RESEARCH WORK

Comparison of three tree legumes planted on an alkaline substratum during the nursery stage. I. Morphostructural variables

María G. Medina¹, D.E. García¹, P. Moratinos² y L.J. Cova¹

¹Departamento de Ciencias Agrarias, Núcleo Universitario Rafael Rangel Universidad de los Andes, Trujillo, Venezuela.

E-mail: dagamar8@hotmail.com

²Instituto Nacional de Investigaciones Agrícolas (INIA), Trujillo, Venezuela

ABSTRACT

A trial was conducted in the Trujillo state, Venezuela, in order to evaluate the morphostructural performance of Albizia lebbeck, Leucaena leucocephala (Trujillo ecotype) and Erythrina fusca under nursery conditions. A completely randomized design was used for repeated measurements in time and ten replications per species. The studied variables were: plant height; stem and branch diameter; number of leaves, branches, roots and total nodules; and branch and root length. At the end of the evaluation (18 weeks) no significant differences were observed between L. leucocephala and E. fusca in plant height (45,2 and 51,0 cm), stem diameter (0,65 and 0,78 cm), number of leaves (43 and 45) and branches (17 and 18) and root length (32,1 and 36,5 cm). A. lebbeck only stood out for the number of roots (61) and average total nodules (6). The results allow concluding that with the utilization of an alkaline substratum in nursery, L. leucocephala and E. fusca showed a similar aerial morphostructural performance and higher than that of A. lebbeck; while albizia showed better results in root morphostructure.

Key words: Albizia lebbeck, Erythrina fusca, Leucaena leucocephala, nursery.

INTRODUCTION

Silvopastoral systems represent an important choice in the development of animal production systems in the tropics and an economically and ecologically sustainable alternative for feeding ruminant herds (Rodríguez et al., 2001). These systems involve the use of local trees and shrubs, which interact with pastures and animals, under an integral management scheme, and emphasize the use of forage ligneous legumes, because their foliage has high protein content.

Among the legume species used as important components of agroforestry systems Leucaena leucocephala Lam. de Wit. stands out (Toral, 1998; Toral, 2000). However, there are other perennial ligneous plants in Venezuela that have not been evaluated under nursery conditions, and which potential is underestimated.

The Albizia genus constitutes one of the legume groups with higher multipurpose nature (García and Medina, 2005) and particularly the species Albizia lebbeck Benth. is of great interest for agrosilvopastoral systems, due to its potential as supplementary protein source in ruminant feeding (Razz et al., 1998; Cerero et al., 2002). It constitutes one of the most widely distributed legumes in the western region of Venezuela and in semiarid regions with little fertile and sandy soils (Medina, 2005).

On the other hand the tree species of the Erythrina genus are much used for shade in coffee and cocoa, particularly in the Trujillo state, due to their easy propagation (Escalante, 2006); as support trees in pepper and vanilla plantations; and as shade in paddocks of tropical zones in India, Puerto Rico and Costa Rica. They ameliorate the soil, because of their capacity to integrate nitrogen and the mulch effect their leaves generate on it (Medina, 2006).

Additionally, due to the high representation of Erythrina fusca Lourd. in plant systems in Trujillo, this species is considered a symbol of the Andean region of Trujillo.

The objective of this study was to evaluate the morphostructural performance of A. lebbeck, L. leucocephala (Trujillo ecotype) and E. fusca under nursery conditions and with an alkaline substratum.

MATERIALS AND METHODS

Location of the experimental area. The trial was conducted at the Experimental Station of the National Institute of Agricultural Research (INIA), located in the central region of the Trujillo state, at an altitude of 345 masl in the Pampanito town, Pamapanito municipality, Venezuela. The experimental area has an annual average rainfall of 1 500 mm and a mean temperature of 27ºC.

Treatments. Three species with potential to be used in agroforestry systems of the Trujillo state were studied: A. lebbeck, L. leucocephala (Trujillo ecotype) and E. fusca.

Characteristics of the substratum used. For nursing the plants perforated black polyethylene bags were used with capacity of 3 kg, which were filled with a substratum composed by 70% alkaline loamy soil (pH: 8,9), 10% sand and 20% composted cattle manure.

