RESEARCH WORK

Recovery capacity of 23 Leucaena spp. accessions after pruning

Hilda B. Wencomo¹ y R. Ortiz²

E-mail: hilda.wecomo@indio.atenas.inf.cu

²Instituto Nacional de Ciencias Agrícolas, La Habana, Cuba

ABSTRACT

Twenty three Leucaena spp. accessions were evaluated for two years, in order to know the variability existing in the population regarding the recovery capacity after pruning. When the plants were higher than 2 m, the uniformity pruning was made at 1 m above the soil level. The measurements were: stem diameter, number of regrowths and their length with a weekly frequency, from which the growth rate of each accession was calculated; in addition, yield was determined. The main component, cluster and simple lineal regression analyses were made through the statistical program SPSS version 11.5 for Microsoft Windows®. Variability was observed in the evaluated indicators (86,11% accumulated in the component) and they all contributed to its explanation. In turn, the cluster analysis allowed the formation of three groups. All the accessions were concluded to show recovery capacity after pruning, with differences among them. The accessions Leucaena leucocephala cv. Cunningham, cv. Peru, CIAT-9119, CIAT-9438, CIAT-751, CIAT-7988, CIAT-7384, CIAT-7929, CIAT-17480, cv. Ipil-Ipil and cv. CNIA-250; L. lanceolata CIAT-17255 and CIAT-17501 and L. diversifolia CIAT-17270 were the ones with higher recovery capacity. For such reason it is recommended, in future studies related to the characterization, evaluation and selection of these species, to make studies including their persistence in time under pruning conditions, as well as their use in the development of new silvopastoral systems.

Key words: Performance, Leucaena spp., pruning.

INTRODUCTION

The physiology of defoliation and regrowth constitutes an essential issue of plant biology, because they are basic elements of pasture management. In the practical order, a response is sought to the influence of defoliation (pruning or grazing) height and frequency on the growth dynamics and yield during the regrowth period. The aerial biomass of the trees and shrubs present in silvopastoral systems provides part of the feedstuffs to the animals developed there, for which the adequate knowledge of its management is essential to achieve a higher level and stability in biomass production throughout the year.

The species Leucaena leucocephala is one of the most studied and utilized in silvopastoral systems (Hernández et al., 2000), which is endorsed by its high protein value, among other aspects; in addition it is used as shade source (Molina et al., 2001), for preserving and improving the soil, producing large biomass amounts and recycling nutrients (Sánchez, 2007); it has regrowth capacity after being browsed or harvested mechanically and capacity to recover rapidly from biotic or abiotic stress (Ruiz and Febles, 2001), a very important aspect to be taken into consideration when this plant is used in such systems. In spite of that, the performance among accessions is different.

Paying attention to the above-mentioned characteristics, an evaluation study of 23 accessions belonging to a collection of the Leucaena genus was started, in order to know the variability existing in the population regarding the performance of the recovery capacity after pruning.

MATERIALS AND METHODS

Location of the experimental area. The study was conducted in areas of the Experimental Station of Pastures and Forages "Indio Hatuey" , which is located at 22º 48' and 7'' latitude north and 79º 32' and 2'' longitude west, at an altitude of 19,9 masl, in the Perico municipality, Matanzas province, Cuba (Academia de Ciencias de Cuba, 1989).

Climate characteristics. The average addition of the rainfall of two years was 2 462,3 mm. The rainfall during the rainy season (May-October) represented, as average, 79,8% of the total annual volume. The evaporation in the zone increased since January, with maximum values in April (220 mm). The average annual relative humidity was 82,6%, with the highest value in July (89,0%) and the lowest one in April (75,5%).

Soil characteristics. The soil has plain topography, with slope of 0,5 to 1,0%, and it is classified by Hernández et al. (2003) as lixiviated Ferralitic Red, hydrated ferruginous nodular humic, of rapid desiccation and deep on limestone. This type is equivalent to the Ferrosol group, in the classification system of FAO-UNESCO (Alonso, 2003).

Plant material. From the 180 accessions existing in the Leucaena spp. collection that are preserved in the germplasm bank of the Station, 23 were taken (table 1), representative of the population (four plants from each one were evaluated for two years).

