Vessel element characteristics of scion overgrowing of the sweet cherryvariety ‘Tieton’ grafted on Cerasus tomentosa

Abstract:【Objective】Scion overgrowing, in which the stock diameter is finer than that of the scion, is acommon phenomenon in cherry cultivation. So far, very little is known about its formation mechanism, inaddition to a mere handful of reports written on the description of scion overgrowing and its effect onbranches and on the results of the fruit tree. To reveal the occurrence mechanism of scion overgrowing, acomparison study on the differences of the anatomical structure between stock and scion is the basis of un⁃derstanding this phenomenon. Vessel elements, one of the main cellular compositions in xylem, are basicstructural units conducting water and mineral nutrition in the plant’s xylem, and their width to a large ex⁃tent influences the coarseness of the stock and scion. This study explores the differences of the vessel ele⁃ment characteristics in secondary xylem among the different parts of the root and stem of Cerasus avium‘Tieton’, a new superior cultivar cultivated through the hybridizing of‘Stella’and‘Early Burlat’byWashington State University, USA, so as to provide a certain theoretical basis for revealing the structuremechanism of scion overgrowing and provide the basis for a further study of the phenomenon.【Methods】Three selected randomly 5-year-old healthy experimental trees with similar growing trends were used asthe experimental materials, in which the cultivar were grafted to C. tomentosa seedlings, grown in the Col⁃lege of Horticulture Science and Technology test base, Hebei Normal University of Science and Technolo⁃gy. The secondary xylems in the sapwood were divided into thin filaments from the root segments of thestock，stem segments of the stock and stem segments of the scion in the experimental trees. All thin fila⁃ments were mixed from the same parts of the trees and used as one sample. Eventually a total of three sam⁃ples were obtained. After segregation of the thin filaments for each sample in the segregation liquid at40 ℃ for 24 h, the segregated tissue cells were stained with 1% safranine O aqueous solution. Temporaryslides were made, observed under an Olympus BX51 microscope and photographed using a DP72 camera.According to the previous method, the vessel element form was described. The length, width and end wallheight of the vessel elements were measured using Photoshop CS6 software. 100 vessel elements were ran⁃domly examined for each sample. The ratio of the end wall height and width was calculated using excel2003. The radian of the end wall slope was also calculated by mathematical and trigonometric functionsATAN applying common functions. The radians were further converted to degrees. The shape and size ofthe vessel elements were compared among the root segments of the stock, stem segments of the stock andthe scion. Relevant data was statistically analyzed using DPS 7.05 software. Significance between meanswas tested using Duncan’s multiple range test.【Results】All the vessel elements in the secondary xylemof the three parts belonged to the pitted vessels, simple perforation and alternate pitting as the basis for de⁃fining the secondary thickening and lignifications of the vessel element’s lateral wall. There were different types of vessel elements, some with long tails, short ones and some in the transitional stage, with bothtails, one tail and no tail, and also some with both sloping end walls, with one sloping end wall and with al⁃most both horizontal sloping ending walls in them. Compared with those of the root segments of the stockand stem segments of the scion respectively，the number of vessel elements of the stem segments of thestock with tails in both end walls were more than 5.3% and 29.8%，with no tail in either end wall less than50.0% and 53.8%, with both slant end walls less than 3.8% and 8.3%, with one slant end walls littlechanged, with smaller end wall slopes more than 22.2% and 66.7%, end wall slopes smaller than 9.4%and 9.5%, with end wall slopes less than 32 degrees more than 75.0% and 95.2%, with end wall slopsgreater than 52 degrees less than 33.3% and 16.7%. The length of vessel elements in the stem segments ofthe stock was shorter than 29.9% and 21.3% in the root segments of the stock and stem segments of thescion respectively, and the number of vessel elements with lengths greater than 260 μm were less than100% both in the root segment of the stock and scion. The width of the vessel elements in the stem seg⁃ments of the stock was wider than 19.5% and 22.4% in the root segments of the stock and stem segmentsof the scion respectively, and the number of vessel elements with a width greater than 50 μm were morethan 60.0% and 80.0% in the root segments of the stock and stem segments of the scion. That is, contrasting with seedling trees reported, the width of the vessel elements did not match from the root systems,stem segments of the stock to the stem segments of the scion.【Conclusion】The differences of shape andsize of the vessel elements between the stem segments of the stock, stem segments of the scion and rootsegments of the stock may be the leading causes for scion overgrowing of the cultivars. However, to deeplyunderstand the mechanics of scion overgrowing, you still need to study cell division, differentiation andgrowth of the fusiform initials in the vascular cambium from hormone regulation, transcriptional regulation, regulation of the posttranscriptional level and protein in several different aspects because vessel ele⁃ments develop from fusiform cambial initials though the development is affected by the growth environ⁃ment selection pressure.