Rubber in guayule occurs as a colloidal suspension in the individual cells in the tissues of cortex and vascular rays of phloem and xylem (Foster et al. 1980). The bark portions of the roots, stems, and branches contain the majority of the rubber (Estilai 1987) and branches contain a higher percentage of rubber and resin than the main stem and roots (Jasso and Kuruvadi 1991). The objective of this investigation was to determine the variability of rubber and resin content in the bark and wood portions of root, stem, and branches and to identify higher rubber yielding lines associated with the production of thick bark.
Rubber percent varied from 3.94 to 11.42% (root), 4.80 to 12.23% (stem), and 4.92 to 11.01% (branches) in the bark, and ranged from 0.38 to 1.55% (root), 0.46 to 4.44% (stem), and 0.72 to 5.00% (branches) in the wood. The mean percent rubber was 7.3% for the bark and 1.4% for wood tissue in the three plant parts; the bark contained 421% more rubber than wood tissue. The total bark and wood portions of these genotypes contain approximately 83.8 and 16.2% of the rubber in the plant. Estilai (1987) estimated that the bark of the plant of guayule contains about 70 to 85% of the rubber in the plant depending on the genotype. Maximum percent rubber was observed in accession 4599 (7.6%), followed by accessions 4580 (5.7%), 4144 (4.8%), 4358 (4.7%), and 4443 (4.3%).
Resin is another important byproduct of guayule; average resin percentage was 11.9 for the bark and 4.3 for wood. The total bark and wood tissue of these accessions contains nearly 73.4 and 26.6% resin in the plant. Superior genotypes identified were 4144, 4443, 4130, 4580, and 4338 which ranged from 8.2 to 9.6% resin.
The thickness of the bark is one of the important characters influencing the total production of rubber and resin in the plant. The mean thickness of the bark was 0.13 cm (root), 0.42 cm (stem), and 0.23 cm (branches). Superior genotypes for bark diameter were 4144, 4161, 4338, and 4597 (root); 4123, 4144, and 4599 (stem); and 4597, 4144, and 4161 (branches). Accession 4144 contained thicker bark in the three parts of the plant studied.
The majority of the rubber and resin accumulation occurs in the bark of the plant. Hence in breeding, high priority should be given to combining bark thickness, higher rubber percent and high biomass yield.