## 4 The Role of Ascorbic Acid to Confer Abiotic Stress Tolerance in Tomato Plants
### The Role of AsA in Mitigating Post-Harvest Losses in Tomato Fruits  
During postharvest life, fruits and vegetables could be exposed to different abiotic stresses [93] such as wounding, phytohormones, temperature, ultraviolet light, storage atmosphere gas composition, and dehydration [94; 95]. In tomato, these losses have been estimated from 10 to 40% [96]. Therefore, suitable postharvest handling is essential to maintain fruit quality and in parallel to extend their marketable period during wholesale until their consumption from consumers [4]. Various treatments have been applied to provide postharvest abiotic stress resistance in fruits and vegetables, but with limited success since tissue response to abiotic stress is a highly complicated system of metabolic processes [97]. Moreover, stress factors could accelerate phytochemical damage in horticultural produce, with loss of AsA as the most sensitive indicator of stress exposure [13; 98; 100; 101].  
Practically, to extend the marketability of tomatoes, it is common to harvest fruits at unripe mature stages and store them at low temperatures increasing the risk of chilling injury disorder [102; 101], resulting in non-uniform ripening, the appearance of surface cracks, rapid loss of firmness, increased respiration rates, and higher water loss [103; 104]. In tomato fruit exposed to chilling temperatures, scavenging of ROS indicates a complex network of molecules and enzymes as a part of an antioxidant response mechanism, correlated to fruit shelf-life [105]. During postharvest storage, the initial content of AsA at harvest is critical for the function of the recycling pathway under stress conditions where reduced AsA is oxidized into the unstable MDHA which dissociates into AsA and DHA (Figure 2) [55]. In tomato, MDHAR has been found to act as the key enzyme in regulating the AsA recycling process under chilling temperature storage [106; 55]. Additionally, AsA levels during storage at chilling temperature could be altered because of APX activity through AsA oxidation [13]. Further study confirms that _APX_ gene expression is up-regulated during chilling stress in tomato fruit [107]. Furthermore, storage of tomato fruit at low temperatures can lead to an increase in GR activity [108; 105]. In cherry tomatoes, most of the genes involved in AsA biosynthesis such as _GME_ and _GalDH_, as well as isoenzymes of APX, MDHAR, DHAR, and GR, were enhanced at chilling temperature storage [108]. On the contrary, a short duration of cold storage enhances only _GPP_ expression [13] suggesting that AsA biosynthetic pathways do not react rapidly under cold stress conditions [108]. Recently, treatments with methyl jasmonate have been proposed as an efficient strategy to regulate tomato post-harvest quality and aroma, by inhibiting ROS accumulation through controlling the AsA-GSH cycle [109].  
All the above suggest a target point towards a better understanding of mechanisms controlling AsA accumulation/degradation and of its role in the postharvest performance of tomato. Recent advances in sequencing technology including RNA-sequencing provide useful tools to approach this important issue [110]. This, in combination with the exploration of tomato biodiversity in terms of better postharvest characteristics and richer AsA content, will put forth the basis for a better understanding of the factors controlling tomato keeping quality [111].