Effects of water stress on grape quality and content of methoxypyrazines in Cabernet Sauvignon

Abstract:【Objective】Northwest China is short of water resources. Previous studies have verified that moderate water stress can limit the over growth of the vine canopy, raising the secondary metabolites in berries. Methoxypyrazines (MPs) are one kind of volatile compounds with strong smells of green grass, green pepper and green pea, and the sensory threshold of these compounds is very low. Therefore, the flavor and quality of wine can be negatively affected when the level of MPs in wine is too high. This study aims to explore the MPs content in Cabernet Sauvignon under water stress during the development of the berries in order to optimize water supply in vineyard management.【Methods】The trial was carried out in vintage 2017 from the period of fruit setting to veraison in a farm of Yuquanying in Yinchuan, Ningxia. Nine- year- old Cabernet Sauvignon vines were treated with different water conditions, achieving three levels of leaf water potential, corresponding to mild (-0.2 MPa≥Ψb≥-0.4 MPa), moderate (-0.4 MPa≥Ψb≥-0.6 MPa) and severe(Ψb≤-0.6 MPa) water stresses. In addition, at veraison, each treatment was further subdivided into three water conditions including mild, moderate and severe water stress, forming a total of 9 treatments. with mild-mild as CK. Based on the rainfall and air temperature, the irrigation amount in each group was determined. The value of predawn leaf water potential was used to reflect the degree of water stress. From 20 days after anthesis, samples were taken every 10 days to determine the variations in physicochemical properties and MPs levels during the development. The hundred-grain weight was collected with an balance at precision of 0.000 1 g. TSS was determined using a WYT 24 hand-held saccharometer. TA was determined with acid-base titration. The berry tannin level was measured using F-D, and anthocyanin content with differential pH method. 3-iso-butyl-2-methoxypyrazine (IBMP), 3- iso- propyl- 2- methoxypyrazine (IPMP), 3- sec- butyl- 2- methoxypyrazine (SBMP), 2-Methoxypyrazine (MOMP), and 3-methyl-2-methoxypyrazine (MEMP) were detected. Approximately 30 g berries were crushed and thawed, and 5 mL of the collected grape juice was added into a 15 mL bottle with brown headspace. The brown bottle was placed in the HS-HPME device, and volatiles extracted with a 50 μm/30 μm CAR/PDMS head space needle for 3 h at 30 ℃ by shading. The extraction head was desorption for 5 mins. The volatiles were detected by GC-FID with a J & W DB wax chromatographic column (50 m × 0.25 mm × 0.25 μm). The temperatures in the injection port and the detector were set at 230 ℃ and 250 ℃, respectively. The external-standard method was applied for relative quantification.【Results】The results showed that the predawn leaf water potential was affected by the amount of irrigation. The value of water potential of each treatment increased due to periodic rainfall and the values fluctuated within the range designed. The hundred-grain weight, TSS and total anthocyanin content increased gradually during the berry development with a relatively slower rate after veraison. At harvest, the hundred-grain weight of all treatments was reduced. T4, T5, T6 and T7 were effective to increase TSS. Except for T6, all the treatments could increase tannin content compared with CK. A similar trend was found in total anthocyanin content in all treatments (except for T1 and T3). And T4 and T5 treatments were most effective to increase total anthocyanin content. Compared with CK, the levels of total phenol, total anthocyanins and tannin in T4 grapes increased by 36.02%, 20.38% and 16.70%, respectively; and T5 improved by 6.99%, 31.17%, and 20.80%, respectively. The significant difference in fruit size caused by water stress, the content of MPs in single fruit was calculated to figure out its accumulation pattern. Under water stress, the contents of various MPs per berry showed a trend of increasing first and then decreasing during the berry development. Water stress decreased MPs accumulation in maturation, especially severe stress (T6, T7 and T8). The degradation rates of different MPs were diverse. At harvest, MOMP and MEMP have been degraded below the detection line, while the contents of IPMP, SBMP and IBMP in berries maintained high. Compared with CK, the contents of SBMP, IPMP and IBMP of all the treatments decreased by a range of 1.82%-76.36%, 2.19%-53.85% and 3.21%-72.82%, respectively. The results showed that T5 not only improved fruit quality but also effectively reduced the concentration of MPs.【Conclusion】In conclusion, moderate water stress treatment from fruit setting to veraison and severe water stress applied to harvest stage can reduce MPs content, improving fruit and wine quality.