PLANT PHYSIOLOGY , Vol 103, Issue 1 105-113, Copyright © 1993 by American Society of Plant Biologists
Effects on Photosystem II Function, Photoinhibition, and Plant Performance of the Spontaneous Mutation of Serine-264 in the Photosystem II Reaction Center D1 Protein in Triazine-Resistant Brassica napus L
C. Sundby, W. S. Chow and J. M. Anderson
Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia
Wild-type and an atrazine-resistant biotype of Brassica napus, in which a
glycine is substituted for the serine-264 of the D1protein, were grown over
a wide range of constant irradiances in a growth cabinet. In the absence of
serine-264, the function of photosystem II (PSII) was changed as reflected
by changes in chlorophyll fluorescence parameters and in photosynthetic
oxygen-evolving activity. The photochemical quenching coefficient was
lower, showing that a larger proportion of the primary quinone acceptor is
reduced at all irradiances. At low actinic irradiances, the
nonphotochemical quenching coefficient was higher, showing a greater
tendency for heat emission. Decreased rates of light-limited photosynthesis
(quantum yield) and lower oxygen yields per single-turnover flash were also
observed. These changes were observed even when the plants had been grown
under low irradiances, indicating that the changes in PSII function are
direct and not consequences of photoinhibition. In spite of the lowered
PSII efficiency under light-limiting conditions, the light-saturated
photosynthesis rate of the atrazine-resistant mutant was similar to that of
the wild type. An enhanced susceptibility to photoinhibition was observed
for the atrazine-resistant biotype compared to the wild type when plants
were grown under high and intermediate, but not low, irradiance. We
conclude that the replacement of serine by glycine in the D1 protein has a
direct effect on PSII function, which in turn causes increased
photoinhibitory damage and increased rates of turnover of the D1 protein.
Both the intrinsic lowering of light-limited photosynthetic efficiency and
the increased sensitivity to photoinhibition probably contribute to reduced
crop yields in the field, to different extents, depending on growth
conditions.