The pruning was done in the dry season, specifically in November of each year according to the proposal made by Hernández (2000). When the plants were higher than 2 m (between 3 and 4 m), pruning was made at 1 meter above the soil level, according to the recommendation made by Francisco et al. (1998).

The measurements related to the recovery capacity of the accessions were: number of branches, stem diameter (at the base, with a caliper) and number of regrowths. For the latter the regrowths produced by the plants were counted and their length was measured (with a graduated ruler), with a weekly frequency.

This procedure was made in the five most developed regrowths (Toral et al., 2006) until the pruned plants reached 2 m of height or more and the yield was also determined.

Statistical processing. The data were processed through the principal component analysis (PCA) (Morrison, 1967), in which those principal components that showed proper values higher than one and sum or preponderance factors higher than 0,70 were taken as analysis criterion.

Afterwards, the cluster analysis was applied for the grouping and selection of the accessions, using as criterion the Euclidean distance, from the results in the PCA (Torres et al., 2006), and the stadigraphs mean and standard deviation were determined for the analyzed variables.

In addition, the lineal regression analysis was used in order to know the best adjustment models and the functional relationship between the variables regrowth number and length. This analysis identifies the model or function that joins the variables, estimates its parameter and, eventually, tests the hypothesis about them. Once the model is estimated, it is possible to predict the value of the dependent variable regarding the other independent variable(s), and provide a measure of the precision of that estimation. The days were considered independent variable and as independent variable: regrowth number and length.

As selection norm of the best adjustment equation, the criteria proposed by Guerra et al. (2003) were taken into consideration: significance level, determination coefficient, R² higher than 0,70, residual variance V(e), residue analysis (el) and the standard error of the estimated parameters SE (âi). All the analyses were made through the statistical program SPSS version 11.5 for Microsoft Windows® (Visuata, 1998).

Selection of the accessions. In the selection process the performance of each accession was evaluated in the variables that contributed the most to the formation of the components (table 2) and some desirable characteristics (in this case agronomic ones) that the trees and shrubs used in silvopastoral systems should have, according to Borel (1997), Gómez et al. (2002) and Simón et al. (2005). It was established that the selected accessions should fulfill all or two of the evaluative criteria considered.

RESULTS AND DISCUSSION

When making the principal component analysis only one component was obtained, which accounted for 86,11% of the total variance (table 3); to this all the evaluated indicators contributed.

The variability shown through the indicators is probably due to the high relationship that existed among them, or the growth dynamics of each particular accession (Dávila and Urbano, 1996), which can be related to the reserve accumulated by the plants and to the utilization capacity and it is expression of the particular genetic potential of the Leucaena species. Such aspect is of high interest for silvopastoral systems, because the capacity to regrow, as well as their adequate development and fast growth, determine their utilization and can contribute as very important indicators to the selection of forage species and accessions.

It is important to emphasize the statements made by Seguí et al. (1989), who declared that the knowledge of the relationships among characters of agronomic interest plays a significant role in the selection process, and indicated that they can be advantageous or not (desirable of undesirable relationships). Thus, the accessions that combine desirable characters, as acceptable regrowth length and number, become a material to be considered for exploitation purposes in silvopastoral systems, in which animal and plants coexist.

The cluster analysis based on the results of the PCA during this stage, allowed the formation of three groups (table 4). According to the results, the accessions L. leucocephala cv. Cunningham, cv. Peru, CIAT-9119, CIAT-9438, CIAT-751, CIAT-7988, CIAT-7384, CIAT-7929, CIAT-17480, cv. Ipil-Ipil and cv. CNIA-250; Leucaena lanceolata CIAT-17255 and CIAT-17501 and Leucaena diversifolia CIAT-17270, belonging to group I, were the plants with higher number of branches, regrowth length, diameter, number of regrowths and yield, with regards to the accessions from groups II and III.

Figure 1 shows a line diagram with an adjustment curve that approaches the relationship between the number of regrowths and the days in each of the groups formed, where the increasing trend is observed in the production of new regrowths in each group, as the days passed. The model that explained with the best adjustment this relationship (R²=0,98***, R²=0,96*** and R²=0,97***, respectively) was the quadratic one. As can be observed, for all the accessions in the first 28 days there was a slow emission of new regrowths, and after 35 days their increase was fast, mainly in the ones that formed group I. In general, these accessions had an increasing recovery capacity after pruning, aspect that could have been influenced by their capacity to recycle the reserves accumulated during the establishment period.

