الفهرس 
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Abstract
This experiment was carried out to study the ameliorating effects of arbuscular mycorrhizal fungi inoculation or/and foliar application of salicylic acid on growth, some physiological and biochemical parameters, and productivity of grafted watermelon plants grown under salt stress during 2020 and 2021 seasons at Balouza Research Station, Desert Research Center, North Sinai Governorate, Egypt.
The experiment was arranged in a split split plot design with three replications in both seasons. Every replicate included twenty-four treatments resulting from the combinations of three salinity treatments: two arbuscular mycorrhizal fungi treatments and four foliar applications of salicylic acid. The main plots were devoted to salinity treatments, while the sub-plots were occupied with the mycorrhiza treatments and the salicylic acid levels were allotted in sub-sub plots.
Grafted watermelon seedlings cv. Aswan F1 onto Star rootstock were used in the two tested seasons. The grafting process was carried out in a private nursery for vegetables in Berqash area, Giza Governorate, by using the tongue approach grafting method. Seedlings of the grafted watermelon were transplanted after the formation of 2-4 true leaves. Thirty plants for each treatment (ten plants per replicate) were transplanted in the first week of April in both seasons of the study. The main plot area was 60 m2 and contained one row with a length of 30 m and a width of 2 m. The distance between the plants was 1.0 m apart. The agricultural management practices (soil preparation for cultivation, fertilization, irrigation, weed control, disease, and pest control) were performed according to recommendations of the Egyptian Ministry of Agriculture for watermelon production.
6.1 The experimental treatments
6.1.1. Salinity treatments
The application of salinity treatments was started directly after transplanting the watermelon seedlings. Three levels of salinity were used, i.e., 1600 ppm (Al-Salam Conduit water), 4000 ppm (underground well 1), and 5000 ppm (underground well 2). Al-Salam Conduit water (1600 ppm) was used as the check treatment.
6.1.2. Arbuscular mycorrhizal fungi treatments
These treatments included inoculated and non-inoculated watermelon seedlings with mycorrhizal fungi (Glomus Gigaspora), which was obtained from the Microbiology Department, Faculty of Agriculture, Ain Shams University. Mycorrhizal inocula were placed directly on the roots of watermelon seedlings before transplanting by soaking for 15 minutes. Mycorrhizal inocula contained 200-300 spores/cm3. Successful inoculation can be detected about three weeks after the transplanting date.
6.1.3. Foliar spray treatments of salicylic acid
Watermelon seedlings in each plot were sprayed with salicylic acid after 15 days of transplanting using four concentrations (0, 1, 2 and 4 mmol/L). The foliar spray was repeated every ten days for four times.
6.2. The obtained results
The obtained results can be summarized as follows:
6.2.1. Vegetative growth
1. Salt stress treatments caused negative impacts on all evaluated vegetative growth parameters where salinity treatment at a level of 5000 ppm recorded the fewest values compared to check treatment (1600 ppm) which recorded the best values, in both seasons.
2. Arbuscular mycorrhizal fungi treatments exhibited excellent effects on the growth of watermelon plants, where the greater values were recorded in the inoculated plants.
3. Salicylic acid treatments had a significant effect on vegetative growth where the best values were recorded with spray at a concentration of 4 mM. The lowest values were obtained from check treatment.
4. Salinity amelioration treatments (mycorrhiza and salicylic acid) upgraded all mentioned plant vegetative growth parameters affected by salt stress. AMF inoculation combined with SA spray at a concentration of 4 mM was significantly the best in mitigating the salt stress impacts at a low level (1600 ppm) but it was moderately neutralizing the destructive effects of the high level of salt stress (5000 ppm) for all growth parameters.
6.2.2. Mineral composition of watermelon plants
1. The results indicated that the progressive salt stress levels decreased N, P and K contents in watermelon leaves but resulted in increases in Na and Cl concentrations. Salinity treatment at a level of 5000 ppm recorded the maximum values of Na and Cl and the minimum concentrations of N, P and K when compared to the check treatment (1600 ppm) which recorded the highest significant values of N, P and K.
