Changes in seed germination, and some physiological and ultra structural aspects of Calotropis procera seedlings under heat stress

Global warming is a problematic for many plants and understanding how the plants cope with temperature stress is an important objective. In this study, two experiments were conducted; the first one explored the effect of different temperatures on Calotropis procera seed germination, and the second disclosed the physiological and ultrastructural aspects of plant seedling. A temperature rise from 25°C to 30°C increased seed germination enormously, whereas seed incubation at 35°C significantly decreased its germination. The treatment of 40°C drastically inhibited seed germination. During the experimental periods, the seeds did not germinate at all at 45°C. Seed incubation at 30°C for 5 days notably enhanced seedling growth. This effect was accompanied by accelerated reserve mobilization and enzymes activity. A mild increase in lipid peroxidation (30% increase) and electrolyte leakage (45% increase) was also observed in response to these temperatures. Additionally, the plasma membrane moved away from the cell wall and became thicker in response to this temperature. Seeds exposure to 35°C for 5 days significantly reduced seedling growth. This adverse effect was accompanied with an increase in lipid peroxidation and electrolyte leakage by 100 and 170%, respectively over the values of 25°C treatment. Furthermore, the plasma membrane was damaged and double stranded with the 35°C application. Cytoplasmic vesiculation appeared in response to 30°C and 35°C treatments. Except for total soluble sugars which was decreased, all other biochemical changes in response to 35°C were comparable with those observed at 30°C treatment.