Growth parameters
The effect of proline or trehalose on growth traits of quinoa when irrigated with different salinity levels in soils supplemented or un-supplemented with compost is presented in Table 3. Compared with the control plants, the irrigation of plants with 3000 and 6000 mg/L led to a significant decrease in all studied morphological parameters (plant height and fresh and dry weights of shoot). These results might be due to the inhibitory effect of salinity through reduced water absorption and metabolic activities or due to Na+ and Cl- toxicity and nutrient deficiency caused by ion interference. The obtained results are consistent with results recorded by Hussein et al. (2019) found that salinity level of 4000 ppm reduced plant height, leaves number, leave area/plant, and dry weight of shoot, leaves, and spike/plant of barley plant. Also, Abdallah et al. (2020) reported that all growth criteria of wheat cultivars are affected with salinity levels due to salt stress effects on plant cells’ functions in terms of different enzymes and metabolism functions in the cell.
On the contrary, plants grown in the presence of compost showed significant enhancement in its growth compared to the corresponding control. The role of compost in increasing quinoa growth may have resulted from endogenous growth promoters produced by microorganism which enhance the mobilization of nutrient through increasing cell divisions and/or increasing the differentiation of the vascular connection between the axillary buds and the main stem. In this regard, Kirakosyan et al. (2006) stated that auxin in forms of IAA produced from fungi (mutant Penicillium) enhanced the growth of rice plant under salinity stress. Also, El Sebai et al. (2016) who worked on quinoa plants reported that the improved growth characteristics resulted from increased application rate of compost and attributed this improvement to the enhanced decomposition of organic matter and mineralization of nutrients.
All used concentrations of proline (0.5, 1.0 mM) or trehalose (2.5 and 5.0 mM) concentrations increased significantly (P < 0.05) the studied growth parameters of quinoa plants in the presence and absence of compost under different salinity levels.
Concerning the effect of proline, Taie et al. (2013) found a similar trend of increased growth parameters (plant height, number of leaves, and shoot dry weight). These increases in growth attributed to low and high proline concentrations that have a role in protecting enzymes, structures of proteins, and organelle membranes (Hoque et al. 2007). These results are also supported by findings of Ashraf and Foolad (2007) who suggested that proline supplies energy for growth and survival thereby helping the plant to overcome stresses. Also, Abdallah and El-Bassiouny (2016) found that exogenous application of proline led to marked increases in growth characteristics (plant height, shoot, root fresh, and dry weight) on quinoa plants. They added that proline treatment induced an increase in apical meristem and cell division (Ali et al. 2008).
For trehalose treatment effect, Zeid (2009) reported that trehalose is nontoxic molecules as compatible solutes able to accumulate at high concentrations in the cytoplasm and participate in turgor maintenance. It also protects the macromolecular structures against the destabilizing effect of anhydrobiotic conditions under salinity stress. Gaff (1996) referred this action of trehalose in transgenic plants in improving water relations and desiccation tolerance to osmoregulation and stomatal closing when drought stress became mild. The same trend of trehalose application was reported by Alam et al. (2014) with different Brassica species.
Photosynthetic pigments contents
Quinoa plants irrigated with saline water (3000 and 6000 mg/L) caused gradual significant decreases in its contents of chlorophyll a, chlorophyll b, carotenoid, and total pigments as compared with their control ones (Table 4). Similar results were recorded by Sadak et al. (2012) on and Abdallah et al. (2020) on wheat cultivars. Kosová et al. (2011) concluded that enzymes’ activities which can be considered as a secondary effect of the reduced CO2 partial pressure resulted from stomatal closure in leaves were responsible for the inhibitory effect of salinity stress on the photosynthetic pigments. Moreover, under salinity stress, the same effect (a reduction in photosynthesis and growth) is induced by the increase in free radicals (ROS) in chloroplasts and destruction of chlorophyll molecules (Sadak et al. 2020). In addition, Rady et al. (2013) referred the adverse effect of salt stress to the chlorophyllase enzymes responsible for chlorophyll degradation.
The results (Table 4) also revealed a significant (P < 0.05) increase in photosynthetic pigments contents in plants grown in soils enriched with compost compared to the corresponding plants grown without compost treatment. This is consistent with compost effects on stomatal opening that is confirmed with Nguyen et al.’s (2012) and El Sebai et al.’s (2016) findings who reported that the adverse effects of salinity stress are ameliorated in terms of increased chlorophyll contents with compost containing microorganism. It is worth noting that carotenoid content was significantly higher in quinoa plants with compost treatment in combination with different concentrations of salt compared to other plants without compost. The increased chlorophyll content accompanied to the enhancement in plant capacity to reduce the damaging effect of ROS may be attributed to the increased carotenoids, which may play a role as a free radical scavenger, with compost treatment.
Results also revealed that plants treated with either proline (0.5, 1.0 mM) or trehalose (2.5 and 5.0 mM) concentrations under normal conditions or under salinity stress with or without compost treatment significantly (P < 0.05) increased its all photosynthetic pigment contents compared to the control and its corresponding control treatments.
