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

Supplemental calcium (Ca2+) fertilization has been reported to enhance plant tolerance against various abiotic stresses, however, its effect on freezing tolerance has not been well studied. We investigated the effect of Ca2+ supplementation on freezing tolerance of spinach (Spinacia oleracea L.) at whole-plant as well as excised-leaf level. Seedlings supplemented with 20 and 25 mM Ca2+ through sub-irrigation efficiently assimilated Ca2+, evident by higher leaf-Ca(2+)content. Supplemental Ca2+ did not impede leaf-growth (leaf-area) but slightly increased dry weight/fresh weight ratio compared to the control. Supplemental Ca2+ significantly improved freezing tolerance as evident by reducing K+/Mg2+/total ion-leakage (i.e. indicative of less membrane damage), alleviating oxidative stress (less accumulation of O-2 and H2O2), and enhancing PSII potential quantum yield /energy trapping efficiency (Fv/Fm) following a relatively moderate or severe freeze-thaw stress, i.e. 5.5 and 6.5 degrees C. Improved freezing tolerance by Ca2+ supplementation was paralleled by enhanced activities of antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). Our data further suggests that, besides a bolstered antioxidant status, improved freezing tolerance of Ca2+-fed seedlings may also be associated with reduced loss of Ca2+ from plasma membrane (due to lower abundance of leaked K+ in extracellular spaces) as well as due to increased mechanical strength of cell wall as evident by cell wall augmentation.

Automated summary

Supplemental calcium (Ca2+) fertilization has been found to enhance freezing tolerance in spinach plants by reducing membrane damage, alleviating oxidative stress, and increasing the activity of antioxidant enzymes such as SOD, CAT, and APX. Additionally, Ca2+ supplementation may also help reduce loss of Ca2+ from the plasma membrane and enhance the mechanical strength of cell walls, contributing to improved freezing tolerance.