Grapes 101

Stomata
Open (left) and closed (right) stomata.  Stomata (plural of stomate) are the portal through which plants regulate transpiration and gas exchange.  When closed, they restrict water use – but also restrict photosynthesis and carbohydrate production.  As water deficits intensify, stomatal closure limits evaporative cooling and can delay ripening.  Leaves heat up, and cellular function can be permanently degraded.  Finally, leaves from basal nodes out start to senesce, dry up, and fall off the vine.  Photos by (L) Bhaskar Bondata, Washington State University and (R) Markus Keller, Washington State University.

How Grapevines Respond to Water Stress

Grapes 101 is a series of brief articles highlighting the fundamentals of cool climate grape and wine production.
By Tim Martinson and Alan Lakso

2016 precipitation totals for Geneva, NY. (1) Monthly rainfall totals up to 8/18/16. (2) Long-term average rainfall for the month (total). Table from the Finger Lakes Grape Program.

Water relations are a key factor in grapevine growth and development.  Plants take up water to maintain cell turgor, to make and expand new tissues, to provide evaporative cooling, and to facilitate gas exchange for photosynthesis (C02) and respiration (O2).  Vines actively regulate the flow of water in response to environmental conditions. 

Water availability dramatically affects vine vegetative growth, fruit composition, and potentially winter hardiness.  An overabundance of water is associated with excessive shoot vigor, canopy shading, and reduced fruit quality - and may delay the cessation of shoot growth and periderm formation, leading to poorer winter hardiness.  

Moderate water stress at the right time can reduce vegetative growth and help vines achieve the appropriate balance between vegetative growth and fruit yield and quality. Severe water stress limits photosynthesis, and can delay ripening, reduce bud fruitfulness, reduce winter hardiness, and result in sudden vine collapse.

How does water move through vines?  Water is taken up by the roots and moved through the xylem tissues (the pipeline) to leaves through water pressure differences.  This process is driven largely by transpiration through stomata in the leaves and green tissue.  As water evaporates into the atmosphere, this places water under tension, setting up negative water potential that draws water up through the vine – much like sucking water through a straw.  Transpiration accounts for 95-98% of a vine’s water use.  Importantly, transpiration also provides evaporative cooling to keep sunlight-exposed leaves close to ambient temperatures.

Evapotranspiration.  Evapotranspiration is a measure of water loss from the soil, including both transpiration (from plants) and evaporative losses from the soil surface.  Evapotranspiration estimates (ET0) use local weather data and a crop coefficient to estimate water use for irrigation scheduling.

When soil water is readily available, the rate of evapotranspiration is driven by temperature, wind, relative humidity, and canopy size.  High temperatures, dry air, and wind drive water use higher.  Large vines have more leaf area and use more water than small vines.  

The stomate.  Transpiration is controlled by stomata (greek word, plural of stomate).  Abundant on the underside of leaves, stomata actively open and close to allow or restrict gas exchange. The two large cells, called guard cells, open and close in response to sunlight, water potential, and hormonal signals to regulate gas exchange.  Stomata are often closed at night when there is no sunlight to drive photosynthesis.  Under adequate moisture during the day, however, sunlight stimulates them to open to allow the vine to take up carbon (CO2) for photosynthesis, and release oxygen (a byproduct of photosynthesis) and water vapor. 

As soils dry out, evaporative demand can exceed the soil’s ability to supply water. This places water under more tension (like a stretched rubber band) – potentially leading to formation of air bubbles or embolisms that interrupt the continuous column of water from root to leaf.  Stomata, by opening and closing in response to environmental conditions, restrict and regulate the flow of water molecules to the atmosphere – and prevent embolisms from forming in the vine’s vascular tissue.

In addition, dry soils cause roots to produce more of a plant hormone called abcissic acid (ABA), which also signals stomata to close, conserving water.

Closed stomata conserve water, but also restrict the vine’s intake of CO2 to use in photosynthesis.  Vines respond in various ways:

Shoot growth slows.  Shoot growth is one of the most sensitive indicators of water stress.  Actively growing shoots (L) have long tendrils that extend past the shoot tip.  Moderate water deficit slows growth, and short tendrils that don’t extend beyond the shoot tip (M) signal slower growth.  Under moderate to more severe stress (R) shoot tips will dry up and fall off. 

Photos by Tim Martinson

Tendrils dry up.  Under moderate stress, tendrils further back from the shoot tip will first wilt (L) then dry out (M & R), rather than persisting and being lignified.

