NaCl stress consequence on the colony diameter and mycelia weight of T. viride:
Salinity decreases the growth of plants and the extent of this decrease may be linked to the interaction between the host, microbe, as well salt level [16, 25]. Thus, it is necessary to study the effect of NaCl on the growth of T. viride. Our results (Fig. 1 and Table 2) showed that, after 5 days of incubation, low salt concentrations had an enhanced influence on the mycelia weight of T. viride as compared to the control one. Also, with increasing NaCl concentrations occurs a decrease in the dry weight of mycelia significantly (p< 0.05) (Table 1). Concerning T. viride colony diameter results, low salt concentrations (50 and 100 mM NaCl) cause a slight decrease in T. viride growth as shown in Fig. 1, however, the decrease is not significant. Nevertheless, the differential inhibitory effects were detected with higher salt concentrations (150, 200, 250, and 300 mM). This is maybe a result of enhancing the water potential of the substrate that reduces the growth of fungal colonies at high salt concentrations . Also, high salt may affect cytoplasmic metabolic activity, such as intracellular proteins that may provide the extra osmotic potential to prevent plasmolysis . Our results are compatible with Zhang et al.  with T. longibrachiatum T6. Moreover, Contreras-Cornejo et al.  indicated that low NaCl concentrations increase the colony diameter and the growth of T. atroviride, although higher salt concentrations caused a significant reduction. This indicates that the effect of NaCl on the growth of Trichoderma spp is dose-dependent, with high salinity inhibiting the growth, and low salinity promoting its growth . Moreover, a study by Guo et al.  reported that the growth of T. asperellum can be promoted by 2% NaCl. Similarly, Rawat et al.  stated that five of forty-five T. harzianum wild-type strains can grow and form spores in growth containing up to 240 mM NaCl. The optical microscopic examination revealed that the treatment of T. viride with high salt concentrations (150 and 200 mM NaCl) caused abnormal mycelial growth and considerable morphological changes, mainly manifesting as deformation, contraction, collapse, globular swellings occurred at the tips of hyphal strands and deformity of the conidium (Fig.2 D, E, F and G). In contrast, the mycelia of the control and that of low salt concentration (50 mM NaCl) were straight and well developed (Fig.2 A, B and C).
NaCl stress consequence on the appearance of cotyledonary leaves:
High salinity is one of the major environmental stresses that cause biochemical changes in plants and limits plant growth, according to earlier studies of Zhang et al.  and Mahmood et al. . Our results showed that salinity reduces the number of tomato seedlings that show the first two cotyledonary leaves after 6 days of salt application by 59.3 and 37.5 % at 50 and 100 mM NaCl over their respective control ones. It means that salinity had negative effects on this parameter (Table 3). Furthermore, salinity increases the time of seed germination by lowering the water potential of the germination media, and gradually reduced final germination as compared to regulation [31, 32]. Similarly, Tanveer et al.  found that salinity decreased the percentage of seeds that germinated since high salt levels result in low water and nutrient uptake, which affects seedling germination and development [33, 34].
Plant growth promotion in T.viride-treated tomato seedlings:
Seedling height, FW, and DW of tomato seedlings were measured 10 days after NaCl application to evaluate the growth-promoting effects of T. viride on tomato seedlings under salt stress (Fig. 3, 4 and 5). Our results showed that these growth parameters were significantly (p<0.05) inhibited with NaCl treatment; where, the seedling FW decreased by 33 and 26%, and DW decreased by 15 and 19% under 50 and 100 mM NaCl, respectively, over their respective control ones. The inhibitory effect of salt stress was in line with the results of Dief et al. , Metwally and Abdelhameed , Zhang et al.  in fenugreek and wheat. The explanations may be the non-availability of mineral nutrients and the outflow of energy to lessen the harmful effects of NaCl [35, 36]. Also, the consequence of osmotic stress, the toxicity of ions, and oxidative stress as a result of salt stress is known to delay growth . However, the effect of salt was alleviated substantially with T. viride application, indicating that T. viride improves these measured variables significantly (Fig. 5). Our results are coherent with Metwally  and Metwally and Al-Amri  results about the improving capabilities of T. viride on onion growth and physiology. Likewise, Zhang et al.  observed that NaCl inhibited wheat seedling growth and that this inhibitory effect was alleviated by adding T. longibrachiatum T6. Compared to NaCl stressed seedlings, the height of tomato seedlings increased by 15 and 34% after being treated with T. viride at 50 and 100 mM NaCl; respectively (Fig. 5), where T. viride attained the maximum seedling height both in control and NaCl treatments. Fig. (4) shows the phenology of control or NaCl stressed tomato seedlings under T. viride fungal application.
