Heterogeneity of chloroplast thylakoids in relation to inhibition by herbicides

This thesis provides data on some aspects of DCMU-induced inhibition of photosynthesis and electron transfer in relation to chloro-plast thylakoid heterogeneity. The work can be viewed on the premise that: (i) inhibition of photosynthesis and eventual death of the plant require much higher concentrations of herbicide than those which prevent oxygen evolution (Ridley S. M. and Horton P. [1984]; Z. Naturforsch 39c, 351-353.) and (ii) the higher plant thylakoid lamellae are known to display extreme heterogeneities, for example cytochrome f oxidation kinetics, lateral segregation of protein complexes, formation of regions of membrane appression and sub-populations of photosystem 2 (« and {3-centres). It was demonstrated that three phases of cytochrome f oxidation existed, possibly showing a spatial separation of sub-populations. By measuring cytochrome f redox kinetics the I50 for electron transfer inhibition by DCMU was found to be similar to that inhibiting water oxidation. Chlorophyll a fluorescence studies revealed three I50's for DCMU: 0"08-*0*19, 2*95-7*84 and ~365juM. Other studies suggested an assignment of the low constant to cc-centres and the rest to p-centres, indicating that a loss of oxygen evolution was seen upon inhibition of a-centres. Effects on photosystem 2p (~5pM) was similar to a DCMU concentration which may relate to photodestruction of the plant (Ridley S. M. [1977); Plant Physiol. 59, 724-732, Ridley S. M. and Horton P. [1984].) Other studies indicated that very high DCMU concentrations affect a photocentre with a long wavelength fluorescence emission, which was predominantly located in the unappressed lamellae. These results suggest that non-cyclic electron transfer is mediated by grana-located oc-centres, which have a strong affinity for DCMU. Beta-centres have a lower affinity for DCMU and may redox poise photosystem 1 mediated cyclic electron transfer. High DCMU concentrations appear to affect a stromally-located sub-population of p-centres whose function at the moment is unclear. Thus a known heterogeneity of the chloroplast lamellae appears to be expressed in the inhibition characteristics of a commonly used herbicide.