# Do Plants Need to Be Sprayed? New Insights into VOC-Mediated Biostimulation by Wood Vinegar

## TL;DR

Wood vinegar vapors alone can boost plant growth as effectively as direct spraying, offering a new way to use this natural product.

## Contribution

The study reveals that wood vinegar's volatile organic compounds can stimulate plant growth without direct contact.

## Key findings

- Plants exposed to wood vinegar vapors showed growth and nutrient levels similar to those sprayed directly.
- Exposure to wood vinegar vapors increased chlorophyll, polyphenols, and key nutrients like calcium and potassium.
- Wood vinegar vapors did not cause plant stress and regulated water loss effectively.

## Abstract

Wood vinegar is a natural liquid obtained during the production of charcoal and is rich in organic substances that can help plants grow. It is usually sprayed directly onto leaves, but because it also releases vapors, it may affect plants through the air as well. This study explored whether wood vinegar can improve plant growth not only by touching the leaves, but also through its vapors alone. Lettuce plants were used as a model crop and were either sprayed with a very diluted wood vinegar solution, exposed only to vapors coming from the same solution, or left untreated. The results showed that plants exposed only to the vapors grew just as well as those sprayed directly. In both cases, plants were heavier, greener, richer in beneficial plant compounds, and contained higher levels of important nutrients such as calcium, potassium, phosphorus, sulfur, and zinc. The plants also showed controlled water loss and no signs of stress. These findings show that wood vinegar vapors alone can stimulate plant growth, revealing a new way this natural product works.

Wood vinegar (WV), a by-product of biomass pyrolysis rich in organic acids and phenolic compounds, has gained increasing attention as a sustainable input for crop production, mainly through foliar application. However, its high content of volatile organic compounds (VOCs) suggests that WV may (also) interact with plants through the gaseous phase, a pathway that has so far been overlooked. This study tested the hypothesis that WV can modulate plant physiological performance, metabolic status, and nutrient accumulation not only via direct foliar contact but also through exposure to WV-derived VOCs. Lettuce (Lactuca sativa L.) was used as a model crop and grown under controlled environmental conditions. Plants were subjected to weekly treatments consisting of either foliar spraying with a 0.2% (v/v) WV solution or exposure to VOCs released from the same solution in a sealed chamber, without direct contact between the liquid and plant tissues, and were compared with untreated controls. Notably, plants exposed exclusively to WV-derived VOCs showed responses similar to those observed following foliar application. Both treatments significantly increased fresh weight, the content of chlorophyll, total polyphenols and the accumulation of key macro- and micronutrients, including Ca, K, P, S, and Zn. For both treatments, the efficiency of photosystem II remained stable, indicating the absence of photochemical stress, while stomatal conductance, transpiration rate, intercellular CO2 concentration, and net photosynthetic rate were markedly reduced, suggesting a regulated stomatal response. Physiological, biochemical, and mineral parameters were assessed using non-destructive optical techniques, gas exchange measurements, spectrophotometric assays, and X-ray fluorescence analysis. These findings indicate that exposure to the volatile fraction released from WV under the exposure conditions adopted in this study can elicit biostimulant-like responses comparable to those observed after foliar application.