Introduction

Mexico, given its geographical position in the Americas, its varied topography and climatic diversity, hosts a wide biodiversity, and is regarded as the major center of diversity of the genus Phaseolus (^{Gepts et al, 1986}; ^{Gepts and Debouck, 1991}), where most of the wild species of this genus grow and some are endemic (^{Delgado Salinas, 1985}; ^{Delgado Salinas et al., 1988}; ^{Delgado Salinas et al., 2006}; ^{Freytag and Debouck, 2002}; ^{Delgado Salinas, 2012}). In the second half of the last century great efforts were made to collect, preserve and use plant genetic resources, as a result, the genetic variability of the cultivated forms of beans is well represented in germplasm banks, but not that of their wild relatives, which do not exceed 10% of the total accessions (^{Cárdenas Ramos et al., 1996}). The genus Phaseolus comprises 75 taxa, most are wild forms distributed in all physiographic regions of Mexico (^{Freytag y Debouck, 2002}; ^{Peña Valdivia et al, 2012}).

The Sierra Madre Oriental region in Nuevo León is known for its diversity of wild species of Phaseolus (^{McVaugh, 1987}; ^{Freytag and Debouck, 2002}), where several accessions of wild beans have been collected (^{Rodríguez Cabrera et al, 1985}; ^{Estrada Castillón et al., 2004}); P. novoleonensis was recently described as a new species (^{Salcedo et al., 2006}). However, in the Phaseolus Catalogue of the Germplasm Bank at the National Institute of Forestry, Agriculture and Livestock (INIFAP), no seed samples are reported (^{Cárdenas et al, 1996}; ^{Freytag and Debouck, 2002}), which has stopped the study of various species (^{Acosta-Gallegos et al, 1996}); also when enough seed is available, the access is very difficult (^{Acosta-Gallegos et al, 2007}).

Based on the above, the present work was carried out to collect seed samples of wild bean germplasm in Nuevo León, for preservation and future use in different lines of research.

The collection was carried out in 12 municipalities of the state of Nuevo León with significant records in Phaseolus wild species diversity in the Sierra Madre Oriental region of Mexico, geographically located between 23° 56' 23" and 25° 42' 59'' north latitude and between 99° 42' 59" and 100° 23' 38" west longitude. The predominant climate is cold sub-humid temperate Cb (w1)(i')w", with summer rains, an average annual rainfall of 600-800 mm and an average temperature of 12-18 °C (^{Medina García et al., 1998}). Acharacteristic feature of the study area the low hills (500-700 m) and the mountain peaks (> 1 500 masl). Soils are composed of several associations of calcaric regosol, calcaric phaeozem rendzina with chromic luvisol and calcic kastanozem in lytic phase, where the vegetation type is oak forests, oak-pine and Juniperus (^{SPP, 1981}).

Wild populations were collected during the period 2010-2012, in two explorations per year, one in August and September, when plants were in the vegetative-flowering stages, and another in October, November and December, when plants were at physiological maturity. In the vegetative-flowering stage, herbarium specimens, mainly stems, leaves and flowers were collected. At physiological maturity, seed samples were collected.

The species identification was done based on the taxonomic treatises of ^{Delgado-Salinas (1985)} and ^{Freytag and Debouck (2002)}. Samples were stored in glass jars, sealed and labeled, with less than 15% grain moisture, and sent to the germplasm bank of the Antonio Narro Autonomous Agricultural University (UAAAN ), for ex situ conservation.

Overall 54 sites were determined with one or more species present, where seed samples were collected and seven Phaseolus species were identified. The distribution of wild species in Nuevo León has been restricted in the foothills of the Sierra Madre Oriental region of Mexico, in the San Pedro, Monterrey, Santiago, Allende, Montemorelos, Rayones, Linares, Galeana, Iturbide, Aramberri, Zaragoza and Dr. Arroyo municipalities (Table 1), where the greatest diversity has been found in Aramberri (five species), Rayones (four species), Santiago, Montemorelos and Zaragoza (three species).

Table 1 Collections of wild species obtained in the period 2010-2012, in 12 municipalities of the state of Nuevo León, Mexico. 

*(1)= P. neglectus; (2)=P. leptostachyus; (3)= P. pedicellatus; (4)= P. albiflorus; (5)= P. novoleonensis; (6)= P. maculatifolius; (7)= P. zimapanensis.

