Research has demonstrated that relationships exist between berry compositional changes during ripening and seasonal vine water deficit variations (here). The purpose of this blog is to highlight the specific relationship between berry sugar accumulation dynamics and seasonal vine water use. The results discussed in this study aim at improving winegrower practices such as irrigation, cover crop selection and canopy manipulation in order to modulate berry ripening.
Sap flow and weather data are collected to compute water deficit index variations in a non-irrigated vineyard, located in southern France, over 2 seasons (cv. Syrah). Thermal time is computed by integrating hourly temperatures above a minimum temperature threshold set at 10°C. Two hundred berries are sampled at eight different dates over the same area of 50 vines, centered on sap flow monitored vines.
1. Climatic context
RAIN: Prior to budbreak (ie. during period 1), year 1 and year 2 are similar in terms of cumulated rain (130 mm for both years). During shoot elongation period (ie. during period 2), cumulated rain is 210 mm for year 1 and 330 mm for year 2.
HEAT: From March 1st to September 1st, 1500 °C.d were accumulated for year 1 and year 2. Over the berry ripening period (900-1500°C.d ), vapor pressure deficit was greater than 3.6 kPa for 13 days during year 1; 0 days during year 2.
For the area, year 1 is considered a “dry year” (less rain and higher vapor pressure deficit) while year 2 is considered a “wet year” (more rain and lower vapor pressure deficit).
2. Vine water use profile analysis
Figure 1 shows the vine water use profile expressed as daily water deficit index variations (more info about WDI here). During a dry year, the level of plant available water is more limited. Thus, WDI reaches low values (ie. < 40%) as observed from 900°C.d onward in year 1. Overall, seasonal WDI is relatively low, which also reduces photosynthetic activity. During a wet year, the level of plant available water is less limited. Thus, WDI is constantly above 50%, which favors a higher photosynthetic activity in year 2.
Figure 1: Comparison of water use profile obtained from sap flow in a non-irrigated vineyard. Year 1 is a “dry” year (ie. less rain and higher VPD). Year 2 is a “wet” year (ie. more rain and lower VPD).
3. Effect on berry sugar accumulation profile analysis
Figure 2 shows the dynamics of sugar amount per berry. During year 1, while a low seasonal WDI is being recorded, initiation of sugar accumulation is late (after 1000°C.d ). The maximal amount of sugar per berry is reached late (after 1500°C.d ). In contrast, during year 2, while a high seasonal WDI is being recorded, initiation of sugar accumulation starts earlier (before 1000°C.d ). The maximal amount of sugar per berry is reached earlier (around 1300°C.d) and is 20% higher than year 1. At the same time, maximal berry weight is 20% higher in year 2 (data not shown).
Figure 2: Comparison of berry sugar accumulation profile in a non-irrigated vineyard. Year 1 is a “dry” year (ie. less rain and higher VPD). Year 2 is a “wet” year (ie. more rain and lower VPD).
CONCLUSIONS: Effect of sap flow variations on sugar accumulation
During period 4 sugar accumulates into the berries (as previously discussed). The synthesis and transportation of sugars from the leaves to the fruit are dependent upon the flow of water. By jointly analyzing sap flow with sugar variations during period 4 it is possible to diagnose the effect of sap flow variations on sugar accumulation and identify whether sugar accumulation disorders are caused by a lack of vine water use. Thus comparing Figure 1 and Figure 2 one can observe the following:
- When WDI is maintained below 50%, sugar accumulation is low. Most likely during year 1, a low water use reduces photosynthetic activity, which in turn affects negatively sugar accumulation in the berry.
- When WDI is maintained above 50%, sugar accumulation is high. Vine water use is sufficient to maintain a high photosynthetic activity. Most likely, the greater water use promotes an earlier rate of berry sugar accumulation and a higher amount of sugar per berry.
PRACTICAL TAKE HOME
During the phase of active sugar accumulation (ie. period 4), it is important to maintain a sufficient level of water supply to meet at least 50% of maximal vine water needs. This, in turn, favors a steady supply of sugars from the leaf area to the fruit. This results in smoother ripening curves as observed during a wet year.
Note: When sugar accumulation is disrupted despite the maintaining of sufficient water supply, something else is causing the ripening disorder. This is typically what was observed in many northern Californian sites in 2017 as reported here.
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