Objectives The phosphorus (P) deficiency is one of the main factors limiting photosynthetic carbon fixation and high-quality yield in peanut production. Calcium can enhance peanut growth and yield in low to medium yielding farmlands. Therefore, we explored the effects of exogenous calcium on alleviating P deficiency-induced photosynthetic inhibition in peanuts.

Methods Peanut cultivar ‘Liaoning Baisha’ was used in a pot experiment conducted in an artificial climate chamber. The P deficiency treatment was imposed by adjusting the P concentration in Hoagland nutrition solution to 0.5 mmol/L (–P) from the normal level of 1 P mmol/L. The treatments were normal P + spraying ddH₂O (CK), –P + spraying ddH₂O, –P + spraying CaCl₂, and –P + spraying trifluoperazine (TFP, a calmodulin inhibitor). We measured the photosynthetic functions, plant growth and thylakoid membrane integrity at 9 and 10 days after treatment imposition in peanuts.

Results Compared with CK, P deficiency reduced the dry matter weight, total leaf area, relative chlorophyll concentration and limited the growth and development of peanuts. The P deficiency reduced the net photosynthetic rate, transpiration rate, and stomatal conductance of peanut leaves. It also reduced the efficiency of PSⅠ and PSⅡ of peanuts by 18% and 5.4%, respectively. Compared with –P treatment, exogenous Ca²⁺ enhanced the dry matter weight and total leaf area of peanuts under P deficiency by 26.7% and 31.9%, respectively. Exogenous Ca²⁺ alleviated P deficiency inhibition based on photosynthetic level and enhanced the net photosynthetic rate and the stomatal conductance of peanut leaves under P deficiency. Compared with –P treatment, exogenous Ca²⁺ enhanced the efficiency of PSⅠ and PSⅡ, alleviating the photoinhibition in peanut leaves under P deficiency. Exogenous Ca²⁺ enhanced the size of the PQ pool, the rate of cyclic electron flow, and the activity of ATP synthase. However, it reduced the ∆pH of thylakoid in peanut leaves under P deficiency. TFP increased the thylakoid membrane damage, reduced cyclic electron flow rate, and ATP synthase activity in P deficiency stressed peanuts compared with –P treatment.

Conclusions P deficiency limited the growth and development of peanuts, reduced the activity of ATP synthase of thylakoid, Y(Ⅰ), Y(Ⅱ), and caused peanut photoinhibition. Exogenous Ca²⁺ alleviated inhibition of the dry matter weight, total leaf area, and relative chlorophyll concentration of peanuts. Exogenous Ca²⁺ ralleviated the Y(Ⅰ) and Y(Ⅱ) inhibition. The peanut CaM (Ca²⁺-modulin) acceptor for exogenous calcium (Ca²⁺) played an important role in the nutritional signalling of Ca²⁺, alleviating photosynthetic inhibition under P deficiency.