Multivariate analyses were carried out on fifty-nine germplasm accessions of cashew derived from both local and exotic populations established at the research plots of Cocoa Research Institute of Nigeria (CRIN), Ibadan, southwestern Nigeria, to assess the extent of variability and pattern of genetic diversity among these cashew populations. Data collected on 36 quantitative and 33 qualitative plant characters were subjected to taximetric tools of Euclidean distance of complete linkage (furthest neighbour) and principal component analysis (PCA). The multivariate analyses tentatively grouped the selections into four distinct morphogenetically diverse clusters. The groupings appear to be a function of origin, eco-geographical distribution, genetic and/or agronomic affinity of the selections. Brazilian populations distinctly clustered together in two major groups while local clones and Indian selections dominated the other two major clusters with each group having its unique fruiting and tree growth habits. The clustering pattern at sub-cluster levels clearly reflects affinity of each genetic population. The principal component analysis and the potency indices showed that fruit characters are the most discriminating parameters for delineating cashew at the varietal level.

The problem of declining tree yield has led to an investigation into the effectiveness of foliar application of exogenous hormones to improve flowering, fruit set, and fruit retention in cashew. Five exogenous hormones, one Gibberellic Acid (GA3) and four Auxins (IAA, IBA, NAA, and 2,4-D) at seven different rates of application (0 mg L -1 , 10 mg L -1 , 25 mg L -1 , 50 mg L -1 , 100 mg L -1 , 250 mg L -1 , and 500 mg L -1 ) were tested on six yield-related components of the two Brazilian cashew genotypes. This trial was a factorial split-split-plot design with each treatment replicated five times within a tree and three replications (three trees) per genotype. Responses varied significantly between exogenous hormones, concentrations and genotypes. The cashew plants used showed hormone-specific and optimum concentration response patterns. Of the five exogenous hormones tested, GA3 was most effective as its application at 50 -100 mg L -1 gave five-fold improvements in flowering (precocity and number of hermaphrodite flowers) and fruiting, and about 69% increase in fruit retention ability and 25% in nut size. Panicles treated with GA3 also produced relatively bigger nuts compared to the untreated. Days to flowering was found to be hormone sensitive, while production of hermaphrodite flowers, fruit set, and nut development tended to be concentration specific. The GA3 exhibited a broad concentration tolerance among the five exogenous hormones investigated. Our data showed that using GA3 at 50 mg L -1 will enhance flowering precocity, shorten flowering duration, increase production of hermaphrodite flowers and fruit set significantly, and resultant nuts develop optimally with high percentage retention. Thus, it suggests cashew yield could be increased by exogenous foliar application of GA3 at 50 -100 mg L -1 at preblooming stage.

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Eleven yield-related plant traits in 33 accessions selected from 3 Nigerian breeding populations were studied to understand the influence of nut size-number trade-off in cashew tree yield. Three-years of data were statistically analyzed for genetic variability estimates and showed that the accessions were significantly different for all the yield-related traits with highest variability in production of hermaphrodite flowers. The pattern of variation was population-specific with sparsely fruiting Brazilian accessions characterized by large nuts and kernels, while the Indian accessions were prolific but with small-medium nuts. Flowering, fruit-set, and fruit-development differed significantly between years suggesting a strong influence of environment on cashew tree production. Consistent relationships were found between nut number and total nut yield at both inter-and intra-population levels, underscoring the importance of the number to nut yield in cashew. However, regression between nut number and nut size confirmed the existence of the size-number trade-off phenomenon in cashew, but the degree varied between populations. Cashew trees with jumbo nuts (>15 kg) were significantly affected by the trade-off, as they exhibited poor flowering and fruit set, in addition to prolonged fruit development. This category of nut size should be discouraged for commercial planting. These preliminary results showed that 342 Nut Size-Number Trade-Off and Cashew Nut Yield 343 cashew cultivars with nut sizes between 9.0 and 12.0 g are better, as such trees produced enough nuts per tree to support profitable investments in cashew farming.