الفهرس 
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Abstract
Soybean represents one of the relatively new crops introduced into Egyptian agriculture which participates in reducing the food deficiency gap because of its high quality ratio of protein and oil found in its seeds reaching up to 40% protein as well as round about 20% oil.
During the last twenty years soybean acreage has dramatically decreased in Egypt which mainly owing to competition with other summer crops, increased production cost, reduced net profit per unit area and difficulties in marketing process. As consequence, the total soybean production become below the requirement of consumption. Therefore, promotion of soybean plant growth, improvement its yield and enhancement its seed quality become an urgent demand to face the shortage of proteinaceous food as well as continuously growing population. For increase of soybean production, several strategies have been adopted such as increase production area, use of chemical fertilizer, organic manure and finally biofertilizer. Use of biofertilizers have many advantages such as their application in farming systems represent a recent trend, newly introduced in agricultural practice for getting clean products and putting an end to harmful effects of unwise doses of chemical fertilizers on human health and environment. Moreover, they are low cost, effective and renewable source of plant nutrients. In addition to their role in enhancing plant growth, they can act as biocontrol agents in the rhizosphere at the same time. In the past, bacteria and fungi have been received more attention compared to yeasts as biofertilizers, but recently rhizosphere soil yeasts have been attained growing attention as biofertilizers.
Obtained results of the current study on the effect of soybean seed coating with fresh yeast cells of some soybean rhizosphere soil yeasts on soybean seeds germination, promotion of its plant growth, improvement its seed yield and increase its seed quality significantly demonstrated the following points.
- Coating of soybean seeds with fresh yeast cells of each of rhizosphere yeast species Saccharomyces cerevisiae and Torulaspora delbrueckii increased percentage of seed germination and stimulated its early germination as well as lengths of seedlings and number of its root hairs in Petri dish experiments. S. cerevisiae was more effective on seed germination percentage and its early germination compared to T. delbrueckii, while T. delbrueckii had superiority for its effect on seedling lengths and number of its root hairs in comparison to S. cerevisiae.
- Obtained results of pot experiments demonstrated that all tested plant growth parameters were positively influenced by treatment of soybean seeds with fresh yeast cells of each of two tested yeast species. The most influenced plant growth parameters were photosynthetic pigments, leaf surrounding followed by lengths of seedling roots and shoots and their dry weights. Getting results also indicated that increase percentages of seedling shoot lengths and its dry weight were more influenced than those of seedling roots.
- Concerning the effect of soybean seed coating with fresh yeast cells on soluble, carbohydrate and protein of seedling roots and shoots and their potassium, sodium, calcium and magnesium contents in pot experiment, obtained results showed that all these traits were significantly positive influenced. Effect of treatment with S. cerevisiae was generally higher than those of T. delbrueckii in this regard. Moreover, soluble carbohydrate and protein as well as K, Ca and Mg, constitutents of seedling roots were higher than those of seedling shoots.
- A similar behaviour for effect of treatment with fresh yeast cells of the two tested yeast species on plant growth parameters and soluble, carbohydrates and protein contents of seedling root and shoot as well as their K, Na, Ca and Mg constituents was detected in field experiments.
- In regard to numbers of nodules per plant root and its dry weights and their influences by treatment with fresh yeast cells of each of S. cerevisiae and T. delbrueckii, getting results demonstrated that S. cerevisiae had superiority for its effect on numbers of nodules / plant root and its dry weight in comparison to its counterpart T. delbrueckii.
- Early flowering of treated soybean plants was obviously observed compared to that of untreated ones. Saccharomyces cerevisiae was more stimulant of early flowering than Torulaspora delbrueckii.
- Concerning yield traits of treated soybean plants with fresh yeast cells of each of S. cerevisiae and T. delbrueckii, earned results of our investigation elucidated that all tested yield traits of treated soybean plants such as number of branches/ plant, number of fruit clusters/plant, number of pods / plant, seed weight / plant and seed yield / feddan were positively influenced by treatments with fresh yeast cells of each of S. cerevisiae and T. delbrueckii. . Saccharomyces cerevisiae was more effective than T. delbrueckii in this respect.
Of interest was formation of four-seed-containing pods on the treated soybean plants which were lacking on the untreated ones. In addition, soybean treated plants exhibited early maturation of their pods and seeds. Saccharomyces cerevisiae also had a high superiority for early maturity as well as high number of four-seed-containing pods compared to Torulaspora delbrueckii.
