Abstract

Dehesas, human-shaped savannah-like ecosystems, where the overstorey is mainly dominated by the evergreen holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.), are classified as a global conservation priority. Despite being Q. ilex a species adapted to the harsh Mediterranean environmental conditions, recent decades have witnessed worrisome trends of climate-change-induced holm oak mortality. Holm oak decline is evidenced by tree vigour loss, gradual defoliation and ultimately, death. However, before losing leaves, trees undergo leaf-level physiological adjustments in response to stress that may represent a promising field to develop biochemical early markers of holm oak decline. This study explored holm oak photoprotective responses (pigments, tocopherols and photosynthetic performance) in 144 mature holm oak trees with different health statuses (i.e., crown defoliation percentages) from healthy to first-stage declining individuals. Our results indicate differential photochemical performance and photoprotective compounds concentration depending on the trees' health status. Declining trees showed higher energy dissipation yield, lower photochemical efficiency and enhanced photoprotective compounds. In the case of total violaxanthin cycle pigments (VAZ) and tocopherols, shifts in leaf contents were significant at very early stages of crown defoliation, even before visual symptoms of decline were evident, supporting the value of these biochemical compounds as early stress markers. Linear mixed-effects models results showed an acute response, both in the photosynthesis performance index and in the concentration of foliar tocopherols, during the onset of tree decline, whereas VAZ showed a more gradual response along the defoliation gradient of the crown. These results collectively demonstrate that once a certain threshold of leaf physiological damage is surpassed, that leaf cannot counteract oxidative stress and progressive loss of leaves occurs. Therefore, the use of both photosynthesis performance indexes and the leaf tocopherols concentration as early diagnostic tools might predict declining trends, facilitating the implementation of preventive measures to counteract crown defoliation.

中文翻译：

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光保护化合物作为早期标志物预测地中海热带稀树草原圣栎树冠落叶

摘要

Dehesas 是人类形状的大草原状生态系统，其上层主要由常绿圣栎 ( Quercus ilex L. subsp. ballota (Desf.) Samp.) 主导，被列为全球保护重点。尽管是Q. ilex作为一种适应地中海恶劣环境条件的物种，最近几十年见证了气候变化引起的圣栎死亡率令人担忧的趋势。树木活力丧失、逐渐落叶和最终死亡证明了圣栎的衰退。然而，在落叶之前，树木会经历叶片水平的生理调整以应对压力，这可能代表了开发圣栎衰退生化早期标志物的有希望的领域。本研究探讨了 144 棵成熟的圣栎树的光保护反应（色素、生育酚和光合性能），从健康个体到第一阶段衰退个体，具有不同的健康状况（即树冠落叶百分比）。我们的结果表明不同的光化学性能和光保护化合物浓度取决于树木的 健康状况。衰退的树木表现出更高的能量耗散率、更低的光化学效率和增强的光保护化合物。在总紫黄质循环色素 (VAZ) 和生育酚的情况下，在树冠落叶的早期阶段，叶片含量的变化是显着的，甚至在下降的视觉症状明显之前，这支持了这些生化化合物作为早期胁迫标志物的价值。线性混合效应模型的结果显示，在树木衰退开始期间，光合作用性能指数和叶面生育酚浓度均表现出急性反应，而 VAZ 沿树冠落叶梯度表现出更为平缓的反应。这些结果共同表明，一旦超过一定的叶片生理损伤阈值，叶子不能抵抗氧化应激并且会发生叶子的进行性损失。因此，使用光合作用性能指标和叶片生育酚浓度作为早期诊断工具可能会预测下降趋势，从而有助于实施预防措施来对抗树冠落叶。