The Friendly Vine: September 2011

"True Facts"

The Hot Weather Question:

“This warm weather has been great for the growth in the vineyards around the valley. Our question to you is, does the HOT weather, above 90 really shut down the vines? Or is it just a rumor?”

This is certainly WAY, WAY beyond the focus of a wine shop blog, don't you think?

Sure, I’ll look into this, I said. How hard can it be to find an answer?

How wrong I was. Just looking in to the topic turns out to be way above my pay grade - AND there's controversy!

hmmmm

Conventional Wisdom

At first blush, it seems that “everybody knows” that grape vines shut down when it gets hot:

“Vines can suffer heat stress, which happens in areas where daytime temperatures approach or exceed 100° F. This is true in many areas of California, Spain, and Southern Italy. High temperatures cause photosynthesis to slow or stop, reducing levels of malic acids in the grapes.”

“In fact, here's the big problem: When temperatures get much above 90 degrees, grapevines 'shut down.' In other words, if it's too hot, the vines go into survival mode and stop ripening grapes.”

“Also, photosynthesis slows around 90 degrees and shuts down after 95, protecting the plant from loss of water.”

“Strange as it may seem, when the heat and dryness hit their typical hellish levels in August, the grape vines go into stand-by mode. Over 35 physiological photosynthetic and enzymatic functions shut down and the plants go into self-induced hibernation. It’s their way of beating the heat!”

“Excessive summer heat will shut down photosynthesis (leaves are temperamental little synthesizers), retarding grape development.”

As I attempted to uncover more than anecdotal rationale for these statements I found, well... nothing. Very few actual studies have been published relating to grape vines and heat. The studies that I do find actually state - that this area has not been very well studied. Sure, if you are a vineyard manager or vine tender, you can walk your rows during a heat wave, see the effects and draw conclusions, but do you know that the vines have “shut down,” that photosynthesis has ceased?

“Indirect evidence indicates that high temperature may disrupt photosynthesis and berry sugar accumulation in commercial vineyards (AWBC 2008; Retallick and Schofield 2008) and phenological windows when high temperature correlates with low wine quality have been identified (Soar et al. 2008). However, these interpretations are speculative in the absence of experiments.”

“Studies that focus on the effect of high temperature at … phenological windows are scarce and highlight important knowledge gaps. Importantly, the widespread notion that vines “shutdown” in response to heat stress…is inconclusive in the absence of studies where heated and control vines are compared.”

Grape Vines are Pussies

Is Something Burning?

When it is really hot out, bad things DO happen to vines - and most kinds of plant life that aren’t freakin’ cacti. A blazing sun and high temps can fry the vines resulting in stomal (stomata=leaf pores) water loss, shoot and leaf burn, desiccation (dried leaves) and leaf drop, stalled development affecting ripening and sugar accumulation, berry sunburn, berry bagging (this just sounds awful!) and berry shrivel. Prolonged exposure to high temperatures AND intense solar radiation damages berries, even those not in direct contact with sunlight.

Berry Bagging - As bad as it sounds

Water plays a key role here and dry-farmed vines fare worse than irrigated vineyards, since strategic watering can mitigate heat effects. Additionally, early season exposure (during key phenological windows) to sunshine seems to act as a sort of SPF 50 for vines since:

“Some plant secondary metabolites (mainly phenolic compounds, flavonoids, and hydroxycinnamate esters) accumulate in the vacuoles of epidermal cells in response to UV-B irradiation and protect plants from further damage and from subsequent or intense radiation and act in the same manner as sunscreen. Vineyards not preconditioned due to cooler early summer temps may have reduced levels and therefore less protection from radiation.”

But because photosynthesis, growth and fruit ripening are all dependent on adequate amounts of sunshine and warmth, most plants love and depend on warmth. Is there a point of warmth prior to things going up in smoke, with plants actually getting cooked and burned, when things just “shut down” per the conventional wisdom?

The Devil's Wine

How hot is too hot?

As you can see from a couple of the comments quoted above, there’s not even any agreement on what constitutes really hot weather. Experiencing the myth of global warming first hand, the growing areas of Australia have had some SEVERE heat waves recently, in 2008 and 2009. Since then, they've actually put some effort and science into understanding what happened to their poor vines. Let’s ask those guys Down Under:

“Temperatures over 40°C (104°F) for longer than 2 days is considered a hot event in Australia, but they have experienced 12 day periods twice in two years. Most growers suggested that more than 2 days over 40°C could be considered a hot event. . . The event was long and challenging! It got hot and stayed hot. . . Both duration and intensity were considered important but duration was more of a factor– '…you can’t keep up with the water for extended periods.' In one case the extended stress on vines caused the leaves to eventually wilt and then the berries became exposed.”

That’s just during the day! If nights are hot too, then things are even worse for vines:

“Significant nighttime stomatal conductance and transpiration (due to nighttime heat) is associated with higher daytime evaportranspiration values and the driving gradient between the leaf and the atmosphere at night.”

