Biological and functional properties of xylem sap extracted from grapevine (cv. Rosario Bianco)

Highlights

• The bleeding sap of grapevine contains abundant protein, amino acids, sugar, organic acids, hormones, mineral elements and other effective ingredients.

• Bleeding sap have a certain resistance to Botrytis cinereal.

• Bleeding sap and MS medium have the same promoting function on sprouting of shoots.

Abstract

Xylem sap exudes from grapevine branches and ensures survival during the dormant period. However, the extensive analysis on the biological and functional characterization of xylem sap has not yet been carried out. Therefore, to comprehend the critical role and potential usage of the xylem sap, we identified and quantified the panel of active ingredients, such as protein, amino acids, carbohydrates, saponins, flavonoids, phenolic compounds, alkaloids, and polyphenols. Results indicates that bleeding sap is slightly acidic, with the accumulation of oxalic acid, tartaric acid, malic acid, lactic acid, and citric acid. Identification and quantification of sugars depicted that xylem sap is mainly rich in fructose, glucose, and lactose. Moreover, distinct amino acids (16), hormones (9), and mineral elements are identified in the bleeding sap. These bioactive compounds are vital for vine growth and development. In addition, xylem sap also has the growth-promoting ability and can be used as a tissue culturing medium for plants. Finally, functional studies suggested that xylem sap possesses antifungal effects that can be considered as a bacteriostatic agent against Botrytis cinerea. Overall, these findings suggested that xylem sap possesses the panel of antioxidants, protein, and growth-promoting and antifungal properties.

Introduction

Grape (Vitis, Vitaceae) is a perennial deciduous vine, typically grown for fruit and vine and are primarily selections of Vitis vinifera L., which are propagated via cutting and grafting worldwide (Wallis et al., 2013a; Haider et al., 2017). According to the Food and Agriculture Organization (FAO, 2018), the total area under grape cultivation is 7.15 million hectares with an annual production of about 79.12 million tons (MT) worldwide. Grapes are highly appreciated due to panel of nutritive compounds, such as glucose and fructose, pectin, organic acids, minerals, vitamins, amino acids, proteins, and crude fiber. In addition to these conventional nutrients, grape is also rich in resveratrol and polyphenols (Conde et al., 2007).

Before onset of spring, the vine is pruned shortly before bud emergence that allows flow of xylem sap from wounded branches, termed as ‘bleeding sap’ (Priestley and Wormall, 2010; Le et al., 2017). During the bleeding period, xylem sap is the source of pabulum that provides energy via nutrient transport for bud sprout (Keller, 2010). The water, minerals, and other organic substances in the xylem are transported from source to sink tissue by positive hydraulic pressure generated in the source tissues (Bruno and Sparapano, 2007; Alexou and Peuke, 2013). The root system is the main source that absorbs water and nutrients, and transport the nutrients in the bleeding sap (Priestley and Wormall, 2010). Therefore, the efficiency of the rootstock can also be characterized by the quantity and composition of the bleeding sap. Constituent analysis can reveal the material circulation, transport, distribution, and plant response to adverse environmental conditions (Wullschleger et al., 2001; Amarante et al., 2006; Ernst and Nelissen, 2008; Keller, 2010). Several studies have shown that grape xylem sap contains sugars, acids, phenols, amino acids, vitamins, and dozens of trace elements (Zheng et al., 2019). It also has vital importance in food and medical care areas (Keller 2010; Le et al., 2017). However, the precise identification and characterization of these minerals, organic compounds, hormones, and acids compounds has not yet been reported extensively in xylem sap during the bleeding period.

Gray mold, caused by Botrytis cinerea, is the most common pathogen of table grapes globally that can develop and spread rapidly in vineyards, leading to severe production losses. It occurs frequently in young leaves and inflorescence during early growth phases and later storage period, with an increasing infection rates yearly (Youssef et al., 2015). As a high-risk pathogen, the control of gray mold mainly relies on the chemical and biological ways of antagonistic microorganisms, mainly Trichoderma, Streptomyces, and endogenous Bacillus subtilis (Toure et al., 2004). With an increase in drug resistance in plants and the instability of biocontrol methods, an effective strategy for combating gray mold resistance is required (Williamson et al., 2010). However, the biological activity of bleeding sap against grey mold has not been verified so far.

As the most promising and highly-appreciated fruit crop, different grapevine organs have been fully utilized to manufacture different products in different industries. Howbeit, the application of xylem sap is still at the research and trial phase. Studies on grapevine bleeding sap are limited and have just focused on the physicochemical components and antioxidant capacity, though, there is no report demonstrating the precise functions of the bleeding sap. Assessing the effective ingredients of the bleeding sap will provide a theoretical explanation for its value in medicine, diet, and cosmetics. This study mainly focuses identification and quantification of effective constituent of grape cv. ‘Rosario Bianco’ bleeding sap, during the bleeding period, evaluation of the substance flow at the beginning of vitality restoration, and verification of the biological activity of the bleeding sap by considering two aspects: the ability of disease resistance and growth promotion on grapevine.