The availability of soybean mutants with altered symbiotic properties allowed an investigation of the shoot or root control of the relevant phenotype. By means of grafts between these mutants and wild-type plants (cultivar Bragg and Williams), we demonstrated that supernodulation as well as hypernodulation (nitrate tolerance in nodulation and lack of autoregulation) is shoot controlled in two mutants (nts382 and nts ll6) belonging most likely to two separate complementation groups. The supernodulation phenotype was expressed on roots of the parent cultivar Bragg as well as the roots of cultivar Williams. Likewise it was shown that non-nodulation (resistance to Bradyrhizobium) is root controlled in mutant nod49. The shoot control of nodule initiation is epistatically suppressed by the non-nodulation, root-expressed mutation. These findings suggest that different plant organs can influence the expression of the nodulation phenotype.The development of N-fixing nodules on legume roots upon invasion of Rhizobium (or Bradyrhizobium) bacteria is subject to regulation by factors both external and internal to the plant host. In particular, the extent of nodulation is restricted by a process termed autoregulation, in which the formation ofnodules on one part of the root systemically inhibits subsequent nodule formation in other root regions (3,15). Nodulation is also severely restricted by the presence of nitrate in the soil (7). Our laboratory has recently isolated several soybean mutants with altered symbiotic features, including some tolerant to nitrate (nts3) which also supernodulate (3, 4), and others which do not form any nodules (nod-) (1, 8).Clearly nodulation is subject to control by plant factors; the sites of this control are unknown, although some experimentation has implicated shoot-root interactions (1 1-13). By means of grafts between these mutant and wild-type plants, we Table I were inoculated with B. japonicum strain USDA110 (approximately 107 bacteria per plant) and were cultured in 25 cm pots filled with vermiculite:sand mixture (1:2 ratio). Glasshouse temperatures were held between 14 and 30°C and incandescent 100 W bulbs extended the photoperiod to 16 h near summer conditions). The pots received 1.2 L of nutrient solution as described by Herridge (10) three times a week (Table I

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1989. Relationship between autoregulation and nitrate inhibition of nodulation in soybeans. -Physiol. Plant. 73: 37^2.Ten of 11 supernodulating mutants of soybean [Glycine max (L.) Merr.] cv. Bragg, in which nodulation was far in excess of that in the wild type, showed pronounced tolerance of nodulation to applied nitrate. Mutant nts (nitrate-tolerant symbiosis) 1116 had an intermediate nodulation response and also showed some inhibition by nitrate. Mutant 1029, a revertant of nts382 (an extreme supernodulator), showed a wild-type nodulation pattern and was equally sensitive to nitrate as cv. Bragg. Grafting experiments with cv. Bragg and nts382 indicated that both supernodulation and tolerance of nodulation to nitrate were dependent on shoot factors. Total leaf nitrate reductase (EC 1.6.6.1 and EC 1.6.6.2) activity of the supernodulating mutants was similar to that in cv. Bragg. We conclude from these results that the inhibitory effect of nitrate on nodule initiation and development in soybean depends on an interaction between nitrate and the autoregulation signal. In the supernodulating mutants, the autoregulation signal is either altered or absent and consequently nodulation in these mutants is not sensitive to nitrate.

Gene flow by pollen dispersal from forestry plantations containing introduced species, provenances or selected elite breeding material may impact on local native forest by changing the genetic diversity, introducing new genes or gene combinations, or causing the extinction of rare genotypes in adjacent native forest areas. Patterns of pollen flow can be used to assess the risk of genetic pollution of native forest areas from nearby plantations. Pollen flow in an artificial population of Eucalyptus grandis was estimated using molecular markers and paternity analysis. Microsatellite genotyping was used to identify pollen parents of progeny arrays from six mother trees. Of 329 progeny analysed, 178 (54%) were assigned to pollen parents within the population. Pollen parents located within the population were between 0-192 m from the respective mother trees, with an average pollination distance of 57.96 m. Pollination of mother trees was outcrossed, not by nearest neighbours, and displayed a preference for inter-provenance matings within the population. Progeny that could not be assigned pollen parents within the population (46%) were assumed to have resulted from pollen immigration from external sources. These pollen flow parameters provide useful information about the dynamics of pollen movement within E. grandis populations and may be used in risk assessment of gene flow from plantations to adjacent areas of native forest.

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Recognition of genetic structure of populations and the ability to identify vulnerable populations is useful for the formation of conservation management strategies for plants. Eucalyptus grandis is a tall forest tree that has a major area of occurrence in subtropical eastern Australia, with smaller populations located in the east coast tropics. Many widespread forest species exhibit population differentiation that corresponds to geographic regions. However, Eucalyptus grandis appears to be an exception based on isozyme and morphological data. This is intriguing given a large discontinuity between northern populations and those in the southern part of the species range. In this study, the distribution of a maternally inherited chloroplast locus was examined because it was more likely to reveal genetic structure due to the slower evolution of the chloroplast genome and limited dispersal of seed in eucalypts. As expected, the G ST for chloroplast DNA was higher than that for nuclear DNA but indicated low population differentiation for a forest tree species. Phylogeographic analysis indicated that the 15 populations grouped into three broad geographical regions; however, overall population structure was weak suggesting that the large geographical disjunction in the distribution of E. grandis may be relatively recent. A paradigm for conservation management of E. grandis based on chloroplast DNA haplotype distribution would take into account the low differentiation among populations.

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