Silicon enhances acquisition of phosphorus in grafted grapevines (cv. Chardonnay) grown under field conditions

Interactions between silicon (Si) and phosphorus (P) in grapevine ( Vitis vinifera ) remain poorly understood. This study examined how Si fertilisation influences rhizosphere P availability by altering root malate and citrate exudation and P uptake in grafted grapevines grown under low-P soil conditions. A field experiment was conducted in a vineyard with cv. Chardonnay grafted onto Kober 5BB rootstock, using rhizotrons that provided direct access to intact roots and their rhizosheath soil. Treatments included unfertilised control (–P, –Si), P fertilisation (+P), and Si fertilisation (+Si). Samples of rhizosheath soil, root exudates, and root/leaf tissues were taken at key growth stages (flowering, pea-sized berry, and veraison). Leaf P concentrations in +Si vines reached levels comparable to those in +P vines. Silicon fertilisation enhanced root exudation of citrate and malate relative to both the control and +P treatments, particularly at pea-sized berry stage. This was accompanied by enhanced expression of VvPEPC and VvALMT1 . In roots, Si supply consistently upregulated VvPHT1;1 , increasing transcript levels several-fold compared with the control across all growth stages, whereas P fertilisation downregulated VvPHT1;1 expression. In leaves, VvPHT1;1 was also upregulated by Si supply at flowering relative to the +P treatment, although its expression remained lower than in the –P vines. Enhanced exudation of citrate and malate under low-P conditions also promoted Si mobilisation in the rhizosheath soil and increased Si accumulation in leaves. This study provides the first field-based evidence that Si fertilisation improves rhizosphere P mobilisation, root P uptake, and shoot P status in grafted grapevine.