Grafting ‘Red Globe’ (Vitis vinifera) onto Multiple Rootstocks: A Systematic, Multi-Year Evaluation Focusing on Graft Compatibility, Vegetative Growth, and Fruit Characteristics

Abstract

Selecting appropriate rootstocks can enhance the adaptability and fruit quality of grafted grapevines. However, grafting studies on ‘Red Globe’, one of the major cultivated cultivars, remain limited, particularly those involving long-term and comprehensive evaluations. The present research grafted ‘Red Globe’ onto four rootstocks—‘101-14’, ‘188-08’, ‘110R’, and ‘3309C’—and systematically compared graft union healing following hardwood grafting, field performance of grafted vines, vegetative growth of mature vines, and fruit phenotypic and quality traits across multiple years. The results showed that ‘101-14’ promoted the accumulation of organic acids, which reached 1.1% in 2023, and caused an increased tendency for berry detachment from the peduncle. The RG/110R combination exhibited a higher CFI, 0.8 on average, at the basal section, and promoted shoot thickening. RG/3309C was found to have a larger shoot length exceeding 600 cm, and a significant increase in fruit weight to nearly 13 g. The grafts on ‘188-08’ showed the highest survival rate of 74% among the graft combinations, and enhanced fruit quality, as evidenced by elevated TSS (16 °Brix) and firmer pulp texture, indicating that ‘188-08’ may serve as a valuable rootstock for enhancing the local adaptability and fruit quality of ‘Red Globe’ grapevines.

1. Introduction

Grafting is a crucial horticultural technique commonly used in viticulture [1,2]. The selection of rootstock significantly impacts scion performance [3,4]. The practice of grafting grapevines onto American rootstocks started mainly due to the widespread outbreak of phylloxera in Europe. Since then, numerous well-known rootstock varieties have been gradually selected, bred, and evaluated for grafting compatibility and performance with various wine grapes [5,6,7,8]. The primary aim of grafting is to improve the adaptability of grapevines to different environmental and biotic stresses [9,10]. In recent years, there has been a growing focus on the effects of rootstocks on fresh grapes [3,11,12].

Rootstocks influence scion growth, yield, ripening time, fruit composition, and vineyard management practices. Although certain rootstock varieties tend to confer specific traits to the scion, for example, we found 101-14M typically promotes higher soluble solids contents [5,12,13,14]—such trait modifications are not always consistent in other 101-14M grafted cases [15,16]. Scion–rootstock interactions, along with soil characteristics and climatic conditions, are all potential contributors to differences in vine and fruit among graft combinations [17,18]. Graft incompatibility is a key factor reducing grafting success, and can occur soon after grafting or several years after planting [19]. Delayed graft incompatibility can lead to a decline in vine vigor and fruit quality in grapevines. Therefore, early evaluation of graft compatibility, along with assessments of vine growth and fruit quality after planting, is essential for the selection of suitable grapevine rootstock–scion combinations.

With the growing focus on rootstock applications, grafting experiments involving various scion–rootstock combinations have been conducted to achieve a range of purposes, such as cultivating grapevines in saline environments or in heavy clay soil [1,20]. According to the USDA, China’s table grape production is projected to reach 14.2 million tons by 2024 [21]. ‘Red Globe’ is one of the earliest grape cultivars introduced to China and is widely cultivated. Evaluating the performance of ‘Red Globe’ grapevines grafted onto different rootstocks can provide valuable insights for targeted rootstock selection. To date, only a limited number of scattered studies have been conducted on the grafting of ‘Red Globe’. These include investigations on grafting affinity coefficients with rootstocks such as ‘41B’ and ‘140R’ [22]; evaluations of fruit quality and leaf mineral nutrient uptake on ‘41B’, ‘SO₄’, and ‘1103P’ [23]; and assessments of vine growth and fruit characteristics when grafted onto ‘Freedom’, ‘Salt Creek’, and ‘110R’ [24]. Among these, ‘Freedom’ and ‘110R’ were found to enhance berry color and total soluble solids (TSS), while ‘140R’ exhibited better grafting compatibility with ‘Red Globe’ compared to ‘41B’. However, these preliminary findings require further validation through multi-seasonal observations under defined ecological conditions, as suggested by Aurand et al. [22]. Despite these initial efforts, systematic and long-term field studies on ‘Red Globe’ grafting remain scarce.

