Whole-Plant Water Use in Field Grown Grapevine: Seasonal and Environmental Effects on Water and Carbon Balance

UV light and adaptive divergence of leaf physiology, anatomy, and ultrastructure drive heat stress tolerance in genetically distant grapevines

The genetic basis of plant response to light and heat stresses had been unveiled, and different molecular mechanisms of leaf cell homeostasis to keep high physiological performances were recognized in grapevine varieties. However, the ability to develop heat stress tolerance strategies must be further elucidated since the morpho-anatomical and physiological traits involved may vary with genotype × environment combination, stress intensity, and duration. A 3-year experiment was conducted on potted plants of Sardinian red grapevine cultivars Cannonau (syn. Grenache) and Carignano (syn. Carignan), exposed to prolonged heat stress inside a UV-blocking greenhouse, either submitted to low daily UV-B doses of 4.63 kJ m-2 d-1 (+UV) or to 0 kJ m-2 d-1 (-UV), and compared to a control (C) exposed to solar radiation (4.05 kJ m-2 d-1 average UV-B dose). Irrigation was supplied to avoid water stress, and canopy light and thermal microclimate were monitored continuously. Heat stress exceeded one-third of the duration inside the greenhouse and 6% in C. In vivo spectroscopy, including leaf reflectance and fluorescence, allowed for characterizing different patterns of leaf traits and metabolites involved in oxidative stress protection. Cannonau showed lower stomatal conductance under C (200 mmol m-2 s-1) but more than twice the values inside the greenhouse (400 to 900 mmol m-2 s-1), where water use efficiency was reduced similarly in both varieties. Under severe heat stress and -UV, Cannonau showed a sharper decrease in primary photochemical activity and higher leaf pigment reflectance indexes and leaf mass area. UV-B increased the leaf pigments, especially in Carignano, and different leaf cell regulatory traits to prevent oxidative damage were observed in leaf cross-sections. Heat stress induced chloroplast swelling, plastoglobule diffusion, and the accumulation of secretion deposits in both varieties, aggravated in Cannonau -UV by cell vacuolation, membrane dilation, and diffused leaf blade spot swelling. Conversely, in Carignano UV-B, cell wall barriers and calcium oxalate crystals proliferated in mesophyll cells. These responses suggest an adaptive divergence among cultivars to prolonged heat stress and UV-B light. Further research on grapevine biodiversity, heat, and UV-B light interactions may give new insights on the extent of stress tolerance to improve viticulture adaptive strategies in climate change hotspots.

Photosynthetic recovery in drought-rehydrated grapevines is associated with high demand from the sinks, maximizing the fruit-oriented performance

To understand how grapevine sinks compete with each other during water stress and subsequent rehydration, carbon (C) allocation patterns in drought-rehydrated vines (REC) at the beginning of fruit ripening were compared with control vines maintained under drought (WS) or fully irrigated (WW). In the 30 days following rehydration, the quantity and distribution of newly fixed C between leaves, roots and fruits was evaluated through ¹³ CO₂ pulse-labeling and stable isotope ratio mass spectrometry. REC plants diverted the same percentage of fixed C towards the berries as the WS plants, although the percentage was higher than that of WW plants. Net photosynthesis (measured simultaneously with root respiration in a multichamber system for analysis of gas exchange above- and below-ground) was approximately two-fold greater in REC compared to WS treatment, and comparable or even higher than in WW plants. Maximizing C assimilation and delivery in REC plants led to a significantly higher amount of newly fixed C compared to both control treatments, already 2 days after rehydration in root, and 2 days later in the berries, in line with the expression of genes responsible for sugar metabolism. In REC plants, the increase in C assimilation was able to support the requests of the sinks during fruit ripening, without affecting the reserves, as was the case in WS. These mechanisms clarify what is experienced in fruit crops, when occasional rain or irrigation events are more effective in determining sugar delivery towards fruits, rather than constant and satisfactory water availabilities.

Rapid stomatal closure contributes to higher water use efficiency in major C4 compared to C3 Poaceae crops

Understanding water use characteristics of C3 and C4 crops is important for food security under climate change. Here, we aimed to clarify how stomatal dynamics and water use efficiency (WUE) differ in fluctuating environments in major C3 and C4 crops. Under high and low nitrogen conditions, we evaluated stomatal morphology and kinetics of stomatal conductance (gs) at leaf and whole-plant levels in controlled fluctuating light environments in four C3 and five C4 Poaceae species. We developed a dynamic photosynthesis model, which incorporates C3 and C4 photosynthesis models that consider stomatal dynamics, to evaluate the contribution of rapid stomatal opening and closing to photosynthesis and WUE. C4 crops showed more rapid stomatal opening and closure than C3 crops, which could be explained by smaller stomatal size and higher stomatal density in plants grown at high nitrogen conditions. Our model analysis indicated that accelerating the speed of stomatal closure in C3 crops to the level of C4 crops could enhance WUE up to 16% by reducing unnecessary water loss during low light periods, whereas accelerating stomatal opening only minimally enhanced photosynthesis. The present results suggest that accelerating the speed of stomatal closure in major C3 crops to the level of major C4 crops is a potential breeding target for the realization of water-saving agriculture.

