Regulation of photosynthesis and energy dissipation in triazine-resistant and susceptible Chenopodium album

As a consequence of the intensive use of herbicides during crop growth, many herbicideresistant biotypes have evolved. One of the first examples is the resistance of Chenopodium album against triazine-type herbicides. About ten years after this discovery, it was observed that triazine-resistant plants (R) have a lower rate of electron flow at the acceptor side of photosystem II. Later, the chloroplasts of the resistant biotype were described as having shadetype characteristics. In addition, the R biotype was found to have an altered lipid composition of the thylakoid membrane, to be more sensitive to high temperature and the thylakoid membrane has a low affinity for bicarbonate. In the presence of high irradiance, R is retarded in growth and development compared to the susceptible (S) biotype and has a higher sensitivity to photoinhibition (this thesis). The goal of this work was to contribute to a better understanding of the mechanistic relationship between light stress and photosynthetic yield, i.e. biomass production.In chapter 2 the effect of photoinhibition on electron transport and photophosphorylation in isolated chloroplasts is described. This research proved that photoinhibition causes a gradual uncoupling between electron transport and phosphorylation. This indicates that photoinhibition causes a proton leakiness of the thylakoid membrane. An important observation was that in vitro the two biotypes do not differ from each other in their sensitivity towards photoinhibition.Because R and S do not differ much in their sensitivity to photoinhibition in vitro, research was performed for the in vivo situation with intact leaves (Chapter 3). When plants have been grown at low light irradiance, little or no differences in dry matter productions appear to exist between the two biotypes. However, growth at high light irradiance causes a significantly lower production of the resistant biotype. Fluoresence studies indicated that the lower productivity of the resistant plants is caused by a higher sensitivity to photoinhibition. The less significant differences in vitro in comparison to the in vivo situation are probably caused by the loss or lower activity of photoprotective mechanisms during the isolation of chloroplasts.Further research into these protective mechanisms (Chapter 4) revealed that R shows more light-induced zeaxanthin formation and a larger change in light scattering than S, especially when grown at high irradiance. The difference in level of non-photochemical quenching (qN) is more pronounced at low light irradiance. Photorespiration acts as an energy dissipative mechanism and appears to be more important in R than in S plants. In conclusion, the increased sensitivity to photoinhibition of resistant plants is not caused by a lower activity of the photoprotective pathways of zeaxanthin formation and photorespiration. The shadetype characteristics of the chloroplasts of the resistant plants are important for the greater sensitivity to photoinhibition.Because qN includes several components, research has been performed to study the role of these components and the role of photochemical quenching (qP) in the differences between R and S in their sensitivity to photoinhibition (Chapter 5). The lower qP in the R plants is explained by a larger absorbance cross section of photosystem II (PSII) in the shadetype chloroplasts of the R plants, which enhances the odds of excitation of PSII. In combination with a reduced rate of electron flow at the reducing side of PSII, a higher excitation pressure causes an increase in the fraction of closed reaction centers. The observed lower qE in the R biotype is suggested to be due to a lower PSII electron flow rate and a lower photosynthetic control in R compared with that in S, leading to a smaller proton gradient across the thylakoid membrane. From these findings, it was concluded that the lower energy dissipation through qP and qE cause the greater sensitivity to photoinhibition of resistant plants in vivo.The last topic of this research, was the study of the effect of several inhibitors of the different energy dissipative mechanisms (qE, qT) to examine of the contribution of these mechanisms to photoprotection (Chapter 6). Addition of these inhibitors was found to lead to increased photoinhibitory damage, especially in the R biotype.In summary, triazine-resistant plants behave like shade-type plants and are adapted to low light irradiances (maize field, forests). The wild-type Chenopodium, a typical sun plant, has more tendency to grow in the open field (road-side, meadow). When R is exposed incidentally or permanently to high irradiance, it will suffer more photoinhibitory damage than the wild biotype (S). The R biotype will be retarded in growth and development due to lack of sufficient activity of photoprotective mechanisms and photosynthetic capacity.