Effects of exogenous brassinolide on flavor quality of Chardonnay and Pinot Noir grape

Abstract：【Objective】Due to unique natural terroir condition and a long history of wine grape cultivation, Hexi Corridor region of Gansu province has the potential to produce high-quality grape wine. As the main cultivars in the local area, the quality of Chardonnay and Pinot Noir are affected by some factors in the growing process, such as ecological conditions, extreme weather and cultivation management practices, which results in a weak varietal aroma profile and unstable flavor quality, and consequently, the loss of commercial value of wine grape. It has been shown that exogenous application of natural or synthetic inducers including abscisic acid, auxin and methyl jasmonate can promote the accumulation of anthocyanins and aroma compounds in grapes and improve fruit quality. As the most biologically active endogenous hormone among the oleuropein sterols, brassinolide is known to improve certain plant growth, development and the formation of secondary metabolite in fruits, but the regulation of this elicitor on flavor- related metabolite accumulation in wine grapes is still poorly understood.Therefore, the objective of this study was to evaluate the effects of preharvest application of brassinolide on the physicochemical parameters, aroma and overall quality of wine grapes. The present study will establish a foundation for flavor improvement in grape berries by exogenous application of brassinolide.【Methods】Chardonnay and Pinot Noir grapes were used as experimental materials and brassinolide at the concentration of 0.4, 0.6 and 0.8 mg ·L^-1 was sprayed twice at the E-L35 (onset of veraison) stage and 7 days later, respectively. Grape samples were collected at the E-L36 (post-veraison), E-L37 (middle maturation) and E-L38 (harvest time) phenological stages respectively for the determination of basic physicochemical parameters. Headspace solid phase microextraction combined with gas chromatography-mass spectrometry (HS- SPME/GC-MS) technology was used to detect volatile compounds, and the evolution regulation of these compounds during fruit maturation was analyzed. Principal component integrated analysis method was employed to screen important quality indexes and the optimum brassinolide treatment concentration.【Results】At harvest, the contents of total soluble solids and reducing sugar, as well as pH value in 0.6 mg · L- 1 brassinolide- treated Chardonnay grapes increased by 5.03%, 4.34% and 7.96%, respectively, while the titratable acid content decreased by 36.42% compared with the control. In addition, the level of total soluble solids, reducing sugar and pH in mature Pinot Noir berries were 19.11%, 11.22% and 1.94% higher than those in the control respectively, and the content of titratable acid was 19.31% lower compared to the control. By contrast, preharvest brassinolide treatment tended to promote the accumulation of total phenols and total free amino acids in Chardonnay grapes, and increased the concentration of yeast assimilable nitrogen (YAN) in Pinot Noir. Notably, the accumulation of species and content of aroma substances showed specific dynamic trends during the berry development and maturation. At three sampling times, a total of 36 volatile compounds were identified in Chardonnay grapes, whereas 49 volatile compounds were identified in Pinot Noir berries. At EL38 stage, (2E, 6E)-nona-2, 6-dienal and nerol were only detected in 0.6 mg·L^-1 and 0.8 mg·L^-1 brassinolide-treated Chardonnay berries. Dodecaldehyde and nerol were not detected in control grapes, while they were found in Pinot Noir berries treated with brassinolide. Compared with the control, preharvest brassinolide application at the concentration of 0.6 mg ·L^-1 significantly promoted the synthesis of amino acid derivatives and isoprene derivatives in both varieties (p＜0.05), and had a better modification effect on these compounds in Chardonnay than that in Pinot Noir. At harvest, the content of straightchain esters, terpenoids, norisoprenes, benzene derivatives and branched-chain aliphatic compounds in Chardonnay grapes treated with 0.6 mg·L^-1 brassinolide was 1.28, 1.83, 1.68, 1.48 and 1.95 times higher than that in the control respectively. In addition, the content of characteristic aroma substances such as hexanol, farnesol, β-damascenone, phenylethyl alcohol, benzyl alcohol and phenylethyl aldehyde in 0.6 mg · L^-1 brassinolide-treated Chardonnay berries were 2.46, 35.75, 2.39, 2.42, 1.32, and 1.64 times more than the control. Pinot Noir grapes that received 0.6 mg ·L^-1 brassinolide were characterized with higher concentration of straight-chain esters, terpenoids and branched-chain fatty compounds, as well as the content of characteristic aroma substances including (E)-2-hexen-1-ol, dodecanol, and phenylethanol than those in the control. These findings showed that the improvement effect of preharvest brassinolide treatment on wine grape quality is dose and variety dependent. The results of principal component analysis and evaluation of aroma characteristics showed that 0.6 mg ·L^-1 brassinolide treatment promoted the biosynthesis of reduced isoprenoids, terpenoids, benzene derivatives and straight chain esters in the berries of both varieties. Notably, 0.6 mg ·L^-1 brassinolide efficiently enhanced the accumulation of most amino acid derivatives, which was conducive to improving the floral and fruit aroma in the berries.【Conclusion】Preharvest brassinolide application at both onset of veraison and one week later could in crease the content of total phenols, total free amino acid and yeast assimilable nitrogen of Chardonnay and Pinot Noir grapes and significantly promote the synthesis of amino acid-derived aroma compounds