04.26.2016  
 

Threat of Nutrient Deficiencies Equal to Drought

Professor outlines minerals' impact on grapevine berry development

 
by Paul Franson
 
vine nutrients
 
The nutrients removed with harvested fruit depend on crop yield in tons per acre, vine size, age and vigor, the variety and rootstock, soil moisture and nutrient status and the weather (temperature, light, etc.). Source: Markus Keller, WSU

Napa, Calif.—Grapegrowers devote a lot of attention to water (hardly surprising in view of recent shortages in California), but most probably don’t focus as much on vine nutrition. While aware that plants obviously need minerals, growers also know that grapevines are relatively undemanding compared to most crops, and it sometimes seems that soil contains an unlimited supply.

Nevertheless, the auditorium at Copia in Napa was filled by growers and winemakers at the Napa Valley Grapegrowers’ third seminar in this year’s series about sustainable vineyard practices, which focused on vine physiology and nutrition.

The first speaker was noted expert Dr. Markus Keller, a professor of viticulture at Washington State University and author of The Science of Grapevines. Keller basically concentrated a semester class into little more than an hour.

He focused on mineral nutrients other than water, commenting that vines are very tolerant of drought. He reminded listeners, however, that nutrients must be dissolved in water for the vine to draw them up and use them.

“The grapes harvested from vines contain 2 to 6 pounds of nitrogen per ton, for example. That has to come from the ground, as vines can’t utilize nitrogen from the air, even though 80% of the atmosphere is nitrogen.”

Plants also require phosphorus, potassium, calcium, magnesium, iron, copper, zinc, sulfur, manganese, chlorine, boron, nickel, molybdenum and silicon, though many in very small amounts.

Keller said that water and nutrient uptake in vines is mostly by very young roots, which die quickly and are replaced by others.

During the early season of bud break, osmotic root pressure drives water up the vines.

During the growing season, however, transpiration of water from the leaves creates negative hydrostatic pressure that draws water up. Water usage is then largely dependent on leaf area—typically half a gallon per square meter per day.

Like people, it is most healthful for vines to drink in moderation, Keller said, typically 12 to 20 inches of water per year from rainfall, plus irrigation. Ideally they drink frequently. Temperature drives water demand: About 5%-10% of water is used between bud break and fruit set, 30%-60% from fruit set to véraison, 1%-30% from véraison to harvest and 5%-25% from harvest to leaf fall.

Water also needs to refill the top 3 feet of soil for freeze tolerance and insurance for the start of next year’s growing season.

Nutrient availability
Nutrients are concentrated in surface soil, but their availability is linked to soil moisture—both diffusion and mass flow. Diffusion is usually dominant, but rapid transpiration favors nutrient movement by mass flow with water flow into the vines. Rapid transpiration enhances nutrient uptake.

The availability of phosphorus, potassium, calcium and magnesium is relatively constant, but nitrogen varies widely depending on the location and timing.

Roots grow in nutrient-rich zones but encounter different nutrients in different locations. NO3- is most easily leached, followed by K+ then H2PO4-.

Shallow roots absorb immobile nutrients, whereas deep roots encounter mobile nutrients like NO3-.

Mycorrhizal fungi extend the root zone and make nutrients more available to the vine.

Protein pores (gates) regulate passage of nutrients into the vines, depending on supply and demand. They’re one-way valves that open and close to prevent nutrient leakage.

Nutrient uptake can be active or passive. The channels for passive transport are usually from high to low concentration of cations (K+, Na+, Ca2+) at high soil nutrient availability.

Transporters (carriers) provide active transport. They act as pumps using energy (ATP) to generate proton (H+) gradient, usually from low to high concentration.

This creates a concentration effect of cations (K+, Na+, Ca2+) and anions (NO3-, SO42-, Cl-) inside the vine when soil nutrient availability is low.

Nutrient uptake competition
Nutrient ions can compete for uptake leading to a deficiency:

High NH4+ (acid soils, pH less than 5.5) limits K+ and Mg2+ uptake

High K+ limits Ca2+ and Mg2+ uptake (especially in young, grafted vines in acid soils)

High P limits Zn and Fe uptake due to complexation, the combination of individual atom groups, ions or molecules to create one large ion or molecule.

High Mg2+limits K+ and P uptake

High pH (>7.5) favors Ca2+and Mg2+ uptake, but limits K+ uptake

High Na+ (salinity) limits K+ and water uptake

High Cl-(salinity) limits NO3- uptake

Adjust the form of fertilizer (like KCl) applied when there is a water deficit as uptake may be affected by other ions.

Growth drives nutrient uptake but uses storage reserves for bud break (through bloom), then uptake occurs mostly during rapid growth (greater than the six-leaf stage). Insufficient nutrient supply will reduce growth.

Nutrient deficiency will reduce cell division, increase the root-to-shoot ratio, increase leaf starch, reduce photosynthesis, increase the root transport system and increase reserve remobilization.