According to related findings, T. harzianum enhanced seedling growth of cucumber after salt application . Moreover, several previous studies have shown that Trichoderma sp form symbiotic relationships with a large variety of host plant roots and promote their growth and development . Besides, Trichoderma sp. is also known to produce a range of antibiotics, including polyketides, trichodermin, peptaiboils, trichodermol, and steroids that promote plant development . Also, the promotion of plant growth under saline condition largely relies on the increase of the activity of ACC-deaminase and the level of IAA production in Trichoderma as Zhang et al.  reported.
Seedling height stress index (SHSI)
The seedling height stress index was reduced under both salt concentrations as compared to the non-saline MS medium (Table 4). However, T. viride increased the seedling height stress index (SHSI) under both salt concentrations with 58.98 and 45.55, respectively. The lowest SHSI was detected in tomato seedlings subjected to 100 mM NaCl. Furthermore, AL-Mutawa  and Rawat et al.  stated that high salt levels result in low water and nutrient uptake, which affects seedling germination and development. However, tomato seedlings treated with T. viride had a significantly higher percentage of SHSI under both salt concentrations. Besides, Zhang et al.  and Rawat et al.  indicated that the symbiotic colonization by Trichoderma enhances root growth and causes solubilization and sequestration of inorganic nutrients, which might be responsible for increased tolerance to osmotic stresses.
Seedling water status:
Regarding relative water content (RWC), our results showed that tomato seedlings have a maximum RWC with T. viride under non-saline MS medium and minimum for seedlings exposed to 100 mM NaCl (Table 4). RWC of tomato seedlings exposed to 50 and 100 mM NaCl decreased by 7.00 and 12.78% compared to control ones grown under non-saline MS medium. Even though, 50 and 100 mM NaCl reduce RWC and WC of tomato seedlings as compared to control; this decrease is not significant. This is in accordance with Metwally and Abdelhameed  and Chaudhuri and Choudhuri  that salt stress affects the water status of fenugreek and jute. Since plants grown under salt conditions are exposed to physiological drought as Na+ and Cl− ions bind water that is necessary for the plants growth [12, 40]. Of particular note, the inhibitory effect of salinity on tomato seedlings was mitigated to some extend by T. viride application. Furthermore, under salinity, WSD was substantially increased; however, these effects were diminished when T. viride was applied (Table 4); as Trichoderma's effects enable plants to more efficiently use water to maintain a lower CO2 concentration within cells.
The consequence of NaCl and T. viride on H2O2 content and lipid peroxidation:
In consequence of superoxide radicals scavenging, H2O2 which is a toxic compound and injurious to plants is produced as a result of salt exposure. Higher concentrations of H2O2 in plants cause lipid peroxidation and membrane injury [23, 34]. Fig. (6 a and b) shows that under 50 and 100 mM NaCl, tomato seedlings treated or not with T. viride had a substantial increase in H2O2 content. Significantly higher levels were maintained in control seedlings under both salinity levels (5.16 and 5.53 mg/g FW). However, T. viride treatment reduces H2O2 accumulation (Fig. 6b), where under salt stress; H2O2 content of control seedlings was significantly higher than that of T. viride treated seedlings. The minimum level of H2O2 was observed in seedlings treated with T. viride (3.30 mg/g FW) followed by non-treated ones (3.42 mg/g FW) grown under non-saline MS medium or control condition. Our findings are coherent with Zhang et al.  and Rawat et al.  in cucumber and chickpea treated with T. harzianum under salt stress conditions. The decreasing levels of H2O2 in T. viride treated seedlings show that, at the cellular level, these seedlings are better fortified with an effective free radical quenching system that brings protection against oxidative stress.
Malondialdehyde (MDA), a product of lipid peroxidation, is mostly considered as an indicator of free radical damage to cell membranes caused by oxidative stress. Our result related to the effect of NaCl in the presence and absence of T. viride on MDA content is presented in Fig.6 a. MDA significantly increased in tomato seedlings subjected to 50 and 100 mM NaCl by 58.3 and 90% relative to the control ones. These results are consistent with Zhang et al.  in wheat seedlings; owing to salt exposure, the ROS formed in tomato cells causes peroxidation in membranous lipids and the formation of MDA [9, 41]. On the other hand, tomato seedlings with T. viride under salt stress were more effective than salt-stressed seedlings in lowering MDA contents, predicting membrane protection. Compared with their respective control, MDA content in tomato under control or 50 mM NaCl decreased by 11 and 14 percent with T. viride application. Our findings are consistent with Dief et al.  and Rawat et al.  that bio-priming wheat seeds with P. chrysosporium or T. harzianum decreased MDA accumulation.
T. viride and NaCl effects on ROS scavenging in tomato seedlings:
ROS act as signaling molecules at low concentration, though it’s excessive accumulation damages plants under stress. These ROS can affect the integrity of cellular membranes and enzyme activities [2, 42]; resulting in oxidative stress which is one of the damaging causes in plants exposed to environmental stresses [9, 41]. For defense from oxidative stress damage, plants depend on non-enzymatic and/or enzymatic systems. Furthermore, inoculating plants with Trichoderma sp. resulted in a number of physiological improvements, including a rise in enzymatic and non-enzymatic antioxidants, which increased plant resistance to stress .
