Evaluated variables

The morphological characteristics were evaluated in leaves: leaf shape, shape of base, colour, margin, shape of apex, fresh weight, dry weight and length of leaf blade and leaf area (cm²) using a leaf area integrator (LIQUOR 1600, NE, USA). Fruit variables were: fruit shape, length and diameter (mm), shape of the base, form the apex, form the pedicel, colour epicarp the exocarp thickness (mm), the epicarp weight (g) weight Fresh pulp (g) and dry matter of the pulp (g). Seed variables were: seed shape, seed weight (g). Descriptors were used of the International Plant Genetic Resources Institute, now Bioversity (^{IPGRI, 1995}). Variable length and diameter were measured with an electronic caliper (Mitutoyo, Inc., Japan) to the nearest 0.1 cm and weight-related variables were determined with a Mettler^® digital balance model AJ150, 0.001 g.

Considered an important variable quality, the percentage of oil in fruits was evaluated though not a morphological characteristic. 100 g pulp were weighed and dried in a forced air oven (LAB-LINE, Imperial II, USA) at 70 °C to constant weight. The percentage of dry matter was obtained through dry/fresh weight and weight multiplied by 100. The oil content was determined by Soxhlet extraction with a distiller, using the official method for determining oil content plant organs relationship USDA (^{Lee, 1981}). Petroleum ether was used as solvent and four replicates were performed for each genotype.

Statistical analysis

A principal component analysis was performed. With the variables that showed more descriptive value of euclidean distances calculating the dendrogram constructed by the Ward grouping method (^{SAS Istitute, 2009}), for maximum separation of groups by their similarities in characteristics of leaf, fruit and seed.

In most of the evaluated quantitative characteristics variability was found as evidenced by the values of the coefficients of variation (CV), which highlights the thickness of the epicarp (CV= 20.2%), the weight of the epicarp (CV= 22.4%), oil percentage (CV= 22.6%), pulp weight (CV= 20.4%) and seed weight (CV= 23.3%).

Table 2 Descriptive quantitative and statistical features of 24 native genotypes evaluated in avocado (Persea americana Mill.) in Nayarit, Mexico. 

As for the quality characteristics, it is considered that the shape of fruit is a morphological indicator of importance because it is one of the features that make it attractive to avocado market consumers (Rodríguez-Medina et al., 2009). In the present study, fruit shape that prevailed was the narrow -obovate with 24.4%. In a study with native genotypes in Uruapan, Michoacan was found that the shape of fruit that stood out was the ellipsoid (32.2%), Dr. Raul Medina (Pers. Comm., 2014). These results show the diversity of existing materials avocado in Mexico and could be used as a source of germplasm in breeding of this species.

The major dimensions of length and diameter (mm) of fruit, presented the AS- 1 and AS-2 (107.41 and 106.98 g, respectively), and the smaller the Tec-1 genotype with 63.68 g genotypes. Similarly, the highest values in fruit weight corresponded to AS-1 and AS-2 (941.72 and 1 042.92 g) selections, and the least significant Tec-1 (188.13 g). ^{Gutiérrez-Díez et al., (2009)} evaluating 42 genotypes of native avocado Nuevo Leon, Mexico reported the highest fruit weight was 251.40 g and 10.53 g minor, for the largest fruit length was 12.76 cm and the lowest was 2.88 cm. Besides the genetic characteristics, these differences may be due to the different climates and altitudes where these native avocados occurred.

The dendrogram, with a cut to a euclidean distance of 0.11 (Figure 1), defined three groups based on morphological descriptors leaf, fruit and seed. Group I consisted of the genotypes Tec-1, 2, 4, 5, 6, 7, 8 and 9 Tecuala, PP -1, CV-2, CV- 1 and JC-1 of Santa Cruz de Miramar and JCC-1, JCC-2 and JC-1 of El Cora, municipality of San Blas, with characteristics of lower weight of pulp, small seed, lighter weight shell, which derives a result of lower total weight.

Figure 1 Ward minimum grouping of 24 native genotypes of avocado (Persea americana Mill.) based on morphological characteristics in Nayarit, Mexico. 

The group II was integrated with MJ genotypes of Zacualpan, municipality of Compostela and CST, CV-3, VC-4, PP-2 and PP- 3 of Santa Cruz de Miramar, San Blas, had as features increased fruit weight pulp, small diameter and leaf. The group III was integrated with JF and JR genotypes Zacualpan and An-1 and An-2 of Santa Cruz de Miramar, and is characterized by greater leaf area, fresh weight and dry weight of leaf, plus greater weight and diameter fruit, and greater weight of pulp.

In the principal component analysis, variability accumulated in the first three components observed 53.8%: 25.8% explained CP1; CP2 and CP3 17.3% 10.7% (Table 3).

Table 3 Eigenvectors and proportion of the variability in the first three principal components, generated with phenotypic characteristics of fruits, seeds and leaves of 24 native genotypes of avocado (Persea americana Mill.) in Nayarit, Mexico. 

CP1 variability was determined by positive values of diameter and fruit weight, weight epicarp weight of pulp and seed weight. These results are similar to those reported by ^{López-Guzmán et al. (2012)}, where the CP1 was related to weight, length, width and weight of fruit pulp in native genotypes of avocado. CP2 was influenced by leaf area, leaf fresh weight, shape of the base of the leaf and leaf dry weight. In the CP3 percentage of oil and dry matter of the pulp exerted a negative influence (Table 3).

Finally, the Figure 2 shows the scattergram of the two main morphological variables of the 24 native genotypes of avocado (Persea americana Mill.) Nayarit, Mexico.

Figure 2 Scattergram of the 24 genotypes of native avocado (Persea americana Mill.) in the first two principal components. 

Conclusions

Three groups with similar morphological characteristics of native avocado genotypes were formed. The characteristics of fruit weight, weight of pulp, fruit diameter, shell weight, seed weight and leaf blade length, characters that better explained the morphological variability, so that these parameters could be used as a selection criterion and differentiation of native genotypes of tropical avocado in Nayarit, Mexico.