Among the species of greater incidence and distribution, P. neglectus (24 collections), P. leptostachyus (15 collections) and P. pedicellatus (eight collections) stand out; also P. albiflorus, P. novoleonensis and P. maculatifolius were important with two collections, respectively, in contrast, P. zimapanensis is a taxon with very limited distribution, only collected at a single site in the Lampacitos town of the Aramberri municipality. Importantly, in this study, no sites were found with presence of P. grayanus, P. altimontanus and P. polymorphus, previously reported taxa in Nuevo León (Rodríguez Cabrera et al., 1985; ^{Freytag and Debouck, 2002}; ^{Estrada Castillón et al., 2004}).

With respect to altitude and species found in this study, it could be noted that P. neglectus had the highest range, found from the 2 170 masl at the top of the Zaragoza municipality, up to 665 masl in the Santiago municipality (Table 2).

Table 2 Characteristics of wild species obtained in the period 2010-2012, in the state of Nuevo León, Mexico. 

P. leptostachyus was the second, collected from 1990 masl in the Santiago municipality, to 714 masl in the Monterrey municipality (Table 2). This species was observed with some frequency in the middle and upper parts of the Sierra Madre Oriental region of Mexico, mainly in open environments with trees, between pine and oak forests, as it is a species with indeterminate growth of creeping type, thriving well in disturbed sites but protected from factors representing threats to its growth and development. These results agree with those obtained by ^{Lépiz et al. (2004a} and ^2004b) in the region of the Verde and Santiago rivers in the state of Nayarit.

P. pedicellatus ranked third, found from the 2 820 masl on the slopes of Cerro del Potosí in the Galeana municipality, up to 1 990 masl in the Agua Fria town of the Aramberri municipality (Table 2). This species is located at higher altitude sites, where wild bean was found and grew up in damp and shade in the oak and pine forest. Similar results have been reported by ^{Lépiz et al. (2004a} and ^2004b) in the slopes of the Nevado de Colima.

The amount of wild forms of Phaseolus collected in Nuevo León, is still incomplete according to previous results (^{Rodríguez Cabrera et al, 1985}; ^{Freytag and Debouck, 2002}; ^{Estrada Castillón et al, 2004}) and yet less evaluated. In addition, the 10Phaseolus species reported for Nuevo León could be considered low compared to the genetic diversity contained in other states such as Jalisco, Durango, Oaxaca and Chiapas (^{Delgado-Salinas, 2012}).

Phaseolus species reported in this study exhibit perennial cycle and indeterminate growth habit, four are climbing (P. neglectus, P. pedicellatus P. maculatifolius and P. zimapanensis) and three creeping (P. leptostachyus, P. novoleonensis and P. albiflorus) (Table 2). The perennial cycle was associated with thickening of the root system, a morphological feature conferring plants the ability to reproduce vegetatively, since it has been observed that growth reaches about 40 cm or more in length. Overall wild beans showed fibrous or fleshy roots, sometimes the size of a yam bean, from whose crowns herbaceous or sometimes somewhat woody stems developed (^{Delgado Salinas, 2012}).

In Nuevo León, wild species of the genus Phaseolus have been seriously affected by the historical exploitation of cattle and goats, which has been indiscriminately managed on mountain pastures, causing great damage to the native flora; another adverse factor has been the road maintenance by local boards of roads, as the case of two small populations of P. novoleonensis found by the roadside between the Allende and Rayones municipalities. Additionally, wild bean populations have been affected by other factors, such as climate change, destruction of natural environments, expanding cultivation lands, increased livestock and intensive agriculture which caused the loss of these populations (^{Brush, 2000}; ^{Acosta-Gallegos, 2012}).

Conclusions

Overall 54 seed samples of wild populations of Phaseolus were collected in 12 municipalities of the state of Nuevo León in the Sierra Madre Oriental region. The observed genetic diversity was represented by seven species (P neglectus, P. leptostachyus, P. pedicellatus, P. novoleonensis, P. albiflorus and P. zimapanensis). Some germplasm maybe missing, therefore there is a need to continue exploring and collecting other wild populations in Nuevo León. Also germplasm collection of wild bean populations is required in the states of Tamaulipas and Coahuila.