Introduction

Cashew (Anacardium occidentale L.) is an evergreen tropical tree crop cultivated in an area of about 9.45 lakh ha in India as against 3.39 mill ha in the world. India is the largest producer (7.28 lakh tonnes) of cashew over an area of 9.82 lakh hectares with a productivity of 772 kg/ha (Saroj et al., 2014). It earns an all time record foreign exchange ($919 million) during 2014-15 from cashew products (cashew kernel-$910 million and shell oil-$9 million) (http://www.thehindu.com/news/national/kerala/cashew-exports-reach-alltime high/article7828878.ece). Other major producers are Tanzania, Mozambique and Kenya. In India, Moharashtra, Andhra Pradesh, Karnataka and Odisha are the major cashew growing states. Cashew breeding programme is constrained by need of large experimental plots, long juvenile period, genetic vulnerability to dreadful diseases and pests due to narrow genetic base. Cashew was introduced to the West coast of India by Portuguese less than 500 years ago (Rao et al., 1998) and large variation in cashew genotypes exist in the coastal region of India (Rao and Swamy, 1994). It generates random variation through open pollination and maintains high level of heterozygosity in the natural population owing to its clonal propagation. Many workers (Cavalcanti, 2000; Singh et al., 2008; Dasmohapatra et al., 2012b; Sethi 2015; Sethi et al., 2015; Sethi et al., 2016a; Sethi et al., 2016b) attempted to exploit such genetic variations and/or heterosis through development of cashew hybrids for genetic improvement in cashew. However, production of cashew in India continues to be low due to non-availability of elite clones (Ferriera-Silva et al., 2009) and suitable management practices. Broadening the genetic base of existing germplasms by hybridization and systematic exploitation of heterosis could pave the way for overcoming the problem of low productivity (Masawe, 1994). Therefore, an ambitious cashew hybridization programme was taken up to develop and identify suitable heterotic hybrids with high nut yield potential. In this pursuit, the authors report the genetic variability among hybrid clones in nut yield and ancillary agro-economic traits as compared to their parents and standard checks.

Materials and Methods

Under cashew hybridization programme, sixty genotypically and phenotypically different hybrids comprising ten cross combinations e.g., Cross A–RP-1X Kalyanpur Bold Nut, Cross B-RP-1xVTH-711/4, Cross C–RP-2xKankadi, Cross D–M-44/3xVTH 711/4, Cross E–RP-1xKankadi, Cross F–RP-2xVTH711/4, Cross G-RP-2xKalyanpur Bold Nut, Cross H-M-44/3xKalyanpur Bold Nut, Cross I-Vittol- 44/3xVTH 711/4 and Cross J-BPP-30/1xKalyanpur Bold Nut were developed in the year 2001 using eight parents (having desirable traits such as bold nut type, profuse flowering, cluster bearing, high shelling percentage (>28%) and nut yield>2ton/ha). These cashew hybrids are designated in terms of alphabetical letters followed by numerical numbers to refer cross combination and hybrid clone number. The nuts of different crosses were collected at full maturity and seedlings were raised in the nursery as per standard package of practices. After attaining desired growth, the seedlings were planted in the main field with a spacing of 4mx4 m in the year 2002. The experimental materials included 71 cashew nut test genotypes comprising above 60 experimental hybrids, eight parents and three standard checks (BH6, BPP8 and BH85) laid out in an augmented design with three blocks (to accommodate 20 hybrids and all parents and checks in each block) for evaluation and selection of promising hybrid(s) over two years (2011 and 2012). Observations on vegetative, yield and yield attributing traits were recorded wherever applicable, as per the standard descriptor of cashew (Swamy et al., 1998) and pooled over two years. The data were subjected to statistical method of analysis of variation following Panse and Sukhatme (1985) and coefficient of variation as per (Burton,1952).

Results and Discussion

The knowledge of genetic variability is imperative for efficient sampling and utilization of genetic resources. With a quest to identify genetic worth and breeding potential of a set of cashew hybrids as compared to their parents and three standard checks, their overall mean values along with range, phenotypic variance and co-efficient of variance for nut yield and its thirteen component traits (Table 1). In the present investigation, major emphasis was laid on identification of superior high yielding genotypes. Many often direct selection based on per se grain yield led to missing of valuable breeding materials which otherwise have potential genotypic worth for some specific traits. The genotypes which represent the favourable extreme boundary of the range variation may occur at very little frequency, but these would provide the necessary base for the desired direction of selection. Some of the genotypes which have merit in relation to specific traits (Table 2).