This performance might be specific of the species (Pezo and Ibrahim, 1999), but it is also possible to ascribe it to the following factors: the distribution of non-structural carbohydrate reserves among the different parts of the plant (aerial and underground), the photosynthetic capacity of the residual leaf area (Greaves et al., 1999), the presence of an adequate area of reserving parenchymatous tissue and active meristematic tissue; and the mobilization and utilization capacity of those reserves. They are necessary for regrowth development (Francisco and Simón, 2001) and in this study they were more efficiently manifested in the accessions belonging to group I than in the other groups.

In previous studies the plants of this genus, especially of the species L. leucocephala, have high regrowth capacity even in the dry season, due to the characteristics of their root system, which has a high component of permanent structural roots, as well as a system of fine roots responsible for water and nutrient uptake in the deepest soil layers (Pezo and Ibrahim, 1999).

In this regard, Ruiz and Febles (1987) referred that the plants of the Leucaena genus, especially the accessions of L. leucocephala, have regrowth capacity after defoliation or browsing. In the case of the other species, it is not exactly known how it occurs, because they are less utilized in research nationally and internationally. Nevertheless, in this study it behaved differently, among species as well as among accessions.

Regarding the above-expressed, there is generalized consensus on the fact that reserves are important in the first regrowth days (between six and eight weeks), and that from that moment on the leaf area is capable of supplying carbohydrates and other substances necessary for the later increase; this is evident when long resting occurs, especially during the dry season. Yet, this performance in the first days can be variable, as it was observed in the accessions belonging to groups II and III, in which the variability was more evident than in the accessions of group I (fig. 1). In studies conducted in this regard in Cuba, Díaz (2006b) proved that the lipid consumption was intense between the first two and four weeks, and varied for the rainy and the dry season.

In turn, the previous elements are linked to the availability of abiotic resources, such as water and nutrients, and nutritional deficiencies and hydric stress decrease the regrowth formation range and the photosynthetic tissue activity. This can accelerate the leaf senescence and decrease the production of edible biomass, aspects that occur independently from the similarities and contrasts that exist among the groups and even among the accessions that form them, as it likely happened in this study.

Figure 2 shows a line diagram with an adjustment curve that approaches the relationship between regrowth length and the days, in each of the groups formed. The model that explained with best adjustment this relationship (R²=0,95***, R²=0,95*** and R²=0,96***, respectively) was the cubic one. It is observed that as the days passed there was an increase in length (growth rate) of the regrowths for the three groups until 56 days, independently from the particular response of the accessions in them; although similarly to regrowth emission, their growth for group I was faster than for group II. However, between 56 and 63 days the growth rate decreased, which coincided with the height of 2,48 and 180,6 cm.

In group I (table 4) the grouping of three accessions was detected (L. lanceolata CIAT-17255 and CIAT-17501, and L. diversifolia CIAT-17270), which are not from the species L. leucocephala, which could indicate that in them the growth rate of regrowths is very similar to that of this species, in which not only the accession in question has influence, but also the integration of several factors. In this sense, Voisin (1963) referred that all plants in their growth dynamics, under stable climatic conditions, describe a sigmoid curve in three stages, to which he called universal biological curve. The organs of any organism also show these characteristics, so that the growth of any plant is the result from the integration of many curves of each of its organs, aspect also mentioned by Stür et al. (1994) for the case of medium-height trees, such as Leucaena.

Under field conditions, due to the climatic elements, these curves usually alternate sometimes, so that the sigmoid character is not clearly observed. In Cuba several authors, among them Blanco (1996), have described the growth curves for tropical pastures, where it can be observed that the time intervals in which their stages are produced differ from the reports for temperate pastures.

In the case of trees and shrubs with forage purposes these principles are also valid. Ligneous plants (especially trees) grow slowly, as compared to grasses, which constitutes a difficulty in the establishment and after this stage in silvopastoral systems.

