Physiological Responses of Cabernet Sauvignon to Dividing Canopies in the Chihuahuan Desert

Abstract

Canopy architecture is fundamental to productivity in grapevines. This research focused on evaluating the impact of opening canopies on the capture of photosynthetically active radiation, photosynthetic activity, and berries’ physicochemical properties in Cabernet Sauvignon grapevines. A completely randomised design was used to compare open and closed canopies, with ten replicates per treatment (20 vines in total), during the vegetative growth period and after harvest. The key measurements included photon flux density (PFD), daily light integral (DLI), photosynthetic rate (PR), stomatal conductance (SC), intercellular CO₂ concentration (IC), leaf area (LA), transpiration, ambient CO₂ concentration, and temperature. Additionally, we assessed berry quality variables, such as total soluble solids (TSS), glucose/fructose ratio, total titratable acidity (TTA), pH, TSS/TTA, and total phenols (TP). During vegetative growth, PFD, DLI, PR, IC, and LA increased significantly (p ≤ 0.05), whereas after harvest, only PR and IC showed variation Closed canopies increased water use efficiency (CO₂/H₂O) by 62.5%, while the temperature was higher in open canopies. Opening canopy increased contacts, gaps and visible sky and reduced leaf area index. Berries from open canopies showed higher TSS, glucose-fructose, pH, TSS/TTA and TP contents. Opening canopy is essential for improving light interception, photosynthetic efficiency, and fruit quality in Cabernet Sauvignon grapevine cultivated in northern Mexico.

1. Introduction

Light quality, intensity, and direction influence not only the photosynthetic process, but also plant development through photomorphogenesis. Light interception by canopies is fluctuating and complex, hence good canopy design and management are critical to optimizing light distribution among foliage [1]. Previous studies have demonstrated the importance of diffuse radiation to productivity in perennial crops [2]. Photosynthesis and biomass increase are maximized at optimal light levels [3], although excessive light can cause photorespiration and irreversible cell damage [1]. Thus, maintaining an optimal light distribution within the canopy is key to maximizing crop yield and quality under varying climate change scenarios [4].

Grapevine (Vitis vinifera L.) is among the crops that enjoy some of the highest cultural, nutritional, and commercial importance worldwide. However, its commercial production requires good canopy management and efficient use of water, light, and CO₂ [5]. This requires the design of appropriate trellis systems (vertical, lyra, pergola, etc.), while winter and summer pruning play an important role [6]. Vine pruning defines the shoots’ spatial distribution and allocation of canopy components (branches, shoots, foliage, and clusters) [7]. In addition, pruning brings about significant improvements in physiological responses through carbohydrate synthesis derived from efficient light use [8].

High productivity is directly linked to the optimal use of photon flux density (PFD) [9]. Canopy management has been shown in previous studies to be a key determinant in berry growth and development [10]. Given that a grapevine is a liana, its distribution of branches, shoots, and leaves is dependent upon mechanical supports [5]. Opening canopies involves hiring trained labor and the removal of up to 50% of shoots, significantly increasing production costs.

Modern viticultural production systems have favored the development and implementation of multiple training systems. In addition, these systems facilitate the mechanization process and cost reduction in terms of manual pruning, shoot positioning, foliar spraying of pesticides and nutrients, as well as harvest [11]. Therefore, the establishment of new vineyards should consider annual pruning and the optimal selection of training systems according to vineyard geographical location, soil type, climatic conditions, and crop destination, always seeking to increase the exposure of the leaf surfaces to solar radiation [9,12].

The Mexican territory is a mosaic of great climatic diversity. This represents a wide opportunity for the establishment of vineyards for the commercial production of grapes for fresh consumption, dehydrated and processed as juices, jams, distillates, and potentially high-quality wines [13]. However, global climate change is causing increased incidences of extreme events in terms of air temperatures and erratic precipitation, conditions that must be considered. In this context, grapevines have been demonstrated to offer improved agronomic performances, including optimal development of oenological attributes such as soluble solids, acidity, pH, phenols, anthocyanins, tannins, etc. [14]. Finally, understanding the relationship between biological and environmental factors modulating shoot growth and their interaction with reproductive organs is essential to optimize the regulation of carbon allocation between vegetative and reproductive organs, as well as to understand canopy microclimate. Therefore, this research aimed to evaluate changes in photosynthetically active radiation, photosynthetic activity-related variables, canopy architecture, and some aspects of physicochemical berry quality in response to increasing light penetration in the canopy. Cabernet Sauvignon grapevines were used given their wide acceptance and presence.

