High rubber yielding selections from a natural population of guayule

Guayule accession 4265 which represents a mass seed collection from five plants occurring in a native stand in Durango, Mexico, was compared with variety 593 (the best of the commercial varieties) on nine characters. Their means for date of first bloom in 1945 were not different. In height and spread of 17-month old and 3-year old plants, and in rubber content and resin content of 2-year and 3-year old plants the means for accession 4265 were significantly greater than those for variety 593. From a comparison of their within-plot variances they were found to be equally variable in date of first bloom, but variety 593 was significantly more variable than accession 4265 in height and spread of plant at both ages. Its great variability in these characters was attributed to aberrant (dwarf) plants and off-type sexual plants which result from its facultative apomictic method of reproduction. Accession 4265 was significantly more variable than variety 593 in rubber and resin content of 2-year old plants. When accession 4265 was classified into three morphological types based on differences in plant habit, and leaf shape and color, the means of these types were found to differ significantly from each other on essentially all of the nine characters. Furthermore, within-plot variances for the three types were homogeneous for most of the characters, indicating that these types probably represented progenies of three of the plants included in the original collection. Two of the types, 4265-I and 4265-II, were markedly superior to variety 593 in plant size and rubber content, both of which affect rubber yield. These two types were also higher than variety 593 in resin content. Evidence from size of variances, chromosome numbers, self fertility, and regression of rubber content on resin content indicates that each of the three types is largely apomictic. Some of the factors affecting the feasibility of selecting for higher rubber content in facultative apomictic strains are briefly discussed.