Italy, September, 2007

Canopy management and resistance to water stress

“Il Chianti e le terre del vino” 

Mario Fregoni
Professor of viticulture at the Università Cattolica del Sacro Cuore Piacenza
Honorary president of OIV
President of the International Academy of Sensory Analysis

 

Without going into detail of the methodology that enables us to determine the state of hydration of the leaves, remember that climatic change and temperature increases have provoked an extraordinary spread of irrigation systems, justified in southern areas but not always so in central and northern Italy. Solving the problem of water stress with irrigation seems to be the most logical strategy, but this is not always the case, either because of lack of water (also for human use!) or because excellent wines don’t tolerate irrigation completely, not, as is believed, because of the sugar content but more for the whole finesse of the wine (which often turns out more tannic and herbaceous) and, above all, for the negative impact on the duration of ageing. However, the great vintages of fine wines have always been made in years with a low rainfall during the ripening period. A minimum of water stress at this stage (especially towards the end) has always been considered beneficial, also with the aim of stopping vegetative growth and increasing deposits in the berries. With the aim of tackling and reducing water stress with a more naturalistic and alternative approach to simplifying the use of irrigation water, water stress has been broached with a different idea, which includes a general project beginning with planting (drought-resistant varieties and rootstocks), training systems that are not loose (experiments with the gobelet with a triangular layout in Montepulciano near Siena have been successful) lower canopies and canopy management with different kinds of green pruning that tend to reduce transpiration, i.e. the consumption of water. On this subject, the strategy of reducing water loss from the soil through evaporation and trying to induce higher water absorption from the soil has also been considered. These ideas are the basis for research carried out during the last three years in two vineyards located on the Soldera estate at Case Basse in Montepulciano, whose plan is summarised in the attached figure, which can lead us to deduce numerous theories. The vineyards used are trained with cordon espalier and simple spur cordon (Institieti) or bilateral (Case Basse), they are rather different in age (Institieti 5 years, Case Basse 25 years) and are situated at different altitudes, exposures and on different soils. The variety used is Sangiovese in both cases, the adult vineyard is grafted onto Kober 5BB and the young one onto SO4. The theory of summer pruning, carried out annually by A. Paoletti from Florence and controlled instrumentally by M. Gatti and F.Bernizzoni from Piacenza, has taken into account the previously expressed physiological knowledge and the possibility of reducing water consumption through transpiration, as well as the known effects of water stress on the synthesis of the berry compounds. Depending on the period when stress is identified, it is necessary to apply the most suitable form of summer pruning, knowing that stress to the green berry, besides reducing the berry weight due to the decrease in cellular proliferation (which needs water), also decreases the anthocyanins and flavanols and stimulates the conjugation between tannins and anthocyanins. Stress during the ripening stage normally increases sugars and the overall quality, but if the stress is severe fewer aromas are produced, particularly those deriving from carotenoids (mentioned above). Few results have been reported from the tests in Montalcino for 2006, awaiting a more complete report in due course. The controls were carried out using an instrument that measures the leaf temperature, photosynthesis and transpiration simultaneously. It is easy to see from the graphs that the adult vineyard on the land called Case Basse (cb) has a higher level of photosynthesis and transpiration than the younger terroir Institieti (int). Photosynthesis in Case Basse in 2006 (cb) turned out to be higher in test A4 (with young and old leaves) and in test A3 (only adult leaves) and B2 (mulching). Test B2 also acquitted itself well in previous years. In the young Institieti vineyard (int) photosynthesis turned out to be better in test A1 (2 base leaves + 7 leaves above the bunch), but with less evident differences between the tests. As far as transpiration is concerned, in Case Basse (cb) it was higher in test B2 (mulching on the row and between rows), whereas in Institieti it was test A1 (2 base leaves + 7 leaves above the bunch) that displayed higher levels of transpiration (though connected to high photosynthesis). The test with the least transpiration in the young Insititieti vineyard (int) seems to be A4 (vines with old and young leaves). The analyses of wines made with microvinification (carried out by M. Vincenzini from Florence) found higher levels of alcohol in test A2 (with 12-14 adult leaves) and in test B3 (superficial working of the soil). All the tests compared showed very high levels of glycerol, quite exceptional (from 6.4 to 8.1 g/l) both in 2006 and 2005. Sensory analysis carried out in Case Basse by a panel of 22 people on a card designed specifically in Piacenza for Brunello di Soldera, showed that the most popular tests were A1, A2, B1 and B2. It must also be taken into account however that the colour of the wine psychologically influenced the tasters and that microvinification is not always significant. The results should, therefore, only be considered examples of an experimental procedure, that will only give conclusive results in a few years’ time. In the meantime we can report that a different method may allow canopy management of a vineyard and make it more resistant to heat and water stress.

In hot, dry areas irrigation is indispensable, but before deciding to use it in temperate areas, under the psychological influence linked to climatic change, it is appropriate to resort to the natural adaptability of the vine and to other tools that can increase resistance to heat and water stress, beginning with the choice of terroir, rootstocks, grape variety and the more or less open structure of the canopy, correlated to the surface area of the leaves. Later on, canopy management with summer pruning enables the regulation of transpiration, photosynthesis and respiration, bearing in mind that this does not always provide univocal answers and has to be modulated according to different parameters, such as climatic conditions, the phonological stage of the berry, the grape variety and so on. Experimental discordances are often due to extending principals acquired in different situations of genetics and environment.