2. Materials and Methods

2.1. Site, Plant Material and Experimental Design

This study was conducted in 2022 (June and September) and 2023 (June–July) in Meoqui, Chihuahua, Mexico (28°23′23″ N,105°37′25″ W) at an altitude of 1200 masl, where mean annual temperature was 18.6 °C and precipitation was 369.8 mm. Grapevines of cv. Cabernet Sauvignon grafted on P-111 were used; the vineyard was 13 years old and planted along north-south rows at 4000 vines ha⁻¹ (1 × 2.5 m). Vines were formed as bilateral cordons, spur-pruned to one or two buds, and conducted on a vertical trellis as a vertical shoot positioning (VSP) system. Therefore, it was feasible to establish a comparison between two (w/o) canopy management treatments named open and closed canopies. To achieve such a goal, the open canopy treatment consisted of spreading the vertical shoots to both sides of the row and keeping shoots in position by means of mobile wires supported on a T structure placed 1 m above the cordon height; that way, the center of the row was divided in two sections, opening the canopy and allowing light penetration. In the close canopy treatment, no further management was applied. During winter, pruning was carried out, leaving on one or two 2 buds per spur depending on shoot vigor. In the summer, shoot thinning was performed by eliminating those misplaced or in a wrong position.

Vines selected as experimental units had average heights of 2.5 ± 0.5 m and trunk diameters of 35 ± 2 cm (measured at 20 cm above ground level). The vineyard was established on a loam/sandy textured soil with 55% sand, 24% silt, 20% clay in the top (0 to 35 cm) layer. The pH was 7.6, organic matter 0.71%, and electrical conductivity 0.4 dS m⁻¹. Soil nutrient contents were: 148 N and 15 P (kg ha⁻¹); and 175 K⁺, 3200 Ca²⁺, 738 Mg²⁺, 0.32 Fe²⁺, 2.86 Mn²⁺, 1.62 Zn²⁺, and 3.46 Cu²⁺ (mg kg⁻¹). Fertilization was applied to the soil by the end of March using a general formula of 80 N:80 P₂O₅:100 K₂O. Management followed standard commercial practices for weed and pest control, and irrigation was scheduled throughout the experiment.

Vines with uniform trunks were randomly selected. Ten replicates were used in the two treatments (close and open canopies), considering each vine as an experimental unit. Photon flux density (PFD) was recorded as well as integrated daily light (DLI), generating data sets of net photosynthesis, stomatal conductance, intracellular CO₂ content, leaf area, transpiration, air CO₂, air and leaf temperatures, numbers and sizes of stomata, and canopy architecture as the number of leaf layers, leaf area index, fraction (%) of canopy gaps and fraction (%) of sky visible from below the canopy. These measurements were made at 08:00, 11:00, 14:00 and 17:00 h. Berry quality was analyzed according to canopy treatment based on sampling dates of June 27 and 29, and July 4, 6, 10, 13, 18, 20, 23, and 27, in 2023.

2.2. Photosynthetically Active Radiation and Daily Light Integral

PFD was determined using a 1 m long linear radiometer LI-191 (Li-Cor, Lincoln, NE, USA) [15]. The radiometer was placed on a horizontal support, along the north/south row. PFD readings were taken at four orientations (east, west, upright, and downward). In each orientation, three measurements were averaged. Data were recorded at 08:00, 11:00, 14:00 and 17:00 h. The results are expressed as µmol m⁻² s⁻¹.

The daily light integral (DLI) was estimated according to the equation ∑PFD (µmol m⁻² s⁻¹) × 0.0144; where the factor 0.0144 corresponds to the total number of seconds of measurement per day, divided by 1,000,000 [16]. The determinations of PFD and DLI were carried out during berry ripening phenophase (on 9 June) and after harvest (on 22 September) in 2022.

2.3. Leaf Area

Leaf area (LA) was determined according to a procedure previously described [1] with slight modifications. Briefly, during the vegetative growth stage, 60 undamaged leaves were selected per treatment (open and closed) following cardinal orientations. Average leaf area was determined using a scanner (Canoscan LIDE 100, Tokyo, Japan) and Photoshop CS1 software (Adobe, San José, CA, USA). The LA results were expressed in cm².

