الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
Two separate experiments were carried out to examine the effect of some hormone derivatives (brassinosteroid, melatonin, and jasmonic acid) and plant growth promoting bacteria (Azospirillum brasilense, Bacillus megaterium, Rhizobium leguminosarum bv. phaseoli, and Pseudomonas fluorescens) on alleviating drought stress on snap bean (Phaseolus vulgaris L.) cv. Valentino plants. The experiment was conducted in the two seasons of 2020 and 2021 at the Experimental Farm of the Horticulture Department (30°06ʹ46ʺN, 31°14ʹ37ʺE), Faculty of Agriculture, Ain Shams University, Shoubra El Kheima, Qalyubiyah Governorate, Egypt. Seeds were sown on the 1st of September in both seasons in cement basins of 2 m2 filled with washed sand soil. Seeds were sown in rows spaced 60 cm apart with the seed spaced 10 cm apart in the row.
In the first experiment, seven foliar-application treatments; including brassinosteroid at 5, and 10 µM, melatonin at 50, and 100 µM, and jasmonic acid at 50, and 100 µM as well as distilled water served as a control treatment, were utilized. The treatments were applied as foliar applications at 21, 38, and 35 days from germination. In the second experiment, four microbial inoculants (Azospirillum brasilense, Bacillus megaterium, Rhizobium leguminosarum bv. phaseoli, and Pseudomonas fluorescens), as well as uninoculation control. Avolume of 5 ml of each microbial was injected around seedlings that were 7 days old and was repeated three times at 15-day intervals. For both experiment, the treatments included three irrigation water levels (100, 80, and 60% of the estimated crop evapotranspiration, ETc), representing well–watered, moderately water– stressed, and severely water-stressed conditions.
The experimental design was a split plot with three replications. The irrigation water levels were randomly distributed in the main plots in both experiments, while the hormone derivatives in the first experiment or the
PGPR treatments in the second experiment were randomly dispersed in the subplots.
In both experiments, vegetative growth parameters (plant length, number of leaves per plant, leaf area, and fresh and dry weights of shoot and root systems), leaf mineral content (N, P, K, Ca, and Mg), SPAD readings, leaf relative water content, leaf membrane stability index, proline content, antioxidant enzyme activities (superoxide dismutase activity, peroxidase, and catalase), yield parameter measurements (pod length, pod diameter, and pod yield) and water use efficiency were recorded. Moreover, yield response factor was computed, and drought tolerance index was estimated. Principal component analysis was also performed.
The obtained results could be summarized as follows:
6.1. Effects of drought stress on the morphological, physiological, and biochemical traits of snap bean plants
- In both experiments, drought stress significantly decreased all measured vegetative growth parameters (plant length, leaf number per plant, leaf area, and fresh and dry weights of shoot and root systems) and resulted in significant reductions in leaf N, P, K, Ca, and Mg contents in both growing seasons. Also, drought stress had a significant negative effect on SPAD readings, relative water content and membrane stability index. The lowest values of all aforementioned parameters were obtained under severe drought stress. Elevated accumulations of proline content, and SOD, POD and CAT activities in snap bean leaves were recorded with increasing drought stress. Moreover, pod yield and its attributes were gradually and significantly reduced by increasing the drought severity.
- Drought tolerance index (DTI percentages) exhibited that proline content and SOD, POD, and CAT activities which have DTI% > 100%, were the traits most sensitive to drought conditions (under both 80 and 60% of ETc).
- Results of PCA demonstrated that there were strong negative correlations between vegetative growth parameters, physiological traits, and yield attributes on the one hand and proline and antioxidant enzyme activities on the other hand under water deficit.
6.2. Effects of some hormone derivatives on the morphological, physiological, and biochemical traits of snap bean plants under drought stress
- Under moderate drought conditions, brassinosteroid at 10 µM highly alleviated the harmful impacts effect on all measured growth parameters. Under severe drought conditions, all hormone derivatives relieved the harmful effect on all measured vegetative growth parameters when compared to the respective control. Jasmonic acid 50 µM was the most efficient treatment in mitigating the negative impacts of severe drought on all measured vegetative growth parameters.
- Under both moderate and severe drought conditions, brassinosteroid at 5µM was the most effective treatment in mitigating the drought impacts on leaf N, P, K, Ca, and Mg contents.
- Brassinosteroid at 5 µM and jasmonic acid at 50 µM were the most effective treatments in ameliorating the harmful effects of moderate drought conditions for both SPAD readings, and relative water content, while brassinosteroid at 5 µM was the most effective treatment for membrane stability index. Moreover, the effect was more pronounced under severe drought conditions since all hormone derivatives significantly minimized the damaging effects of drought stress on these parameters. Brassinosteroid at 5 µM was the most efficient treatment in alleviating the negative effects of severe drought conditions.
