Potassium nutrition of self-rooted pinot noir grapevines (Vitis vinifera L.) grown in pots

Abstract

Berry sugar content and pH are two important quality parameters for wine grapes. Potassium (K) nutrition affects leaf photosynthesis, plant growth and carbohydrate translocation from leaves to fruits, hence, fruit sugar accumulation. Potassium is removed from vineyards in fruit and prunings and needs to be replaced to avoid K deficiency. However, overuse of potassium fertiliser may lead to high berry K accumulation that is related to a high berry pH, which lowers wine quality. Both K deficiency and high berry K content in grapevines become problems in New Zealand, The aims of this study were to investigate the effects of K nutrition in grapevines on vine growth and berry composition, the effects of using magnesium (Mg) to reduce berry K concentration and pH and the relationship between berry K and sugar accumulation.

Pinot noir (Vitis vinifera L.) vines, which were developed from hardwood cuttings and planted in 4-litre pots filled with coarse (80% 0.5-2.0 mm) silicon sand, were grown in a glasshouse. Potassium or Mg treatments were applied using a nutrient irrigation system. The sand was flushed daily with the nutrient solutions to keep constant nutrient levels in the sand.

A range of K treatment levels (0.05 - 31.25 mmol/L), in solution, were used in a preliminary experiment to investigate the effects of K nutrition on vine growth and berry composition. The K treatment influenced (p≤0.001) vine growth. The medium K levels resulted in a maximum vine growth with the highest vine dry weight (DW, >40g), total leaf area and shoot length. Low K levels reduce vine vegetative growth more than berry growth. Low K reduced vine DW (10-20 g), total leaf area and shoot length, but did not reduce berry size nor bunch DW with the low crop load in this experiment. The results indicate that K application to the vines with K deficiency is needed for adequate vine vegetative growth. Higher K levels reduce vine growth with high berry [K] and pH (4.4 units). The juice pH was linearly related (R²=0.83, p<0.001) to berry [K] (%DW) at berry maturity. Therefore, reducing vine K uptake on the vines with a higher K may be necessary in order to obtain a lower berry [K] and subsequent pH.

In an attempt to achieve this, two experiments were conducted to study the possibility of using Mg to reduce berry [K] and pH. In one experiment, increasing Mg reduced berry [K] and pH at high vine K levels. But magnesium did not affect tissue [K] nor vine growth at a low K level. Increasing Mg treatment levels from 0.3 to 30.0 mmol/L at the medium and higher K levels (>1.25 mmol/L) reduced juice [K] from 83 to 35 mmol/L, and pH from 3.8 to 3.4 units, respectively. In a second experiment, high pre-veraison Mg not only reduced preveraison [K] of leaves and berries, but the effect was also carried over to post-veraison. When Mg level was increased from 0.3 to 25 mmol/L, applied at pre-veraison, juice pH was reduced from 3.65 to 3.37 (p<0.001) by berry maturity. The results indicate that addition of Mg to the growing medium can reduce berry [K] and juice pH at higher external K (e.g. > 1.25 mmol/L). Mg application may be an approach used by grape growers to reduce berry K uptake and pH of the vines with high K levels.

The relationship between berry K and sugar accumulation was also investigated in these experiments. Dry weight (DW) per berry was used as a measure of berry sugar accumulation because most of berry DW is sugar, or derived from sugar, imported into the berries. In the primary experiment, low K reduced leaf and berry K uptake, but did not reduce berry sugar accumulation. The berry [K] was closely related to leaf [K] at berry maturity (R²=0.88, p<0.001). However, the berry K content and DW per berry were related in each K treatment during berry growth (R²=0.72-0.96, pl0.01). The results suggest that berry K level is related to the K status of the vines; and that low berry K accumulation does not affect sugar translocation into berries; indicating that berry K and sugar accumulation are indirectly related.

In order to more fully characterise this indirect relationship between berry K and sugar accumulation revealed in the preliminary experiment, two approaches were employed. Berry K accumulation was altered by Mg treatment or berry sugar accumulation was altered through shading the leaves. Increase in Mg level from 0.3 to 30.0 mmol/L reduced (p<0.05) berry K content from 2.93 to 2.34 mg at 6.25 –mmol/L K level, but did not affect berry DW. High preveraison Mg reduced post-veraison berry K content, but did not affect berry DW. Low berry sugar accumulation induced by leaf shading in another experiment reduced berry sugar accumulation, but did not reduce berry K accumulation. Increase in shade levels from 0 to 75% reduced berry DW from 0.20 to 0.15 g, but did not affect berry K content. However, berry DW and K content per berry were related in each shade treatment (R²=0.74-0.83, p<0.001). The results from all experiments suggest that berry K depends on the K status of the leaves, and berry K and sugar accumulation occur together but are not dependent on each other.

In summary, potassium deficiency affects vine vegetative growth more than berry growth, high K reduces vine growth and results in high berry [K] and pH, high Mg reduces the berry [K] and pH on vines with high K levels. Berry K and sugar accumulation are indirectly related. Berry K depends on the K status of the leaves. A model, describing berry K accumulation and its relationship to berry sugar accumulation, is proposed.