Given this context, we grafted ‘Red Globe’ onto four relatively understudied grape rootstocks and conducted multi-year evaluations focusing on graft compatibility, vegetative growth, and fruit quality traits. This study aims to advance our understanding of scion-rootstock grafting characteristics in ‘Red Globe’ and to provide valuable insights for local viticultural practices.

2. Materials and Methods

2.1. Plant Materials and Grafting Details

Following winter pruning, healthy canes of ‘Red Globe’, ‘101-14M’, ‘110R’, ‘188-08’, and ‘3309C’ were placed in a trench for storage. In late March of the following year, segments of ‘Red Globe’ canes bearing a single well-developed bud were selected as scions. Scions were then grafted onto rootstock cuttings with their buds removed and basal ends cut at a 30° angle using the cleft grafting method. The graft union was tightly sealed by wrapping it with grafting tape. A total of 200 grafts were made, and every 10 grafted cuttings were bundled and labeled. The upper ends of the scions in each bundle were quickly dipped in wax.

As we described previously, basal ends of the grafts were dipped into a rooting solution containing 20% naphthylacetic acid (NAA) and 30% indole-3-acetic acid (IAA) (Aibidi Biotechnology, Beijing, China) for 3 min, and then placed on the nursery bed. Fine sand was used to fill the spaces between grafted cuttings on the bed, with temperature and humidity controlled at approximately 25 °C and 60%, respectively, to support callus formation. Grafting experiments and evaluations were carried out annually from 2018 to 2020.

2.2. Healing and Growth Evaluation of Grafts

After a 30-day nursery cultivation, with a bundle including ten grafts taken as one replicate, four bundles for each grafting combination were randomly chosen for the evaluation of graft union formation and early vegetative growth. One bundle as a replicate was used to calculate the rooting rate, budbreak rate, callus forming index (CFI), and graft healing index (GHI). Grafted cuttings with visible root formation were considered ‘rooted’, and those displaying green tissue or shoot emergence were recorded as having undergone ‘budbreak’. We defined the callus formation grade (i) based on the percentage of callus coverage on the basal cut surface as follows: 0, 0%; 1, 1–25%; 2, 26–50%; 3, 51–75%; and 4, 76–100%. We dissected the graft union and, similarly, defined the graft healing grade (j) based on the percentage of callus coverage area on the graft cross-section as follows: 0, 0%; 1, 1–25%; 2, 26–50%; 3, 51–75%; and 4, 76–100%.

CFI = (i × N_i)/(4 × 10),

(1)

where N_i represents the number of cuttings exhibiting the corresponding callus forming grade i (i = 0, 1, …, 4).

GHI = (j × N_j)/(4 × 10),

(2)

where N_j represents the number of cuttings exhibiting the corresponding graft healing grade j (j = 0, 1, …, 4).

2.3. Survival Rate and Graft Compatibility Evaluation Under Field Conditions

Survival rate and graft union healing were assessed 120 days after transplanting (at the end of April) from the nursery to the field at Kongzhuang experimental station of Changli Institute of Fruit Research (39°42′ N, 119°05′ E; 14 m), Hebei Academy of Agricultural and Forestry Sciences. The experimental site is characterized by a typical semi-humid continental monsoon climate. The soil is loam with a pH ranging from 6.5 to 6.7. Meteorological data were obtained from the local meteorological bureau and are presented in Figure 1. For survival rate determination, a random field plot with a capacity of 100 planting sites was selected to assess plant survival. A total of 15 plants (5 plants per replicate) were measured to evaluate growth affinity using the affinity coefficient (AC), which was calculated as follows:

AC = C/A,

(3)

where A is the diameter of the scion measured 5 cm above the graft union, and C is the diameter of the rootstock measured 5 cm below the graft union. For ungrafted vines, the AC value was assumed to be 1 by default. After leaf fall, all the vines were harvested and stored as planting materials for vineyard establishment in the following year.

2.4. Growth Evaluation Under Field Conditions

The grafts harvested in 2018 were planted in the vineyard in 2019 with a spacing of 0.7 m × 4.0 m under an overhead training system. A total of 30 plants for each graft combination, including the own-rooted plants, were randomly assigned to three plots. The rows were oriented north–south, with vine growth directed eastward. After the vine canopy was fully established on the trellis in 2021, growth-related indicators were systematically evaluated during the subsequent two growing seasons (2022–2023). At the end of the growing season, the diameter of the scion was measured 5 cm above the graft union. The number of shoots per vine (NSV) was recorded, and the basal diameter of each shoot was measured. Additionally, the lengths of all the canes (mature shoots) and shoots were measured and summed separately for each vine. A total of 12 vines, 4 vines per combination from each plot, were randomly measured.

