Methoxypyrazines (MPs) are a unique class of volatile compounds containing nitrogenous heterocyclics that impart green bell, vegetal and herbal odors to red grape berries and wines. In this study, the quality and MPs levels of grape berries from six representative red wine grape varieties were determined in the two consecutive years. The results showed that, at maturity, the highest total soluble solid was observed in Petit Verdot grape berries in the two consecutive years. While the anthocyanin content showed the highest in Marselan berries in 2018, in 2019, Petit Verdot berries had the highest anthocyanin content. Moreover, 2-methoxypyrazine (MOMP), 3-methyl-2-methoxypyrazine (MEMP) and 3-ethyl-2-methoxypyrazine (ETMP) levels were relatively lower, with almost no detectable in berries at maturity. The relative higher 3-isobutyl-2-methoxypyrazine (IBMP) content was observed in Cabernet Sauvignon, Marselan, Merlot, and Malbec berries. However, 3-sec-butyl-2-methoxypyrazine (SBMP) and IBMP were only detected in six wines, and their levels were higher than those in the grape berries. Furthermore, correlation analysis showed that there was a statistically positive correlation between the expression levels of VvOMT1 and VvOMT3 and MPs content in grape berries, while the lowest association was found in the VvOMT2. These findings provide a basis for selecting the most suitable grape varieties to improve wine quality.

Malic acid is the predominant organic acid in grape berries, and its content is affected by abiotic factors such as temperature (fruit zone microclimate) and water (vine water status). The objectives of this study were to explore the potential mechanisms behind the effects of vine water status on the biosynthesis and degradation of berry malic acid and the potential downstream effects on berry metabolism. This study was conducted over two growing seasons in 2021 and 2022, comprising three watering regimes: no water stress (CK), light water stress (LWS), and moderate water stress (MWS). Compared to CK, a significantly higher level of malic acid was found in berries from the MWS treatment when the berry was still hard and green (E-L 33) in both years. However, water stress reduced the malic acid content at the ripe berry harvest (E-L 38) stage. The activities of NAD-malate dehydrogenase (NAD-MDH) and pyruvate kinase (PK) were enhanced by water stress. Except for the E-L 33 stage, the activity of phosphoenolpyruvate carboxylase (PEPC) was reduced by water stress. The highest phosphoenolpyruvate carboxykinase (PEPCK) activity was observed at the berry veraison (E-L 35) stage and coincided with the onset of a decrease in the malate content. Meanwhile, the expression of VvPEPCK was consistent with its enzyme activity. This study showed that water stress changed the content of some free amino acids (GABA, proline, leucine, aspartate, and glutamate), two of which (glutamate and GABA) are primary metabolites of the GABA pathway.

Soil burial is a survival strategy for grapevines that can safely overwinter in north-western regions of China. A suitable training system was beneficial for soil burial to provide winter protection. Moreover, the training system can also significantly affect fruit quality during the development of grape berries, such as primary and secondary metabolites. In this study, four-year-old Cabernet Gernischt grapevines were used as experimental material and exposed to four training systems, including the Ningxia traditional vertical trunk (control, CK); the gobelet (T1); single guyot (T2); slant trunk with vertical shoot positioning (STVSP) (T3). The results showed that total soluble solid total phenol content was 12.69%, 57% higher under T3 training systems than in the control, and T3 alleviated the canopy density, leading to improving the leaf photosynthetic efficiency gas chromatography-mass spectrometry (GC-MS) assay used to detect the aroma compounds. The results indicated that the T3 training system enhanced the accumulation of alcohols, carbonyl compounds, C6/C9 and esters, which account for the largest proportion of volatile compounds, and the qRT-PCR reveals that VvEcar, Vvter, VvCCD1, and VvLis were raised under T3 at the transcriptional level. Moreover, T3 contributes to most free amino acid synthesis. Additionally, the PCA reveals the correlation of free amino acids under four training systems, which reflected the mostly amino acid related to T3, and thus, we could speculate that T3 enhances the overall aroma. These results may lead to new strategies to select a new, short trunk training system to achieve mechanized buried soil, to prevent cold and produce high-quality wine in this area.

Water stress can affect plant ecological distribution, crop growth and carbohydrate distribution, impacting berry quality. However, previous studies mainly focused on short-term water stress or osmotic stress and few studies paid attention to the responses of grape to long-term water stresses. Grapevines were subjected to no water stress (CK), mild water stress (T1) and moderate water stress (T2). Hundred-berry weight and malic acid content were reduced under T1 and T2; however, glucose and fructose content showed the opposite trend. Endogenous hormones and polyamines (PAs) can regulate plant growth and development as well as physiological metabolic processes. T1 and T2 could increase abscisic acid content, however, indole-3-acetic acid, jasmonate, gibberellins 3 and 4, cytokinin and trans-zeatin contents were slightly decreased. Three species of PAs (putrescine, spermidine and spermine) were detected, presenting obvious tissue specificity. Furthermore, there was a statistically positive correlation relating spermidine content in the pulp with glucose and fructose contents of grape berries; and a negative correlation with organic acid. In summary, water stress had a profound influence on hormonally-driven changes in physiology and berry quality, indicating that endogenous hormones and the PAs play a critical role in the development and ripening of grape berries under water stress.