Experimental procedure

In each bag three seeds of each species were planted, all newly-harvested and previously selected (discarding those that showed mechanical damage and/or diseases), on which pre-germinative treatments were applied in order to guarantee a satisfactory emergence. The L. leucocephala seeds were dipped in hot water at 80ºC for two minutes (González et al., 2005). For A. lebbeck, immersion in water at room temperature for 24 hours was used and after that time a slight cut was performed on the seed coat in the zone opposed to the embryo. In the case of E. fusca seeds, immersion was used during 24 hours in water at room temperature (Rodríguez and Murgueitio, 1995).

The distribution of the bags that contained each species was randomly made in the nursery area (50 m²), to prevent the possible influence of shade in specific parts of the area and the effect of changes in solar intensity, regarding the time of day.

All the material planted in the nursery received irrigation daily in the morning (4 L/minute/m²) in order to maintain the necessary moisture for the emergence and development of new seedlings. When they reached an average height of 7 cm a thinning labor was performed to leave the most vigorous one in each bag, from the recommendations made by Medina and García (2010) for previous evaluations on alkaline substrata. Neither rooting stimulators nor chemical fertilizers were applied, because the substratum conditions were considered optimum.

Measurements

The measurements were made with a weekly frequency (first measurement: seven days after emergence), by means of destructive samplings in 10 plants per measurement, during the 18 weeks.

The evaluated variables were: plant height in centimeters (with graduated ruler, from the soil level to the apex of the apical branch), stem diameter in centimeters (at the base of the seedling, with caliper), number of branches, leaves, roots and nodules (through visual count), branch and root length (in centimeters with graduated ruler), and branch diameter (at the base interception of the branch and stem, with caliper).

The nodule count was made after the other variables were measured, at each measurement time. The bag and the remnant soil were carefully withdrawn in order to leave the root system of each plant uncovered and count the number of nodules and roots, as well as to measure root length. The absence of nodules in the withdrawn soil due to mechanical detachment at the moment of removal was verified.

Experimental design, treatments and statistical analysis. A completely randomized design, with measurements repeated in time and ten replications per treatment were used. For the data processing the statistical pack SPSS version 10.0 for Windows was used.

The data were processed through a variance analysis. For the mean comparison Duncan's multiple test (Duncan, 1955) was used, with a significance level P<0,05.

RESULTS AND DISCUSSION

Regarding the height variable (fig. 1) the species grew progressively throughout the experiment, although in the first six weeks of evaluation no significant differences were observed among them. Since the seventh week, L. leucocephala and E. fusca showed a remarkable increase in this variable and with that they significantly exceeded A. lebbeck (P<0,05). An identical performance pattern was maintained until the end of the evaluation.

During the whole trial, E. fusca showed high growth rate, and after 18 weeks it was more than 50 cm high; while leucaena, until week eight, showed remarkable increases in height in each measurement and afterwards they were less noticeable, reaching 45,2 cm. A. lebbeck showed the lowest growth rate and showed constant but discreet height increases; this species measured 22,6 cm in the eighteenth week.

The performance pattern of height could have been related to the favorable environmental conditions that prevailed during the trial, seed quality, adequate substratum on which the seedlings were developed and maintenance labors, aspects that have a noticeable influence on the development of tree and shrub species at the initial stage (Medina et al., 2007).

On the other hand, the progressive growth of each species and the differentiated performance of each one, regarding their particularities, suggest that the quantity of substratum used (3 kg), as well as the space provided in the bag for plant growth (2 000 cm³), were adequate for the seedlings to grow without volume limitations in the initial stage; if it is considered that in nursery nutrient absorption is to the conditions and fertility of the substratum used (Medina, 2006).

