There exists both intra- and inter-species variation in relation to the efficacy of the compartmentalisation process – no one individual is able to compartmentalise identically to another.
With regards to intra-specific variation (differences within different genotypes and phenotypes of the same species), studies have highlighted that compartmentalisation is moderately to strongly related to genetics. Such a conclusion was drawn after surveying how similar wounds inflicted to different specimens of the same species in a particular environment brought about marked variation in decay extent and the patterns of decay. Research concluded that the pathogen itself was not the sole influencer of the extent of decay, as certain specimens simply exhibited less resistance than other specimens (Hiratsuka & Loman, 1984; Shigo, 1981; Swanson, 2001). Genetics may likely therefore be the primary driver in determining resistance to decay and effective compartmentalisation.
Research has also suggested that stock that has been grafted is typically stronger at compartmentalising than non-grafted stock, as the strong compartmentalisers, when grafted, are more likely to ‘take’ than poor compartmentalisers (Santamour, 1984; Santamour, 1988). In essence, the grafting process is an unintentional filtering of the gene pool, with only the optimum specimens reaching the point of retail or other utilisation. This can also have an impact inter-specifically, as particular species may be more likely to have been propagated via grafting than others, in the nursery setting. For these species, resistance may be inherently higher, as a result of the indirect genetic ‘engineering’.
Further evidence to demonstrate intra-species variation in compartmentalisation ability is evident within the Populus genus. Certain clones of hybrid black poplars are more effective at healing and compartmentalising than others (Shigo et al., 1977). Difference was, according to the research, not down to the rate of growth, nor the ability to close off the wound (as certain specimens had a fully closed wound but significant internal decay, and others had not closed the wound but had little internal decay). Whilst the results did not suggest a specific reason as to why, genetics may likely have played a role.
It should be stressed that environmental factors also play a role, as whilst the genetic potential of a specimen may be that of being resistant to decay, the environment it is a part of might be less than optimal condition-wise (Lonsdale, 1999; Shigo, 1991; Smith, 2006). To illustrate this, each tree has its own genetic parameters to fight decay (vigour), though the environmental factors may impact upon the vitality of the tree and therefore mean it is running at ‘reduced capacity’. This will have implications on the tree’s ability to compartmentalise decay.
Turning attention towards inter-species variation, one can observe that certain species, such as Betula spp., are less able to block their vessels as a defensive response, thus meaning the reaction zone (Walls I-III) is weaker – tylosis and suberin deposition is less prevalent (Lonsdale, 1999). Typically speaking, when one states a species is ‘less able’, it is meant the abundance of parenchyma cells within the wood and the manner in which vessels within the wood join to one another are not as optimal in relation to species that can compartmentalise well (Dujesiefken & Liese, 2015). For example, a tree with fewer parenchyma cells and larger, more abundant, and more connected vessels, will be able to compartmentalise less readily than one with fewer, smaller, less-connected vessels, and with more parenchyma cells – the fewer connections ultimately serve to provide fewer pathways for fungal pathogens, and mean the tree needs to expend less energy on plugging the vessels that are flooded with tyloses and suberin upon exposure to air following wounding (Smith, 2006).
Additionally, species with low wood densities, such as Populus spp., are less able to compartmentalise decay (Lonsdale, 1999). A lower density inherently reduces compartmentalisation efficacy, and is more typical of diffuse porous species. Diffuse-porous species, unlike those that are ring-porous, do not lay down the very dense latewood – ring-porous latewood is far more problematic for hyphae seeking to penetrate into the wood radially. However, the abundant and very massive early conductive tissues of ring-porous Ulmus spp., for instance, also pose a risk (Dujesiefken & Liese, 2015), particularly if infection is early in the growing season. This is in spite of elm being distinctly ring-porous. In fact, research into the compartmentalisation ability of elm suggests that it may not be so much the inherent capability of an individual to compartmentalise that is under genetic control, as it is the anatomical features and biochemical systems of the individual. The efficacy of compartmentalisation would, as a result, be a direct result of the inherited anatomical and biochemical features of the individual and its forebears (Shigo et al., 1980). In the case of elm therefore, its massive xylem structures facilitate rapid decay and reduce the ability for tyloses to fully plug the vessels (Dujesiefken & Liese, 2015).
Genetics, much like with intra-species variation, also influences on the meta-level. In essence, trees have the genetic capacity to build chemical boundaries around infection, though some species have a high capacity to build these systems and others do not. Species that are not able to build effective boundaries decay rapidly following injury and species that can build effective barriers do not decay as readily (Garrett et al., 1979). For example, a study into compartmentalisation ability of many larch species (Larix spp.) concluded that genetics of different species was a significant factor in successful compartmentalisation (Curnel et al., 2008). It did however not detail how genetics varied between species, so no assumptions can be made on what within the genetic make-up of a species causes such variation. Another study suggested that variation in the abundance of induced antimicrobial compounds between species is a factor that can determine the potential efficacy of the reaction zone (Baum & Schwarze, 2002).
Panning back to assess genetic characteristics of the Plantae kingdom, in particular the life strategy of a species, one can further ascertain why some species are better at compartmentalisation than others. As tree resources are allocated between growth and defence, and the growth-differentiation balance hypothesis states that resources for standard growth functions are allocated for defence purposes only when necessary (Smith, 2015), the life strategy of a particular species may dictate how many resources an individual phenotype might allocate for such defence mechanisms in place of growth. The efficacy of the second wall, for instance, is dependent on the availability of stored carbohydrates (usually starch) within xylem parenchyma cells. The availability of such carbohydrates is, at least in part, genetically-controlled (Santamour, 1988). The genetic qualities of some trees thus enables them to be more efficient in the storage or utilisation of carbohydrates.
It should also be noted that the location of the wound itself on an individual, compiled with the aforementioned intra-specific and inter-specific variation, can cause different specimens to compartmentalise more or less successfully. Root wounds can compartmentalise better than stem wounds, and stem wounds can compartmentalise better than upper branch wounds. This is ultimately down to the fact that cell type proportion and distribution varies significantly throughout an individual (Dujesiefken & Liese, 2015).
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