Coefficient of variation (CV) is a measure of variability per unit mean value of a character. Therefore, the more a character is having wider range the more would be its CV value. The coefficient of variation with respect to different characters ranged from 8.10% in case of canopy spread (E-W) to as high as 50.83% in case of number of perfect flowers. Among the characters studied, number of perfect flowers, sex ratio, apple weight, nuts/panicle and nut yield (ton/ha) revealed considerable genetic variation with CV values more than 25%. In the present investigation, mean nut yield/ha also revealed a spectacular wide array of variation ranging from 1.59 to as high as 4.34 ton/ha.

Wide range of variability was observed in all traits with maximum being in number of perfect flowers (15.41-233.43) followed by sex ratio(0.063-0.563), apple weight(30.69-149.78) and nuts/panicle(2.22-8.02) as also revealed from coefficient of variability i.e., 50.83%, 42.48%, 30.79% and 26.02% respectively. Dadzie et al. (2014) reported significant differences in the performance of the clones in all morpho-economic traits. Harrish et al. (1994) recorded wide variation in height, girth, East-West and North-South spread of plant. In vogue, clones having wider sex ratio are high yielder in Cashew (Dorajeerao et al., 2001). Under Bhubaneswar condition, sex ratio varies from 0.093% to 1.038% in various cashew types (Sen et al., 1995) and it varied from 0.07(Ullal 2)0.73(Vengurla 3) to under transitional tract of Karnataka, India(Hegde et al., 2000). Devi(1981) recorded maximum variability for percentage hermaphrodite flower, nut weight, shelling % and mean nut yield. Aliyu and Awopetu(2011) also reported highest variability in production of hermaphrodite flowers among yield related traits. In contrast, Dasmohapatra et al. (2012a) observed maximum variability of sex ratio followed by nut yield (kg/tree) and nuts/panicle. However, nut weight and flowering shoots/m_² of canopy showed moderate genotypic and phenotypic co-efficient of variability. Aravindakshan et al. (1986), however, observed significant difference in apple weight with the highest value in H3/13 (132.67 g) and lowest in K28-2 (31.33 g).

On an average, the cashew genotypes flowered at five years after planting (from seeds) and grown to a height of 4.11 m averaged over observations recorded in March, 2011 and 2012(at the time of harvest). The height of plants among 71 test genotypes ranged from 2.77 m in M44/3 to a maximum height of 4.97 m in cashew nut hybrids B 5 and I-16(data not shown). In the present investigation, flowering duration ranged from 78days in case of check variety BH 6(Jagannath) to a maximum of 126 days in cashew hybrids e.g., A71 and H6. However, Sen et al. (1995) observed that duration of flowering ranging from 53.26 days to 90 days in different cashew types. Cashew is polygamo-monoecious plant which bears inflorescence having male and hermaphrodite flowers. Flowering season seems to vary depending upon genotypic response to location and it varied from December to March under Bapatla(A.P, India) condition (Dorajeerao et al., 2002). Higher percentage of perfect flower and synchronized flowering for shorter period are desirable characters (Sriharibabu, 1981; Chattopadhyay and Ghosh, 1993). In the present investigation, the cashew hybrids e.g., F28 and F38 bore more than 200 perfect flowers, while short synchronized flowering was observed in BH6(78days) followed by D10 (80days), Kalyanpur Bold Nut (82days) and C30(83days) which came to initial flowering in first week of December, first week of February, second week of January and first week of February respectively.

Some of the cashew genotypes e.g., J13, J14, RP1, BPP30/1, BPP8 and BH6 flowered as early as first week of December, while most of the cashew test genotypes come to flowering during January and are categorized as medium flowering types. A few moderately late flowering types e.g., D10, C30 and D9 revealed delayed initial flowering (first week of February), but such genotypes had shown short synchronized flowering periods e.g., 80, 83 and 87 days respectively.