2.4. Number and Dimensions of Stomata, Stomatal Pore and Guard Cell

Stomatal measurements were made on 9 June 2022, according to a method described elsewhere [17], with slight modifications. Casts were made using transparent nail polish (nitrocellulose). A thin layer was applied to abaxial and adaxial leaf surfaces and allowed to dry for approximately 120 s. Following film removal, transparent adhesive tape was used to transport this film to a slide to obtain the leaf epidermal cast. Stomata density (per mm²) was quantified using an optical microscope (Olympus BX51, Melville, NY, USA) with a 20× objective and 0.15 mm² field of view. Stomata numbers and dimensions (length and width), stomatal pore area, and guard cells were also determined.

2.5. Parameters of Photosynthetic Activity and Canopy Architecture

Determination of photosynthetic rate, stomatal conductance, intercellular CO₂, transpiration, air CO₂, temperature (air and leaf), and stomatal conductance was carried out on previously selected plants with mature leaves, recording treatment (open or closed) with four readings per experimental unit (two on the east and two on the west sides of the row). These variables were recorded during véraison on 9 June 2022, and after harvest on 22 September 2022. Readings were taken at 08:00, 11:00, 14:00, and 17:00 h [12]. A non-dispersive infrared CO₂ analyzer LI-6400XT (Li-Cor., Lincon, NE, USA) was used, equipped with a 6400 LED cool light source assimilation chamber. A camera was used to match the natural ambient light conditions in the foliage at the time of reading.

Prior to harvest on 9 June 2022, the number of leaf contacts (leaf layers), the fraction (%) of canopy gaps (GAP), the fraction (%) of sky visible from below the canopy, and the leaf area index were assessed. These data were recorded using an LI-2000 canopy analyzer (Li-Cor, Inc., Lincoln, NE, USA), equipped with a fisheye hemispherical lens and an optical system with five concentric detectors useful for determining the distribution of canopy foliage at five angles (7, 23, 38, 53 and 68° from the vertical). The equipment was placed on top of the canopy to capture incident ambient light (defined as sky), following readings at the same location, but in the canopy middle height, close to the fruiting zone, taking the four readings recommended by the device manufacturer. These measurements were repeated on four plants along the row on both treatments.

2.6. Berry Quality

A sample of 200 berries was harvested per treatment on each sampling date (27, 29 June; 4, 6, 10, 13, 18, 20, 23, and 27 on July 2023). Berry selection was conducted considering their position in the cluster (shoulder, middle and lower sections) with three replicates per treatment. The samples were transported in polystyrene boxes for juice extraction, which was performed manually by smashing the samples without breaking the seeds, following storage at 4 °C for 1 h while all samples were processed. The juices were centrifuged at 10,000 rpm for 15 min at 4 °C, and allowed to stand for 15 min, after which supernatants were taken. The total soluble solids (TSS) were determined using a temperature-corrected Atago ATC-1 hand-held refractometer (Atago, Tokyo, Japan) at 20 °C. Results are expressed in °Brix. D-glucose/D-fructose concentration was determined using an enzymatic method described elsewhere [18] with modifications. A 2 mL sample of juice was taken, centrifuged at 3000 rpm for 15 min at 4 °C, following absorbance at 340 nm with a UV visible spectrophotometer Y15 (Biosystems, Spain). The results were expressed in g L⁻¹. Total titratable acidity (TTA) in g L⁻¹ of tartaric acid was determined by titration with 0.1 N NaOH, and pH was determined using a HI-2002 phmeter (Hanna Instrument, Woonsocket, RI, USA). Total phenols (TP) were determined by the Folin–Ciocalteu method [19], and the results were expressed as mg GAE L⁻¹.

2.7. Statistical Analyses

The statistical analyses consisted of determining differences between the open and closed canopies treatments with respect to PFD, DLI, leaf area, and photosynthetic variables like photosynthetic activity, stomatal conductance, intercellular CO₂, transpiration, air CO₂, temperature (air and leaf), water use efficiency, and stomatal conductance. Using paired samples, the means of the observations were grouped according to the angle of measurement of the radiometer, plant orientation, and time of recording. Data regarding the number and dimensions of stomata, ostioles, and guard cells and on canopy architecture, the number of leaf contacts or layers, fraction of GAP (%), fraction of visible sky below the canopy (%), and LA were analyzed with Student’s t-test for independent samples. The values of gaps and the fraction of sky visible from below the canopy were subjected to linear regression analysis. Finally, the berry quality data were analyzed with a curvilinear regression model for which the sampling date was considered as the independent variable. All analyses were performed with the software SPSS (Statistical Package for the Social Sciences) 19.0. FP1.