Concerning the effect of proline application, Yan et al. (2011) explored the role of proline in possessing some defensive mechanisms for damage of plants in addition to its function as a nutrient under salt stress. These defense mechanisms played a role in promoting photosynthesis, maintaining enzyme activity, and scavenging ROS. Taie et al. (2013) found that proline exogenous application was capable of overcoming the harmful effects of salinity on photosynthetic pigments.
Regarding trehalose treatments, Abdalla et al. (2016) found that presoaking rice seeds in trehalose produced a significant increase in photosynthetic pigments compared to untreated resultant plants. Moreover, Ali and Ashraf (2011) attributed the increment in biomass and some key photosynthetic attributes to a specific increase in Rubisco activity, resulted from increased amounts of Rubisco protein, chlorophyll, and the light-harvesting apparatus.
Change on osmo-protectants
Perusal of data in Table 5 for quinoa plants irrigated with saline water (3000 and 6000 mg/L) revealed gradual decreases in total soluble sugars (TSS) compared with the control with or without compost addition. Similar results have been reached by Rady et al. (2011) in sunflower cultivars. Salinity stress resulted in a significant improvement in proline and free amino acid levels of quinoa plants. These results are supported by other results recorded by El-Bassiouny and Abdel-Monem (2016) who found that salinity stress was capable of acting as activators of free amino acids accumulation in sunflower plants. Moreover, Abdallah et al. (2016) reported that proline plays crucial protecting roles from drought-osmotic stresses in terms of osmotic adjustment, stabilization, and protection of enzymes, proteins, and membranes and the reduction in lipid membranes oxidation. Mitysik et al. (2002) recorded that increased osmolytes (proline and free amino acids) protect macromolecules via stabilizing protein structure and/or scavenging ROS produced under stress conditions.
Data also showed that compost treatment increased significantly quinoa plants TSS contents compared to untreated ones (compost-free). The processes involved of compost lead to increased rates of photosynthesis and of carbon compounds to the plants (El-Sebai et al. 2016). In addition, it is mentioned that the increase in total soluble sugars induces adjustments in the osmotic balance and increases the chlorophyll contents which increased its rate of photosynthesis and carbohydrate synthesis. Compost supplementation positively improved proline and free amino acid content in quinoa plants with increasing salinity stress level (Table 5). These results are in agreement with plants by El-Sebai et al. (2016) on quinoa and Banerjee et al. (2012) on mustard plant.
Foliar application with different concentrations of proline (0.5, 1.0 mM) or trehalose (2.5 and 5.0 mM) in the presence of compost at different salinity levels induced a significant (P < 0.05) increase in the content of osmoprotectants (total soluble sugars, proline, and free amino acids) in quinoa plants, proline accumulation recorded with stressed plants compared to control plants especially at higher salt concentration. These effects are in line with the recorded results by Noreen et al. (2018) who denoted that copper stressed wheat plants reduced ROS content and increased proline accumulation with proline treatments and Yang et al. (2014) on salt-stressed Arabidopsis plants treated with trehalose which elevated its total soluble sugars. Ibrahim and Abdellatif (2016) found that foliar application of trehalose improved plant osmotic adjustment ( free amino acids, reducing sugars, and total soluble sugars) on wheat leaves under water stress.
Antioxidant enzymes
The results in Table 6 showed the activities of the antioxidant enzymes (SOD, CAT, POX, and APX) in the shoots of quinoa. The irrigation of quinoa plants with saline water (3000 and 6000 mg/L) caused gradual increases in SOD, CAT, and APX and gradual decrease in POX activity as compared with the control in the absence and presence of compost. In terms of salinity effect, these results were supported by the results of El-Bassiouny and Sadak (2015). In this concern, Abdallah et al. (2020) stated that the increase in antioxidant enzymes could be considered as an indicator to the increased production of ROS and stimulation for a protective mechanism to reduce oxidative damage resulted from plant responses to stress. Multiple antioxidant enzyme systems are involved in the enzymatic scavenging of ROS.
An addition of compost to the soil increased significantly the studied antioxidant enzyme activity compared to stressed and unstressed plants. Similar effects were obtained by El-Sebai et al. (2016) who found that compost increases tolerance of quinoa plants to salinity stress through increasing the activity of antioxidant enzymes (CAT, SOD, POX, and APX activities) as compared with the corresponding control. Also, Shaukat et al. (2006) recorded increments in POX activity on Triticum aestivum as a result of inoculating the plants with Pseudomonas strains.
The obtained results showed that foliar application of proline (0.5 and 1.0 mM) or trehalose (2.5 and 5.0 mM) significantly increases the SOD, CAT, POX, and APX activities under salt stress and unstressed conditions. The obtained results came in line with the findings of Nounjan et al. (2012) who found that the SOD activity was considerably increased to convert O2 to H2O2 under salinity stress, which is detoxified thereafter by POX, APX, and CAT. However, enhanced CAT activity in trehalose considered an efficient H2O2 scavenging mechanism that contributes to tolerance of salt-induced oxidative stress (Dolatabadian and Jouneghani 2009). Also, Abdallah and El- Bassiouny (2016) reported that proline plays a regulatory role in the activity and function of SOD, CAT, and POX enzymes in plant cells as well as in their participation in metabolic response development in response to environmental factors. The antioxidant enzyme system (the first defense line) provides vital protection against oxidative damage through scavenging the active oxygen species (O−, OH−, and H2O2) within stressed plant cells (Sharma and Dubey 2007).
