Statistical analysis

The data obtained were analyzed by means of an analysis of variance and a comparison test of Tukey means (p≤ 0.05) was performed. The statistical program SAS (Statistical Analysis System) version 9.1 (^{SAS, 2003}) was used to analyze the data.

Nitrogen

There were differences p≥ 0.5 between species (Table 1). The percentage of nitrogen varied between the species and the organs of the three Rhynchostele orchids. When the species were in a state of development of young leaves and flower buds, the percentage varied from 1.54 to 2.20%. In Table 1, it is observed that the element is required more in the state of young leaves and flower buds than the rest of the stages of development. The N is an essential mobile element in cell division and expansion, as well as in the growth of vegetative structures, such as stems and leaves (^{Sonneveld and Voogt, 2009}).

In leaves, the percentage varied in a range of 1.09 to 2.07%, in the different ages of the leaves (new, mature and old), independently of the species (Table 1). In addition, it was observed that the percentage of N decreased as the leaves senescen. It has been reported that the percentage of N sufficiency in Cattleya orchid leaves ranges from 1.50 to 2.5% (^{Jones et al., 1991}). In the case of the Rhynchostele aptera species, the levels found in leaves were low. However, in the absence of other reports of species similar to the Rhynchostele, these values may be sufficient for this species, since no visual symptoms of deficiencies were detected.

In pseudobulbs regardless of the species, the percentages found were from 0.51 to 1.06% (Table 1). In the epiphytic orchids, the storage organs are the pseudobulbs, which have the capacity to store water, minerals and carbohydrates (^{Ng and Hew, 2000}). However, the percentage found is lower compared to the leaves. ^{Poole and Sheehan (1973)} mention that in leafs of Cattleya the concentration of N foliar decreases with the age of the leaves, since this tends to translocate towards the pseudobulbs of greater age. However, the latter was not observed in Rhynchostele species, since the percentage in pseudobulbs was lower, which means that orchids differ in their nutrient distribution in the plant.

The concentration in root varied in a range of 0.76 to 1.15%, indistinctly of the species. The function of the roots of the orchids is to absorb water and nutrients, as well as fix the plant to the substrate. The roots only grow during a vegetative period and suspend their growth with the development of vegetative shoots (^{Hagsater et al., 2005}). This is consistent since the N is a mobile element and was found in the organs with higher demand such as buds (new leaves and flower buds) and low levels in roots.

Phosphorus

The concentrations in young leaves and floral buds were higher than in the rest of the organs of the plants. This can be attributed to the fact that the P is a mobile element, and is found in a greater proportion in young or recently matured leaves (demand organs) (^{Fernández, 2007}). The concentration interval was 0.26 to 0.35% regardless of the species evaluated (Table 1).

The results in leaves varied from 0.12 to 0.29%, independently of the species and the age of the leaf (young, mature and old). ^{Jones et al. (1991)}, for Cattleya spp. indicates that the P sufficiency values range from 0.13 to 0.75% in the foliage. Only the case of R. aptera in old leaves, is below the sufficiency values, which is attributable to the mobility of the element towards the demand zones (^{Fernández, 2007}).

In general, it was observed that the concentration of P is low in the pseudobulbs of the Rhynchostele species, compared with the leaves. Ranges of 0.06 to 0.14% were obtained regardless of the species and the stage of development of the pseudobulb. As mentioned above, the pseudobulbs function as storage organs for water and reserve substances; however, low levels of P were found, which could have been used to support the production of flowers, fruits or the development of new shoots. ^{Bachiega-Zambrosi et al. (2012)} suggest that remobilization of P reserves is important to meet the demands of new vegetative and reproductive growth. In the roots (0.07 to 0.08% of P) the concentration of P was even lower than leaves and pseudobulbs, which was associated with demand zones such as young growing leaves.

Potassium

In relation to K, the highest concentration was in young leaves and floral buds, as in the previous elements. The concentration of K in young leaves and flower buds was 0.85 to 1.10% (Table 1), which are considered low, compared to the level of foliage sufficiency that ranges from 2.00 to 3.5% in orchids of the Cattleya genus (^{Jones et al., 1991}). Similarly, concentrations were low in the leaves regardless of the species and morphological development, where the concentration varied from 0.45 to 0.9% (Table 1). However, it is inferred that the concentrations found could be of sufficiency and that the genus Rhynchostele could require concentrations lower than 2% of K, since they did not present visual symptoms of deficiency, water stress, cold damage, or diseases (Figure 1).

In this regard, ^{Kumar and Kumar-Sharma (2013)} indicates that plants that have an adequate supply of K make more efficient use of water during stress, having a direct relationship with resistance to frost and disease. In addition, K promotes stem stiffness and is involved in cell growth, as well as in the stabilization of the membrane and the cell wall (^{Armstrong, 2002}). The concentration of K in pseudobulbs was from 0.44 to 0.86%, levels slightly below the foliage (Table 1). Likewise, root levels were low from 0.18 to 0.25%, this effect could be due to the mobility of K inside the plant (^{Marschner, 2012}), being translocated towards the aerial part.

