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Jasso Cantú, D., J.L. Angulo Sánchez, and R. Rodriguez García. 1996. Identification of guayule regions in northern Mexico, based on rubber yield and coproducts quality. p. 336-339. In: J. Janick (ed.), Progress in new crops. ASHS Press, Alexandria, VA.

Identification of Guayule Regions in Northern Mexico, Based on Rubber Yield and Coproducts Quality

Diana Jasso Cantú, José Luis Angulo Sánchez, and Raúl Rodriguez García


  1. METHODOLOGY
    1. Collecting Sites and Sampling
    2. Rubber, Resin, and Biomass Measurements
  2. RESULTS
    1. Morphologic Characteristics
    2. Physiologic Characteristics
  3. REFERENCES
  4. Table 1
  5. Table 2
  6. Fig. 1
  7. Fig. 2
  8. Fig. 3

The development of guayule as a commercial crop has encountered several problems in México, despite a great amount of research and development carried out in the 1970s and mid 1980s (Angulo and Lopez 1978; Lopez and Kuruvadi 1985; Kuruvadi 1985; Lopez and Kuruvadi 1987; Kuruvadi 1988). During this period, a pilot plant established technical feasibility evaluation for the rubber recovery process, and a commercial unit for rubber production was designed (Campos et al. 1978). However, the commercial production units have not been established up to this moment. Among the problems preventing guayule commercial development are the lack of raw product and the low international price of Hevea rubber.

The present guayule areas are based on wild stands, the plants are scarce, widely distributed, and rubber content is variable, ranging from 5% to over 17% (Jasso et al. 1993). This fact makes difficult to pinpoint the potential rubber yields per unit area, hence there is no solid basis to calculate profitability. Furthermore, fluctuations in the rubber price make cost estimations uncertain. Previously, we proposed to consider guayule as a source of more than one product, and to defining new applications in order to improve commercial exploitation (Angulo and Jasso 1995). In addition, we have considered that selection and reproduction of high yielding clones is critical to develop guayule as a commercial crop.

We established a productivity index (PI) in order to identify plants with outstanding features. This index is used in this work to designate high yielding areas and plants, within the Mexican guayule region. Iodine value was determined because the resin might be used for varnish and adhesives (Belmares et al. 1980; Schloman 1988; Schloman and Wagner 1991; Thames and Wagner 1991). This parameter reflects the presence of double bonds required for these applications. Because biomass would be used as a feedstock, bagasse quality was tested by determining protein content after resin and rubber extraction. Although some data on the protein content in the guayule bagasse have been reported (Schloman and Wagner 1991), only one previous work on the resin iodine value determination was found (Belmares et al. 1980). There are no reports on the possible correlation between rubber or resin production in guayule and the iodine value. We suggest that the chemicals possessing double bonds (including fatty acids, terpenoids, and other chemical species) may be related to rubber biosynthesis.

In this study, three zones, in two states, Coahuila and Zacatecas, formerly identified as high rubber yielding areas (Naqvi and Hanson 1983; Lopez and Kuruvadi 1985), were revisited, guayule shrubs samples collected, and analyzed. Biomass, rubber, resin, bagasse, iodine value, and protein were evaluated along with common morphologic characteristics (Kuruvadi 1985; Lopez and Kuruvadi 1987) such as shrubs height and top spread.

METHODOLOGY

Collecting Sites and Sampling

Three sites were selected for plant acquisition: Atenco (site 1), close to Saltillo, Rocamontes (site 2), Coahuila, and Noria de Guadalupe (site 3) in the state of Zacatecas (Fig. 1). These sites have been subjected to rubber yielding studies and the molecular weight characteristics reported. Plant acquisition was carried out during the first week of Sept. 1991. Twenty-five plants, between 35 and 75 cm height, were randomly selected from each site. Measurements of the shrub height and spread were obtained in the field, the results are presented in Table 1. Afterwards, the plants were cut at ground level and transported to the laboratory, where the total weight was determined. The leaves were cut from the shrub and the defoliated plants immersed in liquid nitrogen and grounded in a Wiley laboratory mill with 2 mm screen.

Rubber, Resin, and Biomass Measurements

Part of the grounded material was used for biomass quantification. The tissue was oven dried at 60°C until constant weight was achieved and total dry weight calculated (Table 2). Resin and rubber content measurements were performed on 5 g samples, by soxhlet extraction, using acetone and toluene as the respective solvents. A resin quality determination was effectuated by double bonds titration using the Wijs method. This was carried out by dissolving the resin in chloroform and titrating with iodine-bromine. After resin and rubber extraction, the protein content was determined in the bagasse obtaining the nitrogen percent by the Kjeldahl method. The protein content (%Pr) was calculated using the nitrogen percent (%N2) and the equation (Dintzis et al. 1988): %Pr = 6.25 (%N2). The results from the morpho-physiologic characteristics were analyzed using a statistical random design with three treatments (localities) and 25 replications (plants).

