The arc mutants of Arabidopsis with fewer large chloroplasts have a lower mesophyll conductance

Abstract

Photosynthetic cells of most land plant lineages have numerous small chloroplasts even though most algae, and even the early diverging land plant group the hornworts, tend to have one or a few large chloroplasts. One constraint that small chloroplasts could improve is the resistance to CO2 diffusion from the atmosphere to the chloroplast stroma. We examined the mesophyll conductance (inverse of the diffusion resistance) of mutant Arabidopsis thaliana plants with one or only a few large chloroplasts per cell. The accumulation and replication of chloroplasts (arc) mutants of A. thaliana were studied by model fitting to gas exchange data and (13)CO2 discrimination during carbon fixation. The two methods generally agreed, but the value of the CO2 compensation point of Rubisco (Γ *) used in the model had a large impact on the estimated photosynthetic parameters, including mesophyll conductance. We found that having only a few large chloroplasts per cell resulted in a 25-50 % reduction in the mesophyll conductance at ambient CO2.

Chloride as a macronutrient increases water‐use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

“…Second, a higher S c / S mes is expected to provide more parallel paths for CO 2 diffusion from the intercellular airspace into the chloroplasts, a very important factor promoting higher g m (Evans and Loreto, ; Terashima et al ., ; Galmés et al ., ). Therefore, A. thaliana arc mutants containing smaller populations of larger chloroplasts show important reductions in g m and photosynthetic rate (Weise et al ., ; Xiong et al ., ). Whereas this phenomenon was clearly associated with a lower S c , no significant differences were observed between the mutants and their wild‐types in V cmax and J max values (Xiong et al ., ).…”

Improved method for measuring the apparent CO2 photocompensation point resolves the impact of multiple internal conductances to CO2 to net gas exchange

“…Weise et al . ). Determining a common intersection is complicated by the fact that the intersection of two lines is more sensitive to experimental noise when slopes are similar, since small differences in slope can produce large differences in intersections between in lines that are approaching parallel.…”

Photorespiration in the context of Rubisco biochemistry, CO2 diffusion and metabolism

“…Furthermore, r m is influenced by anatomical traits such as cell wall thickness (Ellsworth et al , ), chloroplast shape and cover (Busch et al , ; Weise et al , ), cell density, and the relative amount of intercellular air space (Lehmeier et al , ). Mesophyll CO 2 diffusion properties, and consequently the CO 2 concentration inside the chloroplast, therefore are mediated by the 3D anatomy of the mesophyll cells and the leaf as a whole (Earles et al , ).…”