Enzymatic responses of the ascorbate-glutathione cycle to drought in sorghum and sunflower plants

To determine if responses of H2O2-scavenging enzymes to drought differ in sorghum (Sorghum bicolor (L.) Moench, C-4) and sunflower (Helianthus annuus L., C-3), we measured activities of the enzymes involved in the ascorbate-glutathione cycle (AGC), ascorbate peroxidase (AP), dehydroascorbate reductase (DR), and monodehydroascorbate reductase (MR), as well as catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD). Sorghum and sunflower were grown in a growth chamber. Water stress was induced by withholding water for 5 days. In sorghum, drought decreased cytosolic activities of AP, MR and POD and increased cytosolic DR activity; however, the chloroplastic activities of AP, DR and MR as well as cytosolic activities of CAT and SOD were not affected by drought. In sunflower, drought increased chloroplastic AP activity and cytosolic activities of DR and CAT, but decreased cytosolic POD activity. Activities of the enzymes involved in AGC were higher in the cytosolic fraction than in the chloroplastic fraction in sorghum. But in sunflower, chloroplasts had higher enzymatic activities than the cytosol. Because of the differential localization of the enzymes in the cell fractions, activities of the enzymes in chloroplasts were generally higher in sunflower than in sorghum, whereas activities of the enzymes in the cytosolic fraction were generally higher in sorghum than in sunflower. These results suggest that, to detoxify H2O2, sorghum mainly uses the cytosolic AGC, whereas sunflower primarily uses the chloroplastic AGC.