Chow Wah Soon, Lee Hae-Youn, He Jie, Hendrickson Luke, Hong Young-Nam, Matsubara Shizue
Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia.
Photosynth Res. 2005 Jun;84(1-3):35-41. doi: 10.1007/s11120-005-0410-1.
Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p = kDeltax, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N = Noexp(-kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N = Noexp(-kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N = Noexp(-kx), the quantum yield of photoinactivation of PS II can be defined as -dN/dx = kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1micromol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10(7) photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely-photoinactivated PS II complexes, able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.
光系统II（PS II）的光致失活，即光诱导的放氧能力丧失，在自然界的任何光照环境下都会不可避免地发生，而修复作用会对其进行抵消。在某些条件下，PS II的光致失活程度取决于光子暴露量（光剂量，x），而非辐照度或光照持续时间本身，因此遵循辐照度与光照持续时间的互易定律，即相等的光子暴露量会产生相同的效果。如果PS II的光致失活概率（p）与光子暴露量的增量成正比（p = kΔx，其中k是单位光子暴露量的概率），那么可以推断出活性PS II复合物的数量会随着光子暴露量呈指数下降：N = N₀exp(-kx)。此外，由于光子暴露量通常是通过在恒定辐照度（I）下改变光照时间（t）来实现的，所以N = N₀exp(-kIt)，即N随时间呈指数下降，光致失活速率系数为kI，其中乘积kI显然与I成正比。鉴于N = N₀exp(-kx)，PS II光致失活的量子产率可定义为 -dN/dx = kN，它会随着剩余活性PS II复合物的数量而变化。通常，在抑制修复的情况下，低光子暴露时PS II光致失活的量子产率约为每摩尔光子0.1微摩尔PS II。也就是说，当叶片组织接收约10⁷个光子时，一个PS II复合物就会失活。一些物种，如葡萄，即使在低温下，其PS II光致失活的量子产率也低得多。对辣椒叶片中PS II光致失活的长期时间进程进行研究发现，当在没有修复的情况下大多数PS II复合物失活时，N的下降偏离了单指数衰减。这可归因于在严重光致失活的PS II复合物中形成了强猝灭剂，它们能够有效地耗散激发能，并保护剩余的活性相邻复合物免受光损伤。
