However, with the wintery weather should come more rain. This isn't really so needed for the grapevines, but for the other farmers out there, it's well past due! Most grapevines around here are entering dormancy, which is evident from their leaves falling off and the shoots going brown, as the periderm formation is completed.
Dormant canes. The red-brown periderm is a protective layer for the canes. The colour of the canes will vary from variety to variety, with some being lighter than others. Pinot Meunier, for example, has a quite reddish colour to its cane compared to Pinot noir or Pinot gris.
The shoot tips may be green still, but with the first hard frost they will freeze and later fall off.
The dormant state is not necessary for normal grapevine growth, as if the weather was fine, the vines would continue growing. But, if you're looking to get synchronized crops so that the fruit can all be harvested at the same time, then having something to stop the current growth and then re-start it from the quiescent compound buds is needed.
The decreasing autumn temperatures are a signal for the vine to start getting ready for winter and freezing, or at least, not conducive to photosynthesis, temperatures. To do this, the vine breaks down as many compounds as it can in the leaves and transports them out into the shoots (now called the canes, since they've lost their leaves and the brown periderm has formed), trunk or roots.
The enzymatic powerhouses of the photosynthetic process (for example, Ribulose-1,5-bisphosphate carboxylase oxygenase (or rubisco), which may be the most abundant protein on the planet) are broken down into their component parts, amino acids, which are transported out of the leaf and converted into more storage-friendly forms in more permanent parts of the vine. Similarly, the green coloured chlorophyll, which harvests the light used in photosynthesis is also scavenged for reuse.
When these compounds are taken away, it exposes other things in the leaves, which remain because they cannot be broken down again. This includes some nicely coloured compounds, such as anthocyanins. So when the leaves are scenescing, the green of the chlorophyll disappears to reveal the other colours, which can be brilliant reds and yellows.
A photograph of the variety Grand Noir* near harvest. This variety is a tienturier type, which have berries that express red juice. The over-expression of anthocyanins carries through to the leaves as well, so they turn a bright red as the leaves scenesce.
This process happens in a lot of woody plants, which is what give some regions around the world their great autumn colours!
*(a.k.a Bahgha, Baga). Red-wine grape widely grown in the Bairada DOC, Beiras region of Portugal. It is apparently directly derived from an Aramon x Petit Bouschet cross. Has several synonym names including Tinta Fina, Tinta Bairrada and Tinta Poeirinha. In France it has the synonym name Grand Noir. In its Portuguese home region it accounts for the vast majority of planted vines. Very susceptible to mildew/fungus diseases, requiring heat and clay-rich soils to thrive. Produces acidic, tannic wine blends capable of ageing well; especially the "garrafeira" (special reserve) versions.
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