Plant water use under heat

Climate change is regularly confronting us with new heat records. Heat waves, often combined with drought, are now three times as frequent and on average 2.3 C hotter than before. Such combined events are particularly hard on plants. Plants transpire to cool themselves, but if they don`t have enough water, this air-conditioning system fails. During dry periods the stomata of the leaves close to prevent them from drying out. As a result, however, the leaf can heat up massively and is then usually much hotter than the surrounding air. The first heat damage occurs from around 40 C. But even when this air conditioner is switched off, the leaves still lose small amounts of water because the leaf skin is not completely waterproof and even becomes leakier when it is hot. The causes of increasing water leakage of the leaf skin under heat are being investigated in the present project. Modern imaging methods are used to make molecular and structural changes in the leaf skin visible. At the same time, various stress physiological methods are employed by which the heat-induced loss of function of the leaf skin can be measured. The leaf skin contains cutin and waxes that melt when exposed to heat - it is not known whether the becoming more permeable to water is reversible and whether a hardening can take place in this regard. It is also the dose that makes the poison. There are practically no studies on the effect of the duration of exposure to heat on plants and the temperature-dependent changes in the water permeability of the leaf skin. The damaging heat dose and the main causes for the heat-induced, increased water permeability of the leaf skin are therefore still largely unknown. Plants from habitats that differ in heat dose are examined. Pronounced short-term midday heat is a phenomenon of the mountain habitat and occurs above all in low statured plant species. Dwarf shrubs, tree seedlings and glacier foreland plants are examined there. In contrast, in the Nepalese tropics, where the heat prevails for months, selected trees and herbs are analyzed. This study includes numerous field tests and laboratory experiments and ranges from the molecular level to the individual level. It therefore promises comprehensive, new knowledge about the heat resistance of plants. The results are of great importance for estimating the future heat risk both for cultivated plants and for plants of natural habitats in a globally warmer world.

Heat and drought are becoming critical challenges for plants under climate change. Our project at the alpine treeline shows that heat damage to leaves depends not only on how hot it gets, but also on how long the heat lasts. The longer the exposure, the lower the critical temperature at which tissue damage occurs-a clear dose-time effect captured by quantitative relationships between exposure duration and LT50 (the temperature causing 50% damage). Moreover, we found that core leaf functions such as photosynthesis (PSII) are impaired at temperatures several degrees below those that cause visible tissue death, underscoring the need for realistic "heat dose" metrics in forecasts and risk models. At alpine field sites microclimate measurements revealed striking contrasts over just a few meters. On south-facing slopes, leaves heat up more (up to 54,4 C), receive more direct sunlight, and experience stronger winds than on north-facing slopes-conditions that push plants close to their thermal limits. How do plants cope? A key protective barrier is the cuticle, the leaf's outer "skin." In the dwarf shrub Kalmia procumbens, plants on the hotter south-facing site developed a thicker cuticle that better limits water loss, especially during heat. In contrast, leaves on the cooler north-facing site invested more in flavonoids-compounds linked to biotic stresses. This demonstrates that protection is not about wax quantity alone: the fine chemical makeup of the cuticle and its thickness work together to control water loss and adapt to local stressors. These principles extend to young and adult conifers. We observed that cuticular water loss (gmin) is higher in the deciduous larch (Larix decidua), consistent with thinner cuticles enriched in flavonoids, while evergreen tree line conifers tend to have thicker protective layers. Crucially, seedling survival near the alpine treeline depends on how well the cuticle withstands heat-an insight with direct implications for regeneration under warmer, drier conditions. To assess heat risk more precisely, we quantified how LT50 shifts with exposure duration and showed that heat damage unfolds stepwise across different leaf functions. PSII performance declines at lower temperatures than those causing irreversible tissue death, providing early warning thresholds that can be used to compare species and guide prevention strategies. What does this mean for forests and conservation? South-facing, hot dry microsites pose the greatest challenge for establishment of young plants. At the same time, our findings highlight a lever for resilience: the fine tuned interplay of cuticle structure and chemistry. Understanding and fostering this protective "leaf skin" will be central to anticipating treeline dynamics and improving regeneration strategies in a warming, drying climate.