Genotypic characterization of cashew (Anacardium occidentale L.) clones using agro‐morphological traits

Abstract High cropping efficiency implies that high yields are obtained from reasonably sized trees. We studied the general and specific combining ability (GCA and SCA) of selected cashew clones of Brazilian (A), Beninese (BE), and Ghanaian (SG) background for cropping efficiency and nut weight in the early years of bearing. Using North Carolina II mating design, four clones were crossed as males to three best clones recommended for farmers. The 12 F1 progenies were evaluated in the field at Wenchi (2012–2018) for increase in trunk cross‐sectional area at the vegetative (TCSAv) and reproductive (TCSAr) stages, canopy spread in the east‐west (CSew) and north‐south (CSns) directions, nut yield (NY), nut weight (NW), and cropping efficiency (CE) using a randomized complete block design with three replications. Cropping efficiencies were in the range of 30.8–67.4 g/cm2/year while nut weight and nut yield varied from 5.9 to 10.5 g/year and 477.8 to 939.4 kg ha‐1 year‐1 in the fourth to sixth years after planting, respectively. The Beninese progenies outperformed the Brazilian progenies for cropping efficiency. GCA effects were more important than SCA effects. Narrow‐sense heritability ranged from 0.47 (CE) to 0.80 (NW). Canopy spread in the north‐south direction correlated (rg = 0.98; p ≤ .001) strongly with cropping efficiency at the genotypic level. Among males, BE203 showed positive GCA effects for cropping efficiency, TCSAv, and nut yield, whereas A2 and SG273 showed positive GCA effects for nut weight. Among females, SG287 showed negative GCA effects for TCSAr. Our study provides evidence that, cashew tree size and nut quality are under genetic control and the identified clones represent a suitable genetic resource pool to increase productivity.


| INTRODUC TI ON
Cashew (Anacardium occidentale L.) is a perennial tree crop and a member of the family Anacardiaceae. The center of origin is north-eastern Brazil but was introduced to West Africa by the early Portuguese settlers in the 16th century (Abdul & Peter, 2010;Mitchell & Mori, 1987). The crop is valued for their nuts and apples and has been a major foreign exchange earner for many developing countries in the tropical and subtropical regions of the world (Oliveira, 2008b). Monteiro et al. (2017) reported that about 45% of the world's cashew production comes from West Africa, with Ivory Coast, Ghana, and Nigeria being major producers. The potential of cashew in alleviating poverty and boosting rural development has been highly emphasized (Dendena & Corsi, 2014;Ingram et al., 2015;Wongnaa & Awunyo-Vitor, 2013). However, low nut yield and nut weight limit the productivity (Adu-Gyamfi, Dadzie et al., 2014). The situation is even made worse by the recent awarenes that nut weight is the major criterion that determines the market value of raw cashew nuts in the cashew global trade. Although the low productivity could be partly attributed to pest and diseases, the high global demand for cashew nut from the ever-increasing world population, coupled with farmers request for varieties that provide high early yield per unit area with "jumbo" nuts that can earn premium price suggest the development of new varieties with high genetic potential for higher nut yield per unit area and improved nut quality.
Cropping efficiency, which is defined as the ratio of the cumulative yield over a period of time to increase in trunk cross-sectional area over the same period (Daymond et al., 2002), is analogous to harvest efficiency in annual crops (Pang, 2006). High cropping efficiency in general term relates to a significant reduction in vegetative vigor and size during production while the adverse effects of high seedling vigor on crop management are minimized (Daymond et al., 2002;Padi et al., 2012). According to , it implies that high yields are obtained in reasonably sized trees allowing crop management practices such as pruning, harvesting, and pesticide application to be possible over the life cycle of the crop. In orchard crops, which includes cashew, a combination of small tree size and high yield efficiency indicates the potential for high nut yield per unit area (Larsen et al., 1992). The trait is being used by plant breeders to increase productivity through high-density planting.
Cashew has been grouped into common and dwarf cultivars based on genetic variability. The common types are widely cultivated with heights ranging from 5 to 8 m and canopy diameter reaching 20 m, whereas the dwarf types are less than 4 m in height, having a homogeneous canopy with diameters less than the common type (Oliveira, 2008a). These imply that the potential size of cashew trees is under genetic control and varieties with different tree sizes can be developed by breeding.
Over the years, a number of genetically diverse cashew germplasm accessions have been introduced from the center of origin to various cashew-growing countries in the West African subregion.
In Ghana, the first introductions were from Brazil, Benin, Nigeria, Mozambique, and Tanzania (Dadzie et al., 2014). The introductions from Brazil were dwarf types while those from Benin were common types with high tolerance to hostile environments (Adu-Gyamfi, Abu . Recently, the narrow genetic base of cultivated cashew clones particularly in Africa and Asia has been attributed to low cashew productivity (Aliyu, 2012;Archak et al., 2003). To counteract this low productivity, many tree crop improvement strategies have strongly emphasized the introgression of desirable exotic alleles into commercially cultivated local varieties which are adapted to local environmental conditions (Ofori et al., 2014;Padi, Ofori, & Akpertey, 2017). Therefore, the occurrence of the local common cashew clones recommended for farmers together with the exotic precocious Benin and Brazilian dwarf germplasm clones indicate a good opportunity to simultaneously reduce tree vigor/size, improve nut weight and subsequently improve cropping efficiency. However, it is crucial to expand on the limited information regarding the combining abilities of these clones of varied introduction background and the type of gene action governing the inheritance of key economic traits for the current cashew breeding program that focuses on progenies as varieties. Earlier studies on cashew have reported significant effects of both GCA and SCA on nut yield (Cavalcanti et al., 1997), kernel weight, and plant height , showing prospects in the improvement of the crop for cropping efficiency and other yield related traits.
In tree crop breeding, extended periods of time in addition to the large amount of resources and labor are required for effective evaluation. Therefore, knowledge of the relationship between traits and parental values in crosses is crucial, as it ensures that effective parental choices are made and genetic breeding progress can be predicted (Ofori et al., 2014a). Over the years, juvenile traits in many tree crop breeding programs have been used as indices for selecting high yielding genotypes in later years (Padi et al., 2012).
The objectives of the present study were to determine (i) the combining abilities of selected Brazilian, Beninese, and Ghanaian cashew clones for vigor, cropping efficiency, and nut weight and (ii) the genotypic and phenotypic correlations between growth and cropping-efficiency-related traits.