In this regard Díaz (2006b) stated that in the case of grasses or herbaceous legumes, the causes of this phenomenon are related, first, to the growth dynamics and leaf expansion (which is very slow in its beginnings) and also with the partition of biomass towards the root system during the first weeks. This last aspect has higher importance for the plants adapted to arid conditions (such as buffel and alfalfa) and for ligneous plants it is linked to the energy cost of lignified tissue production.

In the three groups, until 28 days, there was slow regrowth growth (fig. 2), which could have depended on the species and the intensity degree with which it was defoliated (by cutting or grazing with animals). This performance might be related to the little amount of leaf area, the utilization capacity of the reserve accumulated in the roots and the stem base, the development of the assimilative system and the distribution of photoassimilates in the different organs. According to Díaz (2006a), in this period the photosynthetic rate is high and respiration is low, which establishes a positive relationship in the net assimilation of the plant and, hence, an increase in the accumulation of dry mass with predominance in the leaves.

Since 35 until 56 days the regrowth growth increased for all the accessions of the three groups, but faster in those from group I (fig. 2). In this period, according to Stür et al. (1994), a remarkable increase of the leaf area occurs, with a positive balance between photosynthesis and respiration until reaching the highest photosynthetic production. In this stage growth becomes relatively constant and independent from the variable that is measured, which expresses the maximum growth rate of the crop and reserve accumulation.

This is in correspondence with the report by Bonilla (2002), about the fact that this period is characterized by the delay in the death rate of the morphological structures of the plant, in regards to the corresponding increase in stem growth, which causes that an increase in tissue production and a decrease in the death of the different components are expressed.

For such reason, according to Díaz (2006a) the maximum production per hectare might not be in correspondence with the maximum photosynthesis, because it depends on how the tissue loss rate due to death behaves.

The performance shown by the accessions from group I as compared to those from groups II and III is likely to be due to the interaction of different factors, independently from the response of each accession in particular.

Plant response to climatic conditions should also be considered, especially rainfall, which although not significant, favored the good development of regrowths and were more effective in some accessions than in others.

The cutting height is another element that could have influenced regrowth emission and growth (Benjamín et al., 1999); however, the response of the accessions was different. In this sense, their deep root system and the amount of reserve substances of the plant should be taken into consideration.

It must also be stated that forage cutting in the different seasons (dry and rainy) and in different development stages (vegetative and flowering), has a close relationship and acts on regrowth performance. Pruning at the beginning of or during the dry season, when there is decrease of the photosynthetically active area and water availability, according to description by Llamas et al. (2001), can cause the depletion of organic reserves (as maximum source of their sustenance), which leads to the decrease of the regrowth formation range and plant growth processes, as seems to have occurred in the accessions belonging to groups II and III.

The increase of regrowth emission and length in general, proved the recovery capacity of the accessions; while the differences among the three groups formed showed contrasts among the species, according to their utilization capacity of the accumulated reserves. The regrowth emission dynamics and their growth rate in an accelerated way, in the establishment stage as well as in recovery after pruning, are very important, because they not only allow crop survival, but also allow it to be used as animal feed, especially in the dry season. The accessions showed an acceptable recovery capacity through regrowth increase and growth.

The emission of new regrowths by plants from this genus, like their growth, showed differences among the groups formed; for such reason, the recovery capacity should not be indicated statically, but taking into consideration the interaction of diverse biotic as well as abiotic factors, as well as the possibility or capacity of the individual genome of each accession to respond or not to the surrounding environment.

According to the results it is concluded that there were differences in the indicators and that they explained the variability found. The species and accessions of the Leucaena genus, in general, showed high recovery capacity after pruning, according to their potential and demand for the use of their reserves, in accordance with the measured variables. In this sense, the accessions L. leucocephala, CIAT-9119, CIAT-9438, CIAT-751, CIAT-7988, CIAT-7384, CIAT-7929 and CIAT-17480; L. lanceolata CIAT-17255 and CIAT-17501 and L. diversifolia CIAT-17270, in addition to the commercial varieties L. leucocephala cv. Cunningham, cv. Peru, cv. CNIA-250 and cv. Ipil-Ipil, were selected for being the ones with higher recovery capacity after defoliation.

It is recommended, in future studies related to the characterization, evaluation and selection of these species, to make studies including their persistence in time under pruning conditions, as well as their use in the development of new silvopastoral systems.