2.5. Determination of Fruit-Related Traits

Once a stable yield was achieved, fruit-related traits were assessed across two consecutive growing seasons (2022–2023). To ensure comparability, the yield per vine for each scion–rootstock combination was maintained at approximately 4.5 kg by cluster and berry thinning. Cluster thinning and berry thinning were carried out when the berries reached pea size, corresponding to stage E-L 31 [25]. The thinning standard was to retain typically one cluster per fruiting shoot, with each cluster evenly thinned to approximately 60–70 berries. Each combination included three replicates, with four vines per replicate. Three fruit clusters were randomly collected from each vine for determination. Measurements of cluster and berry size and weight were conducted following the guidelines of the International Organization of Vine and Wine (OIV) descriptors. The berry shape index (BSI) was calculated as the ratio of vertical diameter to transverse diameter of the berry.

Ten berries were randomly selected from each of the three replicates when measuring the following indices. The pulling resistance of the peduncle (PRP) and compression resistance of the berry (CRB) were measured using a mechanical force gauge (NK-50, Algol Instrument Co., Ltd., Taiwan, China) following the manufacturer’s instructions, with values recorded in Newtons (N). PRP was defined as the force reading at the moment the carpopodium detached from the berry, and CRB was defined as the reading when the berry was pressed to crack. Berry flesh firmness (BFF) was assessed by pressing a handheld fruit hardness tester (KM-1, Takemura Electric Works, Tokyo, Japan), equipped with a cylindrical tip (5 mm diameter), onto the center of the berry where the skin had been sliced. After juicing the berries, total soluble solids (TSS) and titratable acidity (TA) were measured using a digital hand-held refractometer (PAL-1, Atago, Tokyo, Japan) and a fruit acidity meter (GMK-835F, G-WON Hitech Co., Seoul, Republic of Korea), respectively, both equipped with automatic temperature compensation. TSS was expressed in °Brix, and TA was presented as the percentage of tartaric acid equivalent.

2.6. Statistical Analysis

Statistical analyses were performed using SPSS 26 (IBM Corp., Armonk, NY, USA). Significant differences among treatments were determined by one-way ANOVA followed by Tukey’s post hoc test at a significance level of p < 0.05. Principal component analysis (PCA) and figure construction were conducted in OriginPro 2018 (OriginLab Corp., Northampton, MA, USA).

3. Results

3.1. Graft Healing and Growth Indicators of ‘Red Globe’ Graft Combinations

Significant variation was observed among the graft combinations across years (

Table 1. Graft healing and early growth traits of ‘Red Globe’ (RG) grapevine hardwood cuttings grafted onto four different rootstocks from 2018 to 2020.

Year	Graft Combination	GHI	CFI	Budbreak Rate %	Rooting Rate %
2018	RG/101-14M	0.63 ± 0.05 b	0.86 ± 0.07 ab	75.0 ± 10.0	10.0 ± 8.2 ab
	RG/110R	0.78 ± 0.05 a	0.98 ± 0.04 a	62.5 ± 5.0	32.5 ± 9.6 a
	RG/188-08	0.48 ± 0.05 c	0.79 ± 0.11 b	69.3 ± 3.8	20.5 ± 19.8 ab
	RG/3309C	0.70 ± 0.00 ab	0.99 ± 0.01 a	65.0 ± 5.8	5.0 ± 5.8 b
2019	RG/101-14M	0.44 ± 0.06 a	0.53 ± 0.09 a	52.5 ± 5.0 b	32.5 ± 15.0 ab
	RG/110R	0.35 ± 0.05 ab	0.50 ± 0.10 ab	36.6 ± 5.9 c	17.0 ± 7.8 bc
	RG/188-08	0.31 ± 0.09 b	0.28 ± 0.05 c	67.5 ± 5.0 a	42.5 ± 9.6 a
	RG/3309C	0.26 ± 0.03 b	0.35 ± 0.04 bc	37.5 ± 5.0 c	7.5 ± 9.6 c
2020	RG/101-14M	0.61 ± 0.06 a	0.89 ± 0.06 a	67.5 ± 5.0 b	70.0 ± 16.3 a
	RG/110R	0.58 ± 0.03 ab	0.92 ± 0.08 a	82.5 ± 5.0 a	27.5 ± 9.6 b
	RG/188-08	0.45 ± 0.08 bc	0.43 ± 0.03 c	77.5 ± 5.0 a	52.5 ± 5.0 a
	RG/3309C	0.39 ± 0.08 c	0.67 ± 0.07 b	80.0 ± 0.0 a	20.0 ± 0.0 b

