Laisk Agu, Eichelmann Hillar, Oja Vello, Peterson Richard B
Tartu Ulikooli Molekulaar-ja Rakubioloogia Instituut, Riia tn. 23, Tartu, 51010, Estonia.
Biochim Biophys Acta. 2005 Jun 1;1708(1):79-90. doi: 10.1016/j.bbabio.2005.01.007. Epub 2005 Feb 19.
The light-dependent control of photosynthetic electron transport from plastoquinol (PQH(2)) through the cytochrome b(6)f complex (Cyt b(6)f) to plastocyanin (PC) and P700 (the donor pigment of Photosystem I, PSI) was investigated in laboratory-grown Helianthus annuus L., Nicotiana tabaccum L., and naturally-grown Solidago virgaurea L., Betula pendula Roth, and Tilia cordata P. Mill. leaves. Steady-state illumination was interrupted (light-dark transient) or a high-intensity 10 ms light pulse was applied to reduce PQ and oxidise PC and P700 (pulse-dark transient) and the following re-reduction of P700(+) and PC(+) was recorded as leaf transmission measured differentially at 810-950 nm. The signal was deconvoluted into PC(+) and P700(+) components by oxidative (far-red) titration (V. Oja et al., Photosynth. Res. 78 (2003) 1-15) and the PSI density was determined by reductive titration using single-turnover flashes (V. Oja et al., Biochim. Biophys. Acta 1658 (2004) 225-234). These innovations allowed the definition of the full light response curves of electron transport rate through Cyt b(6)f to the PSI donors. A significant down-regulation of Cyt b(6)f maximum turnover rate was discovered at low light intensities, which relaxed at medium light intensities, and strengthened again at saturating irradiances. We explain the low-light regulation of Cyt b(6)f in terms of inactivation of carbon reduction cycle enzymes which increases flux resistance. Cyclic electron transport around PSI was measured as the difference between PSI electron transport (determined from the light-dark transient) and PSII electron transport determined from chlorophyll fluorescence. Cyclic e(-) transport was not detected at limiting light intensities. At saturating light the cyclic electron transport was present in some, but not all, leaves. We explain variations in the magnitude of cyclic electron flow around PSI as resulting from the variable rate of non-photosynthetic ATP-consuming processes in the chloroplast, not as a principle process that corrects imbalances in ATP/NADPH stoichiometry during photosynthesis.
在实验室种植的向日葵、烟草以及自然生长的高山一枝黄花、垂枝桦和心叶椴叶片中，研究了光合电子从质体醌（PQH₂）通过细胞色素b₆f复合体（Cyt b₆f）传递到质体蓝素（PC）和P700（光系统I，即PSI的供体色素）的光依赖控制。稳态光照被中断（光 - 暗瞬变）或施加一个高强度的10毫秒光脉冲以还原PQ并氧化PC和P700（脉冲 - 暗瞬变），随后P700⁺和PC⁺的再还原过程被记录为在810 - 950纳米处差分测量的叶片透射率。通过氧化（远红）滴定法（V. Oja等人，《光合作用研究》78 (2003) 1 - 15）将信号解卷积为PC⁺和P700⁺成分，并使用单周转闪光通过还原滴定法确定PSI密度（V. Oja等人，《生物化学与生物物理学报》1658 (2004) 225 - 234）。这些创新使得能够定义通过Cyt b₆f到PSI供体的电子传输速率的完整光响应曲线。发现在低光照强度下Cyt b₆f的最大周转率显著下调，在中等光照强度下这种下调有所缓解，而在饱和辐照度下又再次增强。我们从碳还原循环酶的失活角度解释Cyt b₆f在低光照下的调节，这种失活会增加通量阻力。围绕PSI的循环电子传递通过PSI电子传递（由光 - 暗瞬变确定）与由叶绿素荧光确定 的PSII电子传递之间的差值来测量。在限制光照强度下未检测到循环电子传递。在饱和光照下，循环电子传递在一些但并非所有叶片中存在。我们将围绕PSI的循环电子流大小的变化解释为叶绿体中非光合ATP消耗过程的可变速率导致的结果，而不是光合作用期间校正ATP/NADPH化学计量不平衡的主要过程。
