RESUMEN
A detailed genetic map has been constructed in apple (Malus x domestica Borkh.) in the region of the v f gene. This gene confers resistance to the apple scab fungus Venturia inaequalis (Cooke) Wint. Linkage data on four RAPD (random amplified polymorphic DNA) markers and the isoenzyme marker PGM-1, previously reported to be linked to the v f gene, are integrated using two populations segregating for resistance to apple scab. Two new RAPD markers linked to v f (identified by bulked segregant analysis) and a third marker previously reported as being present in several cultivars containing v f are also placed on the map. The map around v f now contains eight genetic markers spread over approximately 28 cM, with markers on both sides of the resistance gene. The study indicates that RAPD markers in the region of crab apple DNA introgressed with resistance are often transportable between apple clones carrying resistance from the same source. Analysis of co-segregation of the resistance classes 3A (weakly resistant) and 3B (weakly susceptible) with the linked set of genetic markers demonstrates that progeny of both classes carry the resistance gene.
RESUMEN
Exposure of plants of Sorghum bicolor L. line E 1287 and hybrid NK 145, of Digitaria smutsii Stapf, and of Paspalum dilatatum L.-grasses having the C4 pathway of photosynthesis-to temperatures of 4°, 2.5°, 2.5° and-3°, respectively, for a single night caused the formation within 36 h of transverse, irreversibly chlorotic bands on emerging leaves. Chlorosis was associated with the presence of chlorophyll-deficient, structurally abnormal plastids in most mesophyll cells, whereas chloroplasts in adjacent bundle-sheath cells were green and possessed a normal lamellar structure. The ultrastructure of other organelles in the chlorotic mesophyll cells appeared normal and the levels of cytoplasmic rRNA, non-plastid lipids and isocitrate dehydrogenase were similar or slightly higher in chlorotic compared with green lamina. The content of chloroplast rRNA and plastid lipids in the chlorotic tissue was low. Activities of all C4-pathway enzymes examined were lower in the chlorotic tissue but phosphopyruvate carboxylase, NADP-malate dehydrogenase and adenylate kinase were reduced to a greater extent than ribulose diphosphate carboxylase, fructose diphosphate aldolase and malic enzyme. The region of emerging leaves that contained mesophyll plastids susceptible to low temperature was identified and the stages of plastid ontogeny below, within and above this region examined. During normal differentiation rapid increases in plastid size and in the content of chloroplast rRNA, chlorophyll, plastid lipids and in the activities of C4-pathway enzymes occur at a developmental stage subsequent to that at which plastids are susceptible to low temperature.