp had longer, wider and heavier fruits with more abundant pulp as compared to those performant for butter.
The factor site was significant for all fruit quantitative traits. The longest fruits were observed at Nampossela (zone SS), while the widest and heaviest fruits as well as the most abundant pulp were observed at Kaniko (zone SS) and the heaviest nuts were observed at Siby (zone SS). The smallest and slightest fruits were observed at Noumoudama (zone Sa) and Solosso (zone SN) which are the more arid zones, but often, not significantly different from some sites of south Sudanian zone regarding certain fruit variables (Table 5).
Variation of fruit quantitative traits for shea trees performant for butter production
Table 6 shows the variation of fruit quantitative traits according to site for shea trees performant for butter production. For all fruits quantitative traits of shea trees performant for butter production, sites were found to be very highly significantly different (Table 6).
The longest and widest fruits were observed at Solosso (zone SN) and Nampossela (zone SS), while the heaviest fruits were observed at Zanzoni, Siby (zone SS) and Solosso (zone SN). The most abundant pulp was still observed at Zanzoni but the heaviest nuts were observed at Siby and Solosso. For this variable (nut weight), many homogenous groups were displayed (Table 6) yielding a non-obvious trend of difference between sites and it was also the only one for which the two types of performance (all sites confounded) were found not significantly different (Table 5). The variation of the difference between sites did not display any climatic gradient effect for all fruit quantitative traits. However, the site of Noumoudama (zone Sa) had the least of all fruit quantitative traits. In few cases, it was found not to be significantly different from certain sites (Table 6).
Variation of fruit quantitative traits for shea trees performant for pulp production
Table 7 shows the variation of fruit quantitative traits according to site for shea trees performant for pulp production. For all fruits quantitative traits of shea trees performant for pulp production, sites were found to be very highly significantly different (Table 7).
Results showed that fruits from Kaniko (zone SS) were longest, widest, heaviest and with most abundant pulp, while those from Siby in the same zone had the heaviest nut weight. Only for fruit length, the site of Kaniko formed the same group with the site of Nampossela (very close). For nut weight, Siby was not significantly different from Kaniko and Zanzoni in the same zone (Table 7). For shea trees performant for pulp production, like shea trees performant for butter production, the variation of the difference between sites did not display any climatic gradient effect for all fruit quantitative traits.
When comparing Tables 6 and 7, one can also observed that the trend of the difference between sites was different for the two types of performance and the trend displayed for shea trees performant for pulp production was similar to that displayed for all performance confounded (Table 5). This result suggested that, the trend displayed for all performance confounded regarding the variation of the difference between sites (Table 5), reflected the trend due to shea trees performant for pulp production which appears to be more marked than that due to shea trees performant for butter production.
Variation of leaf morphological traits
Shea trees identified for two agronomical performances (butter and pulp) were found significantly different according to leaf morphological traits. Results indicated that shea trees performant for pulp had longer leaf petiole and laminar, wider laminar basis and top. According to this result, leaf morphological traits could be used to distinguish shea trees of different agronomical performances. Kelly et al. (2011) reported that farmers (men and women) were able to recognise shea trees through leaf characteristics. According to farmers, shea trees performant for butter production have thin and smooth leaves while those performant for pulp production have broad and shiny leaves.
Sites also were found significantly different and various trends were observed regarding climatic gradient effect as two variables displayed evident climatic gradient effect, two others displayed less evident climatic gradient effect and one variable did not displayed any climatic gradient effect.
Regarding variables having evident climatic gradient effect (petiole length and laminar top wide), leaves of shea trees from more arid zones (Sa and SN) had shorter petiole and wider laminar top as compared to those of shea trees from more humid zones (SS and NG). It was observed that petiole length decrease progressively from the GN (Fougatiè) to the Sa (Noumoudama). This variable seems to be an interesting morphological descriptor allowing discriminating shea tree according to the climatic gradient. Shorter petiole observed at more arid zones could be an adaptation strategy. This result suggests an important role played by this organ (petiole) in shea plant physiology and in its adaption to local environmental conditions. It was also observed that leaves of shea trees from these more arid zones had wider laminar top. The result suggests that shea trees vary in the dimension of their organs, therefore resist and survive in their environment.
Similar results were observed by Nyarko et al. (2012) who found significant difference between shea trees from three zones (Sudan Savanna, Guinea Savanna and the transitional belt of the Northern Savanna and the Southern Forest) in northern Ghana and observed wider laminar and shorter petiole for more arid zone (Sudan Savanna zone). Nyarko et al. (2012) explained their results as possible effects of climatic conditions, soils characteristics and environmental mutation and stated that, probably, the short petiole of the leaves made them to be positioned at angles that allow maximum interception of sunlight for photosynthesis. The wider laminar top that we observed for arid zone may also play the role of intercepting maximum sunlight for better photosynthesis and better predisposition to other vital and agronomical performance functions like the role that Nyarko et al. (2012) reported for the short petiole.
