During the reforestation process in degraded areas, it is necessary to use quality plants so that they have a greater ability to establish, grow and develop in the plantation sites (^{Muñoz et al., 2015}). Consequently, its use increases the survival percentage, since, once established in the field, it will be able to live and grow, even in places with low productivity.

However, it is difficult to obtain suitable plant for each site. The attributes that determine its quality refer to morphological and physiological qualities (^{Mexal and Landis, 1990}). The former are related to the structure of the plant; while, the physiological ones to the responses it has when being planted (^{Rose et al., 1990}). The morphological criteria are more often used than the physiological ones to assess plant quality (^{Haase, 2008}).

The main morphological attributes are stem height, neck diameter, root system, height-diameter ratio (robustness index), biomass, foliar area and presence of mycorrhizae (^{Escobar-Alonso and Rodríguez, 2019}). In regard to the physiological ones, those used are the concentration of carbohydrates, nutritional status, water potential, electrical conductivity, stomatal conductivity, CO₂ assimilation and chlorophyll fluorescence (^{Escobar-Alonso and Rodríguez, 2019}).

Despite the fact that there is experience on the evaluation of the plant quality parameters in Mexican forest species, the great variability in conditions and production processes in the nurseries means that knowledge in this sense is still limited, so it is necessary to carry out more research in this regard. Therefore, the present work aimed to assess the quality of the plant in the nursery of six species of the Pinus genus produced in polyethylene bags.

At the Héroes Bicentenario Forest Nursery located in Tecámac, State of Mexico (administered by the Protectora de Bosques of the State of Mexico), 90 Pinus plants of six species were randomly chosen (15 plants per species), produced in natural ground and polyethylene bags (10 × 24 cm). The age of the plant material was different: Pinus greggii Engelm. ex Parl. and P. leiophylla Schiede ex Schltdl. & Cham., were 7 months-old; P. pseudostrobus Lindl., 10 months-old; P. ayacahuite Ehrenb. ex Schltdl., 12 months-old; P. hartwegii Lindl., 14 months-old and P. cembroides Zucc., 15 months-old.

Each individual was evaluated for their plant quality by measuring the diameter (mm) at the root neck (DC) with a Mitutoyo 500 - 196 - 30 digital vernier, and the height (cm) from the base to the apical bud (Alt) with a Westcott H-6560 graduated ruler; the data of both variables were used to calculate the Slenderness Index (IE = Alt / DC). Substrate was removed from all plants and the stem and root were sectioned.

The plant material (aerial part and root) was placed separately in paper bags, dried in a Novatech HS35-AIA oven (50 °C, 9 days) and weighed with a Ohaus, Harvard Trip^® two-plates scale to estimate aerial dry biomass (BSA, g), root dry biomass (BSR, g), total dry biomass (BST, g), BSA / BSR ratio and Dickson's quality index [ICD = BST / (IE + BSA / BSR)] (^{Dickson et al., 1960}).

To determine the quality of the plants (high, medium or low), the values obtained for DC and Alt were compared to the morphological indicators of plant quality at the nursery that are stipulated in the NMX-AA-170-SCFI-2016 Mexican Standard (^{Secretaría de Economía, 2016}); while IE, BSA / BSR and ICD with the intervals cited by ^{Rodríguez-Ortiz et al. (2020)} (Table 1).

In regard to DC, all species, except for P. ayacahuite, recorded higher values than those indicated in the NMX-AA-170-SCFI-2016 (^{Secretaría de Economía, 2016}), and thus, they were considered high quality; P. hartwegii in particular, was the best of all, since it more than doubled the value indicated in the Standard (Table 2).

Results suggested that five species of pine evaluated had the potential to establish themselves in the field, since the diameter dimensions indicate well-developed root systems, resistance to physical damage, twists, tolerance to those caused by animals and to high temperatures on the soil surface (^{Mexal and Landis, 1990}; ^{Birchler et al., 1998}; ^{Muñoz et al., 2015}).

