School of Life Sciences, Arizona State University, 427 East Tyler Mall, Tempe, Arizona, 85281, USA.
Department of Environmental Sciences, University of Virginia, 291 McCormick Road, Charlottesville, Virginia, 22904, USA.
Ecol Appl. 2020 Jul;30(5):e02101. doi: 10.1002/eap.2101. Epub 2020 Mar 19.
Drought is among the most damaging climate extremes, potentially causing significant decline in ecosystem functioning and services at the regional to global scale, thus monitoring of drought events is critically important. Solar-induced chlorophyll fluorescence (SIF) has been found to strongly correlate with gross primary production on the global scale. Recent advances in the remote sensing of SIF allow for large-scale, real-time estimation of photosynthesis using this relationship. However, several studies have used SIF to quantify the impact of drought with mixed results, and the leaf-level mechanisms linking SIF and photosynthesis are unclear, particularly how the relationship may change under drought. We conducted a drought experiment with 2-yr old Populus deltoides. We measured leaf-level gas exchange, SIF, and pulse amplitude modulated (PAM) fluorescence before and during the 1-month drought. We found clear responses of net photosynthesis and stomatal conductance to water stress, however, SIF showed a smaller response to drought. Net photosynthesis (A ) and conductance dropped 94% and 95% on average over the drought, while SIF values only decreased slightly (21%). Electron transport rate dropped 64% when compared to the control over the last week of drought, but the electron transport chain did not completely shut down as A approached zero. Additionally, SIF yield (SIF ) was positively correlated with steady-state fluorescence (F ) and negatively correlated with non-photochemical quenching (NPQ; R = 0.77). Both F and SIF , after normalization by the minimum fluorescence from a dark-adapted sample (F ), showed a more pronounced drought response, although the results suggest the response is complicated by several factors. Leaf-level experiments can elucidate mechanisms behind large-scale remote sensing observations of ecosystem functioning. The value of SIF as an accurate estimator of photosynthesis may decrease during mild stress events of short duration, especially when the response is primarily stomatal and not fully coupled with the light reactions of photosynthesis. We discuss potential factors affecting the weak SIF response to drought, including upregulation of NPQ, change in internal leaf structure and chlorophyll concentration, and photorespiration. The results suggest that SIF is mainly controlled by the light reactions of photosynthesis, which operate on different timescales than the stomatal response.
干旱是最具破坏性的气候极端之一，可能导致生态系统功能和服务在区域到全球范围内显著下降，因此对干旱事件的监测至关重要。太阳诱导叶绿素荧光（SIF）已被发现与全球范围内的总初级生产力强烈相关。最近在 SIF 的遥感方面的进展允许利用这种关系进行大规模、实时的光合作用估算。然而，一些研究使用 SIF 来量化干旱的影响，但结果不一，并且 SIF 与光合作用之间的叶片水平机制尚不清楚，特别是这种关系在干旱条件下可能如何变化。我们进行了一个为期 1 个月的 2 年生白杨实验。我们在干旱前和干旱期间测量了叶片水平的气体交换、SIF 和调制脉冲幅度（PAM）荧光。我们发现净光合作用和气孔导度对水分胁迫有明显的响应，然而，SIF 对干旱的响应较小。净光合作用（A）和导度在干旱期间平均下降了 94%和 95%，而 SIF 值仅略有下降（21%）。与对照相比，最后一周干旱时电子传递速率下降了 64%，但当 A 接近零时，电子传递链并没有完全关闭。此外，SIF 产量（SIF）与稳态荧光（F）呈正相关，与非光化学猝灭（NPQ）呈负相关（R=0.77）。归一化后的最小暗适应样本荧光（F）后，F 和 SIF 都表现出更明显的干旱响应，尽管结果表明该响应受到多种因素的影响。叶片水平的实验可以阐明大尺度遥感观测到的生态系统功能背后的机制。在轻度胁迫事件持续时间较短的情况下，特别是当响应主要是气孔响应而不是与光合作用的光反应完全耦合时，SIF 作为光合作用准确估算值的价值可能会降低。我们讨论了影响干旱对 SIF 弱响应的潜在因素，包括 NPQ 的上调、叶片内部结构和叶绿素浓度的变化以及光呼吸。结果表明，SIF 主要受光合作用的光反应控制，而光反应的作用时间尺度与气孔响应不同。
