The influence of global warming (A.Bonfante)

Harvesting is a delicate operation and determining when grapes are ripe is difficult: ripening is a physiological process that has no precise timing and depends on 'fixed' factors such as the genetics of the plant or on variables, such as human action and climate, that are difficult to predict. Despite the rather cold spring, as opposed to the temperature peaks above 35°C in July and August, the grape harvest seems productive, with a forecast of 46 million hectolitres of wine for 2019, about one 16% less than in 2018, as reported in Corriere della Sera. Of course, the vineyard requires a lot of sun and fears spring frost, hail, excess water or drought the most. How, then, does one verify the influence of climatic variations on the complex vineyard-soil-climate system, the so-called 'terroir'? We start with the viticultural destination of a given territory, described by an index, the Amerine-Winkler- A&W. "It is a bioclimatic index defined by the sum of average daily temperatures minus the vine's vegetative zero (10°C), from 1 April to 31 October, expressed in degree days (degrees-day or DD). It is used to get an indication of the possibility of an area's destination for a specific variety,' explains Antonello Bonfante, researcher at the Institute for Mediterranean Agricultural and Forestry Systems (Isafom) of the CNR. 'What happens with climate change, and in particular with global warming, is that areas currently planted with vines tend to increase the value of this index and thus create conditions that may no longer be optimal for current production.

Faster technological ripening, i.e. in fewer days, does not, however, correspond to correct phenological ripening, the process that leads to the required organoleptic characteristics due to the presence of certain aromas, acid profile, sugar content and pigments. "The increase in air temperature leads to an increase in berry temperature, with direct effects on the synthesis and degradation of anthocyanins; it accelerates the degradation of the most thermolabile aromatic compounds and acids, which are essential for good overall harmony," explains the researcher. "The A&W index can help us to understand whether, considering the future climate scenarios of the Intergovernmental Panel of Climate Change-(Ipcc), an area will have the same thermal conditions as today and whether those in the future will be suitable for the vine currently cultivated, with the possibility of identifying new areas that are currently unsuitable for vine cultivation, but will be thermally suitable in the future.

Climate change not only affects the average temperature but also modifies the annual rainfall profile, with tangible consequences on the water regime in vineyards. "The relationship between water stress in the vine and the qualitative response of the berries, from an organoleptic point of view, is well known in the literature. In fact, the vine is like an artist who has to suffer, but not succumb, in order to give its best," continues Bonfante. That is, moderate water stress must occur to induce competition between cytokine synthesis and absissic acid, hormones that regulate the vegetative development of the vine. When the plant stops growing due to the reduced water supply, typical of the summer season, the berries accumulate, especially in the skin, those substances such as anthocyanins, tannins, polyphenols, which give wines special organoleptic characteristics and decrease the malic acid content compared to tartaric and citric acid. "If the water regime of a vineyard changes due to climate change, irrigation may become necessary, to ensure that water stress is useful for quality production," says CNR expert Antonello Bonfante, the researcher, who nevertheless warns: "Not all areas currently planted with vines can switch from dry to irrigated management, due to the difficulty of water supply, the problem of production regulations, and the economic difficulty of companies in sustaining irrigation infrastructure. In the semi-arid areas of Portugal and Spain, for example, the irrigated vineyard areas have increased by about 20% and therefore no longer fall within the DOC zones.

The role of oenologists and winegrowers, already complex in itself, is also to manage stress in the vineyard. However, Bonfante emphasises that it is 'complicated in Italy to identify those terroirs that are best adapted to climate change, since we have very different soil-plant-climate combinations in our peninsula. We can in any case say that within the same vine, given the same variety, management, etc., some portions of the territory are more resilient than others in maintaining their quality standards, mitigating the negative effects of climate change. From the soil to the canopy, the winegrower can intervene on the exposure of the plant with the orientation of the rows, the management of the canopy, the use of nets or shading substances to lower the temperature of the grapes and favour the adaptation of the vines'.

But how is the stress to which a vine is subjected assessed? "Direct methods make it possible to measure, by means of acquisition sensors, the xylem flow, i.e. the liquids that reach the leaves from the roots, estimating the plant's water potential, which is correlated to the stress felt at that precise moment," adds the CNR expert. "Indirect methods include the measurement of leaf temperature, soil water potential or the application of based models to determine actual daily transpiration. Furthermore, through the use of satellite images or multispectral sensors equipped on a drone, it is possible to assess the water content of the canopy, an indirect indication of water stress and its distribution in the vineyard'.

Even in Oxfordshire, England, it is now harvest time (Chardonnay and Pinot Grigio). This seems like an anomaly, since vines grow well at latitudes between the 40th and 50th parallels, but rising average temperatures now make it feasible to grow grapes even at latitudes above the 50th. "England imported wine worth €3.5 billion in 2017, second in the world after Germany. The importance of any domestic development of the wine sector is clear, all the more so after Brexit,' argues Bonfante. 'I would not worry about the future development of viticulture in those latitudes, as the qualitative result is not a foregone conclusion. Producing great wines requires great experience from field to cellar, which translates not only into technology but also into tradition.'

In fact, as often happens, Italy has a tendency to underestimate itself: in the eternal France-Italy challenge, our country wins on biodiversity with 504 varieties entered in the vine register against the 278 of its French cousins, according to Coldiretti data. "The enormous genetic resource, in terms of native grape varieties, could ensure the survival of our wine-growing areas even in climatic conditions that are very different from the current ones," concludes the CNR-Isafom researcher.

Alessia Famengo

Source: Antonello Bonfante, Mediterranean Institute for Agricultural and Forestry Systems, tel. 081/7886701 , email antonello.bonfante@cnr.it -