Drought differentially elicits antioxidant defense systems in two genotypes of Euphorbia tirucalli

Euphorbia tirucalli, a member of Euphorbiaceae, is a drought and salt-tolerant species. It is distributed in subtropical and semi-arid parts of Africa and was brought to Asia and North America. The plant has succulent stems performing CAM metabolism and small non-succulent leaves performing C3 metabolism. Different genotypes of E. tirucalli showed different tolerance to drought stress, especially Morocco and Senegal genotypes. This difference has tempted us to investigate the difference in antioxidant stress mechanisms in stems as compared to leaves. Plants from both genotypes have been subjected to drought (10% Volumetric Water Content, VWC) for 8 weeks and the leaves and stems were investigated for their reactive oxygen species (ROS) and antioxidants levels. Shoots of Morocco genotype retained water more efficiently under drought compared to Senegal shoots. Although H₂O₂ accumulated in stems under drought compared to leaves, however, the stems have elevated levels of various non-enzymatic and enzymatic antioxidants. Stems of Morocco increased their levels of H₂O₂ more under drought compared to Senegal stems, and Morocco stems showed a higher increase in their non-enzymatic and enzymatic antioxidants under drought. Morocco stems are distinguished by increased ascorbate peroxidase (APX) and peroxidase (POX) activities, while in Senegal stems catalase (CAT) activity specifically increased under drought. Levels of H₂O₂ in leaves were higher as compared to stems even under control conditions; however, leaves did not show increased antioxidant enzymes levels under drought stress. This tendency to accumulate H₂O₂ could be used by E. tirucalli as strategy to kill and get rid of leaves under drought to reduce transpirational water loss. These differences in antioxidant systems of Morocco and Senegal genotypes could, at least partly, explain their differences in drought tolerance and may reflect evolutionary adaptations to different local native climates.