Baroli I, Melis A
Department of Plant and Microbial Biology, University of California, Berkeley 94720-3102, USA.
Planta. 1998 Jun;205(2):288-96. doi: 10.1007/s004250050323.
We investigated the effect of photosynthetic electron transport and of the photosystem II (PSII) chlorophyll (Chl) antenna size on the rate of PSII photoinhibitory damage. To modulate the rate of photosynthesis and the light-harvesting capacity in the unicellular chlorophyte Dunaliella salina Teod., we varied the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO3, or with supplemental CO2 bubbled in the form of 3% CO2 in air. The NaHCO3-grown cells displayed slow rates of photosynthesis and had a small PSII light-harvesting Chl antenna size (60 Chl molecules). The half-time of PSII photodamage was 40 min. When switched to supplemental CO2 conditions, the rate of photodamage was retarded to a t1/2 = 70 min. Conversely, CO2-supplemented cells displayed faster rates of photosynthesis and a larger PSII light-harvesting Chl antenna size (500 Chl molecules). They also showed a rate of photodamage with t1/2 = 40 min. When depleted of CO2, the rate of photodamage was accelerated (t1/2 = 20 min). These results indicate that the in-vivo susceptibility to photodamage is modulated by the rate of forward electron transport through PSII. Moreover, a large Chl antenna size enhances the rate of light absorption and photodamage and, therefore, counters the mitigating effect of forward electron transport. We propose that under steady-state photosynthesis, the rate of light absorption (determined by incident light intensity and PS Chl antenna size) and the rate of forward electron transport (determined by CO2 availability) modulate the oxidation/reduction state of the primary PSII acceptor QA, which in turn defines the low/high probability for photodamage in the PSII reaction center.
我们研究了光合电子传递以及光系统II（PSII）叶绿素（Chl）天线大小对PSII光抑制损伤速率的影响。为了调节单细胞绿藻杜氏盐藻（Dunaliella salina Teod.）的光合作用速率和光捕获能力，我们改变了培养基中无机碳的含量。细胞在高光照下培养，要么提供有限的无机碳供应（初始浓度为25 mM NaHCO₃），要么以空气中3% CO₂的形式鼓入补充CO₂。在NaHCO₃培养的细胞中，光合作用速率较慢，PSII光捕获Chl天线大小较小（60个Chl分子）。PSII光损伤的半衰期为40分钟。当切换到补充CO₂条件时，光损伤速率延迟至t1/2 = 70分钟。相反，补充CO₂的细胞显示出更快的光合作用速率和更大的PSII光捕获Chl天线大小（500个Chl分子）。它们还显示出光损伤速率，t1/2 = 40分钟。当耗尽CO₂时，光损伤速率加快（t1/2 = 20分钟）。这些结果表明，体内对光损伤的敏感性受通过PSII的正向电子传递速率调节。此外，大的Chl天线大小会提高光吸收和光损伤速率，因此抵消了正向电子传递的缓解作用。我们提出，在稳态光合作用下，光吸收速率（由入射光强度和PS Chl天线大小决定）和正向电子传递速率（由CO₂可用性决定）调节初级PSII受体QA的氧化/还原状态，这反过来又决定了PSII反应中心光损伤的低/高概率。