| Cashew parental clones, experimental design, and crop management
The plant material used in this experiment involved two exotic and five local parental clones. The exotic parental clones selected for the study were BE203 from Benin and A2 from Brazil while the local (Ghana) parental clones selected for this study were SG273, SG224 ,SG266, SG276, and SG287 (Table 1) Forest Transitional Agro-ecological zone. This zone is characterized by a mean annual rainfall of 1,300 mm with mean annual temperature range of 26.1-28.9°C (Lacombe et al., 2012;Owusu & Waylen, 2013). Soil samples were randomly collected from 16 different spots across the experimental site at a depth of 0-30 cm before establishment of the trial. The samples were bulked together and subsamples were taken to the laboratory for analysis.
The soils were found to be predominantly lithosols with acidic reaction (pH: 5.1) (Table S1). With the exception of soil organic carbon content, the nitrogen, phosphorus, magnesium, and potassium levels were higher than the recommended levels reported by Dedzoe et al. (2001). The total available phosphorus content was approximately 25% higher than recommended levels. Based on the soil chemical composition, the soils at the experimental site appeared to be fertile. Cashew seedlings were transplanted to the experimental site in June 2012 at a spacing of 10 m × 10 m (100 plants/ha) with 15 plants per plot in a randomized complete block design with three replicates. The standard practices for cashew production in Ghana were duly followed. This included the application of pesticides mainly cyperderm (active ingredient -cypermethrin) @ 150 ml/ha ) to control insect pest from August to October annually.

| Measurement of agronomic traits
In general terms, vegetative and reproductive traits were as- where d is the stem/trunk diameter.

