A review of the canopy architecture formation mechanism and regulation technology in fruit trees

Abstract: Understanding the fundamental nature of an ideotype tree’s structural characteristics is often acatalyst for the development of new technologies in fruit cultivation and management. It is widely believed, as demonstrated in the overwhelming majority of fruit tree species, or the same species cultivatedin different natural conditions, that a tree’s structural characteristics such as a training system, branchtype, canopy position and crop load affect the yield and quality of the fruit. Indeed, studies have uncovered multiple overlapping but functionally distinct optimum canopies in fruit trees, including the naturalcanopy (no pruning) and different types of training systems (Spindle shape, Open center shape, Y-shape,T-shape, V-shape, etc.). The characteristics of different training systems and canopy structures of fruittrees can be evaluated by using morphological, cytological, physiological, biochemical and molecular mea⁃surements. There are many parameters that can be measured in fruit trees that have been associated with atree structure. The different parameters reported among the training systems tested could be associatedwith the different capabilities of each system to intercept and distribute the light in the canopy. So naturallight availability within fruit trees is an important factor for canopy and fruit development as light radiation provides energy for photosynthetic organisms and to motivate photosynthetic processes. Actually, when considering the single tree canopy for a given genotype, highly vigorous growth of a canopy graduallyreduces the light penetration from top to bottom of the canopy. That is, the top/outer portions of a canopyintercept more light than the inner/bottom portions, and the photosynthetically active radiation (PAR) transmitted portion gradually decreases when light passes through a fruit tree canopy, which generates thelight’s microclimate and determines the amount of available light for distribution location within the cano⁃py. Furthermore, the capability of light transmitting through a canopy intercepted by the inner/bottom por⁃tions is affected by many factors such as cultivation spacing, canopy architecture and fruit tree management practices including pruning, thinning and laying mulch using reflected light, etc. Consequently, theselection and application optimum training system is important, which is benefited by creating a betterlight microclimate and providing guidance for fruit manipulations, especially for the practicing of high andstable yields, high quality and lower cost production. And this has long been a focus for researchers in thepast few decades. In this paper, we will first review the different training systems, there are more than 20types of training systems applied in 13 genus listed in the table, which is just a part of the various applica⁃tions in fruit trees, but are the most commonly used and some improved training systems have also beenlisted, such as round shape, spindle shape, improved spindle shape, open center shape, Y-shape, Tshape,V-shape, Columnar shape and so on. Through the comparative experiments of different trainingsystems, researchers specifically selected some of the optimum training systems for different species culti⁃vated in different natural conditions. This paper will bring forward a second point, that is, the mechanismof formation and regulation for training systems. We will show that the results of the mechanism, includingthe characteristics of branching and internode length genetics, dwarf gene expression and regulation, hor⁃mone content and their proportion are involved. The researches about the dwarf and semi-dwarfism regula⁃tion gene successfully cloned in peach, apple and pear etc, provide a direct and efficient approach for newcultivar breeding and the study of special gene sites. Especially, semi-dwarfism on root stock can providea long-distance transport of mRNA from grafts and reduce the stature of a scion, in which the result pre⁃sented clearly indicates that it would be applicable for regulating the fruit tree canopy. Though there is nota full understanding about the network for tree structure genes’regulation pathway, it points to further op⁃portunities for studying the tree structure formation mechanism from the perspectives of related genes.Over recent decades, researches have been conducted in obtaining and keeping optimum tree structures toinvestigate the effects of pruning (including pruning period and method) on tree canopy growth, light mi⁃croclimate, fruit yield and quality etc. The researchers stated that many growth and development charac⁃teristics such as photosynthetic efficiency, flower bud formation, fruit set, color, size, soluble solids andfruit maturity and so on are influenced by pruning. Moreover, some of the researchers studied the effect ofa combination of the training system and rootstock (including root system) on light microclimate, fruityield and quality. All of the research about relationships between orchard training systems and fruit yieldand quality would assist in optimizing tree canopy pruning strategies. In this article we aim to provide anoverview of canopy structural characteristics in fruit trees, summarize the mechanism of formation and regulation for training systems, review the current progress in pruning technologies and correlation, and pros⁃pect the potential applications of mechanical pruning, molecular mechanism of tree structural formationand regulation, and provide a reasonable evaluation method for different training systems. Furthermore,the limitation that should continue to make research on these topics are proposed, which will provide abeneficial reference for fruit tree manipulations.