Within T. aestivum species, different cultivars respond variably to biotic and abiotic stresses [32, 33, 34]. Understanding these differences is essential for crop improvement. This can inform the efforts aiming at developing salt-tolerant crops including wheat and can help optimize agricultural practices in salt-affected regions such as Saudi Arabia. The variability in salt stress responses among wheat cultivars might be attributed to differences in the genetic background, these differences control salt stress perception and signalling pathways, osmotic adjustment capacity, ion transport and compartmentalization, and the activation of stress-response genes [35, 36].
Plant growth, Salt stress reduced differentially the growth of the three wheat cultivars.
Salinity alters plant growth and development, increasing NaCl concentrations in the growth medium result in adverse effects on plant growth and survival. Salinity leads to severe impact on the physiology traits of wheat plants including total biomass, however, the sensitivity of salt impact varies greatly among wheat cultivars [37]. In the current study, a distinctive variation in salt tolerance was observed between three cultivars, Najran was the most tolerant to saline cues while Qiadh was the most susceptible cultivar. Under salt stress, a dramatic decline in mean fresh and dry weights as well as lengths of root and shoot tissues was measured in all cultivars, but the reduction was significantly lower in the NaCl-tolerant cultivar Najran than in the NaCl-sensitive cultivar Qiadh. This decline in all growth parameters might be a consequence of the increased salt concentration around root area which in turn causes water deficit, nutritional imbalance and osmotic stress in the plant resulting in stomatal closure [38]. Moreover, prolonged exposure of plants to salinity leads to ion toxicity in the leaves and severely affects photosynthetic reactions, cell division and cell elongation which results in a reduction in root and shoot lengths [39].
Yield, salt-stress impacted differentially the spike and seed number and seed weight and germination.
It is well known that salt stress influences negatively the expansion of plant leaves causing a reduction in photosynthetic capacity which in turn affects the quantity and quality of grain yield [40]. The results obtained in the present study confirm this, where Qiadh cultivar showed the highest reduction in fresh and dry weights of the shoot as well as in yield production compared to the other two cultivars. In contrast, salinity had a positive effect on spike and seed numbers in Najran and Mebiah whereas seed weight had been negatively affected by NaCl. These findings are in line with those of [18], who found a significant decline in yield outputs of all tested wheat cultivars except Sakha 94 and Sids 13 cultivars, suggesting that while most wheat cultivars are sensitive to salinity some cultivars are salt tolerant, our results suggest that Najran wheat is among the salt-tolerant cultivars.
Interestingly, the Najran salt-tolerant cultivar seems to have invested carbon in producing more lighter seeds under salt-stress while maintaining a relatively high rate of germination (around 90%), in contrast to Mebiah which increased the number of seeds without maintaining good germination rate (less than 50%). In the salt-sensitive Qiadh cultivar not only the number of seeds produced under salinity declined but also the germination rate was reduced (around 70%). Choosing to maintain itself under salinity stress via an increase in seed number while maintaining a very high germination rate testifies for an advanced adaptation to salt-stress in Najran wheat. It is crucial to find the genetic determinants of this trait, several genes might be involved in the control of spike and seed numbers, the discovery of these genes is underway and some suspected transcription factors involved in the control of the developmental processes leading to spikes and seeds have already been identified mainly in rice [41]. Considerable effort is being put into identifying the orthologs of these genes as well as other genes potentially involved in the control of yield in wheat using mainly quantitative trait loci mapping.
Osmoregulation, concomitant production of different substances for osmotic adjustment is not essentially required for salt tolerance.
Prolonged high salt concentrations impair plant growth due to the resulting hyperionic and hyperosmotic stresses. Plants respond to these stresses by implementing biochemical mechanisms to facilitate water uptake and therefore maintain cell turgor and plant growth [42]. Osmotic adjustment is one of the crucial biochemical strategies in plant acclimation to salt stress. Proline, soluble sugars, starch and organic acids are of the main organic osmotica which are synthesized within plants to assist survival under salt stress. It is demonstrated in this study that different T. aestivum cultivars might employ various mechanisms to alleviate the harmful effects of saline stress. For example, Mebiah had the highest free Proline content, whereas Qiadh and Najran had the lowest Proline concentration in both root and shoot tissues. These results might be interpreted that Mebiah responded to the high level of NaCl by producing Proline which not only plays a great role in osmoregulation but also protects plants from the damage caused by ROS and toxic ions. Proline has been found to participate in lowering osmotic potential [43], storing carbon and nitrogen [44], detoxifying ROS [45], protecting the enzyme activities of photosynthesis and production of antioxidants [46] and inducing adaptive responses by acting as a stress signal [47] under unfavourable conditions. Another example, Najran wheat exhibited higher accumulation of soluble sugars in the roots and shoots while Qiadh had the lowest content of soluble sugars. This accumulation may participate in keeping the photosynthetic activity leading to maintaining plant biomass as these soluble sugars act as building blocks of macromolecules. These findings were accompanied with what we had found in growth analysis where Najran showed the highest fresh and dry weights under salt treatment compared to the other cultivars. In contrast, Qiadh displayed the highest increase in starch levels in both roots and shoots in comparison with the two other cultivars. Increasing the level of soluble sugars and decreasing starch levels might be a critical trait in salt tolerant cultivars. Similar results were reported by [48], who found a higher accumulation of soluble sugars in salt tolerant rice genotype, Pokkali, suggesting their important role in osmotic adjustment and accumulation of carbon energy reserves in plants. [49] have pointed out a decline in starch concentration in salt-treated leaves of Oryza sativa L. as a result of carbon limitation due to the poor photosynthetic activity under salt-stress, the decline in this case might be a result of the suppression of starch biosynthesis.
Organic acids are ubiquitous metabolites in plants which accumulate in response to salt stress to act as compatible solutes for osmoregulation and as ROS scavenger as well as plant protectors. The accumulation of total organic acids in the salt stressed wheat cultivars was obvious in the roots, however it decreased in the shoot of Najran and Qiadh cultivars. These findings are consistent with a previous study that confirmed the increase of organic acids in root tissues and their depletion in the leaves under saline treatment, suggesting that these different levels of organic acids might be attributed to organ-specific functions [50]. In the case of salt stress, roots uptake excessive amounts of sodium cations which require anions to balance the charge. Thus, organic acids are more accumulated in the roots to enhance the cation–anion balance. Furthermore, their high level in the roots assist plants to osmotically adjust under salinity conditions.
Anti-oxidant, salt stress enhanced phenolics production as an antioxidant response
Salt-stressed plants respond to oxidative stress resulting from the accumulations of ROS by operating an antioxidant-defence systems that prevent damage caused by ROS and detoxify ROS molecules. Phenolics are one of the nonenzymatic antioxidants produced to mainly protect plants against various stresses and act as ROS scavengers. It would seem that the activity of the ROS- defence systems rises under extreme environmental stresses and is more pronounced in tolerant plants than sensitive ones [51–53]. This suggests that the defence systems perhaps work more effectively under unfavourable conditions. In the current study, prolonged saline stress has shown significant accumulation of total phenolics in all wheat cultivars and was more pronounced in Najran followed by Mebiah and Qiadh, confirming that wheat cultivars with different sensitivity to NaCl stress exhibit different levels of metabolites alteration. Previous studies [54] have pointed out possible involvement of phenolics particularly phenylpropanoids in salt-tolerance in wheat. The expression of many of the genes involved in the productions of these substances increase under salt-stress in Najran wheat [54]. It would be of interest to extend transcriptomics analysis to the Qiadh cultivar and compare the results to those previously obtained in Najran.