At the end of April, France was hit by a very strong frost episode. This was the worst frost incident in Bordeaux since April 1991. At this point it seems to us that it is important to write articles about this topic to better inform the winegrape community on the impact of such episodes to the vines. Further on we will discuss available solutions to help assist the plant to get back on track.
Picture 1 : Frozen bud (image: France 3 region)
Two waves of frost touched the “Nouvelle Aquitaine” region in France. The first wave occurred between April 20th to 21st. This episode was not too heavy and frost only occurred in low basin blocks. The second frost occurrence was much more intense and happened between April 27th and 28th in the Bordeaux region. Damages to this region were very severe and widespread. Depending on wine appellation, topography, humidity, varietal, phenological stage, cover crop, …, the intensity of crop damages has varied from 0 to 100%.
The Gironde region agriculture chamber came up with an estimation of crop loss of 40% on average. Fruition Sciences team has obtained the same estimates observing vineyard blocks with all primary shoots completely burned to some vineyards only displaying slight damages on the apex.
To determine what is the best response to have after a frost event, it is important to consider the level of frost damage observed on the vine (from partial to total). Depending upon the severity and the extend of frost damage – from partially affecting the buds to damaging the whole vine-, different decisions will be taken.
In this first article of our frost series, we will focus on biomass and nutrient accumulations. We will try to understand what is the physiological impact of a frost episode on vines reserves and its ability to assimilate nutrients.
To frame our conversation, we split the season in different periods to characterize biomass accumulation phases.
- The first period: from leaf fall to budbreak. This phase contributes to the quantity of available Nitrogen in the ground for plant growth at spring. During that phase, organic Nitrogen is converted into mineral Nitrogen which can be assimilated by the roots. The rate of Nitrogen mineralization is dependent upoon the amount of soil moisture available and soil temperatures.
- The second period : from budbreak to flowering. During this period, vine is mostly heterotrophic which means that vine growth is based on its reserves (starch). This period is the most spectacular in terms of aerial biomass accumulation as the shoot growth rate is the fastest. However, total plant biomass remains almost constant during that period because reserves stored in root biomass decrease to maintain a steady shoot growth at the same time (Schreiner, 2016).
- The third period: from flowering and veraison.
- The fourth period : from veraison to harvest
- The fifth period : from harvest to leaf fall.
In contrast, during periods 3-4-5, the vine is completely autotrophic meaning that the carbon needed for its development is directly obtained from the CO2 in the air. During periods 3 and 4 , total plant biomass increases (root+shoot+ cluster). Figure 1 shows how the timing of biomass accumulation varies for different organs.
Why is it important to analyze frost even in relation to vine period?
From a winemaker and vineyard manager standpoint, it is important to analyze the timing and severity of frost to respond with the best vineyard pruning practices after frost. The pruning practices after frost will be different according to
- the extend of the damage
- the period the vine was under when frost occured.
As a new growth start is desired after frost, the level of reserve still available will directly drive vine response in terms of biomass production and accumulation. THis will in turn affect vine performance for the coming year and for the next year.
Figure 1: time variations of biomass distribution between different organs (adapted from Schreiner, 2016)
Application in Bordeaux 2017?
Frost even in Bordeaux took place as the vine was in transition between period 2 and 3. The frost “resets” vine cycle to start anew. Consequently, it is important to understand that in 2017, frost effected vines will go over two heterotrophic cycles (undergoing twice the second period stage in the same season).
- The occurence of 2 heterotrophic cycles within the same season will deplete the vine reserve to an exceptional low. In other words, experimenting 2 “period 2″ cycles within the same season will draw resources from vine reserves over a period twice as long…and some vines may not have sufficient reserves to sustain this exceptional draw. That is why careful pruning practices should be implemented to help the vine compensate and sustain its growth for consecutive years.
Thus, the timing of the frost will have 2 practical effects on vineyard response:
- Nutrient deficit during grand growth (period 2 and 3) will be more common in 2017.
- Reserves replenishing for 2018 season may be lower than usual, because plant cycle has been reset to a later start. The number of days with a fully active leaf area will be lower and there will be overall less photosynthesis activity over the season.
Last it is important to keep in mind that the postponing of period 2 will not only affect the vegetative organs . Fruit development starting on period 3, will also occur with one month of delay , meaning that vines will mature one month later. Thus the last stage of fruit maturation will be completed under shorter day length . Because light and temperature during the last phase of fruit maturation are critical drivers of fruit chemical composition, we know this late maturity will bear a distinct footprint on the taste of vintage 2017 wines.
In the next article, we will discuss how to fertilize after a frost episode. Knowing nutrition and biomass accumulation cycles, we will discuss how it ispossible to adapt vineyard practices in order to minimize the effect of frost on the 2017 vintage.
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