Exogenous Calcium Improves Photosynthetic Capacity of Pinus sylvestris var. mongolica under Drought

Calcium (Ca), a secondary messenger, plays an essential role in improving drought resistance. We used the Fast Chlorophyll Fluorescence Induction Dynamics technique to investigate the effects of exogenous calcium on electron transport and energy fluxes in an 8-year-old Mongolian pine to investigate the mechanism of action of Ca in regulating drought adaptation in Pinus sylvestris var. mongolica. We found water stress significantly decreased Pn and Gs, but exogenous calcium significantly improved photosynthesis under water stress. The chlorophyll a fluorescence transient (OJIP) analysis revealed that water stress increased Fo and decreased Fm, inactivating reaction centers. Water stress reduced VI and VJ while increasing Mo, destroying the electron transport chain. Exogenous calcium increased Sm while decreasing VI and Mo under water stress, enhancing electron transport from QA to QB. Furthermore, 5 mM Ca2+ increased I-P phase and ψPo, δRo, and φRo, decreasing the drought-induced reduction in electron accepters of PSⅠ. The increase in ABS/RC, TRo/RC, ETo/RC, and DIo/RC caused by 5 mM Ca2+ demonstrated that calcium can regulate photoprotection to promote photosynthetic activity. Thus, exogenous calcium alleviated drought-induced reductions in photosynthetic activity by regulating photoprotection and boosting the electron transport efficiency at the acceptor side of PSⅡ and PSⅠ.