Effect of water deficit on oxidative stress and degradation of cell membranes in needles of Norway spruce (Picea abies)

We studied the impact of mild and severe drought stresses for 42 days and rehydration for 21 days on 4-year-old seedlings of Norway spruce. Water relations in spruce tissues were determined on the basis relative water content of needles and shoot water potential (Psi(shoot)). During the stress, we measured the level of: reactive oxygen species (ROS), antioxidants, and degradation of cell membranes. In the seedlings subjected to severe stress, Psi(shoot) decreased to -2.4 MPa, while in those subjected to mild stress, to -0.8 MPa. After rehydration, shoot water potential increased, but did not reach the control level. Water deficit caused oxidative stress, reflected in an increased production of ROS: superoxide anion radical (O-2(.-)) and hydrogen peroxide (H2O2). Their concentrations in needles were the highest in seedlings subjected to severe stress, where they exceeded the control level by 116 % and 30 %, respectively. During rehydration, the differences in ROS levels between treated and control seedlings diminished. Oxidative stress causing degradation of cell membranes included: de-esterification of phospholipids, oxidation of fatty acids, and increase in concentration of malondialdehyde, as their permeability to ions increased by, 125 %. In the defence against the oxidative stress in needles, an important role was played by low-molecule antioxidants such as glutathione, ascorbic acid, flavonoids, cc-tocopherol and antioxidant enzymes. An increase in intensity of water deficit caused a significant reduction in the level of low-molecular antioxidants, which attests to their utilization during the process of scavenging for free radicals. Water deficit at Psi(shoot) = -1.7 MPa caused a decline in ascorbic acid level by 37 % in needle cells. An effective defensive mechanism removing the excess of ROS was also reflected in the activity of the main enzymes of oxidative stress: superoxide dismutase (SOD) and guaiacol peroxidase (PO). As a result of water deficit, SOD activity increased by 80 %, while PO activity decreased by 82 %.