RESEARCH WORK

Soil quality indicators: A new way to evaluate this resource

Gertrudis Pentón, G. J. Martín, Katerine Oropesa, Yolai Noda y F. Alonso

¹Estación Experimental de Pastos y Forrajes "Indio Hatuey". Central España Republicana, CP 44280, Matanzas, Cuba
E-mail: gertrudis.penton@indio.atenas.inf.cu

ABSTRACT

The objective of this study was to evaluate the morphoagronomic performance of the specie Morus alba L. var. Tigreada during the first establishment stage, since seedling transplant. For such purpose, the response of the crop to seedling management for transplant was determined, growth was characterized and the linear variables that allow quantifying the leaf area of such variety were determined. The seedlings were maintained in the nursery for 120 days and they were transplanted in the rainy season. The rows were oriented from East to West, with a planting frame of 1,0 x 0,5 m. Adequate weed, pest and disease control was maintained. There was no irrigation and organic matter was applied at the moment of transplant. Two management forms of the seedlings for transplant were established: total defoliation, and cutting at 50 cm of height and total defoliation. During the initial growth stage after transplant, a slight increase was obtained in the regrowth capacity of the cut and defoliated seedlings. This did not affect the establishment of the species, because after 135 days the leaf biomass production varied between 100 and 116 g/plant. Growth in the first 155 days described a sigmoid curve, characterized by a slow rate during the first 21 days; this was followed by intense growth between 30 and 135 days, becoming slow again with the beginning of the dry or winter season. It was proven that the measurement of the leaf length and its adjustment through the nonlinear models Y = B0 + B1X + B2X2 and Y = B0 + B1X + B2X2 + B3X3 allow estimating the leaf area, and this is achieved for the condition that the length of the leaf oval measures between 1,3 and 20,4 cm. The high correlation found makes this estimation procedure appropriate.

Key words: growth, establishment, Morus alba L..

INTRODUCTION

Mulberry (Morus alba L.) is a millennial plant, which has become a forage resource in agroforestry systems. The variety Tigreada which is the study object of this work- shows high bud emergence dynamics and growth and development rate of the branches and leaves. It has defined dots on the stem cuticle, and its leaves, lobulated, reach from 1,3 to 20,4 cm of blade length with 3 and 12 months of age, respectively (Pentón
et al., 2007).

The first references to the multiplication methods of M. alba in Cuba are from the 30's of the 20th century. Fernández (1935) commented about the possibilities of planting by seed, cutting and seedling. However, the trend according to the report by Infojardín (2011) is the use of cuttings, due to the easy handling of this propagation means and the high survival percentage of the plantations, and the propagation by cleft or budding grafts on plants obtained from seed. Nevertheless, the advantages of seedling planting cannot be obviated, although it demands more time and dedication during the nursery stage, it constitutes a warrant for the establishment of the plantations, especially when the method of seedling defoliation and pruning is used before the transplant (Cifuentes and Kee-Wook, 1998). It should be also stated that the evaluations of mulberry in the nursery stage have shown high survival rates (80 and 100%) 35 days after planting (Noda et al., 2004).

Because mulberry is highly dependent on the reserves accumulated in the stem during the initial growth stage, and because it shows a seasonal behavior, with a certain semideciduous character in the winter months, it is necessary to conduct further studies about growth and development during the establishment.

It is known that the growth analysis refers to the evaluation of plant production, derived from the photosynthetic process and the result of the performance of the assimilatory system during a certain period of time. According to Jadão et al. (2004), it was developed and first applied by physiologists from the English school, and it is internationally considered as the pattern method for estimating the biological or primary productivity of plant communities. The leaf area is a variable that describes the dimension of the photosynthetic system (Kozlowski et al., 1991), hence the importance of its estimation through simple allometric variables.

Lal and Subba Rao (1951) stated that the leaf area could be measured with complex instruments, such as the optical planimeter, or through simple and arduous methods, such as the mechanical planimeter. It can be estimated with the application of equations and coefficients, which selection depends on the objective of the measurement and the accuracy level desired in the work. Such estimations must start from linear measures and geometrical figures (Simón and Trujillo de Leal, 1990), or from the relation between leaf area and weight (Del Pozo and Álvarez, 2001; Paytas, 2005).

Due to all the above-explained facts, the objective of this study was to evaluate the morphoagronomic performance of the species M. alba during the first stage of establishment. For such purpose, the effect of seedling management at the moment of transplant on the yield indicators was determined; growth was characterized, in general; and the linear variables that allow characterizing the leaf area of the variety Tigreada, independently from the crop to which the crop is subject, were determined.

MATERIALS AND METHODS

Trial location. The study was conducted in areas of the mulberry germplasm bank, located in the EEPF "Indio Hatuey". Its duration was153 days since the seedling transplant.

Soil characteristics. The soil of the site is Ferralitic Red (Hernández et al., 2003), with good surface and internal drainage. The chemical composition is characterized by low mineral availability and low content of P2O5, K2O and organic matter. The pH is slightly acid to neutral (table 1).

