Sun-induced chlorophyll fluorescence (SIF) is a direct indicator of plant photosynthetic activities and can potentially indicate plant physiological changes caused by water stress. However, the direct effect of water stress on the physiological SIF responses in crops at the field level still needs further research to clearly understand the involved mechanisms. To study this relationship, we made use of Unmanned Aerial Vehicles (UAVs), which are flexible and cost-effective to acquire SIF data at a high temporal resolution. We acquired near-infrared SIF (760 nm) and red SIF (687 nm) measurements using a UAV platform over irrigated and non-irrigated sugar beet plots. To represent physiological changes in crops, we calculated the apparent SIF yield (SIF normalized by the absorbed photosynthetically active radiation) at 760 and 687 nm (SIF_760yield and SIF_687yield), the fluorescence emission yield at 760 nm (ΦF₇₆₀), and the SIF_ratio (the ratio between SIF₆₈₇ and SIF₇₆₀). ΦF₇₆₀ was estimated using the recently developed NIRvH approach. For an improved interpretation of the response of these SIF indicators, we also acquired additional UAV-based hyperspectral and thermal data. We found that on June 28, when sugar beets were experiencing water stress, SIF_687yield, ΦF₇₆₀, and SIF_ratio all showed a significant response to the recovery of the irrigated sugar beets (p-value < 0.05). On the other hand, on July 24 when both water stress and heat stress affected the crop, only ΦF₇₆₀ and SIF_ratio weakly tracked the changes induced by the irrigation (p-value < 0.1). ΦF₇₆₀ had similar changes to SIF_760yield in both June and July, but ΦF₇₆₀ was more sensitive to irrigation. This indicates the importance of correcting for the structural effect when interpreting the SIF response. The findings suggest that SIF indicators can indicate water stress at the field level, but its value to detect the changes of photosynthetic activities under severe stress needs more investigation.

太阳诱导的叶绿素荧光 (SIF) 是植物光合活性的直接指标，可以潜在地指示水分胁迫引起的植物生理变化。然而，水分胁迫对田间作物生理 SIF 反应的直接影响仍需要进一步研究，以明确了解所涉及的机制。为了研究这种关系，我们使用了无人机 (UAV)，它灵活且经济高效地以高时间分辨率获取 SIF 数据。我们使用无人机平台在灌溉和非灌溉甜菜地块上获得了近红外 SIF (760 nm) 和红色 SIF (687 nm) 测量值。为了代表作物的生理变化，我们计算了 760 和 687 nm 处的表观 SIF 产量（由吸收的光合有效辐射标准化的 SIF）（SIF_760yield和 SIF _687yield），760 nm 处的荧光发射产率（ΦF ₇₆₀），以及 SIF_比率（SIF ₆₈₇和 SIF ₇₆₀之间的比率）。ΦF ₇₆₀是使用最近开发的 NIRvH 方法估计的。为了更好地解释这些 SIF 指标的响应，我们还获得了额外的基于无人机的高光谱和热数据。我们发现，在 6 月 28 日，甜菜面临水分胁迫时，SIF _{687 产量}、 ΦF ₇₆₀和 SIF_比都显示出对灌溉甜菜恢复的显着响应（p 值 < 0.05）。另一方面，在 7 月 24 日，当水分胁迫和热胁迫都影响作物时，只有 ΦF ₇₆₀和 SIF_比率微弱地跟踪灌溉引起的变化（p 值 < 0.1）。ΦF ₇₆₀在 6 月和 7 月与 SIF 760 产量变化相似，但 ΦF ₇₆₀对_灌溉更敏感。这表明在解释 SIF 响应时纠正结构效应的重要性。研究结果表明，SIF指标可以指示田间水分胁迫，但其在严重胁迫下检测光合活性变化的价值需要进一步研究。