Scientists have discovered that plants exposed to extreme heat leave behind a unique chemical fingerprint, even when they have enough water to survive. The discovery is giving researchers a new way to understand how rising global temperatures silently stress forests and crops long before visible damage appears.
The study, published in npj Science of Plants challenges the long-standing belief that heat becomes dangerous to plants mainly because it is accompanied by drought. Researchers found that heat alone can disrupt the internal chemistry of plants, forcing them into metabolic stress even when water is freely available.
To isolate the effects of temperature, researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research conducted experiments on seven plant species under carefully controlled conditions. The plants were kept fully watered while temperatures were gradually increased from 10 degrees Celsius to 40 degrees Celsius.
Although the plants survived the heat, scientists noticed dramatic changes inside their leaves. The chemical composition of the sugars produced by the plants began to shift as temperatures crossed approximately 30 degrees Celsius.
Lead researcher Philipp Schuler and his team found that photosynthesis in most plants started declining above this temperature threshold. At the same time, respiration increased sharply. Respiration is the process through which plants burn stored sugars to maintain their internal functions and survive stressful conditions.
The study revealed that plants under extreme heat rapidly consumed their stored starch reserves and converted them into sugar to fuel rising energy demands. At cooler temperatures, leaves stored nearly 14 percent of their dry weight as carbohydrates, much of it in the form of starch. Under hotter conditions, that reserve dropped to below 8 percent.
Researchers also detected unusual changes in oxygen and hydrogen isotopes within the sugars produced by the plants. Isotopes are slightly different forms of the same chemical element that scientists often use to study environmental and biological processes.
As temperatures increased plant sugars became enriched with heavier hydrogen isotopes while simultaneously losing heavier oxygen isotopes. Scientists believe this chemical signature reflects the plant’s growing metabolic stress as it struggles to maintain energy production during heat exposure.
The findings were especially important for so-called “C3 plants,” which include most trees and major food crops such as wheat, rice, barley, and legumes. These plants showed clear signs of stress once temperatures exceeded 30 degrees Celsius.
In contrast, the experiment’s only “C4 plant,” sorghum, remained stable even under higher temperatures. C4 plants use a different photosynthesis system that makes them naturally more tolerant to heat. Crops such as corn, sugarcane, and sorghum belong to this category.
The distinction could have major implications for global agriculture as climate change intensifies heatwaves around the world. Scientists warn that staple food crops dependent on C3 photosynthesis may become increasingly vulnerable to rising temperatures.
Researchers say the discovery could also transform the way scientists study past climate conditions. Sugars produced in leaves eventually become part of a tree’s wood, meaning the chemical stress signals may be preserved inside tree rings for decades or even centuries.
Until now scientists generally assumed that the chemical pathways linking leaf water, sugars, and tree rings remained relatively stable across temperatures. The new findings suggest that extreme heat leaves a distinct metabolic signature that can potentially be traced through historical tree ring records.
This could allow climate scientists to identify periods when trees were under severe heat stress, even if forests showed no obvious visible damage at the time.
The study also raises concerns about the hidden impact of climate change on ecosystems. Trees and crops may appear healthy externally while internally struggling with an unfavorable carbon balance burning more energy than they can produce through photosynthesis.
Researchers believe this newly identified chemical fingerprint could become an important diagnostic tool for monitoring forest health, agricultural resilience, and the longterm effects of global warming on plant life.
As heatwaves become more frequent and intense worldwide, scientists say understanding these invisible stress responses will be critical for predicting how forests, ecosystems, and food systems may respond to a rapidly warming planet.