الفهرس 
Plant responses to droughtOnly 14 pages are availabe for public view
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Abstract
Nowadays, the key challenge of agriculture is to secure enough food for the worldwide rapidly-growing populations, especially with the current and aggravating fresh water shortage. The plant growth promoting rhizobacteria (PGPR) can offer a promising, feasible, lowcost and sustainable solution to reduce the yield losses under drought stress. Accordingly, this work mainly aimed to enhance the growth and productivity of a drought-sensitive, but high-yielded and widecultivated wheat cultivar under normal and drought-stressed conditions using selected promising PGPR. The preliminary experiments were conducted to select the most drought sensitive wheat cultivar and the most promising PGPR for inducing drought tolerance in wheat plants. To select the most droughtsensitive wheat cultivar out of three tested ones (Gemiza 9 (G9), Sids1 (S1) and Sids 12 (S12)), two experiments were conducted using five concentrations of PEG6000 (0, 10, 20, 25 and 30%). The results showed that G9 was the most drought sensitive cultivar, as it had the lowest percentage of germination and the highest percentages of reduction in root and shoot fresh and dry masses and water contents. On the other hand, to select the most promising PGPR, 21 isolates/strains were screened for their ability to fix nitrogen, to tolerate drought stress using three concentrations of PEG6000 (20%, 25% and 30%) and to promote growth of wheat seedlings under drought-stressed (17 days of ceasing irrigation) and stress-relief (10 days of reirrigation) conditions. As a result, the isolate B11 and the strain Azospirillum brasilense NO40 (B17) were selected, because both of them showed the highest nitrogen fixation activities and maintained high growth in media supplemented with PEG6000 up to 30%. Moreover, they enhanced the growth of wheat seedlings under drought stress, in addition to inducing significant recovery for them represented by the healthier phenotypes and the higher FMs, DMs, WCs and survival percentage, implying that both PGPR were compatible with the whole system into which they were introduced (the host plant, the soil type, and the imposition and relief of the tested stress) and therefore they induced the desired impact efficiently. The selected PGPR were characterized for colony and cell morphology, Gram staining, motility, pigment production, oxygen requirement and catalase activity. The isolate B11 was identified as Stenotrophomonas maltophilia using the 16s rDNA sequence analysis. Thereafter, both PGPR were subjected to a set of in vitro experiments to study their responses to drought stress by investigating: 1) their survival under higher concentrations of PEG (up to 50%); 2) the effect of 25% PEG on their growth rate, culture pH and their plant growth promoting traits (N2 fixation, ACC deaminase activity, P-solubilization and production of HCN, ammonia, and phytohormones (IAA, GA and ABA) and, 3) their capacity to enhance the tolerance of wheat seedlings to drought stress in sterilized Spermosphere model. The results showed that both A. brasilense NO40 and S. maltophilia strains could survive concentrations of PEG up to 50%. The cultures’ pH was slightly alkaline over time and the cell densities were lower in the PEG-containing cultures. However, the selected PGPR were able to maintain their plant growth-promoting abilities under stress, and consequently enhanced the lengths, FMs, DMs and the relative water contents (RWC) of the wheat seedlings growing under normal and drought-stressed conditions. The main greenhouse experiment was carried out to study the impact of the two selected PGPR (A. brasilense NO40 and S. maltophilia) on the growth and productivity of wheat plants (cv. G9) grown in unsterilized sandy soil under normal and drought-stressed conditions. Samples were collected at seedling stage (15 days after sowing) and at yield stage (120 days after sowing). At the seedling stage, the pH, EC and the activities of dehydrogenase (DHA) and nitrogenase (N2ase) enzymes were determined in the rhizospheric soil. Also, the seedlings’ growth criteria (lengths, FMs, DMs and RWCs of roots and shoots and the area, FM, DM and RWC of the second-node leaf) were measured in addition to determining the photosynthetic pigments content as well. To evaluate the drought-induced damage, the leakage of electrolytes and the contents of H2O2 and malondialdehyde (MDA) were estimated in leaves, while the defense responses was studied by measuring the ascorbic acid (A.A) content in leaves, and the activities of catalase (CAT) and peroxidase (POX) enzymes, and proline content in roots and shoots. At the yield stage, some productivity criteria were measured (the percentage of spike formation/pot, spike weight (g), percentage of grain filling/plant, weight of grains/plant (g) and weight of 100 grains (g)), besides estimating the contents of carbohydrates, total soluble proteins and free amino acids in the yielded grains. The results showed that drought stress slightly shifted the pH of the rhizospheric soil to be more alkaline, especially with the PGPR presence. Meanwhile, the soil EC was reduced under drought stress in all treatments, but the significant reductions were obtained in the soil of the inoculated treatments. Also, drought stress significantly inhibited the activities of DHA and N2ase enzymes in the soil of the uninoculated seedlings, while their activities were less affected in their inoculated counterparts. Furthermore, the DHA activity in the soil of A. brasilense-treated-drought-stressed seedlings was surprisingly increased to record the maximum activity among all treatments. The onset of drought stress during the seedling stage, resulted in a substantial reduction in the growth of wheat seedlings, represented by significant decreases in most of the measured criteria, obvious wilting, as well as the reductions in the contents of the photosynthetic pigments (chl-a and chl-b) and ascorbic acid. Despite the enhanced production of proline and activities of CAT and POX, the drought stress induced a significant damage to the uninoculated wheat seedlings, illustrated by a notable increase in the electrolytes’ leakage, and accumulation of MDA and H2O2, suggesting that the plant defenses were not enough to cope with the damage. Consequently, a remarkable decline was obtained in all of the measured yield parameters in these plants, where the yielded grains had less amounts of sucrose, starch and higher contents of the direct reducing sugars, total soluble proteins and the total free amino acids than that of the inoculated-stressed plants.