Number of flowering laterals/m_², number of panicles/flowering lateral and panicle characteristics e.g., length and breadth of panicle and percentage of perfect flowers/panicle have direct bearing on apple and nut yield. A71, A48, A62, A99, B27 and D19 had shown more than 20 flowering laterals/m_². Among these, B27 exhibited significantly longer and broader panicle as compared to the best standard check (BPP8) while D19 revealed significantly broader panicle. In this context, Dorajeerao et al. (2002) observed maximum 21.25 flowering laterals/m_² under Bapatala (A.P., India) condition while Samal et al. (2002) observed the flowering laterals of 11.62 to 24.88 in different cashew types under Bhubaneswar (Odisha) condition.

Tree size is compared in terms of trunk girth and canopy spread. C41, E3 and E16 exhibited significantly higher trunk girth over the best standard check variety BPP 8. Canopy spread in North-South(N-S) direction is a desirable trait as the genotypes having more of N-S canopy spread receive sunshine for major part of the day compared to canopy growth in East-West direction. A71, B27, C41, D10, G8 and G9, H6, H8 and I12 had revealed significantly large canopy spread in North-South direction. While, none of the test genotypes could surpass the best standard check variety BPP 8 for canopy spread in East-West direction. Falade (1981) studied varietal differences in tree size and yield of cashew under Nigeria condition. The variations in the size of the tree were found to be comparatively narrower than that in the yield.

Varieties greatly differ in their yield potential with regard to year of harvest. Nut yield was assessed over two years. Overall productivity (nut yield) of the present set of cashew genotypes was 2.77 kg/plant/plant (1.71 ton/ha) which resulted from an average 4.75 nuts/panicle with nut weight of 7.83 gm and kernel weight of 2.39 gm. Nuts/panicle is reported to be a primary component trait for improving nut yield (Lenka et al., 2001; Aliyu, 2006). Highest number of nuts/panicle (8.02) was recorded in a cashew hybrid G- 8 which was one of the top five high yielding cashew hybrids. Among 71 test genotypes, all the top yielders were the hybrids which had shown significantly higher productivity (>3.63 kg/plant and 2.25 ton/ha) as compared to the best standard check variety “BPP8” with nut yield of 3.16 kg/plant and 1.97 ton/ha. Cashew nut hybrids e.g., D19, H 6, B27, A71 and G8 (Figure 2) recorded nut yield ≥2.50 tons/ha and among these D19 recorded highest productivity (4.34 g/plant and 2.71 ton/ha). Damodaran (1979) studied variability in the F1 population of cashew and observed considerable variations for mean nut yield, weight of 100 nuts, shelling %, mean weight of apple, size, colour and shape of apple. It was also noted that a large number of productive hybrid progenies were derived from the crosses in which one of the parent was an exotic type.

A wide array of genetic variation was observed for apple and nut characteristics among parents and promising hybrids (Figure 1 and Figure 2). The parent varieties e.g., VTH 711/4, Kankady and Kalyanpur Bold Nut bore very bold nuts. Among hybrids, D19 was shown to have very large size nuts and also recorded highest nut yield (4.34 kg/plant, 2.71 ton/ha). Besides, C 30, D 47, E 16 and F28 also had shown bold nuts with significantly higher nut weight (>9.0 g). Ghatge et al. (2009) evaluated thirty F₁ hybrids of cashew for the variability in cashew apples and cashew nuts. They reported very bold nuts (9.93 g) with highest shelling percentage (32.65%) in H815. Brazilian accessions characterized by sparsely fruiting with large nuts and kernels, while the Indian accessions were prolific but with small–medium nuts (Aliyu and Awopetu, 2011). In general, cashew trees with jumbo nuts (>15 kg) were significantly affected by the trade-off, as they exhibited poor flowering and fruit set, decrease in nut number/panicle in addition to prolonged fruit development. Therefore, cashew nut varieties with very bold nuts are usually discouraged for commercial planting. In this context, cashew cultivars with nut sizes between 9.0 and 12.0 g are reported to be better; as such trees produced enough nut/ tree to support profitable investments in cashew farming (Aliyu and Awopetu, 2011).

