Modern Techniques in Agricultural and Horticultural Sciences

The study of grapevine phenology is very useful for vineyard management and control, as it allows the application of phytosanitary treatments at the phenological stages where the crop is most susceptible to different diseases, mainly cryptogamic. This work describes the phenological behaviour of the red Mencía variety, the most common in the Ribeira Sacra Designation of Origin (Northwest Spain), as well as its thermal requirements throughout its vegetative cycle. It has been carried out in a vineyard located in the highest areas of the Sil River canyon, during the year 2021. The observations were made on 10 vines and the phenological scale used was the one standardized by the BBCH. The vegetative cycle, from the beginning of sprouting to the harvest, took 163 days, during which 5097 GDD (Growing Degrees-Days) were accumulated.

Keywords: Phenology, Vitis, Mencía variety, Thermal requirements

In agronomic, economic, social, historical, and cultural terms, the grapevine is one of the most important crops in the northwest of the Iberian Peninsula, including areas such as Galicia, León, and the north of Portugal. According to the Spanish Wine Market Observatory (OEMV), the vineyards in this area cover an area of some 25,823 hectares. This study focuses on the Ribeira Sacra Designation of Origin, a territory that offers an excellent opportunity to study what has been called "Heroic Viticulture", in reference to the special orographic characteristics of the area, which determine the cultivation methods http://www.cervim.org/.

The aim of the work is to provide information on the direct relationship between phenology and the different meteorological parameters, during a complete vegetative cycle of the crop, in a territory that has recently been included in the UNESCO, World Network of Biosphere Reserves https://es.unesco.org/.

The knowledge of vine phenology carried out during the year 2021 provides essential information on the characteristics of wine-growing regions, making it possible to plan cultural agricultural practices and the timing of phytosanitary treatments.^1,2 Moreover, by taking into account information on geographical variations in phenology in crops located in different bioclimatic zones, we can determine the adaptive capacity of a given variety to different weather conditions.³ The main factors that determine the onset of the vegetative activity of a vine, as well as the development of the successive phenological phases, are meteorological, among which parameters such as temperature, precipitation, and photoperiod stand out.⁴ Of all the meteorological parameters, the temperature is one of the main ones, due to its great influence on the microclimate of the vineyard, having a great significance in the distribution and diversity of cultivars.⁵ Other factors such as photoperiod, soil type, crop orientation, cultivar genetic factors, cultural practices, and inter-species competition also have an important influence.⁶

We must also consider climate change, which has become a major concern for wine production worldwide, as grapevines are very sensitive to variations in climatic conditions, mainly temperature changes.⁷ Factors such as temperature and solar radiation are decisive in fruit ripening, inducing earlier harvests, which implies important modifications in the chemical composition of the fruit, as well as in the organoleptic characteristics of the wine obtained.⁸ Winegrowers are forced to adapt to this new situation, carrying out practices such as reorienting planting lines or modifying the vegetative canopy to ensure greater shade.⁹ According to the winegrowers themselves, harvests have been brought forward by three to four weeks over the last 50 years. Phenological studies have proved to be a very useful tool for wine production, both qualitatively and quantitatively.¹⁰ There are previous studies on the phenology of Mencía in this same Designation of Origin, together with studies on the incidence of phytopathogens on the crop,^11,12 so the current study aims to determine the variations in the phenological behaviour of the main red variety grown in this territory over time.

Location and climatic characteristics of the study area
The present study was conducted at the O Mato plot (332m a.s.l. 4230032.3" N; 73003.02" W), Figure 1 during the year 2021. The predominant cultivar in the O Mato vineyard is Mencía, included in the preferential category according to the regulations of the Ribeira Sacra Designation of Origin (DOG N 194, 2009). The Mencía cultivar has a medium vigour with early sprouting and semi-late maturation.

For climatic characterization, we considered the meteorological data of the maximum, minimum, and mean temperature, relative humidity, and rainfall variables Table 1, provided by one station located very close to the studied vineyard, in Monforte, a station belonging to the weather service Meteogalicia
https://www.meteogalicia.gal.

Fieldwork and phenological protocol
We studied the phenological stages for the cultivar Mencía following the scale adopted by the BBCH (Biologische Bundesanstalt Bundessortenamt und Chemische Industrie) as a standardized scale.¹³ Table 2 The phenological observations were applied to 10 grapevines distributed throughout the plot. The study was carried out during the active grapevine cycle, from the 1st of April to the grapevine harvest date in September. During the sampling period, the vineyard was visited weekly, except at the flowering period, when the number of visits was increased to twice a week. For the purposes of determining the phenological calendar of the vines, the start date of each stage was deemed to be when 50 % of the plants under study reached the stage concerned. From the BBCH scale, a total of 18 phenological phases belonging to the six main stages were selected for observation.

Thermal requirements
The heat requirements were calculated following the criterion proposed by Galan,¹⁴ which considers the daily sum of the maximum temperatures (growing degrees-days, GDD) from the end of the cold period to the start date of each phenological phase.

GDD ºC =∑Timax
Timax= daily maximum temperatures in a number of days i.

The F-test of the statistical analysis determines whether the variability between the means of the groups is greater than the variability of the observations within the groups. If this ratio is sufficiently large, it can be concluded that not all means are equal. In this study, it gives a relatively high value, due to the fact that it is a one-year sample, where the elements sampled are not excessively large Table 3.

