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Little is known about mechanisms that permit or prevent formation of root nodules in which nitrogen gas (N 2 ) is fixed by rhizobial bacteria in association with nursery crops in the legume family. We tested the hypothesis that two genes associated with the fi xation of N 2 could be used as markers of nodulation capacity among legumes because they occur only in species that can form functional nodules. The presence of ENOD2 and ENOD12 was tested in genomic DNA from three non-nodulating legumes [ C rcis canadensis L. (eastern redbud), Gleditsia triacanthos L. var. inermis Willd. (thornless honeylocust), Gymnocladus dioicus (L.) K. Koch (Kentucky coffeetree)] and two nodulating legumes [ Albizia julibrissin Durazz. (silk-tree), Laburnum alpinum (Mill.) Bercht. & Presl. (Scotch laburnum)]. Southern analyses indicated that ENOD2 is present in thornless honeylocust, Kentucky coffeetree, and Scotch laburnum, and that ENOD12 is present in eastern redbud, thornless honeylocust, and Scotch laburnum. These results diversify the group of nodulating legumes in which ENOD2 and ENOD12 have been found and show these genes also occur in legumes considered incapable of nodulation. We conclude that neither gene can be used to screen existing or new leguminous nursery crops for the capacity to form N 2 -fixing nodules. Index words: nutrition, symbiosis, sustainable production. Species used in this study: Cercis canadensis L. (eastern redbud), Gleditsia triacanthos L. var. inermis Willd. (thornless honeylocust), Gymnocladus dioicus (L.) K. Koch (Kentucky coffeetree), Albizia julibrissin Durazz. (silk-tree), Laburnum alpinum (Mill.) Bercht. & Presl. (Scotch laburnum), Maackia amurensis Rupr. & Maxim. (Amur maackia). Significance to the Nursery Industry Efficient management of nitrogen (N) inputs during production of nursery crops is increasingly important as efforts to protect the environment from damage caused by excess N intensify. After nursery crops are installed in the landscape, N often is the only mineral element that limits their growth. Crops that can form root nodules in which bacteria fix N 2 gas from the atmosphere have the potential to be produced with relatively little N fertilizer. Such plants might also be sustained with relative ease in landscapes where soil is of poor nutritive quality and no N is applied. Yet little is known about the genetics and physiology of N 2 fixation in nursery crops that may have this potential, most of which are in the legume family. A broad objective of our research program is to identify and strive to overcome barriers that prevent development of effective nodules in legumes important in the nursery and landscape industries. This paper reports research designed to test for the occurrence of two genes associated with N 2 fixation in several leguminous tree species. Some legumes we studied form root nodules in which N 2 is fixed, whereas others, which never have been observed with nodules, are considered incapable of associating with N 2 -fixing bacteria for reasons unknown. Nursery and landscape proReceived for publication February 26, 1999; in revised form June 17, 1999. Supported in part by The Horticultural Research Institute, 1250 I Street NW, Suite 500, Washington, D.C. 20005 . Dr. Ton Bisseling (Agricultural University, Wageningen, Netherlands) generously provided the clone of ENOD12 cDNA. Journal Paper No. J-18295 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, and supported by Hatch Act and State of Iowa funds. Professor, Department of Horticulture. Postdoctoral Research Associate, Department of Horticulture. Graduate Research Assistant, Department of Horticulture. fessionals will benefit from this research by gaining information on the potential value of N 2 fixation and the present understanding of the nodulation capacity of some common nursery crops. Moreover, this research illustrates a facet of fundamental research that eventually may lead to a broader palate of species that can be produced and maintained with minimal inputs of N.

Two Early Nodulation Genes are not Markers for the Capacity of Leguminous Nursery Crops to Form Root Nodules