With significant tree-to-tree variation evident in economically important traits including kernel mass and kernel recovery, Canarium indium is a suitable candidate tree species for improvement through domestication. This result is particularly important given it is the focus of commercialisation efforts to benefit stakeholders along its value chain (Carias et al. in press). Improvement must be based on individual tree selection rather than provenance-based domestication, because little of the variation was attributed to sampling location. There were significant linear correlations among fruit, nut-in-shell (NIS) and kernel traits, particularly between kernel mass and fruit mass and between kernel mass and NIS mass. This means that when screening for trees with large kernel mass, they can first be selected on fruit and NIS mass before undertaking further detailed assessments. Trees were found to primarily have single-kernel fruits. Double kernel fruits were relatively common and triple kernel fruits were rare. Twin kernels occurred in both single and double kernel fruits. More than half of the trees that were sampled over two fruiting seasons varied significantly between years in mean kernel mass (63.6%), and one quarter (25.9%) of trees varied significantly in kernel number. However, trees that produced larger kernels in the first year of sampling, tended to maintain their rank among the trees and produced relatively larger kernels in the second year of sampling. The results from this study have important implications for further domestication of the species to make selections that are desirable for planting in commercial agroforestry systems. Small-holder farmers participating in the canarium nut extractive economy will also benefit when superior germplasm is disseminated for new plantings.
Characterisation of fruits, nuts and kernels
This study confirmed that tree-to-tree differences in fruit, nut-in-shell (NIS) and kernel dimensions and mass as well as kernel recovery (kernel:NIS) were highly significant. Little of this variation was attributed to the subpopulation level. This suggests that there has been no effective isolation between subpopulations due to anthropogenic seed movement between islands, and human selection has not occurred more strongly in any given location. This is consistent with studies of canarium trees in other locations in Vanuatu (Evans 1999b), Papua New Guinea (Nevenimo et al. 2007; Leaky et al. 2008) and Solomon Islands. Randall et al. (2016) found that variation in economic traits for canarium nut production is high and continuous, and significant tree-to-tree variations exist with no significant differences between populations.
The large variation in the kernel morphology of C. indicum provides the opportunity to select and breed trees with desirable traits for commercial production. Higher market prices for edible nuts can be achieved for larger and less damaged kernel with a kernel recovery above 0.25 (1:3 Kernel:NIS) (Adeigbe et al. 2016; Leakey et al. 2008). Kernel mass is an important trait to the industry because larger kernels generally require fewer relative inputs from both producer and processor. We found that trees with the largest mean single kernels were almost four times greater in mass than the smallest single kernels (4.63 vs 1.28 g), which is very similar to previous studies (1–4 g) (Evans 1999b; Leakey et al. 2008; Randall et al. 2016). Almost 20% of the trees met the threshold (> 3 g kernel mass) considered for improvement in kernel quality and uniformity (Leaky et al. 2008).
Significant differences in fruit mass occurred in approximately 40% of the trees measured between years. Variation between years in kernel traits such as mass, kernel recovery, and kernel number is not uncommon and has been reported in other nut crops such as hazelnuts (Bostan and Günay 2009; Cristofori et al. 2008). Importantly in our study, those trees that produced larger kernels in the first year of sampling, tended to maintain their rank amongst the trees and again produced larger kernels when measured in a subsequent season. This gives confidence that the trees selected in any given season will continue to produce larger kernels compared to other trees in the population across other seasons.
Kernel recovery is of interest for processors seeking to maximise return for effort and is an important selection criterion for other commercialised nuts (Arzani et al. 2008). Just over half (52.6%) of the trees sampled in our study had a kernel recovery of at least 0.25. This means that tree selections based on this attribute can increase efficiency in terms of transport mass per unit of kernel recovered. This is particularly important for canarium since shell mass comprises over one quarter of the total fruit mass.