Photos by Tim Martinson

Leaves droop.  As water stress becomes more severe, leaves will droop (L, M) and face away from the sun (R, looking South to North at noon).  They may be warm, or even hot to the touch, signaling loss of evaporative cooling.   With air temperatures close to 90˚F, leaves with closed stomata may reach 110-114˚F in the sun.   If water is replenished through rainfall or irrigation soon enough, these symptoms are usually reversible.

Photos by Tim Martinson

Leaf bleaching.  If vines lose their evaporative cooling and temperatures are high for more than a few days, they may take on a ‘bleached’ look, due to thermal breakdown of leaf tissues.  This process is irreversible, and will permanently reduce vine capacity for photosynthesis.

Measurements and photo by Alan Lakso

Leaf senescence.  If water stress continues, embolisms (air pockets) may form in leaf vessels or shoot xylem, and leaves will start to break down and senesce.  In severe circumstances, vines or shoots will collapse.

Photos by Tim Martinson

Vine water use varies. Large vines (eg. Concords) use more water than smaller vines (Riesling). Divided canopies (such as GDC Concord) use 20-25% more water than single canopies.

Estimated Mid-summer Water Usage Rates in New York

Variety/Training System Acre-inches/week Gal. per acre/week
Riesling 0.8-1.5 22-40,000

Concord High Cordon (Single Curtain)

1.2-1.5 32-42,000
Concord Geneva Double Curtain 1.4-1.7 38-45,000

Data from A. Lakso; assumes full canopy, no rainfall.

Soils have different water holding capacity.  Soil texture and depth determine how much water soils can supply.  Under estimated usage rates above, silty loams at field capacity can supply water for 6 weeks; sandy soils will run out of water in a week. 

Water stress will affect vines on coarse, shallow soils earlier and more severely than vines on deep, loamy soils.

Water Holding Capacity of Different Soils

Soil type

Inches of water
per inch of soil

Acre-inches of water
in 24 inches of soil

Clay or Silty Loam 0.25 6.0
Sandy or Gravelly Loam 0.15 3.6
Loamy sand 0.10 2.4
Sandy As low as 0.03 0.7

Source: A. Lakso

How much does water stress reduce vine function?  In 2002, we conducted a study that looked at irrigation and foliar nitrogen and its impact on vine function, yield, and fruit composition in a vineyard with shallow soils.  Drought that year brought us seven weeks from late July to early September with no rainfall.

We measured vine water status in irrigated and non-irrigated vine.  Mid-day leaf stem water potential (a measure of water tension due to drought) was -12 to -15 bars from early August through the start of September in unirrigated vines, and -5 bars in irrigated vines. As a general reference, growers in irrigated production regions often start applying irrigation when stem water potential reaches -9 to -10 bars.

We also measured stomatal conductance (a measure of gas exchange through stomata and photosynthesis) in the same vines.  Note that leaf photosynthesis was severely reduced at mid-day for about seven weeks – from a few weeks before veraison through the middle of the post-veraison ripening season.

This missing photosynthesis resulted in 3° Brix lower soluble solids in the unirrigated vines. 

Finally, pruning weights in the irrigated vines (following two dry years) were twice those of the unirrigated vines. 

This example illustrates how water stress – particularly in soils with limited water holding capacity – can reduce vine capacity in a dry year.   Since soils vary greatly in depth and water holding capacity, the impact of the 2016 drought will vary from site to site.  Growers with fertile, deep soils may see a welcome reduction in vine growth that may reduce the crop, but have a modest impact on fruit ripening.  Others with shallower soils and no irrigation may see delayed ripening and possible carryover effects in 2017.

Thanks to Michelle Moyer and Justine Vanden Heuvel for valuable comments and review of this article, and to Bhashkar Bondada and Markus Keller for the photos of leaf stomata.

References:

Hellman, E. 2012. The Evapotranspiration Method for Irrigation Scheduling, eXtension grape community of practice resource article.

Keller, M. 2010.  The Science of Grapevines: Anatomy and Physiology.  Elsevier, Amsterdam.

Martinson, T., L. Cheng, A. Lakso, T. Henick-Kling, and T. Acree. 2003.  Lack of irrigation in 2002 reduced Riesling crop in 2003.  Finger Lakes Vineyard Notes.

Tim Martinson is a senior extension associate in the Section of Horticulture, based at the New York State Agricultural Experiment Station in Geneva, NY.

Alan Lakso is Emeritus in the Section of Horticulture.