Vasile V. Trif and Eleonora Trif
Australian Group of Photosynthesis, Centr. Physiology & Biochemistry of Vegetables, Tarlee 5411, S. Australia.
Manganese - containing complexes are implied in the water photodissociation occurring in PSII reaction centres, during the photo stage of the photosynthesis. To get information concerning this process, mixtures of chloroplasts with solution of MnSO4 of various concentrations, from 0.05% to 0.5%, were investigated by EPR spectroscopy. It was evidenced the simultaneous presence of different types of paramagnetic species : (i) Mn(i) - isolated, hydrated Mn (II) ions, with g = 2.00 and A=10mT; Mn(a) - clustered Mn - containing species, with B = 60mT; FR - photosensible free radicals with g = 2.004 and B = 1mT. We defined the parameter f = (Rl-Rd/Rd), where Rd stands for the intensity of the dark stable free radical signal, whereas Rl stands for the intensity of the free radicals signal induced during illumination in resonator, Fig 1a. The photosensibility of the free radicals was related to the photocatalytic effect of the mixtures on the decomposition of water.
Relations between the concentrations of the different types of paramagnetic species are presented in Fig. 1b: (i) a linear dependence: Rl = a Mn (i)+b; (ii) a square law : Mn (a) = KMn(i)2 with K = K0 ln 2; (iii) the parameter f rearches its maximum value of 1.6 for a concentration of C = 0.15 of Mn(II) ions in mixture, when it is reached the maximum in the photocatalytic activity in mixtures. The effect of different types of gas phase molecules (O2, H2, CH4, and of water stress on plants, as well as the above mentioned relations suggest that the reaction scheme in mixtures would imply several steps: (i) the development of photosensible free radicals in a direct relation with the concentration of hydrated Mn(II) species, and in connection with the water photodissociation.
(ii) the formation of clusters containing Mn with mixed oxidation states, this process being intermediated by free radicals generated previously. The cluster-like signal can also be originated from precipitated Mn - lipoprotein complexes, from the pigment P671 - the chlorophyll - like Mn - pheophytin, Fig. 1c, interacting with the chloroplast tylacoid membranes, as well as from bridged Mn - complexes produced during oxygen evolution in PSII.
Fig. 1. (a) Photosensible free radicals; (b) Relations between the concentrations of the paramagnetic species; (c) The structure of P671 pigment.