Uptake of silica by grapevines from soil and recirculating nutrient solutions

The uptake of silica by grapevines was investigated both in the field and in hydroponic cultures. In aqueous soil suspensions (14 d at 20 °C) of 6 different locations (clay/loam) an equilibrium of 55-71 ppm of soluble silica (expressed as SiO2) was measured; the content of grapevine leaves at harvest time was between 0.44 and 0.73 % of the dry matter, the concentrations being correlated with the silica solubility of the relative soil. Before budburst xylem exudates contained only about 1-4 ppm SiO2 (rising with soil temperature), whereas during summer up to 68 ppm were measured. Six different grapevine cultivars were grown in recirculating nutrient solutions supplied with different amounts (112, 10 and 0 ppm) of SiO2; the solutions were changed weekly. The average SiO2 content of leaf dry matter at harvest time (0.1-2 %) was correlated with leaf age and the SiO2 concentration of the nutrient solution whereas in stems and petioles it was always less than 0.1 %. Significant varietal differences could be found only for cv. Regent which accumulated about 20 % more SiO2 than the other varieties from the 112 ppm solutions. By the end of the vegetation period fresh leaves from plants grown on 112 ppm contained always around 0.1 % of water soluble SiO2, irrespective of the leaves' age whereas the total amount of SiO2 was up to 2 % in old leaves from the basis of the shoots and less than 0.5 % in the apical region. In the leaf center the silica concentration was always around 50 % lower than in the leaf periphery

The influence of silica fertilization on the resistance of grapevines to powdery mildew

Six different Vitis cultivars grown in recirculating nutrient solutions supplied with 0, 10 and 112 mg.l-1 SiO2, respectively, were infected with spores of the powdery mildew fungus Uncinula necator and analyzed after 2, 4 and 6 weeks. Whereas the size of the silica endoskeletons induced by the powdery mildew was influenced by the SiO2 concentration, the cultivar-specific resistance could not be increased. However, after 2 weeks all varieties showed a slightly but significantly enhanced resistance in the 112 mg.l-1 solution which disappeared during the following weeks. The results show silica to be essential for a normal powdery mildew resistance but make evident that the Oidium susceptibility of cultivars cannot be overcome by supplementary silica fertilization in the field, the Si contents of most soil solutions being far above the minimal requirements of the grapevine