Values represent means ± SD. Different letters within the same year and column indicate statistically significant differences at p < 0.05. CFI, callus formation index; GHI, graft healing index.

). In 2018, RG/110R exhibited the highest GHI, significantly higher than RG/188-08, while RG/101-14M and RG/3309C showed intermediate values. Similar trends were observed for CFI, where RG/110R (0.98) and RG/3309C (0.99) were significantly superior. Budbreak rates were highest in RG/101-14M and lowest in RG/110R, whereas rooting rates varied notably, with RG/110R outperforming the other combinations. In 2019, RG/101-14M maintained the highest GHI, while RG/3309C showed the lowest. CFI declined across all the combinations compared to the previous year. RG/188-08 had a notably higher budbreak rate compared to other combinations, particularly RG/110R and RG/3309C (<40%). The rooting rate of RG/188-08 was significantly higher than RG/3309C. In 2020, GHI and CFI values recovered in all the combinations. RG/101-14M again achieved the highest rooting rate, with superior CFI, whereas RG/3309C had the lowest values in both traits. Budbreak rate was highest in RG/110R and lowest in RG/101-14M. Overall, RG/101-14M demonstrated consistently high GHI and rooting rates across years, while RG/110R exhibited superior CFI and budbreak performance in 2020.

3.2. Field Survival Rate of ‘Red Globe’ Grafting Combinations

The own-rooted ‘Red Globe’ vines (RG) consistently exhibited the highest survival rates, ranging from 80% in 2018 to 85% in 2020, with a mean of 82% (Figure 2). Among the grafted combinations, RG/101-14 and RG/188-0 showed relatively better performance, with mean survival rates of 71% and 74%, respectively. In contrast, RG/110R and RG/3309C had lower and more variable survival rates, averaging 64% and 65%, respectively. Notably, RG/110R and RG/3309 exhibited a marked decline in 2019 and 2020, suggesting potential incompatibility or reduced environmental adaptability in those years. These findings indicate that graft compatibility and rootstock selection significantly influence field survival outcomes over multiple growing seasons, and RG/188-08 and RG/101-14M combinations maintained relatively higher and more stable field performance over time compared to the other rootstocks tested.

3.3. Graft Compatibility of ‘Red Globe’ Grafting Combinations as Indicated by Field Growth

The ratio of rootstock to scion diameter, used as a morphological indicator of graft compatibility, where a value closer to one indicates better anatomical matching between graft partners, varied among the different graft combinations and years (Figure 3). Over all three years, all the grafted combinations exhibited ratios significantly greater than one, indicating that the rootstocks tended to be thicker than the scions. Among the tested combinations, RG/110R had the highest diameter ratio in 2018 (2.10), suggesting a marked overgrowth of the rootstock, potentially indicative of lower compatibility. In contrast, RG/3309C showed more moderate diameter ratios in 2019 (1.28) and 2020 (1.65), relatively closer to the ideal 1:1 ratio. RG/188-08 and RG/101-14M maintained intermediate values with less fluctuation, with RG/101-14M displaying the most consistent ratios over time (ranging from 1.59 to 1.78). Overall, RG/3309C and RG/101-14M demonstrated relatively better anatomical compatibility, whereas the pronounced imbalance observed in RG/110R may reflect weaker graft union coordination.

3.4. Vegetative Growth of ‘Red Globe’ Grafting Combinations

To assess the vegetative growth response of ‘Red Globe’ grafted onto different rootstocks, five growth-related traits were evaluated in 2022 and 2023 (Figure 4).

