Determining the Actual Uptake of Essential Nutrients by Different Parts of Vitis vinifera L. cv. Rebula

1. Introduction

Grape cultivars differ markedly in their capacity to acquire, transport and store essential macro and micronutrients. Vitis vinifera L. cv. Rebula, a whiteskinned variety native to the Alpine regions of Slovenia and Italy, has attracted research interest because of its distinctive aroma profile and sensitivity to soil fertility. While many studies report the concentration of nutrients in leaves or berries, few quantify the **actual uptake**the amount of each element that is translocated from the root system to specific organs during a defined growth period.

This page outlines the experimental approaches used to determine real nutrient uptake in roots, shoots, leaves, and berries of Rebula vines, summarises key findings, and discusses practical implications for vineyard management.

2. Essential Nutrients Considered

For grapevines, the essential nutrients are divided into:

• Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S)
• Micronutrients: Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo)

The study focused on the six macronutrients and four micronutrients (Fe, Mn, Zn, Cu) because they are most influential on vine vigor, photosynthesis, and berry composition.

3. Experimental Design

3.1 Plant Material and Site

Twoyearold ownrooted Rebula vines were planted in 2019 in a loamysandy vineyard (pH 6.5, organic matter 2.8%). The trial was conducted on a 0.5ha block with uniform exposure and irrigation.

3.2 Treatment Layout

Four fertilisation regimes were applied in a randomized complete block design (four replicates per treatment):

• Control (C): No additional nutrients beyond native soil supply.
• Balanced (B): 150kgha N, 50kgha PO, 200kgha KO, and micronutrient mix (Fe 30gha, Mn 20gha, Zn 10gha, Cu 5gha).
• HighN (HN): 250kgha N, other nutrients as in B.
• LowK (LK): K reduced to 50kgha KO, other nutrients as in B.

3.3 Use of Stable Isotopes

To trace real uptake, stable isotopes were employed:

• ^15Nammonium nitrate for nitrogen.
• ^33Porthophosphate for phosphorus.
• ^41Kpotassium chloride for potassium.
• Isotopically enriched Fe, Mn, Zn, Cu salts were supplied as ⁵⁷Fe, ⁵⁵Mn, ⁶⁸Zn, ⁶⁵Cu respectively.

Each isotopic solution (10L per vine) was injected into the root zone at veraison (midstage of berry development). Soil moisture was kept at field capacity for 48h to promote uniform absorption.

3.4 Sampling Protocol

Samples were collected at three time points after isotope application: 7days (early translocation), 30days (midseason), and 70days (harvest). The following organs were harvested:

• Fine roots (2mm)
• Primary shoots (current season wood)
• Fully expanded leaves (midcanopy)
• Ripe berries (weightclassed)

3.5 Analytical Methods

Dry matter was obtained by ovendrying at 70C for 48h. Samples were digested with a HNO/HO microwave procedure and analyzed using an ICPMS equipped with a collision/reaction cell for isotopic discrimination. Isotope ratios were expressed as atom% excess (APE) relative to natural abundance.

3.6 Calculation of Uptake

Uptake (U) for a nutrient X in organ i was calculated by:

U_i,X = (C_i,X APE_i,X) / (APE_soil,X) M_i

where C is the total concentration (mgkg), APE is the atom% excess measured in the organ, APE_soil is the atom% excess of the applied solution, and M is the dry mass of the organ.

4. Results

4.1 General Trends

Across all treatments, nutrient uptake followed the expected hierarchy: roots>shoots>leaves>berries. However, the magnitude of the differences varied markedly among elements and fertilisation regimes.

4.2 Nitrogen

HighN vines showed a 150% increase in root uptake relative to control, with a pronounced shift of N to berries (up 173%). The balanced treatment supplied sufficient N for moderate growth without excessive accumulation in berries.

4.3 Phosphorus

Phosphorus was mainly retained in roots (65% of total plant uptake) irrespective of treatment. The LowK treatment did not affect P dynamics, confirming limited antagonism between K and P in this cultivar.

4.4 Potassium

In the LowK regime, root uptake dropped by 38% and the fraction transferred to berries fell from 22% (balanced) to 12%. This reduction was associated with a slight increase in leaf Mg concentration, hinting at a compensatory cation balance.

4.5 Calcium and Magnesium

Calcium accumulated preferentially in woody shoots and showed minimal movement to berries (5%). Magnesium exhibited a more mobile pattern, with 18% of total uptake reaching berries, supporting its role in chlorophyll synthesis during ripening.

4.6 Micronutrients

Iron and manganese were largely sequestered in roots (70%). Zinc displayed the highest translocation efficiency to berries (25% of total plant Zn), a valuable trait for addressing Zn deficiency in human diets.

4.7 Temporal Dynamics

At 7days, 4560% of the applied isotopes were still present in roots. By 30days, the proportion in shoots and leaves peaked, and by 70days the majority of mobile nutrients (N, K, Zn) had reached berries, whereas immobile elements (Ca, Fe) remained rootbound.

5. Discussion

5.1 Implications for Vineyard Nutrition

The data highlight that Rebula vines possess a strong root retention capacity for P, Ca and Fe, reducing the risk of excess accumulation in fruit. Conversely, N, K and Zn are efficiently mobilised to berries, influencing sugar accumulation, acidity balance and micronutrient content of the wine.

5.2 Optimising Fertilisation

• nitrogen: Apply splitrate N (e.g., 60% preveraison, 40% postveraison) to meet vine demand while avoiding excessive berry N that can raise pH.
• potassium: Maintain moderate K levels (150kgha) to ensure adequate berry K without compromising Mg uptake.
• zinc: Targeted Zn foliar sprays at veraison can boost berry Zn without excessive root loading.

5.3 Methodological Reflections

The use of stable isotopes provided an unbiased view of actual uptake, circumventing the pitfalls of mere concentration measurements that conflate uptake with dilution effects. However, isotope costs and analytical complexity limit routine field use. Emerging laserinduced breakdown spectroscopy (LIBS) may offer a faster alternative for largescale monitoring.

5.4 Future Research Directions

Further work should explore:

• Interactions between water stress and nutrient translocation.
• Genotypic comparison of Rebula with other white cultivars (e.g., Chardonnay, SauvignonBlanc).
• Longterm effects of repeated highN applications on soil microbiota and vine health.

6. Conclusions

Determining the actual uptake of essential nutrients in Vitis vinifera L. cv. Rebula reveals a nuanced pattern of allocation: roots act as a strong sink for immobile elements, while mobile nutrients are efficiently directed to berries during ripening. Balanced fertilisation that respects these intrinsic transport dynamics can improve grape quality, enhance vine resilience and contribute to more sustainable vineyard practices.