Assessment of the protective efficiency of nonphotochemical quenching in higher plants

PhDPhotosystem II (PSII) is the primary generator of electrons required for photosynthesis. The reaction center protein of PSII (RCII) is the most susceptible component of the photosynthetic machinery to damage. Photodamage can lead to long-term downregulation of photosynthesis. This occurs because plants are exposed to rapid light fluctuations and high light conditions, leading to the over accumulation of excess energy around PSII. Plants have developed a mechanism to dissipate this excess energy called nonphotochemical quenching (NPQ). In order to quantify the protectiveness of NPQ (pNPQ), a novel methodology was developed and employed. During methodology development, development, it is shown that a variable PSI fluorescence should be taken into account, and how it can be calculated. Application of the procedure assessed the contribution of xanthophylls lutein, violaxanthin, zeaxanthin, and the PsbS protein to pNPQ. Results show that the most important factors governing photoprotection are the PsbS protein and the correct xanthophyll composition in their natural binding sites. The more xanthophyll variation, the greater the photodamage at the end of the pNPQ assessment procedure. PsbS is essential to achieve the maximum pNPQ. PsbS increases the aggregation of LHCII. Arabidopsis with excess PsbS has three-times more aggregated LHCII than wild type levels of PsbS. The phototolerance and pNPQ required for Arabidopsis grown under different conditions and for leaves of different ages was also calculated. Plants grown under low light conditions accumulate disconnect antenna (LHCII), which is inefficient at protecting RCII, despite the high NPQ levels. Investigating plants of different ages, it was found that eight-week old Arabidopsis are the optimum age for pNPQ effectiveness. Younger and older leaves suffer photodamage at lower light intensities and form less pNPQ. This thesis demonstrates the novelty and adaptability of the pNPQ assessment procedure, and offers a sound case for its use in acclimation and photoinhibition experiments.Queen Mary Principal’s Research Studentshi