Water relations in plants treated with growth promoting rhizosphere bacteria

BackgroundPlant growth promoting rhizobacteria (PGPR) often increase leaf area. Since water loss by transpiration is proportional to leaf area, adaptive responses will be required if further growth is not to be compromised by plant water deficits. However, studies of water relations of plants growing in soil enriched with PGPR have yielded conflicting results and an analysis of the causes of this contradiction are needed if PGPR application is to be utilized successfully in practical farming.ScopeWe discuss the effects of PGPR on stomatal conductance, transpiration, leaf relative water content, leaf water potential, root growth and hydraulic conductance. We assess their importance for maintaining water balance and sustaining and promoting plant growth following bacterial inoculation. We also assess the involvement of PGPR on hormone concentrations in planta (primarily abscisic acid, ABA) in regulating these processes.ConclusionsPGPR exert variable effects on plant water relations. For example, both increased and decreased stomatal conductance and relative water content have been reported. Effectiveness of these alternative responses is discussed in relation to soil water availability. Plant responses to PGPR can be classified in terms of: (1) stimulating root growth thereby increasing capacity for water uptake and compensating for increased transpirational load (2) promoting stomatal closure and reduced transpiration, (3) increased activity of water channels (AQPs) in roots resulting in greater hydraulic conductivity; (4) osmotic adjustment that helps maintain leaf turgor under decreased leaf hydration.;