RELATIONSHIP OF WOODY PARTHENIUM ARGENTATUM AND HERBACEOUS P. HISPIDUM VAR. AURICULATUM (ASTERACEAE)

Interspecific hybrids between woody Parthenium argentatum Gray (guayule), native to Mexico and Southwest Texas, and herbaceous perennial P. hispidum var. auriculatum (Britton) Rollins, native to the United States, were obtained successfully. The F, hybrids were intermediate for most morphological characters with the exception of the short woody stem, yellow.pollen color, and the trichome morphology. Chromosome counts revealed the presence of 2n = 36 A-chromosomes in P. argentatum. The same number of A-chromosomes and four B-chromosomes were found in P. hispidum var. auriculatum. Observations of pollen mother cells showed regular meiosis in both parental species. At diakinesis, chiasmata averaged 1.12 and 1.24 per bivalent for P. argentatum and P. hispidum var. auriculatum, respectively. Meiotic behavior of the F1 hybrids was irregular. F, hybrids averaged 4.43 univalents at metaphase I, 1.95 laggards at anaphase I, and 1.62 micronuclei at the tetrad stage. The low pollen stainability (5.1%) in the F, hybrids and the limited number of viable BC, seeds (4.07%) may be reflections of the irregular meiosis. Although these primary hybrids are partially fertile, they can be used to introduce desirable characteristics of P. hispidum var. auriculatum, such as herbaceous perennial habit, regrowth ability, and cold tolerance into guayule. GUAYULE, Parthenium argentatum Gray (Asteraceae), a desert shrub from Mexico and Southwest Texas, is being developed as a domestic source of natural rubber (Estilai and Youngner, 1984; Hinman, 1984). Other Parthenium L. species do not produce appreciable amounts of rubber but are valuable sources of desirable traits, such as disease resistance, cold tolerance, increased biomass, and rapid growth, which may be introduced into guayule via interspecific backcross breeding programs. The 17 known species of Parthenium fall into four sections: Section Parthenichaeta includes guayule and seven other woody species (P. cineraceum Rollins, P. fruticosum Less, P. incanum H.B.K., P. lozanianum Bartlet, P. rollinsianum Rzedowski, P. schottii Greenman ex Millspaugh and Chase, and P. tomentosum D.C.); the caespitose woody section Bolophytum includes P. alpinum (Nutt) T. and G., and P. ligulatum (Jones) Bameby; the herbaceous section Argyrochaeta contains P. bipinnatifidum (Ortega) Rollins, P. confertum Gray, P. densipilum Blake, P. glomeratum Rollins, and P. hysterophorus L.; and the herbaceous peI Received for publication 8 August 1986; revision accepted 29 December 1986. Research supported by the California Agricultural Experiment Station, United States Department of Agriculture, Science and Education Administration, Grant Nos. 82-CRSR-2-2004 and 84-CRSR-2-2368, and National Science Foundation Grant PCM-8310013-01. rennial section Partheniastrum contains P. hispidum Raf. and P. integrifolium L. Hybrids have been produced from crosses between guayule and the following woody arborescent and caespitose species: P. tomentosum, P. incanum (Rollins, 1946), P. schottii (Estilai, Hashemi, and Youngner, 1985), P. fruticosum (Gomez, 1983; Naqvi et al., 1987), P. rollinsianum, and P. alpinum (Hashemi, West, and Waines, 1986). Interspecific hybrids between woody guayule and the herbaceous species of the genus are limited. From 1,297 crosses between tetraploid (2n = 72) P. argentatum and diploid (2n = 34) annual P. hysterophorus, Rollins (1946) obtained a single hybrid. No hybrids were obtained from 867 crosses between diploid guayule and diploid P. hysterophorus (Rollins, 1946). More recently, Naqvi (1982) produced hybrids from interspecific crosses between diploid (2n = 36) P. argentatum and tetraploid (2n = 72) perennial P. integrifolium. In addition to the above hybrids, which included P. argentatum as the woody parent, Rollins (1946) obtained hybrids from crosses between woody triploid (2n = 54) P. incanum and herbaceous P. hysterophorus. Based mainly on the morphological grounds, but also on the limited information from crosses of woody and herbaceous species, Rollins (1950) found it impossible to connect the herbaceous species of section Argyrochaeta with