[Relations between chlorophyll content and chloroplast fine structure in a Chlorina mutant of Arabidopsis thaliana...

In a chlorina mutant (ch 3) of Arabidopsis thaliana the total chlorophyll content was reduced to about 1/10 the amount of the green wild-type. At the same time the ratio of chlorophyll a/b was markedly increased. The electron microscopic analysis of the chloroplasts showed a considerable reduction in the membrane formation. Especially an inhibition of the grana differentiation was apparent, leading in some stages to an entirely abnormal thylakoid stacking. Depending on developmental as well as environmental conditions, chlorophyll content and chloroplast structure varied in the mutant in a different manner. These changes induced by endogenous and exogenous factors were comparatively investigated in order to reveal the relations between pigment deficiency and structural disturbances in the chloroplasts.The characteristically deviating a/b-ratio (i.e. the relative chlorophyll b deficiency) in the mutant was "normalized" under particular experimental conditions in two different ways: 1. Under the influence of continous illumination the relative chlorophyll b content increased together with the total amount of chlorophyll, obviously because a threshold of concentration of chlorophyll a was exceeded and thus a normal synthesis of chlorophyll b (from chlorophyll a?) became possible. 2. In cultures of seedlings under an 8:16 hr light-dark-succession as well as in old bleaching rosette leaves the "normalization" of the a/b-ratio was due to a relatively more rapid destruction of chlorophyll a.In electron microscopic cross sections the number, differentiation, and stacking of the thylakoids increased with an increasing total amount of the chlorophylls, while with decreasing chlorophyll content the membranes evidently disintegrated. However, no relation between chloroplast differentiation and absolute or relative chlorophyll b content could be established. Indeed, the proof that chlorophyll b is actually not necessary for the formation of a normal chloroplast structure was furnished by the electron microscopic analysis of another, chlorophyll b-less mutant (ch (1)), in the chloroplasts of which a membrane differentiation with typical grana piles could be demonstrated.