Figure 9

Visual scheme of symplastic Mn toxicity to suppress CO₂ assimilation. When Mn is excessively accumulated in leaves, the leaf IAA concentration is lowered, which might stimulate IAA degradation activities under Mn toxicity⁵⁷. As a short-term effect, the decrease in IAA concentration affects stomatal opening⁷³. Subsequently, as a long-term effect, the auxin-dependent signal transduction involving ARF transcriptional factor is perturbated in the leaves to cause an IAA-deficient phenotype, changing both the stomatal and leaf anatomical structures^71,72. The change in stomatal function and leaf structure severely limits CO₂ diffusion to the chloroplasts, and CO₂ assimilation by Rubisco is inhibited. In contrast, the photorespiration reaction limits the photosynthetic electron activities to cause ROS production⁸². The decreased sugar production efficiency in photosynthesis suppresses sugar catabolism. Therefore, growth is inhibited under Mn-toxic conditions.