Tommaso Frioni1, Clara Ripa1, Laura Palermo1, Claudia Romano1, Pier Giorgio Bonicelli1
1 Università Cattolica del Sacro Cuore, Piacenza, Italy
IN+VITE is an Operational Group for Innovation funded by Measure 16.1.01 of the Emilia-Romagna Region (Focus Area 4B, call 2022). The goal of the partnership is to identify new solutions to improve the competitiveness and sustainability of non-irrigated viticulture in the hills of Emilia-Romagna. Two actions of the project focus on testing the effectiveness and effects of hydrogels and superabsorbent polymers on both new plantings and established vineyards in relation to issues imposed by climate change. The project is coordinated by Università Cattolica del Sacro Cuore, project partners are Azienda Agricola Lusignani, Azienda Agricola Baraccone and Fabrizio Camorali, Vinidea and Centro Tadini.
Super-absorbent polymers, what are they and how do they work
The term hydroretentive polymers, or hydrogels, identifies materials consisting of polymer chains that are parallel to each other, rich in cross-links, forming a network rich in hydrophilic groups, and therefore capable of absorbing water or solutions (Fidelia and Chris 2011). Therefore, when water comes into contact with these materials, it rapidly migrates within the polymer network in which it is stored. The amount of absorbable water varies depending on the richness of hydrophilic groups, the length of these chains, and the nature of the polymer. Once immersed in water, these can take on the consistency of a gel. In the agricultural sector, their use has been limited, especially in Italy, because of their cost and technical aspects related to their manipulability. However, recently, the cost of producing hydrogels has been significantly reduced new super-absorbent polymers have been developed that are highly effective in absorbing water and totally biodegradable over time in the agricultural environment (Fidelia and Chris 2011). In the current context, such tools stand as promising resources for improving water availability for plants, reducing the percentage of vine establishment failures, anticipating entry into production, and finally limiting irrigation volumes.
As part of the project, a series of experimental trials were conducted in the laboratory, in a controlled environment, and in the field, on hydrogels of various kinds in relation to water-limiting conditions for vine development.
In the laboratory, tests showed that 1 g of a potassium polyacrylate-based hydrogel can absorb up to 84 g of water (Table 1), the rate of release of which was found to be temperature dependent (30 °C and 40 °C). Furthermore, at the end of a second hydration cycle, the maximum water uptake was found to be reduced compared to the first cycle, but to a minimal extent.
Table 1: Characteristics of a hydrogel based on potassium polyacrylate as is and substrate with and without the addition of hydrogel
Different letters indicate significant difference for P<0.05 (SNK test).
Next, the physical characteristics of a sandy loam soil were analyzed as a function of polymer application at a dose of 12.5 mg/g. The substrate with hydrogel demonstrated a higher field capacity of 0.21 g H2O per g, compared with the substrate without Polygreen. This result did not change at the end of a second hydration cycle (Table 1).
Tests conducted in the laboratory also showed that the tested hydrogel makes available about 95% of the absorbed water in a range of water potential (Ψ) compatible with uptake by vine root systems (Figure 1). In general, vines are able to absorb a solution at soil Ψ levels between -0.1 MPa and -1 MPa (Deloire et al. 2007).
Fig. 1: Left, correlation between the water absorbed by Polygreen and the water potential at which it is made available. Right, relationships between water concentration and the water potential at which it is made available in a substrate with and without the addition of Hydrogel.
The same analyses were conducted on the sandy loam-textured soil mentioned earlier. In the absence of hydrogel, this releases 95 % of the available water at Ψ between -0.01 MPa and -0.15 MPa.
This means that much of the water retained by the substrate is then rapidly lost through evaporation or under conditions of no water stress for the vine. For the same substrate with hydrogel added, the correlation between available water and Ψ at which it is released was found to be shifted to the right (Figure 1). Specifically, the range of available water between 0.2 and 0.05 g H2O / g substrate is released at higher Ψ than the substrate without Polygreen, but still below the -1MPa threshold. This means that the addition of hydrogel allows a sandy soil to retain more water that would otherwise be lost through evaporation or in the absence of vine-limiting conditions.
Grapevine response to hydrogel application
In 2023, the efficacy of the two different hydrogels was tested on Sangiovese plants grown in pots under semi-controlled conditions at an outdoor space of the Department of Sustainable Plant Production Sciences (Università Cattolica del Sacro Cuore, Piacenza). Specifically, vines to which potassium polyacrylate-based hydrogel (H1) was applied and vines to which organic derivative-based hydrogel (H3) was applied were compared to an untreated Control.
During these trials, in addition to evaluating vine response as a function of progressive water deficit, the response curve of photosynthesis to increasing values of photosynthetically active light (PAR) was characterized before and after a period of particularly severe water deficit imposed artificially by suspending irrigation.
In the period before the severe water stress, the leaves of H1 vines showed higher photosynthesis values than those of the control leaves. In particular, the difference between theses increased progressively at PAR values > 400 μmol m-2 s-1. At light saturation (PAR > 1200 μmol m-2 s-1), the vines with H1 had a net photosynthesis of 15 μmol m-2 s-1, the vines with H3 had a net photosynthesis of 13 μmol m-2 s-1, while in the control, assimilation did not exceed 11 μmol m-2 s-1 (Fig.2). Following severe water stress, despite restoration of full water volumes, the leaves of the control were unable to take up positive photosynthetic rates, and even at light saturation, net assimilation was less than 1 μmol m-2 s-1. In vines with SH1 and SH3, although leaves did not exhibit net assimilation values comparable to the pre-water stress period, net photosynthesis was still significantly higher than the control for any PAR threshold examined, up to values close to 8 μmol m-2 s-1.
Fig. 2: Correlation between net leaf photosynthesis and light intensity reaching leaves, before a period of severe stress (top) and following the stress period (bottom), in vines to which H1 and H3 polymers were applied at planting and Control vines.
In 2023, the H1 hydrogel was tested at planting in a Sauvignon Blanc vineyard in the Piacenza Hills. The polymer was placed under the root systems of the rooted cuttings at a dose of 30 g/plant. Shoot growth was monitored during the season, and the main physiological parameters and water status of the vines were monitored during the season. Plants on which H1 was applied had higher basal and midday foliar water potentials in July than in Control (+0.04 MPa). At the end of the season, although no failures were found in both theses, the sum of the length of the two shoots left at the green pruning stage was higher in the vines treated with Hydrogel (+21 cm). The trial continues to verify the outcome of the application on vineyard entry into production.
Bibliographical references
Deloire, A. “L’INFLUENZA DELLO STATO IDRICO DELLA VITE SULLO STILE DI VINO.” (2007).
Fidelia, N., & Chris, B. (2011). Environmentally friendly superabsorbent polymers for water conservation in agricultural lands. Journal of Soil Science and Environmental Management, 2(7), 206-211.
