الفهرس 
INTRODUCTION
Effect of salt stressOnly 14 pages are availabe for public view
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Abstract
The present study was carried out during the 2017 and 2018 seasons at the Floriculture Farm, Faculty of Agriculture, Assiut University, Egypt to investigate the effect of foliar application of some biostimulants on growth, volatile oil percentage, oil yield and chemical constituents of sweet basil (Ocimum basilicum, L. cv. Genovese) plants undzer salt stress conditions.
The experiment including 20 treatments which presented the combination between four salinity levels; control “tap water”, 1000, 2000 and 4000 ppm NaCl and five treatments of biostimulants; control, 200 ppm salicylic acid, 3 ml L-1 seaweed extract, 4 g L-1 active dry yeast and 1000 ppm moringa leaf extract. The treatments were arranged in a split-plot design with four replicates. The four salinity levels represented the main plots, while the five biostimulants treatments represented sub-plots.
At the middle of March, sweet basil seeds were planted in earthenware pots 15 cm, filled with clay loam soil at 5 seeds in each one. Uniform seedlings were transplanted after 45 days from the sowing in earthenware pots 30 cm diameter and 40 cm height. Seedlings were irrigated regularly with tap water for one week after transplanting, and then subjected to different salinity levels and foliar application of biostimulants. Two applications repeated for each cut at two week intervals.
Plant samples were collected at 40% flowering for three cuts, 45 days between each and other, the 1st one at the beginning of July. Data were recorded on; plant height, main branch number, leaf number, herb fresh and dry weights per plant as well as leaf area and total leaf area. Essential oil percentage, oil yield per plant, photosynthetic pigments content and total carbohydrates percentage as well as total phenols and free proline contents were determined.
The obtained results can be summarized as follows:
I. Effect of salinity
1. The highest values of vegetative growth characteristics were achieved with 1000 ppm NaCl, followed by non-salinity treatment (control). Contrarily, the lowest values of growth parameters were obtained from plants irrigated with the highest salinity level (4000 ppm NaCl) compared to control and the other salinity levels for all cuts during both seasons.
2. The low salinity level (1000 ppm NaCl) resulted in the highest essential oil percentage, oil yield and photosynthetic pigments content compared to control and the other salinity levels in the three cuts during both seasons. Meanwhile, the minimum values were recorded with 4000 ppm NaCl.
3. Total carbohydrate percentage, total phenols and free proline contents considerably increased by increasing the salinity levels from 1000 to 4000 ppm NaCl.
II. Effect of biostimulants
1. All treatments of salicylic acid, seaweed extract, active dry yeast and moringa leaf extract remarkably improved growth parameters comparing with the untreated plants.
2. Seaweed extract was more effective on increasing the vegetative growth characteristics than the other biostimulants, followed by salicylic acid treatment.
3. Foliar application of seaweed extract considerably increased the essential oil percentage, oil yield per plant and contents of chlorophyll ‘a’, chlorophyll ‘b’ and carotenoids as well as total carbohydrate percentage compared to control and the other biostimulants in the three cuts during both seasons, in most cases. Meanwhile, salicylic acid produced the highest values of total phenols and free proline contents.
III. The interaction between salinity and biostimulants
1. Foliar application of salicylic acid, seaweed extract, active dry yeast and moringa leaf extract considerably improved all the vegetative growth characteristics studied of sweet basil plants and reduced prominently the hazard effects of salinity.
2. The combined treatment of seaweed extract and 1000 ppm NaCl resulted in the best vegetative growth characteristics and the highest essential oil percentage, oil yield per plant and photosynthetic pigment contents compared to the other combined treatments, followed by salicylic acid treatment at the same salinity level. On the contrary, the lowest values were recorded with the high salinity level (4000 ppm NaCl) without any biostimulants application.
3. Treating plants with seaweed extract under 4000 ppm NaCl gave the maximum total carbohydrate percentage.
4. The highest total phenols and free proline contents were obtained from plants treated with salicylic acid and 4000 ppm NaCl compared to the other combinations.
Conclusions and recommendations
from the obtained results, it appears that treating sweet basil plants with biostimulants such as salicylic acid, seaweed extract, active dry yeast and moringa leaf extract could improve the growth parameters, as well as the essential oil percentage, oil yield per plant and photosynthetic pigments content under salt stress conditions. Foliar application of seaweed extract at 3 ml L-1 was the most effective treatment for improving the vegetative growth characteristics and gave the highest essential oil percentage, oil yield per plant and photosynthetic pigments content, under low salinity level (1000 ppm NaCl), followed by salicylic acid treatment (200 ppm). Hence, these treatments are recommended for improving the growth performance and alleviating the adverse impacts of salt stress in sweet basil plants.