Regarding variables that displayed less obvious climatic gradient effect (laminar length and width), leaves of shea trees from more humid zones had longer laminar, while those of shea trees from more arid zones had wider laminar. Shea leaves with longer laminar were observed for more humid zone by Nyarko et al. (2012), strengthening the present study results particularly in the case of shea trees performant for butter production. The broad leaves in more arid area could be explained by a greater need of shea trees in photosynthesis and/or in the regulation of transpiration/respiration mechanism in order to withstand the harsh climatic conditions. This result suggests that shea trees would, in view of the climatic conditions, developed an adaptation strategy controlled by the leaf surface which, through the mechanisms of photosynthesis and respiration/transpiration, would play an important role in the physiology of trees.
The manifestations of the climatic gradient effects suggest that climatic conditions only would not explain the variance for these variables according to the sites. Other factors (management practices, individuals age, genetic, etc.), would contribute to their variation. Djekota et al. (2014) observed variation within and between sites regarding the length and the width of the shea tree leaves in the region of Mandoul in Chad but did not identify the sources of this variation.
Leaf laminar basis width was the variable that did not display any climatic gradient effect. Like the variation of leaf laminar length and width, other factors including the genetic, could explain its variation according to the sites. Gwali et al. (2012) did not observe any climatic gradient effect in the study of morphological variation of shea tree in Uganda.
Variation of fruits quantitative traits
Shea trees performant for butter and pulp production were also found to be significantly different according to fruit quantitative traits. Results indicated that shea trees performant for pulp had longer, wider, heavier fruits with more abundant pulp. These results confirmed that, farmers in designating shea trees performant for pulp production had good appreciation of this agronomical performance of shea trees. It is important to note that the size of shea fruit is the apparent characteristic and the major criteria used by farmers in selecting trees. Studies discriminating shea trees based on their agronomical performance are rare. This study reveals, however, that several fruit quantitative traits differentiate shea trees performant for pulp production from those performant for butter production. Hence, the length, width and weight of the fruits of shea trees are good indicators and descriptors of shea tree performant for pulp.
Sites were also found to be significantly different for all fruit variables (Table 5). The variation of the length and width of the fruit according to the site did not display climatic gradient effect. Other factors that influence the variation of these traits include management practices and/or the size and age of the trees. Highest fruit sizes were found in sites from the old cotton belt, because of the intensification of cotton culture, there is a problem of land which resulted in a long use of the fields and a reduction, or even the disappearance of fallow. Shea trees therefore are beneficial to cultivation and fertilization activities. Elias (2013) reported shea trees benefit from farmers’ management practices of agroforestry parklands.
Observed fruit weights in this study were higher than those observed by Nyarko et al. (2012) and the variability in this study was also higher than that observed by Gwali et al. (2012) who did not observe an effect of climatic gradient in the variation of the weight of shea fruits, but noted a strong positive correlation between the weight of the fruit and the pulp (r = 0.963). This correlation would explain the similar variations of these two variables observed. Fresh nut weights were also higher than those observed by Nyarko et al. (2012). Like other variables, the weight of the nuts did not display a climatic gradient effect.
The current study did not clearly reveal climatic gradient effect on shea fruit quantitative traits like the results reported by Nyarko et al. (2012) according to which the fruits of the Sudanian savanna zone (dryer), were longer, wider and heavier than those of wetter zones. According to Nyarko et al. (2012), variation among sites could also be explained by farmers’ selection criteria, based mainly on morphological traits of fruits and influenced by several factors including social, cultural, economic, biotic and abiotic. They are also referred to environmental mutations as a potential cause of the difference between sites, and also the edaphic conditions that might explain the lower performance of the wettest sites as compared to dryer sites. Ugese et al. (2010) did not observe a climatic gradient effect in comparing nine sites covering three agro ecological zones (Sudanian savanna, Northern Guinea savanna and southern Guinea savanna) in Nigeria, despite the considerable diversity of shea fruit and nut morphological traits. These authors have noted a strong influence of some environmental variables on phenotypic traits of the shea tree fruits. The effect of environment would explain the result observed for Noumoudama (zone Sa) at the extreme north of shea area in Mali. Fruits from this site had the least of all fruit parameters and it is believed that this is an adaptation strategy of the species in the environment.
This study, which is the first regarding description of shea trees according to agronomical performance, shows approaches for shea trees characterization according to shea agronomical performance, highlighting some phenotypic descriptors of agronomical performance through the leaves, but also and especially through the fruits.
The effect of the type of performance varied according to the studied organs and variables. This factor was significant for most of the leaf variables and for all fruit variables. Thus, leaf morphological traits allowed distinguishing shea trees performant for different products and so are agronomical performance descriptors of shea at least for the two types of performance considered by this study. Fruit quantitative traits were found as excellent descriptors by distinguishing clearly shea trees for pulp production (fruits of larger dimensions and a more abundant pulp) from those for butter production.
Site effect was significant for all leaf and fruit variables. Climatic gradient effect was only displayed for few leaf variables, suggesting a possible effect of other factors (management practices, environment, genetics, biology, etc.) which in addition to the climate, would influence leaf and fruit characteristics, regardless of the agronomical performance of shea trees.
This study highlighted the importance of local knowledge in identification and selection of plus trees since all indicated trees were found performant for the criterion of selection. Hence, any improvement and multiplication program must take into account, this aspect for a greater chance of success. The study also confirmed results of several previous studies on the conjunction of several factors which would be the basis for the variability observed in shea leaf and fruit morphological traits.
The authors have not declared any conflict of interests.
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