Interception of copper fungicides by cover crops as a way to prevent soil contamination in vineyards?

The long-term use of copper (Cu) fungicides in viticulture has led to the accumulation of Cu in vineyard topsoils, threatening soil health and ecosystem sustainability. Phytoextraction of Cu using cover crops (CCs) has been tested to decrease the concentration of Cu in vineyard topsoil, but its efficiency is limited by the fact that none of the CCs used in viticulture are capable of hyperaccumulating Cu. The present study investigated the use of CCs as "umbrella plants" to intercept the Cu in Bordeaux mixture before it reaches the soil. The ability of spontaneous CC and Sedum lineare to intercept Cu was evaluated in microplots after five applications, nine applications, and 45 days after the tenth and final application of Bordeaux mixture. To illustrate the mechanisms by which Cu could be retained in leaves exposed to Cu fungicides, drops of CuSO₄ were deposited on germinated grapevine leaves, and Cu internalisation and transformation were assessed using synchrotron-based analyses (μXRF, μXANES). Our results showed that CCs tolerated repeated applications of Bordeaux mixture without growth impairment, and intercepted up to 7% of the amount of Cu sprayed on the surface of their leaves. Spontaneous CC and Sedum lineare accumulated similar amounts of Cu, suggesting that, in our experimental conditions, leaf properties had only a minor impact on Cu interception. Synchrotron analyses suggest that part of the ionic Cu intercepted by a grapevine leaf can penetrate plant leaves, bind to the cell wall in the leaf epidermis and penetrate the mesophyll and vascular tissues, where it associates with sulphur ligands. This study provides proof of concept for using CCs as "umbrella plants" to intercept Bordeaux mixture during grapevine growth. Species with high soil coverage and with low water and nutrient demand would be good candidate CCs to avoid competition with grapevine.