WORK RESEARCH
Comparison of three tree legumes planted on an alkaline substratum during the nursery stage. II. Growth and biomass distribution pattern
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 compare the growth and biomass distribution pattern of Albizia lebbeck, Leucaena leucocephala (Trujillo ecotype) and Erythrina fusca for 18 weeks in nursery. The studied variables were: growth rate related to height (GRH) and DM distribution (GRDM), as well as dry matter percentage (DM) and dry weight per plant of the fractions (leaves, stem, root). A completely randomized design for measurements repeated in time and ten replications per species were used. E. fusca showed the highest average GRH and GRDM (3,7 mm/day and 94,8 mg/day), followed by L. leucocephala (3,1 and 81,3) and, lastly, A. lebbeck (1,2 mm/day and 53,1 mg/day), respectively. In dependently from the species, the highest percentage of average DM was observed in the roots (31,9), followed by the stem (28,0) and leaves (25, 4). A higher DM distribution per plant was observed in the leaves (3,9 g) and root (3,1 g) as compared to the stem (2,9 g). It is concluded that, for these nursery conditions, E. fusca showed the best results; while A. lebbeck would need more time to reach the desirable characteristics for its transplant to the field.
Key words: Biomass, growth, legumes, nurseries.
INTRODUCTION
Many legume tree and shrub species are used under tropical conditions as important components of agroforestry systems, among which Leucaena leucocephala Lam. de Wit., Albizia lebbeck Benth. and Erythrina fusca Lourd. stand out due to their multipurpose nature, forage potential and natural distribution (Razz et al., 1998; Toral, 2005; Escalante, 2006).
In spite of all the advantages of silvopastoral systems, the massive propagation of trees in livestock production systems is limited by the slow establishment after planting, because of the characteristics of these species and the competition with undesirable plants (Medina et al., 2007). In order to counteract such problem the cultivation of the species in nursery, before they are planted in the field, is recommended, so that the plants are more vigorous to face establishment conditions (Medina, 2006). Studying the performance of these ligneous plants in their initial growth stage is important in order to establish viable propagation and establishment strategies within the framework of animal production systems.
Considering the above-explained facts, the objective of this study was to evaluate in nursery the growth and biomass distribution pattern of A. lebbeck, L. leucocephala and E. fusca in the Trujillo state, Venezuela.
MATERIALS AND METHODS
Location of the experimental area. The trial was conducted for 18 weeks 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 municipality, Venezuela.
The average annual rainfall was 1 500 mm and the mean temperature, 27ºC.
Treatments. Three species with potential to be used in silvopastoral systems of the Trujillo state were studied: A. lebbeck, L. leucocephala (Trujillo ecotype) and E. fusca.
Characteristics of the substratum used. For the plants to be put in the nursery, 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. Pre-germinative treatments were applied to the seeds 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 E. fusca seeds, immersion was used during 24 hours in water at room temperature (Rodríguez and Murgueitio, 1995).
The other particular details were described by Medina and García (2010) and Medina et al. (2010) in previous works.
Measurements. The measurements were made with a weekly frequency. The first evaluation was made seven days after emergence, by means of destructive samplings in 10 plants during the 18 weeks.
The growth rate related to height (GRH) was estimated through the difference of growth with regards to each week, and it was expressed in millimeters per day. The growth rate according to dry matter distribution (GRDM), referred in milligrams per day, was calculated by determining the weekly increase of the dry weight of leaves, stems and roots, for which the plant was carefully taken from the bag and the remnant soil to leave it uncovered. Afterwards, it was separated into leaf, stem and root through two transversal cuts, one on the stem base and the other on the base of the leaf petiole. The individual samples per plant were placed in paper bags and introduced in a forced-air stove (Kaltein Trademark, Colombia) for 72 hours at 40ºC, in order to know the dry weight of each fraction. The DM percentage was estimated by weighing each part before and after its introduction in the stove, according to the procedure described by the AOAC (1990).
Experimental design and statistical analysis. A completely randomized design for 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, for a significance level P<0,05.
RESULTS AND DISCUSSION
Table 1 shows the GRH and GRDM in the studied species during the evaluation period.
Regarding the GRH, during the first two weeks E. fusca was different from the other species (P<0,05); between the fourth and ninth measurement L. leucocephala showed higher values (P<0,05), while from week ten, E. fusca showed significant differences (P<0,05) again from the others.
The results proved that E. fusca showed an accelerated growth immediately after emergence and a stable performance throughout the evaluation, while the performance of L. leucocephala was also outstanding until the thirteenth week. In the case of A. lebbeck, it did not stand out as compared to the other species.
The decreasing trend of GRH in leucaena since the ninth week (63 days in nursery) suggests that, under nursery conditions with alkaline substratum, this plant can be transplanted after eight weeks. The same happened with E. fusca, although this species can remain in nursery at least until 18 weeks, because it did not show constant growth. In the case of A. lebbeck, given its slow growth, it should remain longer than the other species under the above-described conditions.
Regarding the GRDM, E. fusca reached higher results (average of 94,8 mg/day) in a higher number of times; while L. leucocepahala and A. lebbeck produced 81,3 and 53,1 mg of biomass per day, respectively.
