The Photosystem II Heterogeneity of Chlorophyll b-Deficient Mutants of Rice: a Fluorescence Induction Study

Abstract

It has been shown that the fluorescence induction curve of DCMU-poisoned spinach thylakoids can be resolved into three kinetically different phases, a rapid sigmoidal phase (±) followed by two slower exponential phases (β and γ), by using a mathematical analysis method previously described (Hsu, B. D., Lee, Y. S. and Jang, Y. R. (1989). Biochimica et Biophysica Acta 975, 44-49). There is evidence suggesting that the a-phase originates from the major 'normal' photosystem II (PSII) centres, while the β and γphases arise from the two minor groups of 'abnormal' PSII centres with low quantum efficiencies due to their slow electron donation systems (Hsu, B. D. and Lee, J. Y. (1991). Biochimica et Biophysica Acta 1056, 285-292). The same kinetic analysis was made on several chlorophyll ²-deficient mutants of rice. PSII_α was identified by its responses to the variation in excitation light intensity, the addition of ferricyanide and the depletion of Mg²⁺, whereas the slower PSII_β and PSII_γ were identified by their reaction to the addition of electron donors like hydroxylamine. It was found that the three types of PSII were present in all the mutants studied, irrespective of the amounts of chlorophyll b and the light-harvesting complexes associated with PSII (LHCII). The results suggest that the PSII heterogeneity cannot be attributed to a difference in the antenna size. The variable content of LHCII in mutants mainly affects a single type of PSII, PSII_α. They also suggest that the presence of sigmoidicity in the a-phase (i.e. cooperation between PSII_α units) does not require LHCII, but the effects brought about by Mg²⁺-depletion (e.g. lowering of fluorescence yield and slowing down of fluorescence rise) are LHCII-dependent.