- Regarding seed quality of treated soybean plants, results of chemical analysis of its seeds demonstrated that its total soluble and insoluble protein contents was obviously increased compared to those of untreated plant seeds. Moreover, the highest increase percentage was detected for soluble protein. In contrast, Tannic acid content of treated seeds was reduced. Concerning mineral content of treated seeds, obtained results showed that sodium, potassium and phosphate contents were higher than those of non-treated seeds. Noteworthy was that S. cerevisiae surpassed T. delbrueckii for improving soybean seed quality.
- Getting results of effect of different concentrations of ammonium molybdate on soybean plant growth in pot experiments demonstrated that different concentrations of ammonium molybdate slightly promoted soybean plant growth with different degrees. This promotion of soybean plant growth by ammonium molybdate became significantly higher when ammonium molybdate was combined with fresh yeast cells of each of S. cerevisiae and T. delbrueckii, especially at 7mmol concentration of ammonium molybdate in comparison to control plant. Saccharomyces cerevisiae was more effective on fresh and dry weight of plant roots and shoots, leaf surrounding and chlorophyll a, b, while T. delbrueckii had higher effect on soybean plant root and shoot lengths and carotenoid pigments. Of interest were increases percentage of soluble, carbohydrate and protein contents of plant roots and shoots, which were more pronounced in the presence of S. cerevisiae than in the presence of T. delbrueckii.
- Treatment of soybean seeds either with 0.1% L-tryptophan or with its combination with fresh yeast cells of each of the two tested yeast species exhibited a significant plant growth promotion which particularly more pronounced in case of the combination between 0.1% L-tryptophan and fresh yeast cells of each of the tested yeast species. Worthy of note was that the combination between 0.1% L-tryptophan and fresh yeast cells of T. delbrueckii was more effective on the promotion of soybean plant growth than its combination with that of S. cerevisiae (except for chlorophyll b). Whilst the combination of 0.1% L-tryptophan with fresh yeast cells of S. cerevisiae surpassed its combination with that of T. delbrueckii for its positive effect on soluble carbohydrate and protein contents of soybean plant roots and shoots. - Both of Saccharomyces cerevisiae and Torulaspora delbrueckii had the potentiality of indole-3-acetic acid production in a chemically defined medium supplemented with 0.1% L-tryptophan or without L-tryptophan. Indole-3-acetic acid production rate depended on yeast species and presence or absence of 0.1% L-tryptophan. Its production rate was higher in the presence of 0.1% L-tryptophan than in its absence. It is worth noting that T. delbrueckii exhibited higher production rate of indole-3-acetic acid in the presence of 0.1% L-tryptophan than S. cerevisiae. In the absence of 0.1% L-tryptophan, S. cerevisiae could surpass T. delbrueckii in this regard. The maximum indole-3-acetic acid production by the two tested yeast species was detected after 3 days incubation period in the presence of 0.1% L-tryptophan and after 5 days in its absence.
In vitro production of polyamine(s) by two tested yeast species in a chemically defined medium was determined and obtained results showed that both of two tested yeast species owned the capability of polyamine production in vitro and T. delbrueckii represented a higher polyamine producer in comparison to S. cerevisiae.
- Solubilization of insoluble inorganic phosphate (tricalcium phosphate) was conducted in vitro in a chemically defined medium using yeast species S. cerevisiae and T. delbrueckii and obtained results demonstrated that the two tested yeast species were capable of solubilizing tricalcium phosphate which was accompanied by dropping of the pH of the culture medium from 6.5 to 5.2-5.3. Torulaspora delbrueckii could dissolve it with higher rate than S. cerevisiae.
- Saccharomyces cerevisiae and Torulaspora delbrueckii could produce ammonia in peptone water broth. The production of ammonia by two tested yeast species can provide soybean plants with a good nitrogen source, particularly, in the soil deficient in nitrogen or with low N- content.
In conclusion, the two tested yeast species Saccharomyces cerevisiae and Torulaspora delbrueckii significantly succeeded in promotion of soybean plant growth, enhancement its seed yield and improvement its seed quality through their potentiality for production of phytohormons such as Indole-3-acetic acid, solubilization of inorganic phosphate, production of plant growth regulators such as polyamine(s), release of ammonia and improvement soil nutrient uptake. These yeast species represent successful candidates for ecofriendly, safe biofertilizers to support sustainable agricultural systems and field crop production.