Correlations of fruit components
Correlating kernel mass to other traits, such as fruit and NIS mass, can help to determine kernel mass by grading fruit. Positive correlation was found between kernel-in-testa (KIT) fresh mass and both fruit mass and NIS mass (r = 0.752 and 0.733 respectively). Research elsewhere suggests that NIS circumference is a moderately good predictor of KIT mass (Evans, 1991). Our results conform with Evans (1999b) who found significant correlation between NIS dry mass and KIT dry mass. This means that high mass fruits and NIS typically also produce higher mass kernels. This can help in grading NIS to accurately cost the purchasing of kernels without the need to crack the NIS (Evans 1996).
These results are in contrast with a study of canarium nut characteristics in young (6 year) plantation trees in Solomon Islands, which found that trees with the heaviest average NIS mass did not produce the heaviest kernel (Randall et al. 2016). It is possible tree age may influence the relationship between NIS and kernel mass, therefore selection for kernel traits may need to occur in only mature trees. Future canarium fruit collections that aim to improve overall kernel mass should initially target trees with high fruit mass to select potential candidates for NIS and kernel trait assessment. Caution must be applied if using fruit characteristic predictors of kernel mass as it is unknown whether the NIS contains a single or double kernel.
Kernel Number
Canarium nuts most commonly contain one kernel with the other two locules remaining undeveloped (single-kernel fruit). However, many fruits contain two kernels and one undeveloped locule (double-kernel fruit), or rarely, fruits contain three kernels (triple-kernel fruit). A combination of single- and double-kernel fruits occurred in approximately 75% of trees sampled and the remaining proportion contained exclusively single kernel fruits. The proportion of single- to double-kernel varied between trees and some trees have a greater frequency of double-kernel fruits than others. Very little is known about the pollination biology of the species and whether kernel number is maternally influenced or is related to pollination levels. However, trees that produced a high proportion of double-kernel fruits in the first year of sampling tended to maintain their rank and produce an equivalent proportion of double-kernel fruits in the second year of sampling. This infers that the factors influencing this trait stayed relatively constant between years. This could be a combination of maternal genotype, resource allocation and/ or pollination success. Further investigation is required to determine what controls this important trait and its heritability.
The mass, length, and width of the kernels was found to be significantly lower in individual kernels from double-kernel fruits compared with single-kernel fruits, but the combined kernel mass of the double kernels was on average higher. Kernel recovery was also significantly higher in double-kernel NIS compared to single-kernel NIS. In an NIS buying model, this makes double-kernel fruits overall more efficient to transport and crack per unit of kernel. However, double-kernel fruits are generally less preferred by large-scale commercial processors because of their smaller individual kernel mass and greater difficulty in removing two whole kernels from the shell (Wallace et al. 2021).
Some NIS contain twins where two individual kernels with separate testas are present within a single locule. Twins occurred in both single- and double-kernel NIS, and in the latter either one or both kernels were twins. This appeared to randomly occur within trees as only occasionally one or two NIS contained twin kernels, although some trees had a greater propensity for twin kernels. The presence of two ovules within each of the three locules indicates that twin kernels are biologically possible in all trees. It is unclear if twin kernels are related to pollination levels when fertilisation of both ovules within a locule occurs, and if so, if maternal barriers may preclude this from occurring more frequently. Twin kernels are undesirable for processors due to the small kernel size and additional testa removal time required (Wallace et al. 2021).
Testa removal and nut cracking
Ease of testa removal is an important consideration for processors as it is a labour intensive process (Wallace et al. 2021). The testa was removed from fresh kernels in this study, and almost always liberated in one piece without breaking. Testa removal was classified as easy on single and double kernels from all trees, with only twin kernels having a limited degree of difficulty. Testa removal was performed on fresh kernels and does not reflect the potential resistance once the NIS or KIT has been dried. Drying NIS or KIT results in the testa shrinking and adhering to the kernel thereby making it more difficult to remove (Wallace et al., 2010).
