## 1 Introduction  
Volatile compounds are naturally produced in plants. At present, a total of 1700 volatile components have been identified from more than 90 plant species [1]. Based on their biosynthetic pathways, volatiles in grapes are commonly divided into three major classes: isoprenoid derivatives, amino acid derivatives, and fatty acid derivatives (Figure 1). These three classes of volatiles have different contributions to grape and wine aroma quality. Terpenoids are biologically synthesized from isopenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). These precursors are formed from three molecules of acetyl-CoA through cytosolic mevalonic-acid (MVA) pathway [2] or pyruvate and glyceraldehyde-3-phosphate under plastidial 2-C-methylerythitol-4-phosphate (MEP) pathway [3]. Terpene profile, especially monoterpene profile, in muscat-type and aroma-type grapes, primarily determines the aroma attributes of respective wines [4]. Norisoprenoids are yielded from the degradation of carotenoids that are biosynthesized from IPP. \(\beta\)-Damascenone and ionone are two important C13-norisoprenoid components and their levels are above their thresholds in grapes. Therefore, they play a primary role in contributing floral odor to many grape varieties, such as Cabernet Sauvignon [4]. Figure 1: Biosynthetic pathways of three classes of volatile compounds in grapes. Metabolism and pathway names are italicized, whereas volatiles are in pink shadow. Abbreviations: G3P, glyceraldehyde 3-phosphate; PEP, phosphoenolpyruvate acid; Acetyl-CoA, acetyl coenzyme-A; TCA, tricarboxylic acid cycle; DMAPP, dimethylallyl pyrophosphate; IPP, isopentenyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; GGPP, geranylgeranyl pyrophosphate; Phe, l-phenylalanine; Val, valine; Leu, leucine; Ile, isoleucine. Amino acid-derived volatiles include benzenoids, branched-chain aliphatics, and methoxypyrazines. Benzenoids are a group of aromatic volatile components derived from l-phenylalanine, a substrate synthesized via shikimate pathway [4]. Branched-chain aliphatic volatiles, including their aldehydes, alcohols, and esters, are mainly synthesized from valine, leucine and isoleucine. Methoxypyrazines (MPs) are nitrogenated heterocyclic products of leucine and isoleucine metabolisms [5]. MPs have extremely low sensorial thresholds and are commonly described as bell peppers or green leaf odor [6]. MPs exist at detectable levels only in Sauvignon Blanc, Cabernet Sauvignon, and Merlot, and they are important contributors to the distinctive vegetative characters of these varieties [7]. Straight-chain alcohols, aldehydes, esters, acids, and ketones are basically originated from fatty acids via \(\alpha\)- or \(\beta\)-oxidation or lipoxygenase (LOX) pathway [8]. In grapes, aliphatic volatiles predominantly include short-chain aldehydes and alcohols [9], most of which are described as \"green\" aromas [10]. These aldehydes and alcohols can be converted into the corresponding acetate esters during wine making and aging processes, which further affects wine aroma quality. Ultraviolet light (UV) is considered one of several major environment factors that affect the development and physiology of plants. A number of studies have been conducted to investigate the impact of enhancing or reducing UV radiation on plant secondary mechanism. For example, the synthesis of flavonoids has been proved as a class of important protective compounds against UV-stimuli. Of these investigations, the UV-B (280-315 nm) effects have been the focus of much attention. This is because only a certain proportion of UV-B can pass through the ozone layer of earth, and its intensity under natural conditions is significantly impacted by regions and climates [11,12]. In contrast, almost all UV-A (315-390 nm) can reach the earth surface, whereas UV-C (\(\leq\)280 nm) is almost absorbed by the ozone layer. In recent years, the intensity of UV radiation reaching the earth surface has increased with a continuous depletion of atmospheric ozone [13,14].