Water-retentive polymers increase nodulation of actinorhizal plants inoculated with Frankia

Actinorhizal plants form a nodular, nitrogen-fixing root symbiosis with the actinomycete Frankia and are economically and ecologically important due t o their ability to improve the nitrogen fertility of disturbed and infertil e substrates. In this study, water-retentive polymer inoculum carriers were applied as a root dip. This treatment significantly increased nodulation a nd in some cases early growth of Alnus glutinosa (L.) Gaertn. and Casuarina equisetifolia var. equisetifolia Forst. & Forst. in a controlled environme nt and also of A. glutinosa under field conditions. Nodule number and nodul e dry weight per plant were at least two to three times greater after 56 to 140 days for plants inoculated with Frankia carried in a water-retentive p olymer base compared with plants inoculated with Frankia in water. Nodules on the roots of the plants that were inoculated with Frankia in a polymer s lurry were distributed throughout the entire root system, rather than conce ntrated near the root collar. When amended with water-retentive polymers, a ctinorhizal plants inoculated with 5- to 10-fold lower titers of Frankia ex hibited early growth and nodule numbers equal to or greater than those plan ts inoculated with standard titers without polymers. The water-retentive, s uperabsorbent polymers clearly increased the nodulation of two actinorhizal plant species.