Intracultivar genetic diversity in grapevine: Water use efficiency variability within cv. Grenache

The selection of genotypes best adapted to environmental conditions has traditionally focused on agronomic and grape composition parameters. However, to classify the genotypes most adapted to climate change conditions, the aim must be to focus on the ecophysiological responses that will ultimately determine their performance. The variability in water use efficiency of 13 Grenache genotypes over three-seasons was assessed under field conditions at leaf, grape and plant level. Results showed a significant effect of genotype at all three levels, and despite the large interannual variability there was a remarkable consistency among levels. Furthermore, using genotype-specific regressions it was possible to identify significant differences in the intrinsic water use efficiency response of each genotype as a function of the vine water status. The relationship between net photosynthesis and stomatal conductance, as well as carbon isotope discrimination in grapes, were also confirmed as reliable physiological indicators for selecting grapevine genotypes to future environmental conditions. Therefore, the proposed multi-level methodology was useful to quantify the intracultivar variability and the identification of more and less efficient genotypes within Grenache.

Modeling Carbon Balance and Sugar Content of Vitis vinifera under Two Different Trellis Systems

Environmental factors might influence the carbon balance and sugar content in grapevine. In this two-year research, the STELLA software was employed to predict dry matter accumulation in Sangiovese vines, comparing the traditional vertical shoot positioning (VSP) and the single high wire (SHW) trellis systems. Every week, vegetative, eco-physiological and grape quality parameters were collected for 15 tagged vines per trellis system to set up the software. Significant differences in photosynthesis were recorded in 2014, with higher values in VSP (23-25% more). Shoot growth was significantly higher in VSP (20-25% more), whereas higher dry matter (30%) and yield (9-11% more) were detected for SHW. At harvest, berry composition suggested a slower ripening in SHW compared to VSP, which was linked to the shading of clusters in SHW. Finally, for the first time, linear regressions were found between measured berry sugar content and STELLA-estimated dry matter (R² = 0.96 in VSP; R² = 0.95 in SHW). This latter evidence allowed the estimation of berry sugar content, showing this software to be a practical tool to support winegrowers in decision making. Other studies are already underway to calibrate and validate the model for other varieties, training systems and environments.

Improving Net Photosynthetic Rate and Rooting Depth of Grapevines Through a Novel Irrigation Strategy in a Semi-Arid Climate

Direct root-zone irrigation (DRZ) is a novel subsurface irrigation strategy initially tested in vineyards for economizing water and securing grape production in arid regions with unstable climatic patterns. However, studies are lacking on the responses of grapevine leaf carbon assimilation and deep rooting patterns to the novel irrigation strategy, which are essential for optimizing grapevine growth and alleviating extreme water stress during periods of heat and drought. Thus, a two-year field study was conducted in a commercial vineyard of Cabernet Sauvignon (Vitis vinifera L.) under a semi-arid climate in Washington, USA to compare the differences in leaf gas exchange and root distribution along the 0-160 cm soil profile, combined with measurements of specific leaf area and total carbon and nitrogen content in leaves and shoots to compare DRZ and traditional surface drip irrigation (SD) under three watering regimes. Compared to SD, significantly higher rates of net CO₂ assimilation, stomatal conductance and transpiration in leaves, which positively correlated to midday stem water potential, were found in grapevines irrigated through DRZ in both years. Meanwhile, DRZ reduced total root number by 50-60% and root length density (RLD) by 30-40% in the upper 60 cm soil at high (0.75-0.80 crop evapotranspiration) and moderate (0.60-0.65 crop evapotranspiration) irrigation rates, but no significant differences were found at low (0.45-0.50 crop evapotranspiration) irrigation rate between DRZ and SD. Higher root number and RLD were detected under DRZ within 60-160 cm soil depths, accompanied by a decreased ratio of total carbon to nitrogen content in leaves with slightly increased specific leaf area. Decreased rainfall and increased temperature in 2018 possibly amplified the positive effects of DRZ. Our study indicates that grapevines under DRZ could develop deeper roots for water uptake, which helps ameliorate water stress and improve the photosynthetic rate as well as enhance grapevine adaptation to semi-arid climates.