| Statistical analysis
For the agronomic data obtained, a two-stage analysis based on plot-level values were used in analyses of variance (ANOVA) following tests for normality (based on the plot of residuals). In the first stage, all 12 F 1 progenies were analyzed to test for significant differences using the average trait values across years, with progenies considered a fixed factor and replicates as random factor, using the GenStat statistical software, version 12 (VSN International Ltd., Hemel Hempstead, UK). In the second stage, progenies were classified into four genotype groups viz, Benin, Brazil, Ghana 1, and Ghana 2 based on the origin of male parent germplasm. The combined means of the three progenies within each genotype TA B L E 1 Characteristics of cashew clones used as parents for the progenies in a 3 × 4 factorial mating design group for cropping efficiency and nut weight were determined. At this stage, genotype groups were considered as fixed factor and replicates as random factor. The general and specific combining ability (GCA and SCA) effect, narrow-sense heritability, additive variance, dominance variance, and environmental variance were estimated using restricted maximum likelihood (REML) methods in AGD-R (Analysis for Genetic Designs in R) software (Rodríguez et al. 2015).
The GCA and SCA effects were estimated using the following model: where Yijk is the observed value; µ is the population mean effect; b k is the block effect; m i is the male GCA effect (i = 1; 2; :::;m); f j is the female GCA effect (j = 1; 2; :::; f); mi * fj is the SCA effect, and eijk is the residual effect. The differences among means were tested by Duncan's Multiple Range Test (DMRT) at the 5% probability level. The genetic and phenotypic correlation coefficients between two traits, i and j, were estimated using METAR-R statistical package (Alvarado et al., 2015). Analysis of the genotype group effects was carried out for only cropping efficiency and nut weight which were the two key traits of interest in this study. The best and worst cashew progenies were concurrently selected based on the overall trait performance using a scale of 1-12 to score for each trait after which the total scores were ranked according to Ofori et al. (2014).

| Analyses of variance
The genotypic effect was highly significant (p ˂ .05) for all traits considered in this study. Partitioning of the progeny effect into male, female, and male × female interaction components indicated that the female effect (GCA effects for female) was not significant for any trait except increase in trunk cross-sectional area at the reproductive stage (TCSAr). Therefore, the contribution of the female effect to the overall variability observed for the other traits among progenies was generally negligible. However, the effect of GCA for males was highly significant for all traits studied except TCSAr and consequently contributed the most to the variation observed among the progenies. On the other hand, the SCA effect was also significant for all traits, with the exception of cropping efficiency and canopy spread in the east-west (CSew) and north-south (CSns) directions.
The ratio of the additive (GCA) to dominance (SCA) variance was also much higher than unity for most traits and indicated the predominance of GCA effects in controlling most traits. Genotype group effect was also significant for the nut weight and cropping efficiency. (1)

| Performance of F 1 progenies for agronomic traits
Among the 12 F 1 progenies evaluated, significant differences (p < .05) were observed for all traits (  Figures 2 and 3). However, the Beninese progenies were superior to the Brazilian progenies for cropping efficiency while for nut weight, the Brazilian progenies were superior to the Beninese progenies. F I G U R E 1 The Beninese cashew progeny SG287 × BE203 at two years after transplanting at Wenchi.

F I G U R E 2
The mean nut weight of four genotype groups estimated from the combined means of 3 progenies planted at 15 trees per plot in three replications. Bars over the mean indicate ± standard error. Significant at p ≤ .05. Means followed by the same letters are not significantly (p ≤ .05). different based on the adjusted p -value for multiple comparisons according to Duncan's multiple range test. Overall, based on nut weight and cropping efficiency performance (obtained using a scale of 1-12 to score clones for each trait after which the total sum of scores were ranked), SG287 × BE203, SG287 × SG273, SG287 × A2, SG266 × A2, SG266 × SG273, and SG266 × SG224 were the top six outstanding progenies (Table 2).

| Combining ability
Analysis of combining ability showed positive significant GCA indicate its suitability to reduce trunk girth at the reproductive stage, whereas the positive GCA effects showed by clone SG266 indicates its unsuitability as a parent for trunk girth reduction at the reproductive stage.