In 2022, the own-rooted ‘Red Globe’ (RG) exhibited moderate performance across most traits, with a scion diameter of 14.4 mm and 4.9 shoots per plant. Among the grafted combinations, RG/3309C showed the largest scion diameter (16.8 mm), the greatest number of shoots per tree (3.07), and the highest shoot diameter (10.33 mm). It also achieved the longest total cane length of 514.73 cm and total shoot length of 620.23 cm, indicating superior vigor. Conversely, RG/101-14M and RG/188-08 showed lower total cane lengths, 388.2 cm and 414.27 cm, respectively, although shoot diameters remained comparable across all the combinations. In 2023, growth performance increased markedly across all the combinations. The self-rooted RG showed a scion diameter of 20.83 mm and 5.93 shoots per tree, but still lagged behind most grafted combinations in terms of total shoot length. RG/3309C again exhibited the most vigorous growth, with the highest number of shoots (7.47), a total cane length of 648.67 cm, and a total shoot length of 656.67 cm. RG/110R and RG/188-08 also demonstrated strong performance, with longer cane and shoot lengths than RG. RG/101-14M, while showing a relatively moderate number of shoots and shoot diameter, exhibited the lowest variability and maintained stable performance across both years.

Overall, RG/3309C consistently promoted the most robust vegetative growth in both seasons, while RG/110R also showed notable vigor. The self-rooted control exhibited moderate growth, highlighting the enhancing effects of these rootstocks on ‘Red Globe’ vine vigor.

3.5. Fruit Morphology Characteristics of ‘Red Globe’ Grafting Combinations

In 2022 (

Table 2. Fruit characteristics of different ‘Red Globe’ (RG) graft combinations in 2021 and 2022.

	Graft Combinations	Cluster Weight (g)	Cluster Length (cm)	Cluster Width (cm)	Single Berry Weight (g)	Berry Vertical Diameter (mm)	Berry Transverse Diameter (mm)	BSI	Peduncle Length (mm)	Peduncle Diameter (m)
2022	Own-rooted	647.9 ± 152.4	19.3 ± 1.5	13.3 ± 2.1	12.8 ± 0.38 ab	30.5 ± 0.7 a	26.5 ± 0.5 a	1.2 ± 0.04	9.2 ± 0.4 a	3.2 ± 0.02 c
	RG/101-14M	656.8 ± 261.5	19.7 ± 4.5	12.3 ± 1.5	11.7 ± 0.20 bc	28.6 ± 0.5 bc	25.0 ± 0.4 bc	1.1 ± 0.02	7.9 ± 0.2 bc	3.7 ± 0.09 a
	RG/110R	684.2 ± 214.2	18.7 ± 4.2	13.0 ± 1.0	13.6 ± 0.10 a	29.7 ± 0.2 ab	26.5 ± 0.3 a	1.1 ± 0.02	8.7 ± 0.3 ab	3.4 ± 0.08 ab
	RG/188-08	723.0 ± 166.3	20.3 ± 2.5	12.7 ± 1.5	12.5 ± 0.44 ab	27.9 ± 0.7 c	24.6 ± 0.5 c	1.1 ± 0.03	9.7 ± 0.5 a	3.5 ± 0.34 ab
	RG/3309C	664.1 ± 133.4	20.0 ± 3.5	12.0 ± 2.0	11.1 ± 0.68 c	28.5 ± 0.3 bc	26.0 ± 0.6 ab	1.1 ± 0.02	7.5 ± 0.5 c	3.1 ± 0.09 c
2023	Own-rooted	601.1 ± 79.1	22.7 ± 1.5	12.3 ± 1.5	12.2 ± 0.12 a	27.8 ± 0.7 ab	26.3 ± 0.9 ab	1.1 ± 0.03	11.1 ± 0.7	2.5 ± 0.16 a
	RG/101-14M	581.1 ± 100.2	21.7 ± 3.1	12.7 ± 1.2	11.1 ± 0.06 b	28.9 ± 0.9 ab	25.8 ± 1.4 ab	1.1 ± 0.04	10.2 ± 0.4	2.3 ± 0.15 ab
	RG/110R	747.7 ± 65.2	24.3 ± 0.6	15.0 ± 1.0	12.4 ± 0.08 a	29.6 ± 1.2 a	27.2 ± 0.6 a	1.1 ± 0.07	10.6 ± 1.1	2.3 ± 0.23 ab
	RG/188-08	810.4 ± 219.2	22.3 ± 4.2	15.0 ± 1.0	11.4 ± 0.09 b	27.7 ± 0.9 ab	24.9 ± 0.6 b	1.1 ± 0.02	10.9 ± 0.2	2.5 ± 0.09 a
	RG/3309C	776.6 ± 192.7	23.0 ± 1.7	16.3 ± 3.5	10.3 ± 0.19 c	26.8 ± 0.7 b	24.4 ± 0.2 b	1.1 ± 0.03	10.8 ± 0.3	2.0 ± 0.03 b

Values represent means ± SD. Different letters within the same year and column indicate statistically significant differences at p < 0.05. BSI, berry shape index.