Yield components
The response of quinoa plants yields parameters by foliar application of proline or trehalose in the presence or absence of compost under different salinity treatment levels (Table 7).
The noticed reduction in yield traits may be attributed to the inhibitory effect of salinity on growth (Table 3) which reflected on the yield as well as the disturbance in mineral uptake (El Sebai et al. 2016).
The use of compost represents an excellent opportunity to decrease the adverse effect of salinity stress and positively improved all studied yield components. El Sebai et al. (2016) reported that soil enriched with compost significantly increased the yield component of quinoa plants under water stress conditions Also, Rady et al. (2016) reported that an addition of compost improved the soil characteristics and consequently pod and seed yields of common bean.
Applying proline or trehalose on quinoa plants at different concentrations showed that both treatments significantly (P < 0.05) increased the yield components as compared with the corresponding salinity levels particularly in the presence of compost. Sadak (2016) found that the treatment of fenugreek plants with different concentrations of trehalose led to an increase in its yield components. Moreover, Abdallah and El-Bassiouny (2016) found that the treatment of quinoa plant with proline at different concentrations stimulated its yield parameters.
Changes in carbohydrate, protein, and oil percentage
The results in Table 8 revealed that irrigation of plants with saline solution reduced the seed nutritive value. The effect of salinity stress was in good harmony with the results obtained Ramadan et al. (2019) on sunflower. The foliar application of proline or trehalose in addition or absence of compost on quinoa plants grown ameliorates the adverse effect of salinity on seed nutritive values in terms of carbohydrate, protein, and oil percentage. Compost addition with trehalose gave the best results compared to that recoded with un-supplemented.
As regards to the effect of compost, similar findings were obtained by El-Sebai et al. (2016) who stated that adding compost to the soil significantly increased total carbohydrate, protein, and oil percentage contents of quinoa yielded seeds.
Similar to the findings with our data, regarding the effect of proline treatment, Abdallah and El-Bassiouny (2016) found also that the application of proline increased the nutritional values of the yielded seed of quinoa plants in terms of carbohydrate and protein percentage. Also, Taie et al. (2013) on faba bean plants showed that treatment with proline significantly increased both total carbohydrates and total soluble carbohydrates compared to the control.
The observed increase in nutritive in seeds in response to trehalose treatments was in accordance with the studied of Elewa et al. (2017) induced that trehalose treatments at different levels caused marked increases in carbohydrate, protein, oil, and antioxidants of the yielded seeds either in non-stressed or drought-stressed plants relatively to the control ones. They added that trehalose serves as a carbohydrate storage molecule as well as a transport sugar (Muller et al., 1999). In addition, underexposure of plants to stress, trehalose can replace hydrogen bonding through polar residues resulting in preventing protein denaturation and fusion of membranes responsible for protein and membrane stability (Iturriaga et al. 2009).
Changes in mineral contents in yielded seeds
Results in Table 9 showed that plants irrigated with high salinity levels (6000 mg/L) reduced the macro-elements content (N, P, and K) in the yielded seeds. In this respect, as reviewed and discussed by Ramadan et al. (2019), salt stress disrupts the availability, competitive uptake, and translocation of nutrients to the plant parts. Interference of other elements with excessive concentrations of Na and Cl ions present in the soil causes imbalanced plant nutrition. These results can be disproved to others on the base that higher concentrations of sodium chloride (NaCl) as the prevailing salt in the soil lowers its osmotic potential that induce a reduction in the availability of water and minerals to root cells resulting in osmotic and ionic effects on plants (Khan et al., 2012).
Soil amendment with compost increases the availability of plant nutrients, improves physical properties, and stimulates biological activities. The data obtained herein are in harmony with those of by El-Naggar (2010) who found that the organic compost applications increased the uptake of N, P, and K of Narcissus tazetta L.
Furthermore, proline and trehalose in the presence and absence of compost generally induced an increase in total N, P, and K contents of quinoa seeds. In support of the above result of increased macro-element content with proline treatment, Dawood et al. (2014) on Vicia faba plants stated that foliar treatment of proline improved the toxicity of stress water by increasing the inorganic nutrients (N, P, K, and Ca) as well as the ratio of K+/Na+. Also, Ali et al. (2008) on maize cultivars stated that proline increased ion uptake and regulated their transportation.
Elewa et al. (2017) found that different levels of treatments of trehalose gave marked increases in nitrogen, phosphorous, and potassium contents of quinoa plants. In another study, a similar effect was obtained by Ajay et al. (2002) on transgenic and non-transgenic rice plants and Yang et al. (2014) on Arabidopsis.