Temperature is not the only variable either. Irradiance, or sunshine, is also a significant factor. Intense sun AND intense heat are a lethal combination for plant life, and solar radiation’s effect on grape berries, in particular, is pronounced:

“The temperature of the grape berry or cluster is dominated during the day by solar radiation, or the intensity of sunlight reaching the fruit. Some heat can be gained or dissipated by convection as warm or cool air passes over the berry. A small amount of evaporative cooling occurs when berries are green and can transpire like leaves, but the berry has fewer stomates than do leaves. The berry stomata are essentially non-functional during ripening. Berry temperature can exceed air temperature by as much as 18ºF to 27ºF when berries are exposed to direct solar radiation in mid-summer. The more intense the solar radiation and the less that air moves through the canopy and across the fruit, the higher the average berry temperature.”

A Heated Explanation

“High temperatures inhibit plant growth and development. Elevated temperatures increase respiration and therefore require greater carbon fixation for sustained growth and survival. Temperatures >35C significantly decrease the activity of Rubisco, thereby limiting photosynthesis.”

That quote appears only to be true to those who have actually studied such things. Everyone else seems to assume the opposite – that respiration slows or shuts down during heat stress. But this point looks to be the key one. High temperatures don’t “shut down” grape vines per se. Given adequate amounts of water, the high heat causes the plants to breath harder, to pant in order to cool off. As temperatures increase the photosynthesis engine kicks into high gear, but at some point chemical reactions begin to fail. Actual experiments have shown that:

“Photosynthesis was also affected on each occasion, with rates declining by 35% and taking 12 days to recover. Up to 10 mg carbon g (berry dry weight)^–1 day^–1 was required for ripening after veraison. For vines heat treated at veraison and mid-ripening, net carbon acquisition rates fell to below 4 mg carbon g (leaf dry weight)^–1 day^–1, which is inadequate to supply berry carbon requirements. This suggests that the impacts of heat on the ripening process can be traced back to the supply of carbon.”

“Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds[2] that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO.”

In controlled conditions and with sufficient water, the vines don’t shut down so much as they lose the ability for carbon fixation and therefore photosynthesis eventually fails rather than shuts down. Stomata on the leaves remain open and the vine keeps “breathing”, however, rather than the plant going dormant.

Broiling in Controversy?

Do grape vines “shut down” in hot weather? Well that depends…

If there isn’t sufficient access to water at the roots, then for sure leaves go limp, pores close and the vine quits breathing. Everything goes flat line as the plant goes dormant.

And its important to note that studies in Australia where the impacts of warming are truly being felt suggest that:

“a short period of high maximum temperature disrupts gas exchange and arrests berry growth and sugar accumulation in established, well-watered vines, and the magnitude of these effects depend on the phenological window, or when in the growth cycle the heat stress occurs”

So it is worth noting that a heat wave can effect grape vines to different degrees depending on when in the growth cycle that it occurs. A blast of heat at the flowering stage or at veraison seems to be the most risky. Studies were done on vines in controlled environments where they were:

"...exposed to temperature (spikes) at flowering, fruit set, veraison and mid-ripening stages. Results: Leaf growth and stem extension were unaffected by heat whereas flowers completely abscised. Berries heat-treated at fruit set developed normally, and those heat- treated at veraison and mid-ripening stopped expanding and sugar content stopped increasing.”

But for my final evaluation I am going with:

“At low water stress, the diffusion resistance for water vapour decreased in response to a gradual increase in temperature. Transpiration increased accordingly. This response was reversible. All plant species studied responded in the same way... At high plant water stress, the stomatal response was reversed, i.e., the stomata closed when temperature was gradually increased.”

High temperatures affect and can inhibit vine growth and berry development. But rather than the vines “shutting down” as is commonly thought, elevated temperatures actually increase respiration. But the requirement for greater carbon fixation to sustain the plant can’t be maintained indefinitely, above certain temperatures. When its hotter than 95F (35C) the activity of Rubisco, a key enzymatic reaction required for photosynthesis, is significantly decreased.

The plant's photosynthesis engine revs up in response to rising temperature, and increased respiration helps to cool the plant. Providing there is enough water at the roots, respiration, or the plant's breathing, continues even above 95F, but photosynthesis begins to fade as key chemical reactions fail. Hardly a plant that is "shut down" or dormant, but of course, after a certain point, temperature and sunshine take their tolls in other ways.

“Contrary to the notion that vines “shut down” in response to heat stress, …trials suggest that (irrigated vines) show otherwise. An important point when drawing conclusions from observation sin the vineyard, however sound in principle, is that the field involves …myriad…correlated factors. For example, if we observe a slowdown in accumulation of sugar in berries after a heat event, can we conclude that heat was the cause? The answer is no. This type of circumstantial evidence is a valuable guide, but no proof of cause and effect. … When heated and unheated vines are compared in specially designed experiments, two seasons of field trials showed a consistent lack of response to three days in a row with day temperature of above 40C (104F). Foliage maintained photosynthetic activity, and the Brix trajectories in berried in heated and unheated treatments were identical. Two important traits, namely photosynthesis and berry sugar accumulation, remained unaffected by short exposure to extreme heat."

"Physiological principles suggest that if the vine is simultaneously hit by water and heat stress, the consequences could be much more negative. Indeed, anecdotal observation of vines shutting down in response to heat episodes might be reflecting the interaction between water and heat." Victor Sadras, principle scientist for crop ecophysiology, at the South Australia Research and Development Institute (SARDI).