), the self-rooted RG vines produced clusters with an average weight of 647.9 g. Among the grafted combinations, RG/188-08 showed the highest cluster weight, while RG/3309C and RG/101-14M produced slightly lighter clusters. RG/110R had the highest single berry weight of 13.6 g, significantly exceeding RG/3309C (11.1 g). Berry vertical and horizontal diameters were generally largest in RG and RG/110R, while RG/188-08 showed smaller berry dimensions. The berry shape index remained stable (around 1.1) across all treatments, indicating consistent berry proportions. Peduncle length and diameter varied, with RG/188-08 exhibiting the longest peduncle (9.7 mm), whereas RG/3309C had the shortest (7.5 mm). RG/101-14M showed the largest peduncle diameter (3.7 ± 0.09 mm), in contrast to the thinner peduncles in RG and RG/3309C.

In 2023, cluster weight generally increased across grafted combinations, with RG/188-08 and RG/3309C producing the heaviest clusters of 810.4 g and 776.6 g, respectively. RG/110R also maintained a high cluster weight of 747.7 g, accompanied by significantly larger cluster length and berry width. RG/3309C exhibited the widest berries (16.3 mm) but the lowest single berry weight of 10.3 g. The self-rooted RG remained moderate in most traits, producing berries with a relatively high weight of 12.2 g and a peduncle diameter of 2.5 mm. Berry shape indices continued to show uniformity (1.1), indicating that rootstock had a limited effect on altering berry proportions. RG and RG/188-08 exhibited thicker peduncles compared to RG/3309C.

3.6. Berry Texture and Taste-Related Traits of ‘Red Globe’ Grafting Combinations

To assess the effects of grafting on fruit force and berry texture, we measured pedicel pulling resistance, berry compression resistance, flesh firmness, berry weight, total soluble solids (TSS), and titratable acidity (TA) across two consecutive years (Figure 5).

In 2022, significant differences in berry detachment force were observed among rootstock treatments. The highest resistance was found in RG/110R (11.7 N), while RG/101-14M exhibited the lowest (8.5 N), suggesting easier berry detachment in the latter. RG/188-08 and RG/3309C also showed relatively high resistance (10.6–10.8 N). Compression resistance remained relatively uniform across treatments (~29.5–30.1 N). Flesh firmness was significantly higher in RG/3309C (0.4 N), while other combinations remained at around 0.3 N. Notably, RG/188-08 yielded the highest TSS of 16.1 °Brix, while TA levels were generally consistent across combinations, except for a slightly lower value in RG/110R.

In 2023, RG/110R had the highest PRP of (13.4 N), followed by RG/3309C and RG/188-08 (11.1 N and 11.5 N, respectively). Interestingly, compression resistance decreased slightly in most combinations, particularly in RG/3309C (24.9 N). Flesh firmness was highest in RG/101-14M (0.3 N), while RG, RG/110R, and RG/3309C had significantly lower firmness values. The TSS was the highest in RG/188-08 with 16.0 °Brix, indicating its consistent enhancement of sugar accumulation. TA levels declined in most combinations, with RG/3309C showing the lowest value of 0.8, while RG/101-14M maintained the highest acidity of 1.1%.

Overall, the rootstocks showed distinctive impacts on berry detachment force and fruit textural traits. RG/110R enhanced berry size and pedicel strength but reduced acidity, while RG/188-08 consistently promoted high sugar content and structural resistance. In contrast, RG/3309C decreased berry size and firmness, which may impact fruit quality and postharvest characteristics.

3.7. Comprehensive Characteristics of ‘Red Globe’ Grafting Combinations

Principal component analysis (PCA) was performed to summarize the variation in agronomic and fruit quality traits among four ‘Red Globe’ graft combinations. The first two PCs (PC1 and PC2) accounted for 48.3% and 30.2% of the total variance, respectively (Figure 6).