3. Results

3.1. Photosynthetically Active Radiation and Photosynthetic Activity Parameters

Efficient canopy management, combining winter and summer pruning, including thinning of clusters, shoots, and leaf removal in the fruiting zone prior to véraison, contributes to improved yield and berry quality [20]. In this regard, at the vegetative growth stage (9 June), opening canopies in Cabernet Sauvignon vines significantly increased (p ≤ 0.05) the interception of PFD, daily light integral (DLI), photosynthetic rate (PR), stomatal conductance (SC), and intracellular CO₂ (CI) and leaf area (LA), with values of 415 µmol CO₂ m⁻² s⁻¹, 5.9 mol d⁻¹ m⁻², 8.9 mmol CO₂ m⁻² s⁻¹, 291 mmol CO₂ mol⁻¹ and 98 cm², respectively (

Table 1. Effect of canopy management on physiological features of Cabernet Sauvignon grapevines at two phenological stages.

Treatments	PFD	DLI	PR	SC	CI	LA
	9 June 2022 (vegetative growth)
Open canopy	415.9 a	5.9 a	8.9 a	150.1 a	291.4 a	97.92 a
Closed canopy	266.0 b	3.8 b	7.2 b	140.2 a	272.1 b	95.20 b
	22 September 2022 (after harvest)
Open canopy	320.7 a	4.6 a	4.2 a	90.34 a	304.5 a	91.39 a
Closed canopy	298.6 a	4.3 a	3.0 b	90.82 a	289.6 b	90.67 a

PFD: Photon flux density (µmol m⁻² s⁻¹); DLI: daily light integral (mol d⁻¹ m⁻²); PR: photosynthetic rate (mmol CO₂ m⁻² s⁻¹); SC: stomatal conductance (mmol H₂O m⁻² s⁻¹); Ci: intracellular CO₂ (mmol CO₂ mol⁻¹); LA: leaf area (cm²). Means with the same letter within a column are not different (Student’s t test, p ≤ 0.05).

). On the postharvest evaluation made on 22 September 2022, both canopies showed no significant differences (p > 0.05) for PFD, DLI, SC, and LA. The only significant difference found was for PR (4.2 mmol CO₂ m⁻² s⁻¹) and CI (mmol CO₂ mol⁻¹) since the closed canopy treatment values fell behind those in open canopy.

Compared to vines under a closed canopy, the leaves expanding prior to fruit ripening in the open canopy vines had a significantly greater (p ≤ 0.05) ratio in water use efficiency. Regardless of phenological, stage water use efficiency (H₂O/CO₂) increased when canopies were open, showing an improvement of 33% (1180 vs. 1760 g L⁻¹). Therefore, it provided an extra tool for managing irrigation schedules, thus avoiding water stress. Transpiration and leaf and air temperatures did not show significant changes. (

Table 2. Changes in transpiration, gas exchange, and leaf and air temperature in grapevine cv Cabernet Sauvignon with an open or a closed canopy structure.

Treatment	Transpiration	H2O/CO2	LT	AT
	9 June 2022 (vegetative growth)
Open canopy	6.4 a	1180.4 b	35.9 a	36.9 a
Closed canopy	6.0 a	1760.4 a	36.2 a	37.3 a
	22 September 2022 (after harvest)
Open canopy	2.9 a	359.5 a	32.9 a	33.2 a
Closed canopy	2.8 a	355.2 b	32.2 b	32.4 b

Tr: transpiration (mmol H₂O m⁻² s⁻¹); H₂O/CO₂: g L⁻¹; LT: leaf temperature (°C) and AT: air temperature (°C). Means with the same letter within a column are not different (Student’s t test, p ≤ 0.05).

).

A remarkable condition was found after harvest, since, while transpiration remained without significant changes between treatments, leaf and air temperatures in open canopies reached higher values, albeit small yet significant (Table 2).

Table 3. Number and dimensions of stomata, stomatal pore and guard cells of leaves of the grapevine cv Cabernet Sauvignon with canopy structure open or closed.