Apple characters (physico-chemical traits) are considered as important cashew descriptors for DUS testing. Considerable variation was observed in mean apple weight, size, shape and colour of apples in the present set of F₁ progenies of cashew. A typical ripe cashew apple represents 90% of cashew fruit and weight between 70 and 90 g (Helosia and Ricardo, 2001). Moura et al. (2001) observed wide variations in nine clones of dwarf cashew in Brazil with respect to firmness, size, shape, color, and fruit weight. Kannan and Thirumaran (2001) observed similar results when studied four varieties of cashew apple. Ghatge et al. (2009) evaluated thirty F₁ hybrids of cashew for the variability in cashew apples and cashew nuts. In the present investigation, the cashew nut parent VTH711/4 had highest apple weight (149.78 g) followed by Kankady (121.99 g); and both had high apple: nut ratio. Besides, I-3 and I-20 also had shown high apple: nut ratio (>10.0). Ghatge et al. (2009) identified large size apple (72.72 g) in H1141 and maximum apple to nut ratio (7.82) in H824.

Appealing yellow apple colour with high TSS fetch consumers’ preference. The test genotypes e.g., D19, H6, A71, C41, C30, F28 and H8 are characterized as high yielding with yellow Apple. Red apple colour was shown to be specific to the parents e.g., RP1 and Vittol 44/3; and the cashew hybrid B27. While, yellow apple with reddish tinges was associated with high yielding cashew hybrids G8 and J12. Among the test genotypes, C30 recorded highest TSS (Brix value 17.45) of cashew apple followed by G16(Brix value 16.70), B 5(Brix value 16.21) and A62 (Brix value 16.04); while F38 revealed lowest TSS (Brix value 9.93). In this context, Sivagurunathan (2010) reported varied TSS value (15-19oB) in apples of different cashew varieties with maximum in Vridhachalam-2.

Among the test genotypes, the popular check variety BH85 and the parent variety M44/3 were shown to have compact canopy. Besides, some of the promising high yielding cashew hybrids e.g., C30, F28 and G8 had characteristic compact canopy. Such compact canopy plant type may serve for restructuring of plant ideotype of popular high yielding cashew nut genotypes. Out of 71 test genotypes, the cashew hybrid A71 had canopy spread (N-S) more than 5.0m. Three cashew hybrids e.g., C-41, E-3, E-16 and a parent variety VTH-711/4 had shown trunk girth more than 70cm. Six cashew genotypes e.g., A 48, A 62, A71, A99, B27 and D19 excelled in number of flowering laterals/m2 (>20) and find their place in the extreme class interval in positive direction.  Total number of perfect flowers and sex ratio determine the nut yield in a cashew genotype. Five genotypes of cashew (F28, F38, F27, F32 and D47) have been identified to have more than 150 perfect flowers while, a single genotype F28 recorded sex ratio more than 0.45. Similarly, three promising high yielding cashew hybrids e.g., G8, G9 and A71; and three popular cashew parental genotypes e.g., RP1, RP2 and M44/3 having more than 6.5 nuts/panicle have been identified. However, the top fourteen cashew test genotypes e.g., A-48, A-62, A-71, B27, C30, C41, D19, G8, H6, H8, H20, J6, J12 and J20 were shown to have nut yield more than 3.5kg/plant and all of these promising high yielding genotypes happened to be the hybrids. All these hybrids except A 62 and J 6 had shown nut yield significantly higher than the best standard check variety (BPP8).

In the present study, B27 could be sorted out to have many desirable agro-economic traits e.g., plant height, trunk girth, canopy spread(N-S), number of flowering laterals/m_², proportion of perfect flowers, sex ratio, nut weight, kernel weight, shelling percentage, apple weight, panicle length, panicle breadth, nuts/panicle and nut yield. On similar consideration, Cashew hybrids e.g., A71, C41, H6, H8 and D19 could be selected for further breeding programme to improve specific agro-economic traits. H-6 had more canopy spread in North-South direction. This was also promising for trunk girth, total number of flowers and nut yield (kg/plant and tons/ha). In contrast J12 had shown more canopy spread in East-West direction along with higher mean performance for other desirable agro-economic traits e.g., trunk girth, number of perfect flowers, percentage of perfect flowers and nut yield (kg/plant and tons/ha). Panicle length and breadth determines total number of flowers. The cashew hybrids e.g., B6, B27 and C44 had shown significantly broad and longer panicles compared to the best standard check variety (BPP8) for the trait.

Thus, in the present set of materials, most of the cashew hybrids revealed one or the other desirable yield attribute(s). But, few cashew hybrids e.g., D19, H6, B27, A71 and G8 had shown their promising status for most of the important nut yield component traits. These elite hybrids may be utilized as donors in future breeding programme.

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