RESULTS

Morphologic Characteristics

Table 1 shows the results for shrubs height and spread, parameters commonly used for plant selection. The range for these parameters is different within sites, but the mean values are very similar. These results reflect the different plants composition of wild guayule stands and are in agreement with several other reports.

Physiologic Characteristics

The results for dry weight, percent rubber and percent resin, iodine value and protein content in the bagasse are shown in Table 2. The weight difference between plants growing in the three sites, contrasted strongly with the similarity observed in the height and spread data. Site 3 (Noria de Guadalupe) and site 2 (Rocamontes) have a difference of only 5%, but that between site 3 and site 1 (Atenco) is approximately 50%, site 3 having the highest plant weight. This behavior is due to shrub thickness, the plants from site 3 and site 2 have thicker stem and branches than those in site 1. Despite height, spread, and weight differences, rubber content are very similar between sites. Large differences in iodine value between the sites were found.

A relationship between the resin content and the iodine value was noted (Fig. 2). As resin content increases, iodine value diminishes. The chemicals constituting the resin are a complex mix (Belmares et al. 1980; Schloman and Wagner 1991) including terpenoids, fatty acids triglycerides (i.e. linoleic, linolenic, stearic, and palmitic), very low molecular weight (VLMW) rubber, and a drying oil, "shellac" type. Several of these chemicals contain double bonds, for example linoleic and linolenic acids, the terpenoids and the VLMW rubber. Iodine value is an indicator of the concentration of these compounds, but it was not possible to differentiate between them with this technique. There was no correlation between rubber and resin content. However when a ternary diagram was constructed with the rubber, resin and iodine value (Fig. 3) all the samples lie within a narrow region. This suggests a relationship between these three variables and rubber biosynthesis.

In order to consider differences in all the physiologic parameters a global productivity index (PI) was defined, as PI = [%Rb + (%Rs x I2) + (%Bg x Pr)]W, where Rb = rubber, Rs = resin, I2 = iodine number, Bg = bagasse, and Pr = protein content. Resin and the bagasse are multiplied by a "quality factor" which is the double bonds or protein content, respectively. The results indicate a productivity index of 12.8 for site 1, 23.3 for site 2, and 29.0 for site 3.

Site 3 possessed the highest index based on rubber yielding and resin and bagasse quality. Differences in the parameters did not follow the same trend in the three sites. Data in Table 2 indicated that for rubber content site 1 > site 3 > site 2, for iodine value site 3 > site 2 > site 1, and for resin content site 2 > site 1 > site 3. However, the PI results show clearly that when all factors are considered the difference in value between the sites is wide. Site 3 has the highest value due to its higher mean total weight and iodine value; protein and resin content showed smaller differences but its contribution was limited. Rubber content was similar, and hence is not a decisive factor. The rubber molecular weight has been considered as a quality parameter for the rubber. However, previous studies show that differences in MW are not significant, for plants reaching this height. We consider that PI might be a valuable selection index to detect outstanding shrubs for genetic improvement. Although the numbers allow the identification of high yielding plants (rubber and coproducts) we do not know the highest or lowest values that will define, the maximum PI or the acceptability limit for guayule. The analysis of plants from regions with high and low rubber production will provide this information. According to these results, shrub height and spread seem not to be the best parameters for plant selection, intended for integral commercial use, as the product and coproducts yielding is very different for similar sized plants.

REFERENCES


Table 1. Guayule morphologic characteristics at the three sites.

Height (cm) Spread (cm)
Site Locality Mean Range Mean Range
1 Atenco, Coahuila 50.5 42-61 57.1 46-71
2 Rocamontes, Coahuila 52.3 48-58 55.4 48-63
3 Noria, Zacatecas 53.5 42-64 57.9 37-66
Significance nonsignificant nonsignificant


Table 2. Mean values for guayule physiologic characteristics at different sites.

Site Total dry weight (g) Rubber (%) Resin (%) Iodine value (%) Bagasse (%) Protein (%)
1 492 10.6 12.9 29.0 76.4 6.5
2 723 9.6 13.7 35.7 76.8 6.2
3 761 9.7 11.9 42.8 78.4 6.9
Significance ** NS ** ** ** **
** NS = Significant at 1% level (**) or nonsignificant (NS).


Fig. 1. Map of the guayule growing region in Mexico and localization of the three analysis sites.


Fig. 2. Relationship between resin content and iodine value for the plants from the three sampling sites.


Fig. 3. Ternary diagram correlating rubber, resin and iodine value.


Last update August 21, 1997 aw