Abstract
The photosynthetic, biochemical, and anatomical traits of accumulation and replication of chloroplasts (arc) mutants of Arabidopsis thaliana were investigated to study the effects of chloroplast size and number on photosynthesis. Chloroplasts were found to be significantly larger, and the chloroplast surface area exposed to intercellular air spaces (Sc) significantly lower in the mutants than in their wild-types. The decreased Sc and increase cytoplasm thickness in the mutants resulted in a lower mesophyll conductance (gm) and a consequently lower chloroplast CO2 concentration (Cc). There were no significant differences between the mutants and their wild-types in maximal carboxylation rate (Vcmax), maximal electron transport (Jcmax), and leaf soluble proteins. Leaf nitrogen (N) and Rubisco content were similar in both Wassilewskija (Ws) wild-type (Ws-WT) and the Ws mutant (arc 8), whereas they were slightly higher in Columbia (Col) wild-type (Col-WT) than the Col mutant (arc 12). The photosynthetic rate (A) and photosynthetic N use efficiency (PNUE) were significantly lower in the mutants than their wild-types. The mutants showed similar A/Cc responses as their wild-type counterparts, but A at given Cc was higher in Col and its mutant than in Ws and its mutant. From these results, we conclude that decreases in gm and Cc are crucial to the reduction in A in arc mutants.
Highlights
The chloroplast is one of the most important plant organelles and carries out many important functions such as fatty acid synthesis, nitrogen (N) and sulphur fixation, and especially photosynthetic carbon fixation[1]
CO2 molecules diffuse from the atmosphere into chloroplasts by overcoming a series of diffusion resistances, including the boundary layer, stomata and mesophyll resistances, which results in a remarkable drawdown in concentration in the chloroplast (Cc) compared to the atmospheric CO2 concentration
Our results suggest that Rubsico and chlorophyll concentrations in chloroplasts may tend to be conservable, which has been mentioned in previous studies[24, 30]
Summary
The chloroplast is one of the most important plant organelles and carries out many important functions such as fatty acid synthesis, nitrogen (N) and sulphur fixation, and especially photosynthetic carbon fixation[1]. The important role of chloroplast number as well as chloroplast shape in gm decreasing was suggested in the study of Weise et al.[16], the reasons of low A and gm in arc mutants are still unclear. As described above, both leaf structural properties, which were not quantified in their study, play the key role in determining A. We used two Arabidopsis arc mutants and the corresponding wild-type plants to investigate the effects of chloroplast size and number on photosynthesis. Our objective was to evaluate whether a small number of enlarged chloroplasts is less beneficial to photosynthesis than a large population of small chloroplasts using leaf structural and biochemical analysis and 1-D mesophyll conductance model
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