Calcium

Ca concentrations of 0.35 to 0.51% were found in young leaves and flower buds (Table 1). In the foliage, values of 0.34 to 0.75% were found regardless of the species and the age of the leaves. However, there is a marked difference between the study species R. aptera and R. macula had low levels in almost all their leaves, in comparison to the optimal reports of Ca in foliage by ^{Poole and Sheehan (1973)} and ^{Jones et al. (1991)} ranging from 0.5 to 2%. Ca is characterized by low transport ability within the plant because once it is deposited in plant tissues it will be very difficult to remove it. That is why young tissues are the first to be affected when there are deficiencies of this nutrient (^{Marschner, 2012}).

Regarding the pseudobulbs, the concentrations ranged from 0.33 to 0.84% (Table 1). In the root the concentrations were from 0.25 to 0.37%. In this regard, ^{Poole and Sheehan (1973)} consider as an optimum level the concentration of 0.1 to 0.4% of Ca in Cattleya roots.

Magnesium

It was found that the concentrations of Mg in young leaves and floral buds varied in a range of 0.08 to 0.19% and in young, mature and old leaves from 0.07 to 0.19% regardless of the species. Levels of foliage sufficiency have been reported at concentrations of 0.30-0.70% for Cattleya (^{Jones et al., 1991}), so the values found were lower than those reported in the literature. However, the plants did not show visual symptoms of Mg deficiencies, so it is inferred that the Mg level required by Rhynchostele may be lower. Additionally, it was observed that the highest accumulation of Mg occurred in the pseudobulbs, where the concentrations were from 0.11 to 0.26%. This may be because the pseudobulbs function as nutrient storage organs ^{Uma et al. (2015)}. In the case of the roots, the values were from 0.09 to 0.13%, similar to the foliage.

In general, the decreasing sequence of nutrient concentration was N> K> Ca> P> Mg. This tendency of nutritional absorption was mainly in the foliage of R. macula and R. aptera. In R. rossii the concentration changed in mature and old leaves in the following way N> Ca> K> Mg> P. In pseudobulbs the tendency of accumulation also changed and was N> K> Ca> Mg> P. The root was the only organ that presented the same tendency in the three species, where its decreasing sequence of accumulation was N> Ca> K> Mg> P. The N and K or Ca were the most required elements in the three species of Rhynchostele, the absorption order changed with the stage of development and genotypes. As pointed out by ^{Jiménez-Peña et al. (2013)}, orchids have different nutritional requirements, these same authors point out the importance of providing the nutrients according to the stage of development and of the species, for a better development.

It was observed that the mobile elements were translocated towards the area mainly in leaves and flower buds. This coincides with that reported by ^{Hew and Ng (1996)} in a study of Oncidium goldiana, where the mineral reserves, mainly in pseudobulbs, are remobilized to support the development of a new outbreak and inflorescence. It has also been reported in Catasetum viridiflavum that the minerals in pseudobulb reserves are important in determining the number of flowers produced (^{Zimmerman, 1990}). Therefore, it is recommended that special attention be given to Rhynchostele species in the fertilization regime, especially during the development period of new shoots, since in their natural habitat they required smaller amounts than what is reported in orchids such as Cattleya and Phalaenopsis (^{Jones et al., 1991}). It is important to consider the nutritional requirements, since the distribution of these during their development is fundamental to satisfy the specific needs in the periods of highest demand, especially of nutrients such as nitrogen (N) phosphorus (P) and potassium (K).

It should be noted that, in general, low values of nutritional concentration were found in the three species studied compared to those reported in the literature, due in part to the fact that the genotypes that were evaluated grew and developed in their natural habitat. It is inferred that if grown under conditions with optimal fertilization doses and with adequate agronomic management, the nutrient concentration values would be higher as has been found in the Laelia anceps orchid (^{Jiménez-Peña et al., 2013}); however, despite the fact that the species grew under natural conditions, no visual symptoms of deficiency were detected, probably this response was due to the fact that the Orchidaceae family members form associations with mycorrhizal fungi that give them greater radical exploration and eventually greater capacity of nutrimental absorption, especially in the initial stages of growth and development, as reported in Eulophia epidendraea and Malaxis acuminata (^{Uma et al., 2015}).

Finally, the importance of the results of this research lies in the fact that they constitute the first report on the concentration and distribution of nutrients in the organs of the orchids R. maculata, R. rossii and R. aptera developed under natural conditions, so that the information generated could be useful in conservation programs and for the agronomic management of these species.