Treatment	NS mm2	LS	WS	LSP	SPO	WGC
Open canopy	47.5 a	31.62 a	24.10 a	13.62 a	1.88 a	8.42 a
Closed canopy	47.5 a	31.35 a	23.15 a	15.2 a	1.55 a	7.40 a

NS: number of stomata; LS: length of stomata; WS: width of stomata; LSP: length of stomatal pore; SPO: stomatal pore opening; WGC: width of guard cell. Means with the same letter within a column are not significantly different (Student’s t test, p ≤ 0.05). The data are expressed in µm.

shows no significant differences in number of stomata per mm² nor for their dimensions (length and width), stomatal pores, and guard cells. Although without significant differences, stomatal measurements in open canopy leaves show slightly higher values in most variables.

Herein, canopy architecture is described by the number of leaf contacts or layers, gaps within the canopy, visible sky below the canopy, and leaf area index (Figure 1). Contacts were consistently higher in open canopies, showing a decreasing number from 6 to 1 as the angles from the zenith increased, while closed canopies ran from 3 to 1, intersecting both curves at around 60° from the zenith. Gap, on the other hand, showed an almost parallel fashion but greatly different, since at an angle of 68° from zenith, open canopies doubled the number of gaps, meaning better chances for light penetration in the canopy.

This means that opening canopies has a strong effect on plant development. The open canopy showed a higher percentage of visible sky with a lower leaf area index and no significant interaction (p ≤ 0.05).

3.2. Berry Quality

Cluster quality is closely linked to berry composition and is defined by several factors, such as foliage density and cluster thinning. In this study, the effect of canopy management, open vs. closed, was assessed on several berry quality variables important for determining appropriate harvest and processing, including TSS, d-glucose/fructose ratio, TTA, pH, TSS/TTA and total phenols (Figure 2). Significant responses between the developmental period and canopy management were found for all berry quality parameters: TTA (p ≤ 0.023), pH (p ≤ 0.020), TSS/TTA (p ≤ 0.000), total phenols (p ≤ 0.041), respectively. Where canopy treatments (open vs. closed canopy) were analyzed individually, the results were also significant (p ≤ 0.000, in all cases). Excepting the pH and TSS/TTA, the phenological stage significantly affected the TTA content, d-glucose/fructose ratio, TTA and total phenols (p ≤ 0.026, 0.026, 0.027, 0.037, respectively).

In general, clusters harvested from open canopies showed higher TSS values (between 14.09 and 21.8°Brix, r² = 0.637) and d-glucose/fructose ratios (between 129.97 and 214.93 g L⁻¹, r² = 0.636) than those harvested from closed canopies (Figure 2). Likewise, the pH 2.82 was higher in open canopies at the first sampling (27 June 2023, 178 Julian days) with an increasing trend, but at the end of the evaluation period (26 July 2023, Julian day 207), the open canopy pH 3.77 was less than the closed canopy pH 4.03. The TTA data started with significant differences, with the highest values for closed canopies (around 17 g TA L⁻¹), although constantly declining in both treatments and reaching similar values at the end of the study with ca. 4.5 g TA L⁻¹. A proportion related to flavor is linked to the dimensionless TSS/TTA ratio, which results from respiration and ripening.

High correlations were observed throughout the sampling period for TSS and TTA (r² = 0.853 and 0.794). The TSS/TTA values were higher in open canopy fruits (0.95/5.50) than closed canopy fruits (0.69/3.97). As far as secondary metabolites, phenols are in great abundance in berries. They play a key role in sensory properties (color, astringency, and bitterness) and have been highly correlated with antioxidant activity [21]. Berries in open canopies showed a steady, although diminishing, increase during the period between 177 and 202 Julian days, reaching values up to 2650 GAE L⁻¹. Meanwhile, in closed canopies, although starting a bit below, the values reached 3020 units at the end of the study. Coefficients of determination ensure a good fit with r² values of 0.89 and 0.81, for open and close canopies.

4. Discussion

4.1. Photosynthetically Active Radiation and Photosynthetic Activity Parameters

This study clearly demonstrates the benefits of canopy opening by increasing PFD, DLI, PR, and CI values in Cabernet Sauvignon vines grown under the conditions of the Chihuahuan Desert. In this sense, solar radiation plays an essential role in biomass production, but excess can cause photorespiration, organ and tissue damage, and, consequently, irreversible damage to the photosynthetic apparatus [22]. Therefore, the determination of the total amount of PFD (400–700 nm) received at the plant surface is of paramount importance, since it represents a useful tool for predicting the yield of any horticultural crop, including deciduous perennials, such as the grapevine [23]. In this study, opening the canopies of Cabernet Sauvignon vines increased the interception of photosynthetically active radiation and the daily light integral, as compared with dense closed canopies, which is likely to bear a direct relationship with biomass accumulation, growth, irrigation needs, and fruit composition and quality [15,24]. Cloud cover increases the intensity of diffuse radiation the rates of gas exchange, and, consequently, canopy photosynthesis [3]. This is confirmed by our results for photosynthetic rate, levels of intercellular CO_2, and canopy architecture.