Deficit irrigation creates low soil moisture, but vines need water for cell expansion; low water status inhibits growth: Shoot growth stops at mid-day if water potential is too low.

Vigor and tendrils indicate water status. Low water potential increases the root-to-shoot ratio.

Therefore, it’s important not to stress vines too soon. “Know your soil,” emphasized Keller, notably field capacity and the permanent wilting point. “Shoot growth and yield is maximized at 3 to 4% below field capacity.”

He warned, too, “If there’s no sap flow (bleeding) in spring, irrigate!”

And, of course, water demand increases with canopy size.

Water deficit acts as a virtual divining rod: Roots grow downward, following the path of least resistance (soil pores, cracks). They grow towards moist soil regions but away from high osmotic pressure. They may find water even below bedrock.

Roots prefer moist soil patches like the concentration beneath drip lines.

Water deficit may increase nutrient status. It decreases nutrient supply and demand, which slows growth. This reduces overall nutrient requirement but requires more nitrogen to support photosynthesis and nitrogen in the atmosphere is useless for grapevines (not legumes used as cover crops). The available nitrogen is mostly nitrate (NO3-) in soil water.

Uptake of NO3- requires boron, so insufficient boron causes nitrogen (and potassium) deficiency. Assimilation of NO3- into amino acids and proteins also requires Mo, Mg, Mn or Co and sucrose. So deficiency in Mo, Mg, Mn or Co can lead to of NO3- accumulation in the vines’ tissues.

Nitrogen requirement
With nitrogen, moderation is desirable. More nitrogen leads to higher yield but more lateral shoot growth and denser canopy. Growing shoot tips compete with fruit, delaying ripening.

Nitrogen suppresses secondary metabolism (phenolics), and nitrogen (and sulfur) enhance volatile thiol precursor production.

You can decrease wine color by adding nitrogen fertilizer, then hedging away excess growth. It’s better to practice “regulated deficit nutrition,” but while some stress is good, more is not necessarily better. Nitrogen deficit increases root growth and reduces shoot growth and leaf photosynthesis and can even lead to leaf senescence.

Early stress of nitrogen plus water deficit cause low yield and nutrient stress exacerbates drought stress. Deficit of N, K, P, Ca, Fe, B, Zn, Mo or Cu or salinity can reduce fruit set and result in “hens & chicks,” especially with low B, Zn and Mo.

Among amino acids, arginine is more responsive than proline to water and nitrogen. Since yeast can use arginine but not proline, more nitrogen fertilizer is needed as food for yeast with deficit irrigation (RDI).

Post-harvest nitrogen is fine if the post-harvest period is long to build reserves. Véraison nitrogen is as good as bloom nitrogen for yeast-available nitrogen.

Red and white wines differ somewhat on their need for nitrogen. Berry size and sun exposure are more important for red than white grapes as phenolics are responsible for astringency and bitterness.

Moderate nitrogen (and sulfur) fuel the aroma potential, notably volatile thiol precursors (mercaptans) responsible for blackcurrant, passion fruit and grapefruit aromas, but at high concentrations even skunky smells.

Low nitrogen leads to low yeast available nitrogen (under 150 mg/L) and sluggish/stuck fermentations, which breed H2S (rotten egg smells).

More nitrogen may delay ripening and flavonols, encouraging acid/flavor retention, an advantage for hot sites or years.

Other deficits
Phosphorous deficit can encourage shallow root growth but decrease deep root growth leading to susceptibility to drought. Mycorrhiza help with phosphorous supply.

Phosphorous deficit also reduces photosynthesis and increases anthocyanins in the leaves, and leads to leaf senescence.

Potassium deficit is common in high-pH soils. This reduces root and shoot growth, phloem loading (sugar trapping), photosynthesis, fruit set, ripening and storage reserves. It causes berry shrinkage and reduces xylem sap flow leading to drought stress and leaf senescence as well as vulnerability to powdery mildew.

Juice and wine pH is sensitive to potassium, but juice pH is not very responsive to soil potassium. High soil pH may even lead to lower juice pH due to Calcium uptake.

Iron deficit is common in high-pH soils and causes lime-induced chlorosis that is worse in wet, cold soil during the spring and with high soil phosphorus. However, iron is the earth’s most abundant metal ion and can cause dust contamination leading to incorrect tissue analysis results. Iron deficit results in oxidative stress with chlorosis in young leaves, lower photosynthesis, decreased sugar export, lower growth with stunted laterals and poor fruit set.

Iron deficiency can also lead to Zn, Mn, Co and Cd accumulation.

If you’d like more details on the subject of vine anatomy and physiology, see Keller’s book, The Science of Grapevines.

Following Keller's talk, Stan Grant of Progressive Viticulture outlined different methods for managing mineral nutrients in vineyards and viticulturist Patrick Riggs of Domain Chandon discussed his measurements of fruit nutrient content at harvest.

 

 

SHARE »
Close
 
Currently no comments posted for this article.
 
CURRENT NEWS INDEX ยป