TA B L E 4
General combining ability for trunk cross-sectional area, canopy spread, cropping efficiency, nut yield, and nut weight of 12 cashew progenies derived from a factorial mating design of three females and four males at Wenchi (n = 15), Ghana

TA B L E 5
Genotypic and phenotypic correlation coefficients for trunk cross-sectional area, canopy spread, nut yield, cropping efficiency, and nut weight among 12 cashew progenies evaluated (n = 15) at Wenchi from 2012 to 2018

| D ISCUSS I ON
The productivity of cashew over the years has been constrained by low nut yield with smaller nut weights (Adu-Gyamfi, Rabany et al., 2015). This has been attributed to the narrow genetic base of commercially cultivated varieties (Archak et al., 2003) in addition to the change in production environment (long dry spells, erratic rainfall patterns, and extreme temperature) (Bello et al., 2017). In tree crop breeding, improving the cropping efficiency of existing varieties has been considered to be a viable strategy to improve productivity through high-density planting (Daymond et al., 2002;Padi, Ofori, & Akpertey, 2017). The emphasis of the present study was to identify clones with good combining ability for reduced tree size and high nut weight in the early years of bearing. Glendinning (1960) has emphasized that selecting for vigorous tree crop varieties at the vegetative stage may lead to high yielding trees that are too large to manage. This has been counteracted by the use of cropping efficiency which is an index that integrates yield with vegetative growth. Cropping efficiency has been found to be a better indicator of productivity than yield itself (Daymond et al., 2002). Many tree crop improvement programs have reported significant GCA and SCA effects on vigor, bean weight, and cropping efficiency (Daymond et al., 2002;Ofori, et al., 2014;. In the present study, significant GCA and SCA effects existed for most of the tested traits including vigor, cropping efficiency, and nut weight, which implied that both additive and non-additive allelic effects were important in controlling these traits as stated by Griffing (1956). However, the much greater than unity ratio of GCA:SCA variances suggest the predominant role of additive gene effects in controlling most traits. Our results are consistent with what has been found in cashew (Cavalcanti et al., 2000;Wunnachit et al., 1992) and other tree crops such as cocoa (Pang, 2006;Tan, 1990) and almonds (García et al., 1994).
Nevertheless, reports on GCA effects on cropping efficiency and nut weight in cashew are rare. The positive GCA effects of the male parent BE203 for cropping efficiency, nut yield, canopy spread, TCSAv and A2 together with SG273 for nut weight implied that they could possess favorable alleles and contribute to the improvement of these traits in an additive fashion, respectively. The significant positive GCA by the Brazilian clone A2 for nut weight in our study agrees with the reports of Aliyu and Awopetu (2011) who emphasized that the Brazilian Dwarf germplasm accessions were storehouses of genes for larger "jumbo" nut weights. Nevertheless, GCA for A2 and BE203 were negative for TCSAv and nut weight which suggest that they have limited potential as male parents in the improvement of these traits, respectively. Interestingly, for any given trait measured in this study, at least one male parent showed a significant GCA effect and this observation demonstrate the amenability of all the tested traits to genetic manipulation through conventional breeding.
But, surprisingly, none of the female parents showed significant GCA effect on any given trait except TCSAr where a significant negative GCA effect was observed with SG287, whereas a positive effect was observed for SG266. This suggests that while SG287 partitioned small amount of assimilates to reproductive growth, SG266 partitioned large amounts of assimilates to vegetative growth. Similar observations in cocoa (Theobroma cacao L.) has been reported (Padi, Ofori, & Akpertey, 2017). Enhancing this trait in the future would require a wide array of parental clones with good combining abilities.
The greater narrow-sense (˃45%) heritability estimates observed for all the tested traits implied that environmental influence was minimal and considerable selection responses can be expected.
In Brazil,  reported similarly high narrow-sense heritability estimates for nut weight (63.2%) but low heritability estimate for nut yield (21%). However, Masawe et al. (1999) reported low narrow-sense heritability range for nut yield (3%-40%) and trunk cross-sectional area (14.7%-38.9%) in Tanzania. In the present study, high heritabilities for nut yield (62%) and trunk cross-sectional area (55%) were observed. The observed differences in the magnitude of heritability estimates for these traits have been attributed to genetic constitutions of varieties tested, environmental conditions, and genotype × environment interactions effects (Crossa et al., 1990).
With the exception of the non-significant correlation between TCSA and cropping efficiency, the genetic correlations observed in our study are consistent with what has been found in other tree crops. Genetic correlations were reported between cropping efficiency and yield (Daymond et al., 2002;Padi et al., 2012) and between yield and canopy spread (Aliyu, 2006;Masawe et al., 1999).
The non-significant correlation between TCSA at the reproductive stage and cropping efficiency and the significant correlation between canopy spread in the north-south direction and cropping efficiency in this study indicates the importance of breeding for wider canopies rather than focusing on reduced TCSA at the reproductive stage to achieve high cropping efficiency. As the GCA for BE203 was also significant and positive for canopy spread (CSns) and TCSA at the reproductive stage, it would be an excellent resource to be deployed in future breeding programs to produce cashew varieties with high cropping efficiencies with better field establishment as high vigor (TCSAv) in tree crops is associated with high seedling survival rates especially under marginal field conditions (Ofori et al., 2015). This finding supports the report of Adu-Gyamfi, Abu  who indicated that the Benin clones could possibly possess alleles that ensures higher water and nutrient-use efficiency with better tolerance to high temperature stress. Nevertheless, in cashew breeding, varieties with compact canopies are preferred as it affords the opportunity to increase yield through increasing plant densities on per hectare basis (Aliyu & Awopetu, 2007). Therefore, further traits improvement would require a wide array of parental clones with good combining abilities.
On the other hand, the negative significant correlation between nut weight and nut yield and canopy spread indicates that the lower the nut weight, the higher the corresponding nut yield and the wider the canopies. Both positive and negative correlations between nut yield and nut weight has been reported in cashew (Aliyu, 2006;Sena et al., 1994). The conflicting reports on these associations have been attributed to differences in the advanced stage of the population utilized and the nature of the previous selection method employed in the previous yield improvement program (Paikra, 2016;Sundararaju et al., 2006). Madeni (2016) has therefore indicated that their use as a selection criterion in the breeding program under all ecologies has therefore demonstrated significant indirect effects on yield through the number of nuts per tree without any adverse compensation effects. The significant and positive GCA for A2 and SG273 for nut weight suggest that they could constitute a suitable genetic resource for future breeding programs that aim to develop cashew varieties with higher nut weights. To attract better prices for premium kernels, nut weights in the range of ~9-12 g have been recommended for farmers (Aliyu & Awopetu, 2011). The ideal variety in this study is therefore considered as one that combine high cropping efficiency with high nut weights. A comprehensive evaluation of progeny performance based on multiple traits has been found to be more stable and The potential of the identified progenies could further be validated under different high planting densities in multi-location tests under farmers' production conditions across the cashew production belts in the West African cashew producing countries to determine those best adapted to specific agro-ecological regions.

