Cell sizes are linked across multiple tissues, including stomata, and this variation is closely correlated with genome size. These associations raise the question of whether generic changes in cell size cause suboptimal changes in stomata, requiring subsequent evolution under selection for stomatal size. We tested the relationships among guard cell length, genome size and vegetation type using phylogenetically independent analyses on 67 species of the ecologically and structurally diverse family, Proteaceae. We also compared how genome and stomatal sizes varied at ancient (among genera) and more recent (within genus) levels. The observed 60-fold range in genome size in Proteaceae largely reflected the mean chromosome size. Compared with variation among genera, genome size varied much less within genera (< 6% of total variance) than stomatal size, implying evolution in stomatal size subsequent to changes in genome size. Open vegetation and closed forest had significantly different relationships between stomatal and genome sizes. Ancient changes in genome size clearly influenced stomatal size in Proteaceae, but adaptation to habitat strongly modified the genome–stomatal size relationship. Direct adaptation to the environment in stomatal size argues that new proxies for past concentrations of atmospheric CO2 that incorporate stomatal size are superior to older models based solely on stomatal frequency.

The paper reviews a decade of progress with breeding new polyploid varieties of Acacia mangium and other tropical acacias in Vietnam, and outlines plans for future strategies to complement conventional diploid breeding. Polyploid breeding is a means of introducing diversity into breeding populations, of making step-change improvements in wood fibre and other properties, and reducing reproductive output of these potentially weedy taxa. To date we have demonstrated that A. mangium tetraploids have larger and thicker wood fibres and we have strong preliminary evidence that the reproductive output of triploids is indeed reduced. Other potential benefits such as wind and stress tolerance and disease resistance are under field evaluation. Trials demonstrate that colchicine-induced tetraploids, whether derived from pure A. mangium seeds or from commercial hybrid clones, are substantially slower growing than diploids. While not the sole determinant of crop value, volume production is hugely important to growers, so long-term success of the program is critically dependent upon demonstration that this problem can be resolved in advanced generations. Future emphasis will be on production of triploid and tetraploids from the interspecific hybrid A. mangium  A. auriculiformis. The better rooting ability contributed by A. auriculiformis maximises prospects for operational cloning of select trees. Highly heterozygous hybrids also offer the better prospect for exploiting heterosis. A strategy for developing a diverse breeding population of neo-tetraploid F 1 hybrids is presented.

No abstract

The frequency of polyploid trees in 10 populations of the predominantly diploid species Acacia dealbata subsp. dealbata Link in south-eastern Tasmania was determined using flow cytometry. At seven of the sites, all trees were diploid. At two sites, single triploid genets were found and, at a third, two tetraploids. Microsatellite markers were used to confirm that triploid trees distributed over an area of at least 930 m2 at the major study site were all ramets of a single genet. Three diploid clones were also confirmed at this site. The 16-grain polyads from the triploid genet were significantly larger than those from diploids, but only the diploid pollen showed any viability in vitro. At three months the green pods on diploids averaged 36 mm and contained four developing seeds per pod. Pods on the triploid were only 13 mm long, with no developing seeds. In spite of maturing pods, two of the diploids did not yield full seed, whereas two other diploids averaged only 0.1 seeds per pod. Seeds were not produced on triploid trees. The low reproductive output is discussed with reference to the breeding system and the impact of clonality on effective cross-pollination.

Diploid clones of the hybrid acacia (Acacia mangium × A. auriculiformis) are widely planted in Vietnam because of their high productivity, adaptability, and commercial wood yields. Polyploid breeding offers possibilities for further enhancing hybrid vigor and generating new high value genotypes. In a field trial, we compared three diploid hybrid clones with their respective colchicine-induced tetraploid lines. Flowering and seed production of each cytotype were observed and open pollinated seed collected for determination of outcrossing rate and ploidy, inbreeding depression and marker inheritance in the progeny. Comparisons are also made with published characteristics of autotetraploids derived from A. mangium. Compared with their corresponding diploid cytotypes, the allotetraploids flowered slightly later but more intensely; produced the same number of seeds per pod but larger seeds; and showed a greatly reduced level of outcrossing (an average of 14% compared with 87%). Inbreeding depression for height growth was less for progeny from the allotetraploid lines (17%) than for those from the original diploids (33%). 96% of seeds from the allotetraploid clones were also tetraploid, but we observed triploids at low frequency at both the seed and field progeny stages. The segregation of the molecular markers in outcrossed allotetraploid progenies demonstrated both disomic and tetrasomic inheritance, indicating that the hybrid behaves as a segmental allotetraploid. Results suggest that an open pollinated breeding strategy is a practical option for improving polyploid acacia hybrids.