The coherence among growth rates (according to height and biomass) is an aspect that has been reported in similar studies (García, 2007). Nevertheless, some authors emphasize the need that species growth under nursery conditions is evaluated regarding DM production, because plant height is more influenced by environmental changes (Guevara and Guenni, 2004); while the DM distribution constitutes an intrinsic feature of each species (Pineda, 2004). Maybe the coincidence between both performances, observed in this trial, is due to the fact that the light, light intensity and humidity conditions did not influence the height reached by these plants during the evaluation.
With regards to the DM percentages of the growing fractions, the results are shown in tables 2 and 3.
In the first week of evaluation, leucaena showed the highest DM content in the leaves as well as the stem and root. In the second measurement leucaena differed from A. lebbeck in terms of DML, unlike the DMS and DMR. Since the third and until the fourteenth evaluation, these species did not differ statistically from each other for any of the three growing fractions and were statistically higher than E. fusca (P<0,05) during that period.
In general, E. fusca showed the highest water content in the tissues of all the plant parts, as compared to the other species during the nursery stage.
The DM content was higher in the roots, with regards to the stems and leaves; a trend that was maintained throughout the trial. Although the DM values constitute one of the variables with lower intraspecific fluctuation in forage plants, it is known that during the growth process of vascular plants the cell water content depends on the hydric conditions under which the plant is cultivated and on specific physiological processes which occur during tissue development (Pineda, 2004). Hence the need not to generalize about DM contents of ligneous species and make characterizations in the nursery stage, as well as the growth phase in the field, in order to report representative values.
During the first nine weeks of evaluation, the moisture content of the leaves and stems was similar and lower to that of the root in all species. However, since the tenth week until the end of the trial the DM contents were stratified in a more defined way and the leaves showed the lowest values. These results coincide with the water content trend reported in several studies for a large number of ligneous plants in the initial growth stage (Yágodin, 1982a), in which because of the continuous process of nutrient absorption, the mineral concentration in the aerial biomass and transpiration, the DM content varied almost homogeneously regarding the plant part; in this sense the highest values were observed in the basal or underground part and the lowest ones in the aerial zone in accelerated growth, which have specialized organelles for transpiration (Yágodin, 1982b).
The most marked stratification was observed since the ninth week which is explained by the fact that continuous plant growth presupposes a higher cell lignification (García, 2007), which brings about a considerable water loss and/or translocation to the leaves, thus increasing the tissue DM content in the aerial support organ.
Although no antecedents were found in literature of similar trials with the studied species, to validate the results of this study, Pineda (2004) described that in the initial growth stage the species tend to differentiate more remarkably the moisture content in tissues, related to the elements they require in higher amount; on the other hand, Ricardi (1992) reported that the species from the Erythrina genus are more succulent than other forage legumes from the Mimosoideae and Faboideae subfamily, on ecophysiological bases and precepts. These two statements support the results obtained in this trial regarding variability in the DM content of the root in the first weeks of evaluation and the superiority of E. fusca in terms of biomass moisture.
The DM content, in addition to its link to the biomass age, is also related to the physiological age of the plant, because it has been proven that during maturation the tissues are rearranged and, in some cases, membrane integrity is lost; this favors the irreversible water loss and, thus, the DM contents are more homogeneous (Yágodin, 1982a).
Regarding the weight accumulation during plant growth in nursery, E. fusca showed the highest value in the leaves, stem and roots, as compared to A. lebbeck and L. leucocephala (P<0,05), except in the first two weeks, in which no differences were found between E. fusca and A. lebbeck for DWL and DWS.
During the trial, the DM accumulation for the three species increased. In the second half of the evaluation an average of 6,7; 4,9 and 5,6 g was obtained for the leaves, stem and root, respectively; this differs from the results reported by Guevara and Gueni (2004), who obtained higher accumulation in the stems, followed by the leaves and roots. These differences in DM accumulation by the organs could have been due to the ecotypes used by these authors and/or the environmental or substratum conditions that prevailed in each trial, considering that they are factors which influence plant performance after emergence (Medina, 2006).
In addition, García (2007) reported that during the initial growth stage and the establishment of some forage species, plants tend to show variable patterns of biomass distribution, regarding the edaphoclimatic conditions. When they are planted under favorable climatic conditions, but there is any chemical limitation in soil composition, the species tends to develop higher leaf area through photosynthesis and the absorption of specific nutrients. Nevertheless, when the contrary occurs, root growth and development is favored in order to counteract the adverse effects of the aerial environment. Perhaps for such reason, under alkaline substratum conditions (pH>8,0) the evaluated plants will produce higher leaf biomass, as compared to stems and roots.
Likewise, E. fusca and L. leucocephala showed high aerial biomass and their roots also had a good capacity of rapid growth. Such aspects are very important, even in nursery, because thus a satisfactory establishment in field is likely to be guaranteed (Ansari et al., 1995).
It should be emphasized that, seemingly, the evaluated species destined a large amount of energy to leaf formation, which could mean, approximately, an increase in forage biomass production during later growth stages.
On the other hand, the results of leucaena in week 11 were lower than the ones reported by González et al. (2005), who after 75 days obtained an average aerial and root total biomass accumulation of 17,5 and 8,3 g/plant.
The substantial differences in such results are likely to be related to the ecotype used in this study (Trujillo) and the one used by González et al. (2005) (cv. Cunningham), in addition to the alkalinity conditions of the substratum used in this case, which could have restricted growth.
CONCLUSION
ACKNOWLEDGEMENTS
The authors would like to thank the support staff belonging to the Experimental Station of INIA in the Trujillo state, Venezuela, for the aid provided in t