T. viride induces salt mitigation through a non-enzymatic mechanism (total soluble protein and proline content):
To assess whether T. viride induces salt tolerance in tomato seedlings due to a non-enzymatic mechanism, proline and soluble protein contents as substances capable of altering osmotic potentials (Fig. 6 c and d) were detected. Osmotic adjustment by lowering the osmotic potential plays a key role in cellular water retention and turgor maintenance, thereby minimizing the adverse effects of salt stress through balancing the solute potential , which then contributes to cell growth.
Proline stabilizes the membranes and prevents the degradation of proteins and enzymes under stress conditions . We observed that T. viride application in MS medium with tomato seedlings resulted in a significant (p <0.05) increase in total soluble protein and proline contents, regardless of the severity of salt stress. Soluble protein content in tomato significantly decreased after NaCl treatment (% of decrease were 20.58 and 40 after 50 and 100 mM NaCl salt treatment compared to those under non-saline MS medium). This finding backs up the hypothesis that high Na+ levels damage plants through disturbing protein synthesis . Additionally, Rasool et al.  and Ahmad et al.  stated that plants cope with overawed osmotic stress caused by salinity build-up osmolytes such as proline, soluble proteins, soluble sugars, and glycine betaine. Nevertheless, the protein content increased by 11.2, 25, and 23.04 %; respectively with T. viride application under control or 50 and 100 mM NaCl treatment, compared to their respective control seedlings. T. viride plays a key role in plant tolerance; where proteins serve as an energy reservoir or possibly an osmotic potential adjuster in plants that are exposed to salinity [2, 48]. These results are in line with Metwally  findings with onion plants inoculated with T. viride. Similarly, our results were supported by Zhang et al.  with T. longibrachiatum application in wheat seedlings. Moreover, Dief et al.  indicated that protein considerably decreased in the wheat seedlings after NaCl treatment; however, with Phanerochaete chrysosporium application its content increased greatly.
On the contrary, both NaCl concentrations cause a conspicuous increase in proline content of tomato seedlings, where its highest value was observed at 100 mM NaCl (Fig. 6 c). In harmony with this finding Ueda et al.  and Khomari et al.  stated that under salt stress, plants accumulate compatible osmolytes, such as proline, to facilitate osmotic adjustment leading to increased dehydration tolerance. Our findings are in line with those of Dief et al.  and Rawat et al.  that seed bio-priming with P. chrysosporium and T. harzianum increased proline content in wheat seedlings. Moreover, previous studies of Zhang et al.  and Khomari et al.  demonstrated that Trichoderma sp. and T. longibrachiatum had a highly significant effect on proline and protein contents in wheat and soybean under control and salt stress conditions. The augmentation in proline contents may be due to the enrichment in proline synthesizing enzymes activity and reduction in catabolizing ones or its circumscribed assimilation in protein synthesis . As well, the further increase in their contents with T. viride indicates that Trichoderma could confer systemic resistance to the treated tomato seedlings by up-regulating the substances capable of causing major osmotic adjustments  as well as energy storage . Moreover, increased proline content enhanced the ability of plants to detoxify the accumulated ROS and protect the seedlings from oxidative damage [24, 41].
Salt mitigation induced by T. viride is dependent on an enzymatic system:
In the direction of estimating whether the enzymatic system plays a role in T. viride prompted tolerance to salt by scavenging ROS, we examined the activities of some antioxidant enzymes in tomato seedlings such as POD, PPO, CAT, and APX as well as total antioxidant capacity (TAC) (Fig. 7 a-e). Results indicate that NaCl stress significantly induced an increase in all the assayed antioxidant enzymes besides TAC in tomato seedlings (Fig. 7). Furthermore, their activities were significantly increased after T. viride treatment under both saline and non-saline stress conditions, compared to their corresponding control. Under non-saline MS medium, T. viride increases POD, CAT, and APX by 15.03, 9.33, and 14.88%; respectively. Whereas, under 50 mM NaCl it increases these values by 60.2, 74.6, and 61.6% as compared to the control ones. Where, these antioxidant enzymes in tomato seedlings perform the main role in ROS scavenging and therefore preventing the oxidative stress prompted damaging effects on several sensitive molecules like nucleic acids, proteins as well as lipids [51, 52]. Our results corroborate with Zhang et al.  and Zhang et al.  that T. harzianum and T. longibrachiatum T6 enhanced the tolerance of cucumber and wheat to salt stress through the increased antioxidative defense. Also, Dief et al.  reported a remarkable increase in CAT and APX enzymes activities in wheat leaves bio-primed with P. chrysosporium under salinity stress. These results indicate that T. viride could confer systemic resistance to tomato seedlings by improving the antioxidant enzyme activities, to result in higher FW, DW, and seedlings length (Fig. 5) in tomato seedlings.