The reestablishment’s post-grafting of fruit plants: Biochemical and physiological factors

The use of grafting techniques and the combination between the different graft and rootstock species has long been the target of investigations proposing solutions to possible issues regarding the adaptation of commercial plants to numerous biotic and abiotic factors in the field (e.g. pathogen resistance, temperature influence, shading conditions, heat stress and salinity). Grafting can also be used as a tool in the study of various physiological effects and has contributed to the advancement in biochemistry and plant physiology. Despite the long history of rootstock use and numerous studies regarding their effects on crown growth, the biochemical mechanisms by which a rootstock exerts control over crown growth and development have not yet been elucidated. In recent years, a growing number of studies has attempted to elucidate the biochemical mechanisms responsible for the incompatibility of certain graft/rootstock combinations in fruit plants, with the enzymatic activity of UDP-glucose pyrophosphorylase being a potential candidate (UGPase). The enzyme UGPase is important in regulating metabolism because it catalyses the interconversion of glucose-1-phosphate sugar with uridine triphosphate (UTP) to form uridine diphosphate glucose (UDPG) and pyrophosphate (PPi), which is a key precursor in the biosynthesis of cellulose, hemicellulose, pectin, glycolipids and glycoproteins in plants and is directly responsible for carbohydrate metabolism in the cell wall biosynthesis of xylem tissues. The involvement of antioxidant enzymes has been analysed during cell development in the early stages of tissue formation at the graft/rootstock interface, although their specific role and effect on compatibility is not yet elucidated. Notably, antioxidant enzymes such as peroxidases (PODs) and superoxide dismutases (SODs) are involved in the lignification of xylem tissues and are important in the early stages of post-grafting since the cell walls of xylem tissues are dynamic structures composed of polysaccharides and phenolic compounds (PC). The activity of PODs correlates with PC concentration, since PODs use PCs, especially low molecular weight PCs, as a substrate for the synthesis of new PODs, thereby presenting an inversely proportional balance and suggesting an interrelationship between the two compounds. Both are important in the early stages of the graft-rootstock connection since the cell walls of xylem tissues are dynamic structures composed of polysaccharides, PCs (e.g. lignins), minerals and proteins. Furthermore, the reestablishment of vascular connections in grafted plants is of fundamental importance for water flow. Grafted plants forming a callus at the graft/rootstock interface enables the flow of water from the rootstock to the crown; however, the insufficient connection between them leads to a decrease in water flow and subsequent losses in carbon assimilation, stomatal conductance and other parameters of gas exchange. Gaseous exchanges in grafted plants suffer from the direct actions of the rootstock since the rootstock can modify the vigour and productivity of the crown species, thus playing a direct role in the ratio of graft/rootstock compatibility, as evidenced by responses. photosystem II (PSII) efficiency. These changes can also be observed in vegetative development and ionic accumulation. In general, plants control the absorption of mineral elements through their root systems, while their biochemical absorption mechanisms signal the aerial portion that governs the demand for nutrients. The quantification of leaf nutrients is another important factor in incompatibility studies.