Supplemental calcium improves freezing tolerance of spinach (Spinacia oleracea L.) by mitigating membrane and photosynthetic damage, and bolstering anti-oxidant and cell-wall status

•Ca2+-supplementation improves in situ freeze-tolerance of spinach seedlings by.•Membrane stabilization, i.e. reducing ion-leakage, specifically K+, Mg2+-leakage.•Improving oxidative stress-tolerance, i.e. bolstering antioxidant enzymes activity.•Improving PSII efficiency as evidenced by chlorophyll fluorescence.•Quantitative and qualitative augmentation of the cell wall.