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Introduction: Communities dominated in the canopy by pinyon pines harbor species richness and have complex physiognomy because they form an ecotone between arid and temperate zones. Objectives: To describe the floristic attributes that distinguish four communities dominated by pinyon pines; define units based on floristic composition and structure; and determine the influence of edaphic factors. Materials and methods: The flora of four communities in the state of Zacatecas was recorded. The arboreal and shrub vegetation was analyzed quantitatively using the point-centered quarter technique. Semi-realistic physiognomic profiles were made for each site. The groups resulting from cluster analysis were ordered with edaphic factors through a canonical correspondence analysis. Results and discussion: Floristic richness was 244 species; each community contains between 60 and 80 species. Structurally, three physiognomic variants were recorded: forests dominated by Pinus cembroides Zucc.; association of P. cembroides - P. cembroides var. bicolor Little; and forest dominated by P. maximartinezii Rzed. The pH, electrical conductivity and soil texture have a great influence on the structure and plant composition of the communities. The P. cembroides - P. cembroides var. bicolor association and its accompanying species develop in nutrient-poor soils, while P. maximartinezii forests prefer shallow soils. Conclusion: The floristic similarity among communities was low. Each community must be managed differently, meeting the particular requirements of the species that develop there.

Pinus maximartinezii (Rzedowsky) and P. pinceana (Gordon), two endemic species of Pinus from Mexico, were successfully regenerated in vitro by organogenesis starting from mature embryos. In both species, 100% of the explants produced adventitious buds after an 18-day induction period on media containing 6-benzylaminopurine (BA) and a subsequent 8 weeks of incubation on growth regulator-free media. The most successful treatment (13.3 µM BA) resulted in an average of 14.6 and 25.9 buds per explant in P. maximartinezii and P. pinceana, respectively. Rooting was achieved by auxin pulses or by Agrobacterium rhizogenes inoculation. In the first case, roots formed on 13% and 7% of the auxin-treated shoots of P. maximartinezii and P. pinceana, respectively. In the second case, A. rhizogenes induced root formation on 60% and 67% of the inoculated shoots from P. maximartinezii and P. pinceana, respectively. The presence of the foreign genes transferred by A. rhizogenes into these roots was demonstrated by histochemical staining for β-glucuronidase activity, the polymerase chain reaction and Southern blot analysis.

Pinus pinceana) has a restricted and fragmented range, trees are widely scattered within populations, and reproduction is limited. Nevertheless, genetic diversity was high; based on 27 isozyme loci in 18 enzyme systems, unbiased expected heterozygosity averaged 0.174. Differentiation also was high (FST 5 0.152), reflecting isolation between southern, central, and northern fragments of the range. Among populations in the northern fragment, FST was only 0.056, and the number of migrants per generation (Nm) was 4.21, which should preclude fixation. Nm between central and southern populations or between them and populations in the northern fragment was lower, 0.99‐1.66, indicating a degree of genetic isolation. Multilocus outcrossing rates ( tm) ranged from 0.836 in the south to 0.897 in the north. Therefore, selfing is low but statistically significant. The equilibrium inbreeding coefficient

Wood from 92 trees of Pinus culminicola, Pinus nelsonii, Pinus pinceana and Pinus maximartinezii from nine localities and oleoresin from several trees of Pinus nelsonii from one Iocality were analyzed for monoterpenes by gas liquid chromatography. Monoterpenes of Pinus nelsonii were composed mainly of α- and β-pinenes, while the main constituent of the monoterpenes of Pinus pinceana and Pinus maximartinezii was limonene, present in amounts of about 90 percent. The similarity in composition of the monoterpenes of Pinus pinceana and Pinus maximartinezii strongly supports the contention of Rzedowski [12] of the close evolutionary relation of the two pines. Pinus culminicola wood produced monoterpenes composed mainly of α-pinene, sabinene, terpinolene and limonene, with other monoterpenes occurring sporadically in sizeabie amounts. The variability between individual populations was relatively high, probably due to their long-time isolation by the intervening low-elevation barriers. Appreciable differences were found in compositions of monoterpenes from stem and from twigs of Pinus culrninicola.