2. As for the effect of mycorrhiza, it was found that there were increments in N, P and K contents with inoculated plants, while Na and Cl contents were decreased. These results were significant in the two growing seasons.
3. Referring to the effect of salicylic acid on mineral contents, the maximum contents of N, P and K were recorded by spray at a concentration of 4 mM followed, in decreasing order, by 2 mM
compared to check plants which recorded the fewest values. Meanwhile, check unsprayed treatment gave the highest significant values of Na and Cl concentrations compared to other treatments in the two seasons.
4. Mycorrhiza and salicylic acid applications positively reduced the harmful effects of salt stress on mineral contents of watermelon plants compared to non-inoculated unsprayed plants. Generally, the low salinity level (1600 ppm) and mycorrhiza inoculation combined with spray at a concentration of 2 mM or 4 mM enhanced the concentrations of N, P and K and reduced Na and Cl concentrations. The differences were statistically significant.
6.2.3. Physiological and biochemical measurements
1. Salinity treatment at a level of 5000 ppm increased proline, catalase and peroxidase enzymes compared to other treatments, whereas leaf relative water content, leaf membrane stability index and chlorophyll content were decreased. The highest significant values of leaf relative water content, leaf membrane stability index and chlorophyll content were obtained from check treatment (1600 ppm).
2. As for the effect of mycorrhiza, it was found that there were significant increments in leaf relative water content, leaf membrane stability index, chlorophyll contents, proline content, and catalase and peroxidase enzymes with AMF application.
3. Salicylic acid treatments induced many positive effects on the physiological and biochemical characteristics. The highest values were recorded by salicylic acid spray at a level of 4 mM compared to check unsprayed plants which recorded the lowest values.
4. Applications of mycorrhiza and salicylic acid alleviated the damaging effects of salinity on the physiological and biochemical traits. AMF
inoculation and SA spray at a level of 4 mM recorded the best values of leaf relative water content, leaf membrane stability index and chlorophyll content under check treatment of salinity (1600 ppm), and recorded the highest values of proline content, and catalase and peroxidase enzyme activities under the highest level of salinity (5000 ppm).
6.2.4. Yield and its components
1. There were severe decreases in the fruit yield parameters with increasing salinity levels where there was a 25.86 % and 23.24 % decrease in fruit weight, a 14.60 % and 15.76 % decrease in fruit diameter, a 10.51 % and 8.99 % decrease in flesh thickness and a 39.21
% and 37.17 % decrease in total yield per plant and per feddan under high salt stress conditions (5000 ppm) in the first and second seasons, respectively, compared to the check treatment (1600 ppm) which recorded the best values in both seasons.
2. AMF exhibited excellent effects on yield characteristics. The results indicated that greater values were recorded in the inoculated plants during both seasons compared to non-inoculated plants.
3. Referring to the impact of salicylic acid spray treatments, the highest values were obtained from the spray at a rate of 4 mM compared to other treatments. The check unsprayed treatment recorded the fewest values in both seasons.
4. The interactions impact between anti-salinity applications positively improved the fruit yield parameters under saline stress conditions. Hence, the highest values were significantly observed with AMF inoculation combined with SA spray at a concentration of 4 mM,
compared with check non-inoculated unsprayed plants at all salinity stress levels.
6.2.5. Fruit quality parameters
1. Salinity had a positive impact on fruit quality traits represented as total soluble solids, total sugars, and lycopene contents. The best values were recorded at salinity level of 5000 ppm compared to other treatments in both seasons.
2. AMF significantly affected the quality of watermelon fruits where the inoculated plants recorded the better values in the two tested seasons.
3. Salicylic acid spray at a concentration of 4 mM gave the highest values in the two studied seasons compared to other treatments whereas check treatment recorded the fewest values.
4. The salinity amelioration treatments led to a significant increase in fruit quality characteristics under salt stress conditions. The best values were observed by salinity level of 5000 ppm with inoculated plants combined with SA spray at a concentration of 4 mM compared to 1600 ppm salinity level combined with check non-inoculated unsprayed plants.