• Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate.
• Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient.
• Heat tolerance acclimated to maximum leaf temperature (T_leaf) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest T_leaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest T_leaf. The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling.
• More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.

中文翻译：

---

处理热量——热带树木的光合热应力

• 气候变暖增加了陆地植物有害叶片温度的风险，导致热应激和生产力损失。在适应热稳定气候的赤道热带树种中，热敏感性可能特别高。
• 在卢旺达原产的三种热带山地树种（Harungana montana、Syzygium guineense和Entandrophragma exselsum）中研究了暴露在阳光下的叶片光合系统的热阈值。叶绿素荧光、叶片气体交换、形态、化学和温度的测量是在三个常见的花园沿海拔/温度梯度进行的。
• 适应整个物种的最大叶温（T_叶）的耐热性。在最温暖的地点，尽管具有较高的耐热性，但在具有最高T_叶的物种中超过了光系统 II 正常功能的热阈值。在蒸腾速率最高和T_叶最低的物种中，情况并非如此。结果指出了管理热应激的两种不同有效策略：通过生理调节叶片渗透压和类囊体膜脂质成分的耐受性，或通过形态适应和蒸腾冷却来避免。
• 与具有更有效叶片冷却的高蒸腾先锋物种相比，低蒸腾山地高潮物种中更严重的光​​合热应激可能导致竞争劣势。