# The Significance of Calcium in Photosynthesis
## 2 Mechanisms of Ca\({}^{2+}\) Involved in Stomatal Movements  
Stomata are important channels for plants to communicate with the environment, especially during photosynthesis. Each stoma comprises a pair of guard cells with a small amount of chloroplasts, and these chloroplasts are related to stomatal movement [46]. _Arabidopsis thaliana_ mutants with no chloroplasts in guard cells show that the closure of stomata is greater than that in wild-type _A. thaliana_[47]. Chloroplasts are different between guard cells and mesophyll cells [48; 49]. The chloroplasts in guard cells have many large starch grains, and their volume is larger than that in mesophyll cells [48]. Stomatal movement is regulated by the water content (e.g., soluble sugars) of guard cells, and osmotic substances also play an important role. The presence of amylase in guard cells can regulate water content through soluble sugars produced by the degradation of starch to regulate stomatal movement [48; 50]. As a key signal of stomatal regulation [51], ROS is mainly produced in chloroplasts [52]. The production of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) induced by abscisic acid (ABA) in guard cell chloroplasts is earlier than that in other locations [53]. The accumulation of H\({}_{2}\)O\({}_{2}\) can inhibit amylase activity and reduce the sugar content [54]. Ca\({}^{2+}\) sensor (CAS), which is located on the thylakoid membranes of a chloroplast, is a Ca\({}^{2+}\)-binding protein [12; 13; 14] associated with the production of H\({}_{2}\)O\({}_{2}\) and nitric oxide \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Protein** & \begin{tabular}{c} **Function Related to** \\ **Photosynthesis** \\ \end{tabular} & **References** & \begin{tabular}{c} **Function Related to** \\ **Calcium** \\ \end{tabular} & **References** \\ \hline CAS & \begin{tabular}{c} stomatal regulation; \\ photosynthetic electron flow; \\ Regulate CCM \\ \end{tabular} & [12; 13] & Ca\({}^{2+}\)-binding & [12; 13; 14; 15] \\ RbohD/F & \begin{tabular}{c} stomatal regulation \\ \end{tabular} & [16; 17] & Ca\({}^{2+}\)-binding & [18] \\ PsbO & The OEC subunit protein; & [19] & The Mn\({}_{4}\)CaO\({}_{5}\) cluster as & [20; 21] \\ PsbQ/PsbP & The OEC subunit protein; & [19; 22] & The Cl’ and Ca\({}^{2+}\) as & [19; 23] \\ PsaN & \
\begin{tabular}{c} Regulate photosynthetic \\ electron flow \\ \end{tabular} & [24] & Regulated by Ca\({}^{2+}\)/CaM & [24] \\ PsaA/PsaL & The PS1 subunit proteins; & Possibly a Ca\({}^{2+}\) & \\  & & & coordinate the two & [25] \\ FD &
\begin{tabular}{c} Electron transport of PS 1 \\ The Calvin cycle key enzymes \\ \end{tabular} & [26] & High affinity with Ca\({}^{2+}\) & [27] \\ FBPase/SBPase & The Calvin cycle key enzymes & [28; 29] & Regulated by Ca\({}^{2+}\) & [30] \\ TKL & The Calvin cycle key enzymes & [31] & Ca\({}^{2+}\)-dependent & [31] \\ CP12 & Regulate the Calvin cycle & [32; 33] & Ca\({}^{2+}\)-binding & [34] \\ D1 protein & Regulate NPQ & [35] & Ca\({}^{2+}\)-binding & [36] \\ PsbS & Regulate NPQ & [37] & Regulated by Ca\({}^{2+}\) & [38] \\ VDE & Regulate xanthophyll cycle & [10] & Regulated by Ca\({}^{2+}\) and & [10] \\ STN8 & Phosphorylate thylakoid & & [39] & Interaction with CAS & [7] \\  & membrane proteins & & & \\ Tic110 & Chloroplast inner envelope & [40] & Regulated by Ca\({}^{2+}\) & [41] \\  & protein & & & \\ Tic32 & Chloroplast inner envelope & & & \\  & protein & & & \\ (p)ppGpp & The regulator in chloroplast & [42] & Ca\({}^{2+}\)-binding & [42] \\ ch-CPN10 & Assist chloroplast protein & & & \\  & folding & & & \\ SAMTL & Chloroplast inner envelope & & & \\  & protein & & & \\ ACA1 & Chloroplast inner envelope & & & \\  & protein & & & \\ AFG1L1 & Chloroplast protein & [45] & CaM-binding & [45] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photosynthesis-related proteins associated with calcium. (NO) in the stomatal closure pathway [55]. In CAS deletion mutations, the cytoplasm Ca\({}^{2+}\) ([Ca\({}^{2+}\)]\({}_{\text{cyt}}\)) concentration decreases and stomatal closure is prevented [56].