Lettuce (Lactuca sativa) and benth (Nicotiana benthamiana) leaves were illuminated with 720 nm background light to mix S-states and oxidize electron carriers. Green-filtered xenon flashes of different photon dose were applied and O₂ evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flash-induced PSII electron transport (= 4·O₂ evolution) exponentially increased with the flash photon dose in any differential layer of the leaf optical density. This proved the absence of excitonic connectivity between PSII units. Time courses of flash light intensity and 680 nm chlorophyll fluorescence emission were recorded. While with connected PSII the sigmoidal fluorescence rise has been explained by quenching of excitation in closed PSII by its open neighbors, in the absence of connectivity the sigmoidicity indicates gradual rise of the fluorescence yield of an individual closed PSII during the induction. Two phases were discerned: the specific fluorescence yield immediately increased from F_o to 1.8F_o in a PSII, whose reaction center became closed; fluorescence yield of the closed PSII was keeping time-dependent rise from 1.8F_o to about 3F_o, approaching the flash fluorescence yield F_f = 0.6F_m during 40 μs. The time-dependent fluorescence rise was resolved from the quenching by ³Car triplets and related to protein conformational change. We suggest that Q_A reduction induces a conformational change, which by energetic or structural means closes the gate for excitation entrance into the central radical pair trap—efficiently when Q_B cannot accept the electron, but less efficiently when it can.

用 720 nm 背景光照射生菜 ( Lactuca sativa ) 和本氏 ( Nicotiana Benthamiana ) 叶子，以混合 S 态并氧化电子载体。应用不同光子剂量的绿光过滤氙闪光并测量由闪光引起的O ₂释放。在数学上考虑了整个叶子的光强度梯度后，在叶子光密度的任何不同层中，闪光引起的PSII电子传输（= 4·O ₂演化）随着闪光光子剂量呈指数增加。这证明了 PSII 单元之间不存在激子连接。记录闪光强度和 680 nm 叶绿素荧光发射的时间进程。虽然在连接的 PSII 中，S 形荧光上升可以通过其开放邻居对闭合 PSII 中的激发猝灭来解释，但在缺乏连接的情况下，S 形荧光表明在诱导过程中单个闭合 PSII 的荧光产量逐渐上升。观察到两个阶段：PSII 中的比荧光产率立即从F _o增加到 1.8 F _o ，反应中心关闭；关闭的 PSII 的荧光产率保持随时间的增长，从 1.8 F _o到约 3 F _o ，在 40 μs 内接近闪光荧光产率F _f = 0.6 F _m 。时间依赖性荧光上升由^{3 个}Car 三联体的猝灭解决，并且与蛋白质构象变化有关。 我们认为Q _A还原会引起构象变化，通过能量或结构手段关闭激发进入中心自由基对陷阱的大门——当Q _B不能接受电子时，效率会降低，但当可以接受电子时，效率会降低。