Plant water status and genotype affect fruit respiration in grapevines

An understanding of fruit gas exchange is necessary to determine the carbon balance in grapevines, but little attention has been paid to the relationships among fruit respiration, plant water status and genetic variability. The effect of plant water status and genotype on cluster respiration was studied over two seasons (2013 and 2014) under field conditions using a whole cluster respiration chamber. Whole cluster CO₂ fluxes were measured in growing grapevines at hard-green, veraison and ripening stages under irrigated and non-irrigated conditions, and under light and dark conditions in two grapevine varieties, Tempranillo and Grenache. A direct relationship between cluster CO₂ efflux and plant water status was found at hard-green stage. Genotype influenced the fruit CO₂ efflux that resulted in higher carbon losses in Tempranillo than in Grenache. Fruit respiration rates decreased from the first berry developmental stages to ripening stage. The integration of fruit respiration rates under light and dark conditions showed the magnitude of fruit carbon losses and gains as well as interesting variety and environmental conditions effects on those processes.

Trunk Girdling Increased Stomatal Conductance in Cabernet Sauvignon Grapevines, Reduced Glutamine, and Increased Malvidin-3-Glucoside and Quercetin-3-Glucoside Concentrations in Skins and Pulp at Harvest

Girdling is a traditional horticultural practice applied at fruit set or other phenological stages, and is used mostly as a vine management. In grapevines, it is used primarily for table grapes to improve berry weight, sugar content, color, and to promote early harvest. The objective of this study was to evaluate the effect of trunk girdling applied at veraison, in 'Cabernet Sauvignon' wine grapes (Vitis vinifera L.), on agronomical and physiological parameters during vine development from the onset of ripening (veraison) to harvest, and additionally to quantify the effect of girdling on primary and secondary metabolism. Girdling was applied 146 days after pruning (dap) at veraison, when berry sampling for metabolomics and agronomical evaluations commenced, with a further three sampling dates until harvest, at 156 dap (30% maturation, 10 days after girdling-dag), 181 dap (70% maturation, 35 dag), and 223 dap (commercial harvest, 77 dag). Skin/pulp and seed tissues were extracted separately and metabolomics was performed using one-dimensional proton nuclear magnetic resonance (1D 1H NMR) spectroscopy and high performance liquid chromatography (HPLC-DAD). At harvest, girdling significantly increased stomatal conductance (gs) in vines, decreased glutamine concentrations, and increased anthocyanin and flavonol concentrations in the skin/pulp tissues of grape berries. Berry weight was reduced by 27% from 181 dap to harvest, and was significantly higher in grapes from girdled vines at 181 dap. Sugars, organic acids, and other amino acids in skin/pulp or seeds were not significantly different, possibly due to extra-fascicular phloem vessels transporting metabolites from leaves to the roots. Using a metabolomics approach, differences between skin/pulp and seeds tissues were meaningful, and a greater number of secondary metabolites in skin/pulp was affected by girdling than in seeds. Girdling is a simple technique that could easily be applied commercially on vine management to improve berry color and other phenolics in 'Cabernet Sauvignon' grapes.

Effects of fertigation by elicitors enriched in amino acids from vegetal and animal origins on Syrah plant gas exchange and grape quality

Nutrition management of vines is a key factor that regulates production and determines grape quality. Currently, one of the main objectives of fertilization is to apply eco-friendly and balanced nutrition. In this case, fertigation is the most efficient strategy. The objective of this work was to assess the effect of innovative fertilizers on Syrah grapevines. The tested products contained an elicitor, macro- and microelements and amino acids from animal and vegetal origins. Fertilizers were applied by fertigation under field conditions in 2016, 2017 and 2018. The influence of the treatments on the grapevine gas exchange, element assimilation and oenological composition of must was evaluated. Additionally, the amino acid contents and volatile composition were determined. The results suggest that application of fertigation does not directly influence the water status of Syrah vines under the conditions of the study but can improve the gas exchange balance and N assimilation. Moreover, the leaf assimilation of Zn, Ca and, to a lesser extent, B were improved at the bloom stage, and the assimilation of Mn was improved at veraison. Small changes were observed in the basic parameters of musts, and an increase in yeast assimilable nitrogen was detected in treated vines. The aromatic profile of grape musts was enhanced in treated vines, and the elicitors enriched in vegetal amino acids could be a sustainable alternative for improving certain vegetative and grape quality parameters of Syrah.