Photosynthesis, Carbon Dioxide and Higher Alcohol Wines

Let me preface this piece by saying that I'm a fan of lower alcohol wines. I was first introduced to wine in 1980s Upstate New York, drinking 11.5% Rieslings and, yes, Cayuga Whites. Red wines from the area, like Marechal Foch and De Chaunac (for an overview of hybrid varieties like Cayuga White, Marechal Foch and De Chaunac, see here) were in the range of 12-12.5%. At the time, this wasn't considered low alcohol, it was considered the norm.

Perhaps because of this early exposure to lower alcohol wines, I've always preferred them, and I actively look for them when considering a purchase. New Zealand, despite being a cool climate winegrowing region, now regularly produces 13-13.5% whites and Pinot noirs are often seen with 14.5%. This goes against my, personal, ideas of what these wines should be.

I look forward to wines from the really challenging vintages where grapes struggle to get as ripe as the winemakers want them. The 2012 vintage in Hawkes Bay had a really cool and extended ripening period and grapes were brought in at much lower sugar levels (measured in degrees Brix in New Zealand and other places) than usual, but it resulted in some very good, and more elegant, in my opinion, wines as a result, with the alcohol being in balance with the other aspects of the palate. New Zealand's most recent vintage was also a challenge due to rainfall in the ripening period, resulting in grapes being brought in before target Brix were hit. I tasted my first of this vintage's wine in August, a Marlborough Sauvignon blanc, that had all the hallmarks of the region, but with only 12% alcohol: it was a much more harmonious assemblage than with the usual higher alcohol wines of the region.

Having got that out of the way, what about climbing alcohol percentages in wine?

Rising average alcohol levels in wines has been a topic of discussion for some time, and various reasons for this have been put forth over the years. A useful study to look at was published in 2011 by Alston et al. where data from California was examined to show that average harvest Brix levels between 1980 and 2010 increased. Sugars in red wine varieties increased by an average of 0.23% per year over that period, and notably, in their Figure 1, Brix for red varieties was pretty much flat from 1980 to the mid 1990s, rising from there to the 2010 average of around 23.8°. This was particularly noticeable for the North /Central Coast and Delta regions, where the rate of increase was 0.72, 0.75, and 0.96%, respectively, between 1990 and 2008, compared to 0.53% for California as a whole (Alston et al. 2011 Table 1).

So fruit sugars are going up (at least in California) and the wines made therefore have higher alcohol. But why are the sugars going up?

A different take on this has come about recently, where people are starting to look at rising carbon dioxide (CO₂) concentrations in the atmosphere and linking this to increased plant productivity (e.g. here and here). This isn't a difficult link to make, as the process of photosynthesis takes CO₂ and water and with the help of the enzyme Rubisco, releases oxygen and sugar (in the form of glucose). It stands to reason that if you increase the availability of a starting material, you can end up with more product. This is assuming that other starting materials (Rubisco, water, sufficiently warm temperatures and light energy in this case) aren't limiting, and that the products don't start piling up in the area where they're being produced - if glucose and oxygen keep building up in the cell, the rate of photosynthesis will slow through a process called feedback inhibition.

So increasing CO₂ should mean more efficient photosynthesis and more sugars to go around? As we usually find, things aren't that simple.

There has been plenty of research into the effects of raising CO₂ concentration and its positive effects on plant growth and productivity, however, much of this has been with relatively short duration experiments. When plants have a longer time and a chance to adapt to the changed conditions there is more talk of photosynthetic down-regulation, or acclimation, resulting in relatively little change.

A review by Makino and Mae notes that longer term plant adjustment is a complicated system. For example, if sugar is being produced more quickly, but the plant does not have the capability of moving the sugars out of the cell fast enough, photosynthesis will be slowed by feedback inhibition. This kind of makes sense, too, as the plant would change things so that a balance remains between production and utilisation of photosynthetic products.

There is also a suggestion that seedlings have a greater response to high CO₂ compared to older plants, possibly because seedlings are generally carbohydrate supply limited, whereas older plants have a store of carbohydrates that are used when needed. Grapevines, being perennial plants, have decent carbohydrate stores even from a reasonably young age.

