The influence of some Bradyrhizobium and Rhizobium strains as plant growth promoting rhizobacteria on the growth and yield of sorghum (Sorghum bicolor L.) plants under drought stress

Abstract

Pot experiments were carried out during two successive seasons (1999 and 2000) to evaluate the efficiency of two Bradyrhizobium (B. japonicum) and Rhizobium (R. leguminosarum) strains as plant growth promoting rhizobacteria (PGPR) on the growth and yield of two sorghum cultivars (‘Dorado’ and ‘Shandawell’) under drought stress (100, 70 and 50 % field capacity — FC). The experimental treatments were inoculation of the strains with half dose of standard fertilizer (NPK), standard treatment and an untreated control. Two samples were taken to determine morphological parameters and chemical constituents (total and reducing sugar, N, P, K, chlorophyll a and b, IAA and GA₃) at 45 and 70 days after sowing. In this study, qualitative assessment of IAA, hydrogen cyanamide (HCN), siderophores production and solubilized phosphorus were tested by both strains under investigation. An electrolyte leakage (Na and K ions) and cell viability tests were used to measure the physiological effect of drought stress on sorghum plants in conjunction with morphological and yield. The results revealed that Rhizobium and Bradyrhizobium strains at 100 and 80 % FC did increase total count of bacteria and decrease total fungi on rhizosphere of sorghum. ‘Dorado’ cultivar did respond to R. leguminosarum as PGPR better that B. japonicum at 80 and 60 % FC, while ‘Shandawell’ gave higher response with B. japonicum at 80 %. This somehow, appears to agree with results of morphology and yield, as well as electrolyte leakage. Qualitative and quantitative of IAA and GA₃ indicated a greater presence of IAA in roots and GA₃ in shoots as a result of both strains inoculation if compared to control. Generally, it can be suggested that promoting effects of Rhizobium and Bradyrhizobium inoculation should be considered not only as symbiotic N₂-fixers for legumes but also as PGPR producers for legume and non-legume to increase shoots and yield under drought stress. This increase might be attributed to changes in many metabolic and physiological processes. Finally, cell viability and electrolyte leakage tests as well as chemical constituents can be used to select drought tolerant cultivars.