Seed germination, seedling growth and antioxidant system responses in cucumber exposed to Ca(NO3)2

Abstract

This study investigated the effects of varying calcium nitrate (Ca(NO3)2) supply on seed germination, seedling growth, and antioxidant responses during cucumber seed germination. Five and 20 mM Ca(NO3)2 stimulated seed germination, while 10 and 40 mM Ca(NO3)2 inhibited it. Germinating seed weight was clearly promoted by 5 mM Ca(NO3)2, but decreased under 40 mM Ca(NO3)2. Ten or 20 mM Ca(NO3)2 caused no marked change. Addition of 10 or 40 mM Ca(NO3)2 increased the activity of many enzymes in germinating seeds, such as superoxide dismutases (SOD), peroxidases (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR). On the other hand, 5 and 20 mM Ca(NO3)2 markedly decreased CAT activity. Among all the treatments, only 10 mM Ca(NO3)2 increased malondialdehyde content. Similarly, the production rate of O2.− was only higher in 20 mM Ca(NO3)2. Compared with the control (0 mM Ca(NO3)2), protein content significantly increased in all treatments except for 20 mM Ca(NO3)2. Calcium nitrate strongly inhibited the growth of seedlings, and damaged leaf and root microstructure. The inhibition and damage were more severe as the Ca(NO3)2 concentration increased. Calcium nitrate promoted the accumulation of photosynthetic pigment, but led to a decrease in chlorophyll a/b. These results suggest that the effect of different Ca(NO3)2 levels on seed germination was variable, while the inhibition effect on seedling growth enhanced with increase of Ca(NO3)2 concentration. This effect is closely associated with Ca 2+ and NO3- concentration, antioxidant enzyme activity, and the different growth and development stages of cucumber.