Phenological behaviour of grapevines
The vegetative growth period of the Mencía variety was calculated from the end of bud swelling, when the buds were swollen but not green (Ph 03), to the berries ripe for harvest phase (Ph 89). In 2021, the vegetative cycle of the vines took place between April and September. The longest stage was the ripening of berries stage (St 8), with 45 days, while the phenological phase with the longest duration also took place at this stage, Ph 85 (softening of berries) with a duration of 30 days. The shortest stage was flowering (St 6) with a total of 11 days, in which also the two shortest phenological phases of the whole vegetative cycle took place, Ph 65 and 69 (Full flowering: 50 % of the flowerhoods fallen and end of flowering); with a duration of two days per phase. The difference between the longest and the shortest stage was 34 days (St 8: ripening of berries and St 6: flowering), while the difference between the longest and the two shortest phenological phases was 28 days (Ph 85: softening of berries and Ph 65: Full flowering: 50 % of flowerhoods fallen; Ph 69: end of flowering). The Mencía variety in the O Mato vineyard only needed two days for Ph 65 and Ph 69 (the shortest), whilst for Ph 85 (the longest) it needed 30 days. For the rest of the phenological phases, no notable differences were observed Table 4, Figure 2.

Analysis of meteorological parameters
The phases included in the main stage 0 (St 0) take place from the beginning of April to the beginning of May Table 4, Figure 2. During this stage, the lowest mean maximum and minimum temperatures (19.29°C and 9.81°C, respectively) are recorded. Stage 1 (St 1) takes place between the beginning of May and the end of May. From this time until mid-June, stage 5 (St 5) takes place. The highest precipitation value (44.5mm of rainfall) is recorded in stage 1. The next stage 6, (St 6) runs from mid-June to the end of June. Stage 7 (St 7), runs from the end of June to the beginning of August and has the highest mean maximum, minimum, and average temperatures (31.41°C, 14.05°C, and 22.22°C, respectively). This stage also has the lowest annual precipitation (1.0mm rainfall) (Table 1). The last main stage 8 (St 8), occurs between early August and mid-September. The heat units accumulated for the ripening of berries (St 8) reached levels of around 5097 GDD in the year 2021. Regarding the heat requirements needed to overcome the various phenological stages, the Mencía variety needed from 1164 GDD for stage 0 (St 0) to 3447 for stage 7 (St 7) Table 3.

The active vegetative period of the vine (from sprouting to harvest) lasted 163 days. Previous studies carried out in this wine region for the same Mencía variety reported very similar results to those obtained here, in 2018 (165 days), 2017 (167 days), and lower than those obtained in 2016 (183 days).¹⁵ The shorter vegetative cycle in 2021 can be attributed to the higher minimum temperatures recorded throughout the vegetative cycle in this area, leading to a shorter development time of the different phenological phases. The lower minimum temperatures in 2016 would explain the longer cycle during this year. Studies prior to 2016 were also carried out in this geographical area and for the Mencía variety also reported values similar to those obtained in this study.¹⁶ The longest cycle length was observed in parallel studies in the same wine region carried out with other red varieties, such as Brancellao and Merenzao, where a vegetative cycle length of 200 days has been found.¹ However, other red varieties such as Maturana and Tempranillo, grown in one of the areas with the highest wine production in Spain, Rioja, have cycles with a similar length to those of our study (158 and 172 days, respectively).⁸ A significant difference has been observed in the date that defines the beginning of phenological stage 0 (St 0). The beginning of sprouting (St 0) normally takes place in March, but as a consequence of the low spring temperatures recorded during 2021, it was delayed to the beginning of April, however, the high temperatures throughout the vegetative cycle allowed the phenological phase (Ph 89) to take place at the beginning of September, a more frequent and usual date in this wine-growing region.

Thermal heat requirements, the daily sum of maximum temperatures, fluctuated from sprouting stage (St 0) to ripening of berries (St 8), in values between 1164 GDD and 5097 GDD. In previous studies carried out in this geographical area, the heat accumulation values recorded for the same variety showed similar values for the sprouting stage (St 0), between 1104 and 1601 GDD, in line with those obtained in our study (1164 GDD). The thermal requirements needed to induce flowering (St 6) were 2763 GDD, also similar to those obtained in previous studies, which ranged between 2892 and 3218 GDD. The date of initiation of the flowering stage is very important in determining the harvest date, as there is a constant number of temperature units accumulated between the two events.¹⁷ Finally, the accumulated heat units for ripening of berries (St 8) reached values of 5097 GDD, a value slightly lower than the 5395 and 5432 GDD reported in Ribeira Sacra in 2017 and 2018.1 In previous studies conducted in this same wine region during the period 2009-2012, more significant differences were reported in the heat units required for the ripening of berries according to the Ribeira Sacra subzone.¹² Vineyards located on the banks of the Sil river required the lowest thermal demand in the 2012 campaign, while other studies in the upper Ribeira Sacra subzone (Amandi) recorded the highest thermal demand in 2007.¹²

The phenological behaviour of the Mencía variety is linked to climatic conditions, with the temperature being the main meteorological parameter responsible for the inter-annual differences that occur in the vineyard, with the delay in the start of the phenological phase (Ph 3) to the month of April, compared to other years, which usually occurs in March. The flowering phenological stage (St 6) was the shortest of the vegetative cycle, while the longest was the ripening of berries (St 8). In terms of thermal requirements to complete the ripening of berries, more than 5,000 heat units (GDD) were necessary.

To the winery that owns the vineyard for the facilities provided to carry out the research.

This research received no external funding.

The authors declare no conflict of interest.

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