- Under moderate drought effect, all hormone derivatives resulted in significant accumulation of proline content compared with the respective control and brassinosteroid at 5 µM gave the significant highest values. For SOD activity, only brassinosteroid at 5 µM gave the significant value
in the first season, while there was no clear effect of hormones used on SOD activity in the second season. For POD activity, brassinosteroid at 5 µM, and jasmonic acid at 50 or 100 µM gave the highest significant values without significant differences. For CAT activity, only brassinosteroid at 5 µM had significant effect in the activity of this enzyme. Under severe drought conditions, all hormone derivatives showed significant effects on proline content and enzyme activities compared with the respective control. Brassinosteroid at 5 µM gave the highest significant values in both seasons.
- Brassinosteroid at 5 µM was the most effective treatment in ameliorating the negative effects of moderate drought conditions on pod length and pod diameter, while brassinosteroid at 10 µM was the most effective one in mitigating the harmful effects on both plant yield and WUE. Under severe drought conditions, brassinosteroid at 5 µM was the effective treatment in alleviating drought effects on pod length and diameter, while melatonin at 100 or jasmonic acid at 50 µM were the most effective ones for plant yield and WUE.
- The Ky factor values of all hormone derivatives used were smaller than those of the non-treated plants under the two water deficit levels, indicating the effectiveness of all applied hormone derivatives in mitigating the detrimental effects of drought stress.
6.3. Effects of some plant growth promoting rhizobacteria on the morphological, physiological, and biochemical traits of snap bean plants under drought stress
- PGPR inoculants significantly increased all recorded growth parameters compared with the check plants. Bacillus megaterium application significantly increased all these parameters and was more effective in attenuating the negative effect of severe water deficiency on vegetative growth characteristics in both seasons.
- PGPR application significantly enhanced N, P, K, Ca, and Mg contents compared with the check plants in both tested seasons. Bacillus megaterium significantly increased all elemental contents under well- watered conditions. Under moderate water level, P. fluorescens gave the highest N, Ca, and Mg contents compared to its respective control. P. fluorescens gave the highest N and P contents, while Bacillus megaterium gave the highest contents of K, Ca, and Mg under severe water conditions.
- PGPR inoculation positively minimized the harmful effects of drought stress and generated a stimulatory effect on SPAD readings when compared to the uninoculated plants. Inoculation with B. megaterium was more effective in comparison with the other rhizobacteria used. Compared with the respective controls, application of B. megaterium significantly enhanced SPAD readings under well-watered and severely watered conditions, while either R. leguminosarum or Pseudomonas fluorescens inoculation had the highest significant SPAD readings under moderate water level in both seasons.
- PGPR inoculation resulted in significant enhancements in LRWC and LMSI compared with non-treated plants. The application of Bacillus megaterium significantly improved LRWC and LMSI compared with other PGPR treatments. Either R. leguminosarum or Pseudomonas fluorescens significantly gave the highest values of LRWC and LMSI at moderate drought stress, while the application of B. megaterium significantly enhanced LRWC and reduced the damaging effect of drought on LMSI under severe water stress.
- PGPR inoculation significantly increased proline accumulation in comparison with the non-treated plants. Pseudomonas fluorescens application gave the highest significant accumulation of proline under moderate drought stress, while Bacillus megaterium significantly accumulated proline content under severe drought stress in both seasons.
- The application of PGPR markedly resulted in significant increments in SOD, POD, and CAT activities compared with the non-PGPR treatment. It is noteworthy that B. megaterium application resulted in the highest significant values of the antioxidant enzyme activities compared with the respective controls under all water treatments.
- PGPR applications positively enhanced the pod yield and its components under all water regimes. Bacillus megaterium inoculation showed the best values of these parameters under all water regimes, and its effect was more noticeable under severe drought. WUE progressively increased when decreasing the water application from 100 to 80% of ETc and then significantly decreased under severe drought stress (80% ETc). Bacillus megaterium application gave the best values of WUE under all water treatments and its application exhibited more pronounced under the severe drought condition.
- The Ky factor values of all PGPR were smaller than those of the non- treated plants under the two water deficit levels, indicating the effectiveness of all applied PGPR in alleviating the detrimental impacts of drought stress.
Conclusion and Recommendation
The results of both experiments showed that the growth and physio- biochemical attributes of snap bean were strongly affected under different deficit irrigation patterns that negatively affected the productivity.
In the first experiment, foliar applications of the used hormone derivatives enhanced all studied traits and mitigated the detrimental impacts of drought on the growth, yield, and quality of snap bean and improved drought stress tolerance of the plants. Brassinosteroid at 5 µM or jasmonic acid at 50 µM were the most effective treatment in ameliorating the negative effects.
In the second experiment, the addition of PGPR improved all studied traits and alleviated the adverse effects of drought on the growth, yield, and quality of snap bean and enhanced drought stress tolerance of the plants. In addition, the current study evidenced that Bacillus megaterium was the most efficient alleviator in mitigating the detrimental effects of drought.
These enhancements via hormone derivatives or PGPR applications resulted from enhancing root and shoot systems which promoted water and nutrient uptake, improving plant photosynthetic efficiency, enhancing leaf relative water content and leaf membrane stability index, and stimulating oxygen radical scavenging by proline and antioxidant enzyme accumulations.