**3. Tall fescue biochemical response to short drought stress**

In response to water stress, plants exert adaptive modifications in their morphological, physiological, and biochemical properties. Photosystem II (PSII) is more vulnerable than PSI during drought stress in which inhibition of CO2 assimilation, coupled with the changes in photosystem activities and photosynthetic electron transport capacity, results in accelerated production of reactive oxygen species (ROS). The damaged targets by ROS attack are recovered by repair or by replacement via de novo biosynthesis. However, under severe drought stress, cell death would be inevitable. The fate of stressed cells is determined by the duration of stress as well as the protective capacity of the plant. Reactive oxygen species play a crucial role in causing cellular damage under drought stress [25]. Reactive oxygen species not only play a signaling role in coordinating nuclear gene expression in order to protect cells during biotic and abiotic stress responses, but can also cause lipid peroxidation and consequently membrane injury, protein degradation, and enzyme inactivation [26, 27]. All plants have ROS detoxifica‐ tion mechanisms, which are enzymatic, with superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), glutathione reductase (GR), and monodehy‐ dro ascorbate reductase (MDAR), and nonenzymatic detoxification mechanisms through flavanones, anthocyanins, carotenoids, and ascorbic acid (AA). The activity of antioxidant enzymes under drought stress will be enormously flexible among several plant species and even between two cultivars of one species. It has been demonstrated that enzymatic break‐ down of ROS is one of the major processes the plant uses to scavenge this signaling molecule [28]. Superoxide dismutase, the first enzyme in the detoxifying process, converts superoxide anion radicals (O2?) to hydrogen peroxide (H2O2), and APX reduces H2O2 to water using ascorbic acid as a specific electron donor [29–31].

It has been stated that ROS not only cause damage to membrane, but also disturb the correct functions of DNA and proteins. Enzyme-mediated disruption of ROS effectively quenches ROS in plant cells [28].

To underlie the key processes concomitant with drought tolerance in tall fescue, four genotypes (Jaguar, h–d, Pixie, and Mini-mustang) were assessed for enzymatic and nonenzymatic antioxidant activity as well as *SGR* expression during water shortage. These cultivars were selected from a larger set of 11 tall fescue cultivars based on the genetic relationships delineated via intersimple sequence repeat (ISSR) analysis [22]. These cultivars were thus selected for preliminary experiments in which drought stress was induced by ceasing irrigation in plastic pots for up to 8 days. Two antioxidant enzymes' activity (markers of drought stress) increased in all four cultivars due to drought stress—APX and SOD. Increased SOD activity of Minimustang, Pixie, jaguar, and h–d (6 days only) is in agreement with [32], who observed similar patterns when heat stress was applied on two annual and two perennial cool-season turf‐ grasses. A strong correlation between SOD activity and drought-induced oxidative stress tolerance has been well established [33]. At day 8, SOD activity slightly declined in h–d. Such decline in SOD activity in severe drought stress has been previously reported [34]. Chlorophyll stability was associated with leaf APX activity in the Jaguar and h–d leaves. APX is known as a chlorophyll protector in leaves [35, 36]. The connection between SOD and APX activity in most genotypes suggests that most of the superoxide that was produced as a result of drought stress was converted to H2O2 and subsequently detoxified by APX into H2O [37]. In transgenic tall fescue, high levels of APX and SOD were considered as defense mechanisms during stress conditions [38]. Regardless of Jaguar, all cultivars showed a nonsignificant decline in CAT activity at 8 days. Such decline in CAT activity has been reported by several authors in different stress conditions [33, 34, 39, 40]. Pixie and h–d genotypes experienced a nonsignificant decline in catalase activity, which does not seem to be in accordance with chlorophyll changes which is in line with the reports of [41, 42]. Identification of some chloroplast proteins that potentially can be involved in the regulation of drought stress in *F. arundinacea* has been reported by [43] during short water scarcity in tall fescue. In another report, the higher photosynthetic capacity of an intergeneric hybrid of *L. multiflorum/F. arundinacea* was likely due to higher efficiency of Calvin cycle during the drought stress [44]. The kinetic variation observed among the antiox‐ idant activity most likely depends on species and cultivars, time and type of stress, tissue or organ types, as well as severity of stress [28, 45].