Experimental procedure. Mulberry seedlings which had been growing for 120 days in nursery and were 1,20 m high as average, were planted. The plots (5 x 15 m) had five rows oriented from East to West, with 30 plants in each (frame 1,0 x 0,5 m). Organic matter was applied at the moment of seedling transplant, at a rate of 15 t of manure/ha. Adequate control was maintained of weeds and pests and diseases. No irrigation was applied.

In order to determine the effect of seedling management at the moment of transplant on yield indicators, two forms were established: total defoliation, and cutting at a height of 50 cm and total defoliation. The samplings were conducted 15, 21, 135 and 150 days after planting.

Experimental design. A randomized block design with four repetitions was established. The evaluated variables were: number of buds emerged (u), percentage of regrown leaves, leaf biomass production (g/plant), number of branches (u), branch length (cm), branch diameter (mm), length of the main stem (cm) and diameter of the main stem (mm).

In the seedlings pruned at 50 cm and defoliated, growth in general was characterized and the linear variables to quantify the leaf area of the variety were determined. For such purpose, evaluations were made weekly, since 15 until 150 days after planting. The evaluated variables were: number of branches (u), branch length (cm), branch diameter (mm), length of the main stem (cm), diameter of the main stem base (mm), leaf length and width (cm).

The samplings to establish the estimation model through linear variables were conducted after 90 days of regrowth and 365 days after the mulberry plantation was established; previously, five representative leaves had been selected per individual (a total of 100 leaves). In the case of the determination of blade length (L) and width (W), with a millimetrical ruler the length of the central nervure was measured and, on the middle point of the leaf length, the length from outer extreme to extreme perpendicularly to the central nervure was also measured. To measure the leaf area (LA), each oval was placed beneath a transparent glass, and without causing damage or tissue destruction the area was measured with an optical planimeter (PZO WARSZAWA PL1).

Statistical processing. A variance analysis was made through the general linear model. The means were compared through Duncan's (1955) multiple comparison test, for a significance level of 0,05. The correlation and simple regression analyses were used to learn the interrelation among the variables and the best adjustment models. The statistical pack used was Infostat, free version.

RESULTS Y DISCUSSION

It is known that in the transplant process of different plant species, the cutting of the apical zone and/or the defoliation of the seedlings stimulate the action of growth hormones regarding the production of new shoots and leaves, depending on the site where the growth spots are located. In the case of M. alba L., although the production of new shoots and the percentage of emerged plants were higher in the cut and defoliated seedlings, the differences were not significant (figs. 1 and 2). Such response is explained by the fact that this species produces buds all along the stem and concentrates a large amount of reserves in its base.

In general, the results coincide with those reported by Milera et al. (2003), who observed, under similar edaphoclimatic conditions that mulberry plants produce 71% of the buds during the first 15 days of the plantation.

In this regard, Satoh (cited by Yamashita, 1985) stated that, in general, the regrowth of new organs after a pruning or defoliation is determined, to a large extent, by the action of cytokinin (originated in the roots), which, by being stored in the remaining leaves, promotes the chloroplastic multiplication and protein and chlorophyll synthesis. This author found an increase in the chlorophyll concentration (30 mg/cm²) until 40 days after plant cutting. At the same time he observed that after the opening of the first leaf, 7 days after the propagules were planted, the chlorophyll content increased rapidly to 3 mg/g fresh weight, in a 14-day interval, independently from the location of the shoots with regards to the soil level.

Leaf biomass production 135 days after transplant did not significantly differ between treatments, and varied between 100,87 and 116,35 g/plant (fig. 3), which coincided with the indifferent performance of the growth attributes after 155 days (table 2). This allows considering that the advantage of seedling cutting and defoliation lies on facilitating the transfer and transplant works of the nursery seedlings.

As a trend, the values of main stem length and diameter of the stem base are in correspondence with the observations made by Pentón et al. (2006), who stated that the described growth pattern, from the planting of mulberry cuttings in July and September, allows establishing between eight and ten months as adequate moment for the establishment. In this period a fast growth is observed, which is shown in heights of 0,60 and 2,00 m after 7 and 12 months, respectively.

A high correlation was found between the branch and stem length and diameter, with coefficients higher than 0,63 (table 3). It should be stated that in the Eastern world high importance is assigned to this type of analysis for predicting the mulberry productivity. It has been empirically used for hundreds of years by those who cultivate this species. The quantitative analysis of growth was defined by Domínguez et al. (2003) as "dimensional analysis technique". It is based on finding the indicators or morphological variables of best adjustment and, from this principle, developing different formulas to estimate production (Murray and Jacobson, 1982).

