RESUMENUna primera mirada al suelo muestra una homogeneidad relativa, sin embargo, existe una comunidad subterránea con cadenas tróficas complejas y diversas, mantenidas por los productos de las raíces en la rizósfera. Las raíces son una fuente de complejos recursos que varían química y morfológicamente, en interacción con la microflora y fauna del suelo encargados de la descomposición y mineralización de la materia orgánica. El estudio de los microorganismos en el suelo es un reto, pues son variadas las técnicas y metodologías que se requieren para ello. Este documento presenta una revisión de las fracción orgánica del suelo, su rol y experiencias de la incorporación de materia orgánica al suelo.Palabras clave: Materia orgánica, cadenas Aróficas, comunidad subterránea, microflora y fauna del suelo. ABSTRACT Soils appear to be relatively homogeneous at first glance; however an underground community exist with complex, long and diverse trophic webs, supported by the rhizosphere (root and root by-products INTRODUCCIÓNEl uso de materia orgánica se ha convertido en la base para el desarrollo de agricultura orgánica. Sin embargo, es un error considerar que agricultura orgánica es simplemente "no usar productos sintéticos". La agricultura orgánica debe considerar dos aspectos esenciales: (a) la diversidad estructural y de procesos, y (b) el manejo ecológico del suelo y nutrición (Brenes, 2003). Por ello, teniendo en cuenta la importancia del suelo en este proceso, este documento presenta una revisión sobre el rol de la fracción orgánica y las experiencias de la aplicación de materia orgánica en los suelos agrícolas.El hombre ha aplicado toda clase de materias orgánicas a los suelos cultivados. Durante 150 años los fisiólogos mantuvieron la teoría húmica, que consideraba que las plantas se nutrían directamente del humus del suelo y la presencia de este material marcaba su fertilidad . Sin embargo, la revolución agrícola promovida en el siglo XIX por Justus von Liebig (1843) demostró que las plantas precisan de agua y sustancias inorgánicas para su nutrición y puso en duda que el humus fuera el principio nutritivo de las plantas. Además, fomentó el desarrollo de los fertilizantes inorgánicos, que son de 20 a 100 veces más concentrados en elementos básicos como N, P, K, que los abonos orgánicos (Arens, 1983), lo que supuso

Responses to prolonged drought and recovery from drought of two South American potato (Solanum tuberosum L. ssp. andigena (Juz & Buk) Hawkes) landraces, Sullu and Ccompis were compared under field conditions. Physiological and biomass measurements, yield analysis, the results of hybridisation to a potato microarray platform (44 000 probes) and metabolite profiling were used to characterise responses to water deficit. Drought affected shoot and root biomass negatively in Ccompis but not in Sullu, whereas both genotypes maintained tuber yield under water stress. Ccompis showed stronger reduction in maximum quantum yield under stress than Sullu, and less decrease in stomatal resistance. Genes associated with PSII functions were activated during recovery in Sullu only. Evidence for sucrose accumulation in Sullu only during maximum stress and recovery was observed, in addition to increases in cell wall biosynthesis. A depression in the abundance of plastid superoxide dismutase transcripts was observed under maximum stress in Ccompis. Both sucrose and the regulatory molecule trehalose accumulated in the leaves of Sullu only. In contrast, in Ccompis, the raffinose oligosaccharide family pathway was activated, whereas low levels of sucrose and minor stress-mediated changes in trehalose were observed. Proline, and expression of the associated genes, rose in both genotypes under drought, with a 3-fold higher increase in Sullu than in Ccompis. The results demonstrate the presence of distinct molecular and biochemical drought responses in the two potato landraces leading to yield maintenance but differential biomass accumulation in vegetative tissues.

No abstract

Orange‐fleshed sweetpotato [Ipomoea batatas (L.) Lam.] (OFSP) breeding populations have gained importance for food security and health reasons. This study's main objectives were to determine genetic diversity in parental material of two OFSP populations developed in Peru (Jewel [PJ] and Zapallo [PZ]) relative to mega‐clones (MCs) using agronomic traits and simple sequence repeat (SSR) markers and to determine whether PJ and PZ are mutually heterotic by developing a PJ × PZ hybrid population (H0). Field trials were performed with clones for PJ (n = 49), PZ (n = 31), MC (n = 21), and H0 (n = 6,898) in Peru. Traits recorded were storage root yield (RYTHA), number of commercial roots per plant, foliage yield, biomass, harvest index, and dry matter (RDM), β‐carotene (RBC), protein, starch, sucrose, iron, zinc, and calcium content of storage roots. Sixty‐six pairs of SSR primers were used to determine molecular diversity. Statistics used were linear mixed models, principal component analysis, and standard procedures for molecular data. New genetic variation was found in PJ and PZ (e.g., RDM ≥29% with RBC ≥25 mg 100 g−1 dry weight basis). For most traits, genetic variance in PJ and PZ was as large as in MC. The SSR marker data clearly separated PJ and PZ into two gene‐pools, together covering nearly the entire MC molecular diversity. Average RYTHA in H0 was high (40.7 t ha−1) with average heterosis increment of 21.8% and range −30.6 to 139.4%. The PJ and PZ lend themselves to study of the efficiency of reciprocal recurrent selection in sweetpotato population hybrid breeding.

Andean tuber crop species oca (Oxalis tuberosa Molina), ulluco (Ullucus tuberosus Caldas), and mashua (Tropaeolum tuberosum Ruiz & Pav.) play major roles in Andean communities. These species show high variability but are threatened with genetic erosion. To study the management of genetic resources of neglected vegetatively propagated crop species, we studied genetic diversity and structure of these species in an in situ diversity microcenter (Huanuco, Peru). A sample of 15 varieties of oca, 15 of ulluco, and 26 of mashua was analyzed with the inter simple sequence repeats (ISSR) molecular markers. Mean genetic distances and global genetic diversities were high for the three species, with higher values for mashua than for oca and ulluco. Assignment technique divided both oca and ulluco samples into two genetic clusters; the mashua sample probably belongs to a single genetic cluster. Inter simple sequence repeats (ISSR) technique showed intravarietal genetic variability for most varieties, suggesting an underestimation of the in situ genetic variability. These results are discussed considering how variation in breeding systems and farmers' practice influenced patterns of genetic diversity. Our findings confirm the hypothesis of a considerable amount of variability found in neglected Andean tubers and are essential to deserve adequate conservation strategies and to maintain genetic resources of neglected Andean tuber crop species under a threat of genetic erosion.