The plant height values in this study coincide for leucaena and erythrina, with the ones obtained by Toral (1998), who reported 37,7 cm as average for this variable, when studying the performance of A. lebbeck, Albizia saman, Bauhinia purpurea, Bauhinia variegata, Cassia grandis and Erythrina indica in nursery after 12 weeks. They also coincide with the reports by Wencomo et al. (2003) and Wencomo (2004) when evaluating 145 and 50 Leucaena spp. accessions, respectively, which average height was 25,0 and 21,7 cm, in shorter evaluation periods than this experiment.

Considering that the nursery period for plants with silvopastoral purposes ends when they reach between 30 and 40 cm of height (Toral, 2000), it is inferred that for the conditions under which the trial was conducted, it is necessary to increase such period for A. lebbeck. In the case of L. leucocephala and E. fusca, the transplant can be made since the eighth week, when these plants begin to measure more than 30 cm. Similar results were obtained by Medina et al. (2007), who evaluated the performance in nursery of leucaena and Moringa oleifera, on alkaline substratum and recommended two months as transplant time. Nevertheless, the performance of the tree and shrub species in the initial stage depends not only on edaphoclimatic factors and substratum characteristics, but also on the performance of each species and accession (Medina, 2006).

The evolution of stem diameter of the species is shown in figure 2. Significant increases (P<0,05) were observed in week 11, moment from which erythrina and leucaena differentiated from albizia until the end of the experiment.

These results prove that, since emergence, the evaluated tree species showed a similar performance, characterized by a slow and progressive thickening of the stem.

The average of this variable was considered higher than the one obtained by Wencomo (2004) for Leucaena spp. accessions (0,3 cm) after 10 weeks, maybe related to the environmental conditions that prevailed in the study areas or the genetic particularities of the accessions used for this essay.

Regarding leaf number, significant differences were observed since the seventh week (fig. 3) between albizia (which showed slow leaf formation) and the other species evaluated (P<0,05). Since that week an accelerated leaf production stage began in leucaena, which had statistical differences with erythrina (P<0,05) between weeks nine and twelve. However, in the three final weeks of the trial no differences were observed between both species.

The dynamics of the number of primary branches is shown in figure 4. Until the seventh evaluation no significant differences were found among the species. Since week nine and until week 11, the species were different among themselves, in favor of leucaena (P<0,05); in the following weeks leucaena and erythrina did not show differences between themselves.

The higher values of this variable constitute a desirable characteristic for tree species, because their higher leaf production and accelerated lateral growth are translated into higher foliage availability for the animal under silvopastoral system conditions (Ruiz et al., 1992). However, it is precocious to state that an outstanding performance in nursery (totally atypical conditions) presupposes a similar performance in the silvopastoral system, because it does not have to be maintained when the plants are transferred to the field, as the plant phenotype is a response to the environment and the phenotypical response can be different when the environment changes.

Likewise, the values obtained at the end of the evaluation (12-18 branches) are considered low as compared to the ones reported by Medina and García (2010) (22-27 branches) in studies that preceded this research (based on the use of alkaline substrata), as well as to those obtained by Angulo et al. (1997) (maximum value, 44 branches).

Figures 5 and 6 show the evolution of diameter and length of primary branches, respectively. In the first five weeks of evaluation no significant differences were observed among the species for these two variables. In the remaining weeks there were statistical differences in favor of erythrina (P<0,05).

Throughout the trial, no statistical differences were observed between leucaena and albizia. In the case of erythrina the branch diameter and length increases were continuous at all measurement times; while the other two species showed discreet increases.

In this regard, the importance of branch morphology and disposition in the architecture of the plants used for browsing is known. The best morphostructural performance of erythrina, regarding branch diameter is perhaps related to the development of this tree which has branches of 40 cm diameter and 14 m long as average in adulthood and it is considered one of the giant tropical trees (Medina, 2006). Considering this intrinsic characteristic of E. fusca, it is possible that since the nursery stage differences are likely to be observed regarding other lower-size trees, such as albizia and leucaena.

In this sense, the average branch length of the three species at the end of the evaluation was higher than that obtained by Medina et al. (2007) (16 vs. 12 cm).

The root system is an important aspect for the plants that are used in silvopastoral systems, because mid- and long-term species survival is related to root number and length, which allows higher water and nutrient absorption, as well as counteracting the physical impact caused by the animal when grazing and browsing. Hence the importance of the nursery stage, when the plant begins to form and develop such system.

