Aleatico grapevine characterization: physiological and molecular responses to different water regimes

Aleatico grapevine is a variety cultivated along Tuscany coasts and in Elba Island (Italy), from which a typical dessert wine ‘Aleatico passito’ is produced after partial post-harvest dehydration of berries. The research project was focused on this variety because the knowledge of the morphological traits and technological characteristics of Aleatico as well as the response of this grape variety to different environmental conditions and climatic changes, including reduced rainfall and water stress, is scarce. Therefore, to obtain a high quality wine that recently received the Denomination of Controlled and Guaranteed Origin and to be more competitive on the market, it is quite useful to conduct thorough studies on this variety and on its responses to different water regimes, especially in terms of secondary metabolites biosynthesis (phenolic compounds and flavours) during ripening. Field trials were carried out, in 2008 and 2009, at ‘La Bulichella’ Winery (Suvereto, Livorno, Italy) in order to study physiological responses (midday stem water potential, gas exchanges and sap flow) and berry composition of non irrigated (WS) and irrigated (IR) Aleatico plants. The climatic trends for the 2008 and 2009 seasons showed that in the 2009 season relative humidity was higher (30-80%) in comparison with 2008 (10-40%). Global radiation in 2009 was also higher against to 2008. The air temperature frequently exceeded 26°C during 2008 season, while in 2009 this occurred and was concentrated during the second part of the season. Relative humidity and air temperature measured at the grape level did not markedly differ from those of the meteorological station. These climatic conditions influenced the midday stem water potential and the gas exchanges that reached lower values in 2009 than in 2008. In particular, in 2008 the photosynthetic activity and conductance of IR plants leaves increased during véraison and maintained higher values than in 2009, during which both parameters were decreasing, apart from the partial recovery due to water supply on August 15th. In 2009 at harvest berry weight was reduced of about 20% in WS plants. An effect of similar magnitude was detected for skin weight, while seed weight was not affected. The sugar accumulation process resulted more pronounced in WS berries and this was paralleled by higher titratable acidity values both at véraison and harvest. In 2009 the total phenolic content of the berries was influenced by water stress only at the end of the trial, with a reduction of the seeds phenolic compounds. The incidence of seeds on total phenolic content was higher than that of skins, confirming that this behaviour is a varietal characteristic. Accumulation of anthocyanins (Anth) on whole wine grape bunches attached to the vine was studied using a non-destructive fluorescence-based sensor, extremely useful for a rapid and non-invasive determination of phenol compound-related parameters in the vineyard. The very same 50-60 bunches were monitored during the seasons at a weekly frequency from véraison to harvest. For each date of measurements, chlorophyll fluorescence signals under different excitation wavelengths were collected to derive Anth, flavonols (Flav) and chlorophyll (Chl) indices. The ANTHR, that is the Anth index based on a single fluorescence signal excited with red (R) light, and the FLAV index increased and decreased with time from véraison to harvest, respectively. The Chl index was monotonically decreasing, while the ANTHRG, based on two fluorescence signals excited with red (R) and green (G) light, followed a biphasic behavior increasing to a maximum at about complete véraison and then decreasing to harvest. All the indices suggested an early ripening process in 2009 compared to 2008, in agreement with other standard indicators such as véraison occurrence, technological maturity and berry development. Calibration of the fluorescence sensor was performed in 2008 by destructive HPLC analysis of phenolic compounds in berry skin extracts. Starting from complete véraison, the ANTHRG index was found to be fairly inversely correlated (r2 = 0.875) to the Anth surface-based concentration (mg/cm2) through an exponential function. On the contrary, the Flav index was uncorrelated to the Flav content, because of the interference of Anth on the fluorescence signals. The ANTHRG non-destructive index was able to detect differences in the Anth accumulation between seasons in accordance with the standard destructive analysis of Anth berry skin content. Water stress imposed in 2009 increased Anth accumulation in berries due to a reduction of berries in size but also to an increased Anth biosynthesis. This effect was observed by both destructive and ANTHRG non-destructive measurements. In order to study at molecular level the expression of specific genes involved in the anthocyanin biosynthetic pathway and water stress-related responses in Aleatico berries, a research stage at the Wine Research Centre of the University of British Columbia (Vancouver, Canada) was carried out. The transcript accumulation of several putative water stress-sensitive genes was preliminary analyzed by qRT-PCR to identify possible common biomarkers in leaves and in berries. Among these genes, Dehydrin1a showed significant changes in transcription in WS samples. Considering genes involved in the Anthocyanins pathway, the expression of UFGT (flavonoid 3-O-glucosyltransferase) and FLS1 (flavonol synthase) appeared to be up-regulated by WS. A similar response was also observed for two genes involved in the anthocyanin hydroxylation and methoxylation processes. Taken together results indicate that the variety Aleatico appears to be tolerant to water stress condition and this information could be useful also for setting up targeted post-harvest dehydration strategies to produce dessert wines and to allow its cultivation in territories where irrigation is not available or saving water when the irrigation must be used in severe dry conditions