Heat Stress Changes Plant Sugars Before Tree Death

Heat-stressed trees are commonly understood through the lens of water. During heatwaves, trees often wilt and die because heat is accompanied by drought. Remove water scarcity from the situation, conventional thinking suggests, and most plants should cope with high temperatures without major difficulty.

A research team tested that idea by eliminating drought and raising the temperature, while ensuring seven plant species received ample water at temperatures reaching 40°C. All of the plants survived.

However, their leaf sugars revealed an unexpected result. Despite having plentiful water, the leaves already showed a chemical signature associated with metabolic distress.

Sugar reveals a heat-stress signal

The researchers set out to examine what takes place within leaves when heat, rather than drought, is the sole source of stress. Scientists at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) designed an experiment that separated temperature from other factors.

They selected seven adequately watered plant species and increased air temperature in 5-degree stages, from 10°C to 40°C. Humidity was held stable throughout. The chemical evidence came from the sugars in the leaves.

Following five days of acclimatisation at every temperature level, Philipp Schuler, the study’s lead author, and his colleagues collected leaf samples and measured each plant’s respiration and photosynthesis.

Heat exposure without drought

C3 plants, a group containing most trees as well as wheat, rice, barley and the vast majority of plant species, coped well with temperatures of up to around 30°C. Once temperatures exceeded that point, their performance began to deteriorate.

Photosynthesis declined, while respiration - the process through which plants burn their own sugars to remain alive - continued to increase. In almost every C3 species examined, the cellular system that turns sunlight into useful energy started to fail above 30°C.

Previous research reviews had found that heatwaves reduce carbon gain in trees. Before this work, however, researchers had not clearly documented the effects on a plant’s internal sugar stores when temperature was the only changing condition.

The point where the balance changes

Under cooler conditions, carbohydrates accounted for almost 14 percent of leaf dry weight. More than half of these reserves consisted of starch, effectively the plant’s savings account.

At higher temperatures, the overall carbohydrate pool fell below 8 percent, while starch represented only about one-fifth of it. Much of the starch had been converted into sugar: immediately available fuel for a leaf using energy faster than normal during overheating.

That shift is consistent with a plant trying to sustain respiration. The important question is what the process leaves behind.

The experiment reached its upper limit at 40°C. Most barley plants failed to survive the heat, and one tomato relative also died. Yet the sugar-related warning signal had emerged long before the leaves failed.

Heat alters plant sugar isotopes

Both water and sugar contain hydrogen and oxygen, elements that occur in slightly lighter and heavier forms called isotopes. Normally, the ratio between heavy and light isotopes in leaf sugars remains relatively consistent.

In the experimental plants, heat disrupted that balance. Once temperatures rose beyond 30°C, leaf sugars became richer in heavy hydrogen but poorer in heavy oxygen. These were opposing chemical shifts caused by the same factor.

Schuler and his colleagues propose that, as heat speeds up respiration, sugars containing lighter hydrogen may be used first, leaving sugars with heavier hydrogen behind. The precise mechanism responsible is still under investigation.

C4 plants remained stable under heat

Sorghum was the sole C4 species among the seven plants and acted as the experiment’s comparison species. C4 plants use a different form of photosynthesis and are more tolerant of heat. Maize, sugarcane and grasses from hot climates are among the examples.

At 35°C and above, sorghum continued photosynthesising at a stable rate. Its leaf-sugar hydrogen isotope ratio did not change. This indicated that the C3 signal reflected heat-driven metabolic stress rather than temperature alone.

The divide is directly relevant to global food production. Wheat, rice, barley and most legumes are C3 plants, whereas maize, sorghum and sugarcane are C4 plants. A warming climate therefore produces two markedly different responses across everyday crops.

What tree rings can preserve

Sugars produced by leaves are ultimately incorporated into wood. Tree rings create a yearly chemical record, including the hydrogen and oxygen isotopes passed on from the leaf sugars from which the wood was formed.

Earlier studies had suggested that hydrogen ratios in tree rings increase when trees experience difficulty, such as defoliation, stress or environmental mismatch. These findings provide a possible explanation for that observation.

The authors note that, if comparable isotope shifts are stored in tree-ring wood, they may help identify trees with an unfavourable carbon balance - in other words, trees burning more carbon than they produce.

A diagnostic tool for heat stress

Climate researchers have long used tree-ring chemistry to reconstruct historical temperatures and rainfall. This approach assumed that the chemical processes linking leaf water to sugar remained broadly stable at different temperatures.

The study challenges that assumption. Above 30°C, the paired signal of increasing hydrogen and decreasing oxygen produces a distinct marker of metabolic stress that previous models did not include.

For forestry specialists and climate scientists, this offers a potential new diagnostic method. Tree rings extending back decades or centuries may be re-examined for the chemical evidence of heat stress.

Damage that never became visible in the bark or canopy may have been recorded all along in the sugars’ meticulous chemical account.