During the whole trial albizia showed the highest development of the root system. Statistical differences were observed in favor of this species regarding root number (P<0,05), as compared to leucaena and erythrina in 12 of the 18 weeks of evaluation (figs. 7 and 8).

Regarding root length the species did not show statistical differences among themselves at any moment of the evaluation, and showed a constant elongation of the root part during the essay.

Soils or substrata with contrasting pH are known to influence drastically root production. However, not all ligneous species respond equally, regarding root development, to soils and substrata of different composition. Albizia is considered a legume that adapts very well to contrasting conditions of acidity and nutrient-poor soils (Medina, 2006). Yet, although the leucaena ecotype used is native from zones with alkaline soils of the Trujillo state and E. fusca has shown excellent growth under the same edaphic conditions, albizia showed a better performance. This result should be considered relevant to develop later studies, aiming at incorporating representatively this species in silvopastoral systems from Trujillo or in zones within the state and the country which are eroded or under bad natural fertility conditions.

The number of roots as well as their length, at the end of this study, indicates that the species showed a progressive development of the root system; this is important, because the root is an essential organ to guarantee a vigorous aerial development (Pineda, 2004) and the anchorage function of the plant in later stages (Medina, 2006). Nevertheless, in general albizia showed better integral results, because although no differences were observed in root length among species, their higher quantity in albizia conditioned a better underground structure of the root system.

As a comment, the fact that in the first evaluation weeks all the species showed similarity regarding the increase of the descriptor variables of the aerial and root performance could have occurred because, in addition to the taxonomic relationship they have, they show a similar architecture regarding the pattern of branch formation and arrangement after the emergence.

Yet, in general, leucaena and erythrina were better than albizia, which coincides with the reports by Toral (1998) when evaluating six forage tree species in their first development stages, and by Gómez et al. (2004) when studying the establishment of L. leucocephala, Gliricidia sepium and A. lebbeck; in both studies it was reported that in most of the evaluated variables albizia showed lower values as compared to the other species.

On the other hand, the presence of nodules was evident in erythrina from the fourth week; while in the other treatments they were detected a week later (fig. 9). Depending on the measurement, each of the species stood out at different times. Nodule emergence in albizia was significantly higher (P<0,05) at seven moments; while in leucaena and erythrina superiority was found only in four and three measurements, respectively.

In general, albizia showed higher abundance of nodules (six as average); on the other hand, erythrina showed as average five nodules and leucaena had the lowest quantity (four).

The fact that increases and decreases were observed in the number of total nodules in these species during the experimental period occurred because they show a constant formation and senescence dynamics, even at the nursery stage (Medina, 2006). On the other hand, the results indicated that the nodulation process did not occur immediately after the emergence, but after the first month post-planting, when the root system began to develop. Likewise, it was proven that, for the three species, symbiosis with the microorganisms present in the substratum was spontaneous, if it is taken into consideration that no previous inoculation was performed on any species.

The quantity of total nodules shown by leucaena in this work was higher than the one obtained by Razz et al. (1995), when evaluating the effect of fertilization with N and P on the nodulation of two ecotypes of L. leucocephala.

CONCLUSIONS

The species evaluated in nursery using an alkaline substratum, showed a differentiated performance in most of the measured variables. Leucaena and erythrina showed a similar performance between themselves, as compared to albizia.

Under the described conditions, the seedlings of L. leucocephala (Trujillo ecotype) and E. fusca were optimum for transplant since the eighth week, because they showed adequate height and optimum aerial and root morphostructural development. For A. lebbeck more time in nursery is suggested, although the root system is well developed.

Since the fifth week the species started to show morphostructural particularities in their growth. Leucaena and erythrina showed higher aerial morphostructural development; while albizia had a more developed root system, with higher nodule production.

ACKNOWLEDGEMENTS

The authors would like to express special thanks to the support staff of the Experimental Station of the INIA in the Trujillo state, Venezuela, for the collaboration provided to conduct this research.