Photosynthesis is a complex biochemical process and is the basis of productivity in all autotrophic organisms, where the assimilation of atmospheric CO₂ is central to the functioning of ecosystems and agroecosystems [9]. The efficiency of the photosynthetic systems in higher plants is very sensitive to variations in atmospheric (temperature, CO₂ concentration, etc.) and edaphic conditions (water, salinity, etc.) [1]. These abiotic factors affect photosynthetic electron transfer and photophosphorylation, key processes in the preservation of thylakoid membranes and the ultrastructure of chloroplast organelles [25]. As demonstrated by [26], Leibar in grapevine cv. Tempranillo clone RJ-43 grown under altered climatic conditions (700 ppm CO₂, 28/18 °C, and 33/53% RH) compared to the current climate of the experimental site (375 ppm CO₂, 24/14 °C, and 45/65% RH), with values of 12.28 ± 0.95 and 11.40 ± 0.98 µmol CO₂ m⁻² s⁻¹, which are very low in comparison to what was found in this study. This could indicate higher efficiency in the use of photon flux density and a greater accumulation of biomass and fruit quality. However, drought has been shown to be the main abiotic stress factor, limiting photosynthetic processes and thus crop plant productivity [24,27]. Recent advances in genomics, spatial modeling tools, and canopy conduction systems have facilitated improvements in photosynthesis and yield in horticultural crops under climate change scenarios [23,28].

In deciduous perennial crops, such as Cabernet Sauvignon grapevines, optimal flower and cluster development is generally related to current photosynthetic rates and to photo-assimilate reserves held in the secondary tissues of stem, branches, and roots [1]. Leaves have been shown to sense and respond not only to their own microenvironment but also to that of other leaves on the same plant [15]. This behavior is related to some signal of systemic irradiance where the shading of mature leaves causes significant changes in the anatomy of developing young leaves, including reductions in stomatal index and in leaf area [11,12]. In this study, significant changes in leaf area were observed with canopy opening during vegetative growth; however, after harvest, both types were similar for this variable. This could explain the effect of the assimilate demand by clusters; once harvested, the assimilates are retained in the leaves and then transferred to storage structures, including stems and roots. Furthermore, one aspect of agronomic management in grape growing relates to the management of the canopy. Here, cordon training systems, shoots number, spacing, vigor, and leaf removal in the fruiting zone [29] are all cultural practices aiming to reduce excessive shading within the canopy.

Canopy structure also affects soil microbial stability and microclimate [21], but the central objective is to maintain optimal levels of light interception and uniform light distribution within the canopy to increase the biological productivity of the plant, as well as the quality of the harvested product [25]. The different grapevine genotypes exhibit a wide range of physiological responses with respect to drought tolerance across the isohydric–monohydric spectrum, showing variations in stomatal control responses over evaporative demand and soil moisture with large fluctuations in leaf water potential [30]. However, in the case of Cabernet Sauvignon, it has been demonstrated that it modifies its stomatal regulation in each growing season in coordination with the adjustment of water potential to more negative values [31]. In our study, the fluctuations in stomatal conductance values were associated with the growing season and phenological stage, but not with canopy management.

In field studies of radiation interception, photosynthesis, transpiration, and growth analysis, leaf area estimation is a useful tool for identifying fast-growing and early flowering genotypes [32]. Cabernet Sauvignon vines have been shown to respond strongly to variations in agronomic management, including irrigation and mineral nutrition, by varying their leaf area, but this response has the potential to cause excessive shading, which has direct effects on berry physicochemical quality, and, consequently, on wine [33].

