Introduction

Rootstocks are an important means of adaptation to environmental conditions while conserving the typical features of the currently used scion genotypes. Rootstocks allow tolerance to phylloxera, but also play a dominant role in water and mineral nutrients absorption¹. In viticulture, mineral deficiencies and toxicities are related to low vegetative development and yield reduction². As well as a lack of a certain mineral, mineral deficiency can be induced by an excess of a competitive mineral, an inappropriate soil pH or the presence of active limestone for iron. The first written work on the impacts of rootstock on foliar deficiency symptoms was produced by Bovay and Gallay (1956)³. Since then, rootstocks have been classified according to their ability to satisfy scion mineral requirements, but only based on petiole mineral content^{4 5}. The objectives of the present study are i) to evaluate the relative effects of the scion and the rootstock on scion petiolar mineral content, using a very wide selection of rootstocks, ii) to determine whether scion mineral content is related to rootstock genetic backgrounds, and iii) to characterise the relationship between petiole concentrations and mineral deficiency symptoms.

Material and methods

The studied plant material is part of the experimental design GreffAdapt located on the “Domaine de la Grande Ferrade” near Bordeaux, France (44° 47' 26.7" N, 0° 34' 26.5" W)⁶. Vitis vinifera cvs. Cabernet-Sauvignon, Pinot noir, Syrah and Ugni blanc were grafted onto 55 different rootstock genotypes and characterised under field-grown conditions. 30 rootstocks used commercially in France and 25 rootstocks used in other countries were selected for relevant traits related to lime-induced iron deficiency chlorosis, drought tolerance and conferred vigour. Parentage assignments were made according to the dominant genetic background present (Figure 1).

In the soil of the experimental vineyard, there is a disequilibrium between Mg and K (K/Mg >> 1) which leads to a plant Mg deficiency through an excess of K. The top soil contains about 2 % of organic matter, thus the plot is considered well supplied. The cation exchange capacity (CEC) of the soil is low (< 7 cmol⁺/kg), emphasising Mg deficiency.

In 2020 and 2021, the petiole concentrations of 13 mineral elements (N, P, K, Mg, S, Na, Ca, B, Zn, Mn, Fe, Cu, and Al) at veraison were quantified by Waypoint Analytical Virginia for 1220 plants aged 5 and 6 years respectively. Mineral element concentrations were determined by an inductively coupled plasma mass spectrometer, except for N concentration, which was determined by a Leco FP-528 instrument, an N determinator.

In 2021, magnesium deficiency severity was visually scored on each plant using the GreffAdapt experimental design. The observations were qualitative and a score between 0 and 3 was assigned, as described in Figure 2.

The rootstock effect on scion mineral content was strong and stable over the 2-year study

The effect of the rootstock genotype is stable from year to year, being mostly stronger or equivalent to the scion (maximum 10-fold) for most macro-element contents except nitrogen.

Over the two years, the percentage of variance explained (PVE) by rootstock varied from 7 % for N to 45 % for S, and for the scion it varied from 2 % for P to 24 % for K. The strongest double interaction occurred for rootstock × scion, with a range of between 11 and 20 %. The block and the block interaction with other factors seemed to be negligible in this study.

The genetic background influenced mineral status over the two years

The rootstock genetic background (Figure 1) particularly influenced the mineral status of P, K and Mg over the two years of analysis; i.e., the proportion of plants which showed deficiency, optimum or excess. In two years of the study, four out of seven genetic backgrounds modified significantly the P and Mg status, such as V. riparia which induced higher Mg and P deficiency.

From a relationship between petiole content and plant symptoms to a mineral nutrient classification of the rootstocks

The rootstock effects on scion Mg deficiency symptoms and petiolar Mg concentrations were highly variable (Figure 3). In most cases (43/55 rootstocks), Mg visual deficiency symptoms were consistent with petiole Mg concentrations. However, this might not be the general rule, as mineral petiole content at mid-veraison did not always reflect the mineral status of the plant. Thus, this work strengthens the existence of a different Mg deficiency sensitivity depending on the rootstock.

Figure 3. Rootstock classification based on scion Mg deficiency symptoms (A), sorted by increasing proportion of Mg deficient plants with a score of 3 and 2, and rootstock classification based on petiole mineral content (B), sorted by increasing proportion of Mg deficient plants.

Conclusion

Overall, this study provides valuable insights into the rootstock effect on scion mineral status. In our growing conditions, the joint evaluation of Mg levels by petiole analysis and symptoms observations underlined the variability of satisfactory mineral nutrition thresholds depending on the rootstock. Finally, visual observations of symptoms reflected the mineral satiety of the plant better than the processing petiole mineral analysis.

As a result of this study, we can propose a rootstock classification for conferred Mg status in the scion to adapt the rootstock genotype to soil characteristics or fertiliser management.

Acknowledgements: This study was carried out with support from Jas. Hennessy & Co. (16100 Cognac, France). The RootLoireValley Project was supported by the InterLoire, and the VitGrefSec Project was supported by the Conseil Interprofessionnel des Vins de Bordeaux (CIVB). This work was also granted by Plant2Pro® Carnot Institute in the framework of its 2019 call for projects. Plant2Pro® is supported by ANR (agreement #19 CARN 0024 01 2019).