When comparing the production of these pine trees with that registered in other nurseries, its superiority is evident; for example, in P. greggii and P. pseudostrobus a medium quality is reported [3.5 and 3.4 mm, respectively] (^{Rueda et al., 2012}).

In regard to Alt, P. greggii and P. leiophylla were above the value suggested in the NMX-AA-170-SCFI-2016 Mexican Standard and P. cembroides had values within the proposed interval; however, P. ayacahuite and P. pseudostrobus had lower height than advisable (Table 2). For P. hartwegii, the NMX-AA-170-SCFI-2016 Mexican Standard does not include a maximum or a minimum value, as it is a cespitose species; however, the record obtained was considerably higher than that documented by ^{Bernaola-Paucar et al. (2015)} (4.3 cm) in 12-month-old trees.

In general, height in pines is a variable that has little value as an indicator of quality, and it does not even correlate, or it does so negatively, with survival in the field (^{Birchler et al., 1998}). However, this characteristic combined with other variables, such as the diameter of the neck (Slenderness Index), acquires greater importance.

For IE, all the species, except for P. greggii, recorded a value lower than six (Table 2), which indicates that the plants are more robust, of better quality, that can resist abuse from transfer maneuvers to the plantation site, to drying out by the wind, frost damage and with a better chance of survival and growth in sites with water limitations (^{Haase, 2008}; ^{Escobar-Alonso and Rodríguez, 2019}). In the case of P. greggii, excessive growth in height (36.5 cm) was observed, despite the fact that its diameter corresponded to the NMX-AA-170-SCFI-2016 Mexican Standard (> 4 cm); this ratio would improve with a lower density in the growth bed during the production phase at the nursery, or with pruning to the aerial part prior to going to the field (^{Muñoz et al., 2015}).

In the BSA / BSR ratio, P. cembroides, P. ayacahuite and P. hartwegii recorded the lowest proportions (Table 2) which indicated high quality, from a good balance between the biomass of the aerial part and the root (^{Haase, 2008}), as well as greater possibilities of success when planted in places with low rainfall (^{Thompson, 1985}).

Conversely, P. pseudostrobus, P. leiophylla and P. greggii had the highest ratios (Table 2), which suggested that they should be planted in places where there are no water availability problems; from its poorly developed root structure, its ability to absorb nutrients and water will be limited to supply the needs of the aerial part (or of the plant) (^{Escobar-Alonso and Rodríguez, 2019}). This can lead to stress due to lack of moisture in dry places before the roots are established, and with this occurs the supply of water and nutrients to the aerial part of the plant.

For ICD, P. hartwegii showed the highest value; while P. greggii and P. pseudostrobus had the lowest; P. cembroides, P. ayacahuite and P. leiophylla had intermediate values. It was determined that all species, except for P. pseudostrobus, show high quality, since their ICD was > 0.5 (^{Rodríguez-Ortiz et al., 2020}).

When combining the values of the different evaluated, variables four species, P. hartwegii, P. cembroides, P. leiophylla and P. ayacahuite registered the best plant quality, with which it can be predicted that they will have greater development possibilities, even if they are established in sites whose conditions are unfavorable for them.

Part of the success of reforesting degraded areas lies in the use of quality plants, with morphological characteristics that allow them to survive and grow, even in unsuitable sites. However, the use of reference values, as well as the evaluation of the morphological attributes of the species produced in the nursery, are still limited in the country (^{Rueda et al., 2014}). The results of the present work showed that P. cembroides and P. greggii have high plant quality; P. ayacahuite, low quality; P. hartwegii, P. leiophylla and P. pseudostrobus, medium quality. This information is crucial to analyze the survival that will be obtained in the field, depending on the characteristics of each site, and will favor timely decisions to be made in the forest management of the reforestation areas in which the plant production lots of the plants of the assessed species are intended.