RG/101-14M clustered near the origin but pointed in the direction of traits, including TA, rooting rate, peduncle diameter, and berry shape index, indicating it produced lengthened berries with higher acid content. RG/110R, positioned in the upper-left quadrant, was primarily associated with increased shoot diameter, berry horizontal diameter, and CFI, suggesting this rootstock combination enhances shoot development and berry expansion. RG/3309C, located in the lower-left quadrant, was closely aligned with cluster size, cluster weight, scion diameter, and total shoot length, indicating that this rootstock promotes vegetative growth and cluster development. RG/188-08 was associated with traits such as TSS, flesh firmness, budbreak rate, and survival rate, suggesting that rootstock ‘188-08’ had a better affinity with ‘Red Globe’ under field conditions and produced berries with a sweeter taste and crisper texture. Overall, the PCA biplot differentiates the graft combinations based on their trait profiles. From the perspective of grafting compatibility and fresh and postharvest fruit quality, 188-08 appears to be the most suitable rootstock for the ‘Red Globe’ grape.

4. Discussion

Hardwood grafting is a traditional option for grapevine propagation that has been preferred by growers because it can be carried out indoors and its a higher establishment success rate [15]. These results on grafting indicate year-specific interactions between scion and rootstock combinations, influencing graft healing dynamics and vegetative responses (Table 1). In 2019, GHI, CFI, and budbreak rate were lower than those in the other two years, although the grafting and nursery were conducted by skilled workers in controllable conditions following standard procedures. Otherwise, the canes from the previous year used for rootstock or scion may be less healthy due to unexpected incidences of pests or diseases. Regardless, both ‘110R’ and ‘101-14M’ exhibit a higher propensity for callus formation with ‘Red Globe’. The overall field survival rate of grafted vines in 2019 was relatively low, which was likely associated with the excessive rainfall during that growing season (Figure 1F and Figure 2), which may have resulted in waterlogging, causing irreversible damage to the root system [26]. In the absence of harsh environmental stress, own-rooted ‘Red Globe’ vines appear to benefit from their physiological integrity, exhibiting relatively higher survival rates (Figure 2), whereas RG/188-08 exhibited a higher survival rate as the own-rooted vines during the rainy season. But the underlying cause remains unclear, although it is possible that it possesses a relatively strong adaptability to water stress. Among the surviving grafts, those on rootstock ‘188-08’ as well as ‘3309C’ grew more coordinatedly (Figure 3), and this affinity might contribute to the integrity of grafted vines to achieve a higher survival. In addition, the affinity coefficients of these graft combinations were relatively high in the present study; for instance, RG/110R exhibited an average affinity coefficient (AC) of 1.7, which is much greater than the values (0.99–1.02) reported by Jogaiah et al. [27] in a green grafting conducted in India. Similar AC values below 1 have also been reported in green grafting of ‘Pusa Urvashi’ grape on four rootstocks [28], while in another research on hardwood grafting of ‘Syrah’ showed that stem diameters below the graft union were much larger than those above the union [29], which may be attributed to factors such as the grafting materials and the grafting method employed. An earlier study showed that when ‘Red Globe’ was grafted onto 41B and 140R, the AC values over two years ranged from 1.2 to 3 [22]. While AC values of ‘Cabernet Sauvignon’ grafted on eight rootstocks, including ‘101-14M’ and ‘110R’, were lower than 1.0 across two years [30]. Therefore, this might be due to the relatively weaker growth vigor of ‘Red Globe’ compared with the rootstocks. However, the scion diameters of the grafted combinations were comparable to or even greater than the basal stem diameter of own-rooted vines (Table S1), indicating that the rootstocks effectively enhanced the basal thickness of the scions during the establishment season.

