Differential effects of coinoculations with Pseudomonas jessenii PS06 (a phosphate-solubilizing bacterium) and Mesorhizobium ciceri C-2/2 strains on the growth and seed yield of chickpea under greenhouse and field conditions Angel Valverde 1 , Araceli Burgos 2 , Tiziana Fiscella 1,4 , Rau´l Rivas 3 , Encarna Vela´zquez 3 , Claudino Rodrı´guez-Barrueco 1 , Emilio Cervantes 1 , Manuel Chamber 2 & Jose´-Mariano Igual 1, * 1 Instituto de Recursos Naturales y Agrobiologı´a-CSIC, Apartado 257, 37071, Salamanca, Spain. 2 CIFA Las Torres-Tomejil, 41200, Alcala ´ del Rio, Sevilla, Spain. 3 Departamento de Microbiologı´a y Gene´tica, Universidad de Salamanca, 37007, Salamanca, Spain. 4 Agriculture Faculty, University of Catania, Via Valdisaboia 5, 95123, Catania, Italy. *Corresponding author Received: 31 May 2006 Key words: chickpea, Mesorhizobium, PGPR, phosphate-solubilizing bacteria, plant yield, Pseudomonas Abstract In the course of a project carried out in two regions of Spain, Castilla y Leo´ n and Andalucı´a, aiming to find useful biofertilizers for staple grain-legumes, an efficient rhizobia nodulating chickpea (termed as C-2/2) and a powerful in vitro phosphate-solubilizing bacterial strain (termed as PS06) were isolated. Analyses of their 16S rDNA sequence indicated that they belong to the bacterial species Mesorhizobium ciceri and Pseudomonas jessenii, respectively. Greenhouse and field experiments were carried out in order to test the effect of single and dual inoculations on chickpea (ecotype ILC-482) growth. Under greenhouse conditions, plants inoculated with Mesorhizobium ciceri C-2/2 alone had the highest shoot dry weight. The inoculation treatment with P. jessenii PS06 yielded a shoot dry weight 14% greater than the uninoculated control treatment, but it was not correlated with shoot P contents. However, the co-inoculation of C-2/2 with PS06 resulted in a decrease in shoot dry weight with respect to the inoculation with C-2/2 alone. Under field conditions, plants inoculated with M. ciceri C-2/2, in single or dual inoculation, produced higher nodule fresh weight, nodule number and shoot N content than the other treatments. Inoculation with P. jessenii PS06 had no significant effect on plant growth. However, the co-inoculation treatment ranked the highest in seed yield (52% greater than the uninoculated control treatment) and nodule fresh weight. These data suggest that P. jessenii PS06 can act synergistically with M. ciceri C-2/2 in promoting chickpea growth. The contrasting results obtained between greenhouse and field experiments are discussed. Introduction A substantial number of bacterial species, mostly those associated with the plant rhizosphere, may exert a beneficial effect upon plant growth (Glick, 1995). This group of bacteria has been termed ‘‘plant growth promoting rhizobacteria’’ or PGPR (Kloepper and Schroth, 1978). Although plant growth promoting rhizobacteria occur in soil, usu- ally their numbers are not high enough to com- pete with other bacteria commonly established in the rhizosphere. Therefore, for agronomic utility, inoculation of plants with target microorganisms * FAX No: +34-923-219609. E-mail: igual@usal.es Plant and Soil (2006) 287:43–50 Ó Springer 2006 DOI 10.1007/s11104-006-9057-8