It's not just photosynthesis that can change, either - under climate change scenarios, increasing temperatures will also increase the respiratory activity of Rubisco. Yes, this enzyme goes both ways: it can help convert CO₂ into sugar, but the same enzyme also latches onto oxygen in the process of photorespiration. This opposes photosynthesis and makes the process less efficient. Photorespiration increases faster than photosynthesis as temperatures increase, so photosynthetic efficiency suffers.

And as Jamie Goode has pointed out (here in an article where he points out a whole bunch of interesting things on the subject) with higher CO₂ plants don't need to open their stomatal pores as much, because a lesser amount of air holds the same amount of CO₂. This can lead to less water use, as with less air movement in and out of the leaf, there is less water vapour lost, too. A side effect of this, however, would be a rise in leaf temperature due to less evaporative cooling (you can experience this by spraying your arm with water - it feels cooler right away because the water is evaporating, and to do that your body heat is used). Higher leaf temperatures could mean more photorespiration, and more time when the leaf gets too hot to keep the enzymatic machinery going. Higher temperatures, associated with climate change, will only make this problem worse.

The multiple changes to the environment will cause plants to respond, but exactly how they respond is really too complex for us to say at the moment, especially when you start to take into account that these changes will have an influence on all the other living creatures around and on the vine (disease organisms, insect pests, and don't forget the soil ecosystem!).

So the overall effect on grapes and wine gets hazy pretty quickly, with lots of factors, and responses, involved. Having said this, I agree with Jamie in that the rise in CO₂ concentration is not really what's responsible for increasing wine alcohol - that has more to do with consumer preference and technological advances.

For those that are thinking about strategies for dealing with high Brix and high alcohol wines, we have a number of tools in the viticultural toolbox, but this is a topic for another article!

Climate change and eco-mindedness

TizWine recently sent out a link to an article about climate change in California and its effects on the wine industry (Napa Valley Register).

It's not really focused on climate change in terms of  global warming, but more the climate of perception and ideas about vineyard management.

There is beginning to be more emphasis on looking at more than just dollars in the bottom line. The article describes how some parts of vineyards that were prone to flooding were ripped out to improve flow through it for wet times of the year. Native plants are being planted near vineyards to encourage native fauna (much like the Greening Waipara programme here in Canterbury) and increase biodiversity.

Then, the use of solar panels to generate electricity, the excess of which can be fed back into the grid (There are reasonably good rebate programmes in the States that assist companies to put in alternative energy generations - would that New Zealand had a similar stance), is described - these are sitting on the surface of a pond, which can have multiple benefits in that it would also reduce evaporation from the pond's surface.

Napa is cooler than one might expect from its latitude, primarily because fogs roll in off the ocean during the night, and don't burn off until later in the day. The change in climate will affect this, and I think it would be rather sensitive, too. If the fogs burned off a couple of hours earlier each day, there could be significant effects on the number of heat units gained in those areas. Likewise, if the fogs hang around for longer, things could be a bit cooler. On reflection, that could be a good thing - maybe the alcohol levels in the wines won't get so high! :-)

So to combat this, they're changing the orientation of the vine rows (typically they run north-south, so there is sun on the east side in the morning and on the west side in the afternoon) to more northwest-southeast, so that in the hotter parts of the day the sun-side of the vines don't get the full brunt of the light and heat.

As well, more leaf cover also can reduce fruit temperature and minimise the amount of berry burn:

In this picture, the leaves were removed over a shaded cluster and shoot, and the resulting exposure to the sun has done considerable damage to parts of the berries but also the shoot and rachis!

So keeping some leaves on to get dappled light on the clusters can be a good thing if you are growing in a hot area. Trellising choice also has an influence, as those systems where the fruit is below the canopy (e.g. pergola (overhead) or T-trellis)are better off in those conditions.

 A Parronal vineyard in Chile. Also called a pergola or overhead trellis system. Here shoots have recently been thinned from the vines to create a more dappled light effect on the grapes and ground beneath.

A T-trellis system in Australia. Here the grapes hang beneath the canopy, too, but there doesn't need to be an elaborate overhead wire system to support the vines.

Trellis systems like there are more commonly used for table grapes, where the appearance of the fruit is especially important.

There is no doubt in my mind that we will need to alter management of our vines to suit changes in climate over the next 40 years - as with all things viticulture, planning for it beforehand is the best approach. So read through the climate projections for your area, and make a plan to deal with the possible changes - before they happen and its too late!!!