| CON CLUS ION
Our study demonstrated that the potential vigor of cashew tree is under genetic control and the Brazilian (A), Beninese (BE), and Ghanaian (SG) clones had different merits as potential parents for TCSAv, TCSAr, cropping efficiency, and nut weight. The comparatively higher nut yields in the range of 477.8-939.4 kg/ha and cropping efficiency varying from 30.8 to 67.4 g/cm 2 /year and nut weights from 5.9 to 10.5g underscores the importance of broadening the genetic base for production. The Ghanaian progenies (standard variety) were comparable to the Brazilian and Beninese progenies for nut weight and cropping efficiency, respectively. However, based on these two traits, six progenies, SG287 × BE203, SG287 × SG273, SG276 × SG224, SG266 × A2, SG266 × SG273, and SG276 × BE203, were outstanding, revealing the ability to combine targeted traits into a single background. Additive effects seemed to play a dominant role in the inheritance of most traits and breeding should therefore focus on cashew parents that are general combiners. Among the males, BE203 showed significant GCA effects for cropping efficiency, nut yield, TCSA at the vegetative stage, and canopy spread in the north-south direction, whereas A2 and SG273 demonstrated significant positive effects on nut weight. Among the females, SG266 and SG278 showed positive and negative GCA effect on TCSA at reproductive stage respectively. Cropping efficiency correlated strongly (r g = 0.98, p < .001) with canopy spread in the north-south direction at the genotypic level. The identified clones constitute a suitable genetic resource pool for improving cashew productivity.

ACK N OWLED G M ENTS
The authors thank Mr Gabriel Boahen and the field staff of the

CO N FLI C T O F I NTE R E S T
The authors declare that there is no conflict of interest.

AUTH O R S' CO NTR I B UTI O N
PKKAG and MB conceived the manuscript. PKKAG collected and analyzed the data, wrote the manuscript. AA, AO, and FP all assisted with data collection and provided significant editorial and analytical advice.

FU N D I N G I N FO R M ATI O N
The authors acknowledge the support of the Deutsche Gesellschaft