The regulation of CO₂ assimilation and transpiration are two physiological processes central to determining the rate of biomass accumulation and temperature control in plant tissues, for which stomatal density and distribution vary along with leaf surface structure [34]. Grapevines are hypostomatic, with both stomatal density (number of stomata per unit leaf area) and stomata dimensions (width-length) varying among clones [13]. Our results show that stomatal density and stomatal dimensions (length and width) behave similarly in both open and closed canopies. Similar results were reported [35] when evaluating stomata density and size over two growing seasons and among a range of grapevine cultivars, including Cabernet Sauvignon, with density values between 36 and 41 stomata mm². The factors that determine variation in stomatal density are not clear. Some authors suggest it is genetically determined, while others conclude it is associated with environmental factors such as light intensity, humidity, air temperature, etc. [3,17]. These variables were not modified in our study on Cabernet Sauvignon canopies.

Previous studies report an unclear relationship between stomatal dimensions (length and width) and stomatal density, because these may vary between growing seasons, species, cultivars, and even clones [13]. However, it is known that cultivars with high stomatal densities tend to have higher stomatal conductance and water use efficiencies. Stomatal density is regulated by hydraulic and chemical signals as a response to water deficit and signaling linked to abscisic acid and leaf water potential [14]. When evaluating different irrigation regimes and atmospheric vapor pressure deficits over two growing seasons in Cabernet Sauvignon grapevines, [36] reported values of stomatal conductance that varied between 84.6 ± 23.5 and 456.6 ± 41.2 mmol m⁻² s⁻¹, where the reductions in stomatal conductance were linked to lower water availability in the and to increases in abscisic acid and symplastic fluid levels.

Canopy management can be defined as adjustments of canopy dimensions at boundaries in space (i.e., canopy volume = width height length) and the numbers and areas of leaves within this volume. Canopy management still represents an active area of research in viticulture that, over the years, has generated multiple options for training systems, pruning (winter and summer), shoot positioning, leaf removal, and vigor control [32]. This set of techniques seeks to improve the efficiency in solar radiation interception, to minimize shading, to improve crop production and/or quality, to reduce the incidence of diseases, and to facilitate mechanization of pruning, pathogen control, weed control, fertilization, harvest, etc. [37]. The leaves of most plants (including grapevines) absorb solar radiation between 400–700 nm. Only a small proportion (~6%) of this radiation is transmitted by the leaves, so light levels at the center of a dense canopy is low, often only ~1% of that at the surface of the canopy [28]. To minimize shading, one of the most common agronomic practices is to increase the proportion of gaps in the canopy [8], especially in the cluster/renewal zones, where uniformity in microclimate is sought [21].

Light and air temperature modify the biology and physiology of the grapevine [32]. Under a scenario of climate change, we hypothesized that, with increasingly extreme fluctuations in air temperature and solar radiation, the photosynthetic apparatus could be subjected to conditions of light saturation, and drastic reductions in photochemical efficiency are induced, leading to the phenomenon of chronic photoinhibition, acceleration of phenological phases, and early ripening. These conditions do not favor obtaining quality fruit and, consequently, quality wine [11,23]. Interception of photosynthetically active radiation is related to leaf area and so to the level of leaf shading [38]. In our study, data for number of contacts and the percentage of visible sky and gaps were higher for the open canopy with a leaf area index (LAI) < 2, showing better light capture according to the Jefferies and Heilbronn model [39]. This model is used to describe the relation between LAI and percent ground cover to estimate intercepted radiation in multiple crops, including Cabernet Sauvignon vines [40].

The open canopy modification allows grapevines to acclimatize (i.e., alter their growth) in response to shade conditions by sensing different portions of the (PFD) spectrum (ultraviolet, blue, red, and far-red) through a complex phototropic sensory system made up of different components including UVR8, cryptochromes, phototropins, ZTL-type receptors and phytochromes [41]. Hence, Cabernet Sauvignon, under open canopy conditions, is sensitive to variations in light levels and quality. However, under closed canopy conditions, they cope with dim light and optimize light capture by increasing the leaf area, leaf thinning (reductions in leaf mass per unit area), and accumulation of photosynthetic pigments, maximizing efficiency in a light interception and transformation of photons to photoassimilates [42]. All these morphological and physiological responses are linked to changes in water relations and atmospheric CO₂ management.

Available evidence indicates that shading reduces transpiration, stomatal conductance, photosynthesis, stomatal density, hydraulic conductivity, and water use efficiency [35]. Under the conditions of our study, differences were detected only for gas exchange and leaf temperature. However, after harvest, a non-significant reduction was observed for all parameters assessed in both canopy types. The LAI, which was higher for the closed canopy, may indicate lower light interception, affecting critical parameters such as transpiration, root development, and photosynthetic capacity. Such physiological behaviors modify the optimal frequency of irrigation application and nutrient supply and may be yield-limiting [4].

