Tree grafting: an introspection with reference to breeding, production and productivity in Hevea brasiliensis

Grafting has transformed the propagation and productivity of perennial tree crops, but genetically variable rootstocks and functional stock-scion interactions can limit the uniform expression of elite scion potential. Evidence from Hevea brasiliensis highlights the need for compatible and uniform rootstocks, self-rooted planting material, and emerging graft assisted biotechnological approaches to improve plantation uniformity and productivity. Due to high heterozygosity and difficulty in fixing desirable traits, grafting has long been the preferred method of propagation in many perennial crops. Hevea brasiliensis, a perennial tree and the primary source of natural rubber (NR), is widely propagated through grafting. As an outbreeding, highly heterozygous species, grafting preserves traits of hybrids developed through prolonged selection. Combined with genetic improvement, grafting has substantially increased rubber productivity worldwide, from ~300 kg ha^-1 to ~3000 kg ha^-1. However, actual plantation yields remain below potential due to significant tree-to-tree variation in latex production, even among genetically uniform clones. In addition to environmental factors, stock-scion interactions are recognized as important determinants of intra-clonal variation, although their underlying molecular mechanisms remain poorly understood. Research integrating anatomical, physiological, and genetic approaches generating genome, metabolome, and proteome data has identified several key contributors to this phenomenon. This review discusses the current knowledge on factors influencing stock-scion interactions in rubber and provides a general framework to address hetero-grafting related variability issues in perennial crops propagated by grafting. The issues discussed are of use to breeders and geneticists to optimize growth and yield in such crops via more precise and sustainable exploitation of their elite genotypes.