Among other uses, this technique is used as an indicator of the habitat value (Harniss and Murray, 1976), as a factor to evaluate the utilization degree of resources (Jonson et al., 1988) and as a tool to estimate directly the growth (Roundy et al., 1989). It is based on a non-destructive procedure which remarkably reduces the evaluation costs (Uresk et al., 1977). Authors such as Tikader and Roy (1999) found highly significant positive correlations, for example: total length of primary branches per plant, total length of secondary branches and edible biomass yield. In addition, they observed, in 10 exotic mulberry accessions in Bengal, that the rooting percentage had a positive correlation to stem length, number of leaves per sample and their dry and fresh weight, fresh and dry stem weight, biomass weight, root weight, length, volume and number of roots per sample. Sahu et al. (1997), when analyzing 15 mulberry varieties, observed highly complex interactions between stem weight per plant and leaf weight per plant. Besides, they noted an indirect effect on the yield of variables, such as the number of branches per plant and the internode length.

All the growth variables described a curve in time which coincides with the Gompertz regression model (table 4). In this regard, it is known that the sigmoid-shaped growth curve, as in the case of this regression model, is related to metabolic phenomena linked to the redistribution, through plants, of the energy porters in the first stages. Three moments were defined during the initial establishment of mulberry from transplanted seedlings (fig. 4). The first occurred between 0 and 21 days and was characterized by a slow growth rate, in correspondence with the fact that, during the first 30 days after planting, a fast decrease of carbohydrates and aminoacids accumulated in the ligneous stems occurred, related to the new developing organs (Yamashita, 1985). Between 30 and 135 days the growth rate increased and, from135 days, the increase of the growth dynamics in length became remarkably slow, which coincided with the beginning of the dry or winter season. Pentón et al. (2006) determined that the species M. alba L. is characterized by a vigorous growth during May to November and a decrease of the fresh leaf and stem production during the period from January to April.

When evaluating the linear variables which allowed quantifying the leaf area of this crop, the results pointed towards the quadratic and cubic equations as the best models, due to their higher contribution (table 5). From the length of the leaf blade as well as the width or the length by width product (L x W), the quadratic and cubic equations exceeded in, at least, one R2 unit the other equations.

In the equations, from the L x W product, the correlation coefficient (R2) did not exceed 80%. The estimations based on the measurement of the leaf width (W) were below 50% contribution. However, the equations derived from the variable leaf blade length (L) had a contribution higher than 91%, with higher significance.

The close relation of the blade length with regards to leaf area differs from the results obtained by Pentón et al. (2006) with M. alba L. var. Acorazonada, which is determined by the distinctive varietal characteristics shown by the species regarding leaf shape. In the var. Acorazonada, they are distinguished by being serrated, heart-shaped; while in the var. Tigreada, they are lobulated. These authors recommended, for the variety Acorazonada, equations from the parameter L x W with R2 higher than 0,99.

In this regard, Simón and Trujillo de Leal (1990) observed in Xanthosoma sagittifolium (L.) that in the clones where the leaf lamina showed a discreetly undulated margin, the R2 of the simple linear model, from the independent variable L x W, was higher than 90%. Nevertheless, in the clones in which the number of undulations in the lamina caused the formation of folds, the R2 values were lower than 90%. These authors suggested that such modifications in leaf shape interfered in the estimation, by introducing an error by underestimation in area measurement.

Leaf morphology is an aspect of relevant importance in the selection of the measures used. When plants are characterized by having leaves with regular shapes, simple mathematical relations can be used from the L x W product. Such methodology is adjusted to crops such as banana, sugarcane and corn (Ascencio, 1985), as well as cherry tree (Cittadini and Peri, 2006). However, there are species such as the medlar tree (Manilkara achras Millar), strawberry, cucumber and tomato, in which equations are adopted from a unique parameter, in some cases due to the irregular shape of the leaf blade, in others, because of the simplicity and speed of their application, although the equations originated by the combination produce moderately superior adjustments (Demirsoy et al., 2005). The results of the study of the mulberry varieties Acorazonada and Tigreada also show that there is remarkable disadvantage of the parameter leaf width as compared to length. Coincidently, Cittadini and Peri (2006) observed that in different cherry tree cultivars the data were better adjusted by using the leaf blade length as independent variable (R2 = 0,863) as compared to W (0,787).

The best adjustments for the estimation of leaf area are described in figure 5. The best relation L vs. LA was shown through the third-degree nonlinear model, where the equation reached a slightly higher accuracy with regards to the second-degree model.

CONCLUSIONS

In this first stage of establishment of the species M. alba var. Tigreada, seedling management at the moment of transplant did not affect yield indicators. Growth in this 150-day stage described a sigmoid curve, characterized by a slow growth rate during the first 21 days, followed by intense growth between 30 and 135 days, which became slow again with the beginning of the dry or winter season. Under such circumstances, it was proven that the measurement of the leaf length and its adjustment through the nonlinear models Y =B0 + B1X + B2X2 and Y = B0 + B1X + B2X2 + B3X3 allow estimating the leaf area of M. alba var. Tigreada, and this is achieved for the condition that the length of the leaf oval measures between 1,3 and 20,4 cm. The high correlation found makes this estimation procedure appropriate.