## 4 The Role of Ascorbic Acid to Confer Abiotic Stress Tolerance in Tomato Plants
### AsA as a Key Molecule under Abiotic Stress  
Abiotic stresses such as drought and salinity are within the main environmental challenges that limit the global productivity of major crops [71; 72]. Plants receive a variety of environmental pressures from the ecosystem, such as water shortage, extremely high or low temperatures, excess salt, or toxic metals in the soil affecting their growth adversely. One of the most important reactions to abiotic stress is the production of ROS consisting of both free radicals (superoxide radicals, O\({}_{2}\)\({}^{-}\), hydroxyl radical, OH\({}^{-}\)) and non-free radicals (hydrogen peroxide, H\({}_{2}\)O\({}_{2}\), and singlet oxygen \({}^{1}\)O\({}_{2}\)), which causes oxidative damage to cellular components, proteins, DNA, and lipids and degrades cell structures when in excess [73]. The accumulation of ROS causes various damages including the peroxidation of lipid membranes and the production of malondialdehydehyde, which destroys membrane integrity [16]. To detoxify ROS, plants have developed enzymatic and non-enzymatic antioxidant mechanisms, including the accumulation of AsA [8; 74], as well as the activation of antioxidant enzymes [67]. Through its roles in cell division and expansion, in acting as an enzyme cofactor, in participating in photosynthetic apparatus and hormone biosynthesis, AsA can protect cells and tissues by detoxifying stress-induced ROS accumulation [74]. Furthermore, as AsA can directly donate electrons totocopherol radicals, it contributes to the reduction of lipid peroxidation and membrane protection [8]. A broad number of studies demonstrated that abiotic stress induced the activity of the enzymes involved in the AsA-GSH cycle, such as APX, MDHAR, DHAR, and GR in many species [67]. AsA serves as a specific electron donor for APX in the conversion of H\({}_{2}\)O\({}_{2}\) to H\({}_{2}\)O. In this reaction, APX has a high affinity for H\({}_{2}\)O\({}_{2}\) scavenging, effectively removing this non-free radical even at low doses, orchestrating ROS signaling under oxidative stress. In contrast to the cellular AsA pool that remains largely at a reduced state even under stress exposure, a high accumulation of the AsA oxidized form is usually observed in the apoplast, providing a putative regulatory mechanism for plant growth and cell-wall loosening and lignification under stress [40; 75; 76; 77; 78]. Thus, through regulating the redox state in the extracellular matrix, AO serves as a modulator of both AsA and ROS accumulation in the apoplast [47; 76], having important implications on signaling under unfavorable environments [45; 76]. Additionally, AO has a vital role in auxin degradation in response to osmotic stress [79], highlighting its diverse roles in mitigating stress injury. Similar to other plant species, tomato plants suffer from a wide range of environmental stress factors, with the most important being drought, salt, and high temperature/high light. Under these conditions, AsA levels alter dramatically. For example, drought stress significantly increases the accumulation of osmolytes such as proline and soluble sugars, and of antioxidants including AsA to protect them from oxidative damage [8; 80]. Tomato can be classified as a moderate salt-sensitive crop, possessing a salinity threshold measured as electrical conductivity (EC) at 2.5 dS\(\cdot\)m\({}^{-1}\), with relative yield losses of 50% when the substrate salinity is approximately at EC 7.6 dS\(\cdot\)m\({}^{-1}\)[81]. Under salinity stress, AsA is an essential compound of non-enzymatic antioxidants in plants, functioning in plant growth and hormone signaling and playing especially critical roles in the fine control of ROS homeostasis to improve salt tolerance [3; 82]. Despite the increased demand for AsA accumulation to alleviate oxidative damage, AsA biosynthesis can be differentially regulated depending on the variation and intensity of stress factors.