Morphological and anatomical factors such as aerenchyma formation in roots and the development of adventitious roots are considered to be amongst the most important developmental characteristics affecting flooding tolerance. In this study we investigated the lengths of adventitious roots and their capacity to form aerenchyma in three-and four-week-old seedlings of two maize (Zea mays ssp. mays, Linn.) inbred accessions, B64 and Na4, and one teosinte, Z. nicaraguensis Iltis & Benz (Poaceae), with and without a flooding treatment. Three weeks after sowing and following a seven day flooding treatment, both maize and teosinte seedlings formed aerenchyma in the cortex of the adventitious roots of the first three nodes. The degree of aerenchyma formation in the three genotypes increased with a second week of flooding treatment. In drained soil, the two maize accessions failed to form aerenchyma. In Z. nicaraguensis, aerenchyma developed in roots located at the first two nodes three weeks after sowing. In the fourth week, aerenchyma developed in roots of the third node, with a subsequent increase in aerenchyma in the second node roots. In a second experiment, we investigated the capacity of aerenchyma to develop in drained soil. An additional three teosinte species and 15 maize inbred lines, among them a set of flooding-tolerant maize lines, were evaluated. Evaluations indicate that accessions of Z. luxurians (Durieu & Asch. Bird) and two maize inbreds, B55 and Mo20W, form aerenchyma when not flooded. These materials may be useful genetic resources for the development of flooding-tolerant maize accessions.

Using a 141 F 2 population generated from maize inbred B64 £ teosinte Zea nicaraguensis cross, quantitative trait loci (QTLs) controlling aerenchyma formation in roots under non-Xooding drained soil conditions were identiWed. Seedlings of Z. nicaraguensis formed clear aerenchyma in the cortex of adventitious roots in non-Xooding conditions, whereas the maize inbred line B64 did not. In the F 2 population, the capacity to develop aerenchyma exhibited wide and continuous variation, suggesting the trait was controlled by multiple genes. A linkage map was developed using 85 SSR markers, covering 1,224 cM across all ten chromosomes. Composite interval mapping analysis revealed that four QTLs for aerenchyma formation under non-Xooding conditions were located to two regions of chromosome 1 (identiWed as Qaer1.02-3 and Qaer1.07), chromosome 5 (Qaer5.09) and chromosome 8 (Qaer8.06-7), and these explained 46.5% of the total phenotypic variance. The multiple interval mapping approach identiWed additional QTLs on chromosomes 1 (Qaer1.01) and 5 (Qaer5.01). Using these results, it may be possible to use SSR markers linked to aerenchyma formation in a marker assisted selection approach to introduce aerenchyma formation in drained soil conditions into maize for the eventual development of Xooding tolerant maize hybrids.

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Quantitative trait locus (QTL) analyses were performed to map the genes controlling adventitious root formation on the soil surface (ARF-SS) under flooding conditions in seedlings of 317 BC 3 F 1 individuals derived from a cross between elite maize Mi29 x teosinte Zea nicaraguensis. An SSR-based linkage map was developed using 94 markers, covering 896.3 cM of the ten chromosomes. The ability of ARF-SS under flooding conditions showed continuous variation in the BC 3 F 1 population. By single point regression and interval mapping analyses, the QTLs for ARF-SS were located on chromosomes 3 (bin 3.04), 7 (bin 7.04) and 8 (bin 8.03). Alleles of Z. nicaraguensis, which has a high ability of ARF-SS, increased the level of ARF-SS for all the QTLs. By comparing chromosome positions of ARF-SS loci to previously reported loci, the region on chromosome 3 was shown to be unique to this teosinte. A possible application of the new QTL to breed flooding tolerant maize is discussed.

1. A practical chamber method has been devised for enumerating the megakaryocyte concentration of aspirated bone marrow. 2. Marrow aspirations from 23 healthy young adults had a mean megakaryocyte concentration of 4.0 ± 1.1 per 10,000 total nucleated cells or 6.1 ± 1.6 per 10,000 granulocytes. 3. Of the three principal cell types in marrow, i.e. granulocytes, erythrocytes and lymphocytes, the megakaryocyte content seemed to parallel most closely the granulocytes.