4.2. Berry Quality

Berries ripening for the production of high-quality wine represents a complex process that involves accumulations of some compounds (glucose, fructose, organic acids, polyphenols, and aromatic compounds) paired with reductions in others (tyramine, phenylethylamine, putrescine, and cadaverine) which can impart undesirable aromas to wines [21]. When harvesting aims to promote maximum quality, expressed in target sugar and organic acid contents, it is necessary to subject vines to a certain intensity of water stress [2]. However, this does not necessarily correspond to oenological maturity; therefore, to establish the optimal harvest time, it is necessary to consider soluble solids, acidity, aromas, and polyphenols, i.e., phenolic maturity [43]. In red grape cultivars, such as Cabernet Sauvignon, phenolic maturity is related to the evolution in the concentrations of anthocyanins and tannins in the epidermis ‘pomace’ and seeds [8]. However, training systems, vine age, watering regime, soil type, environmental temperature, and solar radiation are a set of biotic and abiotic factors that intervene in the accumulation and synthesis of phenolic compounds, responsible for color, aroma and texture—the main sensory characteristics for any high-quality red wine [38]. In a previous study conducted with an open canopy (Lyre) and two pruning systems (Royat and Guyot), [44] reported significant variation in yield and fruit composition (anthocyanins and total phenols). This behavior coincides with the total phenolics data observed in this study, which serves as an indicator for producing wines with greater color and flavor.

Cabernet Sauvignon has outstanding characteristics for red wine production with excellent quality but has shown varied behavior with respect to its genetic characteristics, productivity, and phenotypic quality, so a gradual selection of clones has been made to improve cluster size and fruit with greater homogeneity in aroma, color and flavor [40]. Therefore, specific assessments of the effects of climate change on the wine industry are important for prioritizing adaptation strategies [20], as most wine regions worldwide are exposed to increased occurrence of extreme events—temperature, hail, floods, drought, etc. [1]. These weather phenomena affect the mesoclimate and often cause a lag in vegetative growth and alterations in ripening and fruit composition. However, the intensity of the physiological response between vineyards can vary according to slope, altitude, surrounding vegetation, and the primary and secondary growths [21].

In these climatic conditions, a vertical shoot positioning (VSP) training system is widely used due to its greater compatibility with the mechanization of practices (pruning, pathogen control, harvest, etc.) and commercial production at the regional level [10]. However, with more extreme seasonal variations in air temperature and solar radiation, Vertical shoot positioning may favor cluster overexposure and, consequently, the presence of sunburned berries, lower yields, pigment color degradation (anthocyanins, flavonols, and proanthocyanidins), and variation in acidity [45].

Foliage modification prior to flowering by removing basal leaves around the cluster aims to improve cluster exposure to solar radiation and increase the content of soluble solids, anthocyanins and flavonols [46]. Traditionally, in wine production systems, canopy management through shoot removal has made a significant contribution to secondary metabolism, affecting the numbers and concentrations of secondary metabolites (phenols, flavonoids, anthocyanins, etc.) with high biological and antioxidant activity [47]. In addition, canopy management helps maintenance of the source–sink balance and improves the cluster microclimate [25]. The evaluation [15] of the effects of leaf and shoot removal on Merlot, reported significant increases in the quercetin and kaempferol ratio in detriment of myricetin derivatives. However, the phytochemical and antioxidant composition of the fruit depends on a complex balance between compounds of primary and secondary metabolism, which are highly sensitive to environmental factors, including solar radiation. On the other hand, post-bloom leaf removal in Merlot [48] reported a significant improvement in Botrytis incidence reduction and herbaceous aromas without affecting yield and cluster weight. In general, berry quality is the net result of a multiplicity of agronomic practices, including pruning, irrigation, mineral nutrition, rootstock, and, above all, training system [7,49].

5. Conclusions

This study highlights the significant benefits of modifying canopy architecture for enhancing productivity in Cabernet Sauvignon vineyards. Opening canopies notably increased the harvest of photosynthetically active radiation, photosynthetic rates, and key berry quality parameters, such as total soluble solids and total phenols. This underscores the importance of canopy management by optimizing light interception and photosynthetic efficiency. However, further research should incorporate additional variables, including soil evaporation, root-stock effect, and diverse growing locations to fully understand the impacts on grapevine productivity.