The capability of antioxidant enzymes to scavenge the ROS issomewhat correlated with plant resistance to water deficit Anjum et al.
2011. Relative activity of SOD1 and SOD3 isoforms wereconsiderably increased in borage under moderate (I3) and severewater deficit (I4) (Figure 4), indicating that borage can remove theROS under stress mainly by SOD increment. SOD is a strong antioxidant andobliterates the first compound which is produced from one-capacity reduction ofoxygen, i.e. radical superoxide, so it is the primary defense against ROS.
Asuperoxide radical has a half-life of less than 1 sec and is rapidly convertedto H2O2 by SOD, a product that is relatively stable andcan be detoxified by catalase and peroxidase Apel and Hirt 2004. An increasein SOD activity is most likely related with the increased production of ROS,leading to enhanced expression of genes encoding SOD Abedi and Pakniyat 2010.H2O2 is toxic to cells. Therefore, it is important that H2O2to be scavenged rapidly by the antioxidant defense system to water andoxygen Guo et al. 2006.
If the over-expression of SOD accompanied byenhanced H2O2 scavenging mechanisms like CAT and PODenzymes activities, has been considered as an important anti-drought mechanismto cope with oxidative stress during water deficit conditions McKersie et al.1999. The increment of SOD activity (Figure 4) could also increase the abilityof the plants to scavenge O2- radicals, which could causemembrane damage Aydin et al. 2011. The high level ofantioxidant enzymes activities induced by drought stress can increase thecapacity against oxidative damage Sharma and Dubey 2005. CAT is an importantenzyme that potentially can convert H2O2 to H2Oand O2, thereby detoxifying ROS in peroxisomes during water stress Sairamand Srivastava 2001. H2O2 inhibits the Calvin cycleenzymes and reduces photosynthetic carbon dioxide assimilation Takeda et al.
1995, which can lead to the formation of ROS through the misdirection ofelectrons in the photosystems. According to our results, CAT activity in boragewas very high under well watering, while it was significantly reduced withincreasing irrigation intervals. Mean CAT activity under mild, moderate andsevere stress were 25.3%, 19.7% and 28.
7% less than that under normalirrigation (Figure 5). This reduction of CAT activity may be associated withdegradation caused by induced peroxisomal proteases or may be due to the photo-inactivationof the enzyme Abedi and Pakniyat 2010. Despite the increasing SOD activityunder water limitation (Figure 4), borage growth and yield decreased withincreasing water stress (Dastborhan and Ghassemi-Golezani 2015), which may bedue to decreasing activity of CAT and high levels of H2O2. Our previous report Dastborhanand Ghassemi-Golezani 2015 showed that seed priming enhanced seedlingemergence rate and leaf area index (LAI) leading to an improvement in fieldperformance of this medicinal plant. Rapid and uniform field emergence ofseedlings is two essential pre-requisites to increase yield, quality andultimately profit in annual plant Finch-Savage 1993.
Early emergence andproduction of vigorous plants by primed seeds were the possible reason fornon-significant effect of seed priming on proline and soluble sugar contentsand antioxidant enzymes activities in borage leaves (p ? 0.05).