Ease of cracking the NIS remained consistent within a tree and most (60.78%) trees had NIS that were easy to crack and extract whole kernels. Whole kernels are preferred by the market and kernels must be whole to conform to premium-graded nuts (Wallace et al. 2021; Walton and Wallace 2005). Around 16% of trees produced NIS that formed a ‘coffin-lid’ and were rated very-easy to crack. Such nuts can be considered desirable among resource owners that consume the kernels immediately or sell them fresh in local markets (Nevenimo et al. 2008). However, this is an undesirable trait when storing the NIS as they can open during storage and cause premature spoiling of the kernel (Evans 1999b). Very-easy to crack kernels also had a greater propensity for ‘water kernels’ where a clear liquid was found inside the intact testa.
Ease of cracking the NIS is an important trait that needs to be considered in conjunction with other selection criteria. For example, in this study, Tree MAL022 had the greatest kernel mass (4.63 g) of all the trees surveyed yet most of the NIS had a very-easy to crack ‘coffin-lid’ form rendering them unsuitable for long-term storage. Moreover, although the ‘coffin-lid’ was removed easily, the kernel was retained in the shell and difficult to extract whole, often resulting in a damaged kernel. Another tentatively elite tree that met the strict selection criteria (MAL038) was also difficult to extract whole kernels due to the way the shell broke during cracking. This can possibly be ameliorated by mechanical cracking and would be acceptable with an NIS buying model, but not a KIT buying model, where manual cracking by producers is likely to produce a high proportion of kernel pieces.
Selection and improvement
Natural populations of canarium trees have sufficient phenotypic variation to permit more intense selection and propagation of commercially desirable traits (Evans 1999b; Leakey et al. 2008). Amongst the traits of interest that identify superior canarium mother trees are kernel mass, high kernel recovery (kernel:NIS) and fruit traits such as an easy to crack shell and easy to extract kernel (Bunt and Leakey 2008; Leakey et al. 2008; Randall et al. 2016), which were investigated in this study.
The productivity of a canarium planting may depend on the seed sourced for its establishment. It is important to identify superior mother trees as the seed source to use in selection (progeny) trials. At least two kernel traits, kernel mass and kernel recovery, have been recommended when selecting superior trees (Leakey et al. 2008). Large kernels with a mean fresh mass of greater than 3 g are generally preferred by processors and considered to improve the economic viability of entities along the value chain (Leakey et al. 2008). In our study around 20% of trees produced kernels with a mean mass of 3 g or more. Further research is required to determine if a trade-off exists between kernel size and fruit set and prolonged fruit development like in other nut crops such as cashews (Adeigbe et al. 2016; Aliyu and Awopetu 2011).
Kernel recovery is an important consideration for kernel processors, though, it should not be considered in isolation from other desirable fruit characteristics to select mother trees. Moreover, these two traits, kernel mass and kernel recovery, alone may not capture market requirements. For example, the largest kernel tree in our study (4.63 g) were mostly a very-easy to crack operculum form (‘coffin-lid’) which are unsuitable for long term storage as NIS. However, as we are employing a seed-based improvement program due to the difficulties of asexual multiplication, this tree was still selected as it is unknown if coffin-lid NIS is a heritable trait.
Commercial nut industries rely on consistent and high yielding cultivars (Walton and Wallace 2011). When considering artificial selection requirements of trees with a higher propensity of single-kernel fruits (i.e., 20 or more of the 25 fruits sampled per tree were single kernels), a mean kernel mass greater than 3 g, a mean kernel length of > 30 mm, a mean kernel width > 13 mm, a kernel recovery > = 0.25 and without a ‘coffin-lid’ form, very few (n = 5) of the 256 trees surveyed met these standards. This either suggests that the search for trees meeting these selection criteria should continue, since founding a tree improvement program on five trees is unwise, or the selection criteria are too strict in the Vanuatu context and should be relaxed. Whilst some confidence is given by trees maintaining their superior ranking in terms of relative kernel mass, year-to-year variability in fruit and nut characteristics was demonstrated and it is necessary to undergo field trials to determine the stability of commercially important traits.