Vegetative growth observations further support that the rootstocks conferred enhanced vigor to the scion, a trend that was particularly pronounced in 2023 (Figure 4). The more vigorous growth observed in 2023 may be attributed to the increased precipitation during the growing season (Figure 1H). Rootstock ‘3309C’ conferred the greatest vegetative growth to ‘Red Globe’, primarily due to the higher number of shoots sprouting from the scion (Figure 4). This is consistent with the findings from Schmid et al., who reported that ‘3309C’ can more readily increase the vine size of ‘Concord’ grape [31]. Rootstock ‘3309C’ has also been reported to positively influence trunk diameter, shoot length, and leaf area index in hybrid grape cultivars in the North America region [32]. Meanwhile, we observed that ‘3309C’ led to a noticeable reduction in berry size and peduncle size, suggesting a potential trade-off between vegetative vigor and fruit development. Similar effects of ‘3309C’ on vegetative growth and berry mass of scions like ‘Chardonnay’ and ‘Cabernet Sauvignon’ were observed when comparing it with those of ‘101-14M’ and ‘110R’ [4]. For own-rooted vines, the opposite pattern was evident, with reduced vegetative growth accompanied by enhanced fruit development. A similar balance also appears to exist between berry weight and quality. Smaller berries of ‘Chardonnay’ have been reported to exhibit higher TSS [16], a trend also observed in the present study. For example, the smaller berry weights of RG/101-14 and RG/188-08 in 2023 may have contributed to their high TSS. However, berry weight may be only one of the factors affecting TSS, as some cases, like RG/3309C in 2022, do not follow this trend. The RG/188-08 combination exhibited higher berry sweetness and demonstrated superior postharvest traits such as flesh firmness, crush resistance, and peduncle detachment (Figure 5). This effect observed in ‘188-08’ appears to result from a scion–rootstock interaction, as our previous evaluation on graft combinations involving ‘Yueguangwuhe’ grape yielded conclusions similar to those in the present study [13], whereas opposite results were obtained for grafts involving ‘Summer Black’ [12]. Rootstock ‘101-14M’ appears to promote fruit ripening, and its relatively low peduncle detachment force may lead to increased fruit drop, which is consistent with our previous observations in fresh seedless grapes ‘Summer Black’ and ‘Yueguangwuhe’ [12,13]. Given that all these mentioned studies were carried out within the same geographical region, the consistency in performance of certain rootstocks may, at least in part, be attributed to regional environmental conditions, which cannot be excluded as a contributing factor. However, this observation differs from findings in ‘Chardonnay’ that when grafted on ‘101-14M’ in a semi-humid zone (40.14 °N, 116.19 °E), which is similar to that of the present study, maturation tended to be delayed as indicated by increased TA and decreased TSS/TA [16], indicating the substantial influence of the scion on the rootstock. On the other side, the fruit of ‘Red Globe’ grafted onto ‘110R’ showed a lower degree of maturity, featured in higher PRP and lower TSS. Consistently, Wang et al. also observed notably reduced TSS in ‘Ruidu Xiangyu’ and ‘Ruidu Hongyu’ when grafted onto ‘110R’, further supporting its potential delaying effect on fruit ripening [33]. Migicovsky et al. reported inconsistent effects of ‘110R’ on total soluble solids content in ‘Cabernet Sauvignon’ and ‘Chardonnay’ [4], a phenomenon believed to result from the scion-mediated modulation of rootstock performance [34]. In a multi-location grafting study involving two scion cultivars grafted onto eight rootstocks, ‘101-14M’ consistently enhanced the total soluble solids of ‘Chardonnay’ across different environments, whereas its effect on ‘Shiraz’ was less pronounced [35]. It demonstrates that the scion has a substantial influence on the actual impacts of the rootstock. Tandonnet et al. demonstrated that the scion can influence the root vigor of the rootstock [36], which may partially explain the variations in different scion cultivars on the same rootstock.

Rootstocks tend to maintain their characteristics; however, their effects on scion traits are evidently influenced by the environment and by the scion itself. Unlike other rootstocks, ‘188-08’, a moderately vigorous rootstock [37], has been scarcely documented in previous studies on grafting, especially in grafting studies using ‘Red Globe’ as the scion. We found that compared to other rootstocks, it exhibited advantages in grafting success and fruit quality of ‘Red Globe’ grafted vines. With reported moderate resistance to phylloxera and drought [5,38], this rootstock may be particularly suitable for the cultivation of ‘Red Globe’ in environments where such stresses are prevalent.

5. Conclusions

In this study, a multi-year comprehensive assessment was performed on the grafting performance of ‘Red Globe’ onto four rootstocks, encompassing graft compatibility, field survival, vegetative growth of mature vines, as well as fruit phenotypic and quality traits. The findings revealed significant differences among these scion–rootstock combinations. Despite exhibiting relatively poor graft union healing in the early stages, the successfully grafted RG/188-08 vines achieved high field survival. Although the vigor of the mature vine was moderate, this combination yielded fruit with better sensory qualities, including higher soluble solids content and firmer flesh than own-rooted vines, indicating that ‘188-08’ is a suitable and promising rootstock for ‘Red Globe’ cultivation.