In Vitro Characterization of Apomictic Reproduction in Guayule

Guayule (Parthenium argentatum Gray, Asteraceae) is a latex-producing perennial desert shrub native to southwestern Texas and north central Mexico. It is potentially an economically viable new crop for the desert Southwest, with the advantages of having low water requirements and producing non-allergenic latex (Thompson and Ray 1988). Diploid guayule plants reproduce predominantly sexually, and possess a sporophytic self-incompatibility system (Powers and Rollins 1945; Gerstel and Riner 1950; Gerstel 1950). Polyploid plants are self-compatible, and can also reproduce through facultative or obligate pseudogamous apomixis, predominantly through mitotic diplospory (Powers and Rollins 1945; Esau 1946). The frequency of apomictic reproduction can vary from plant to plant, and even from flower to flower on a single plant (Esau 1946). The factors that control the mode of reproduction are not known. Breeding programs for P. argentatum would be facilitated if a relatively rapid and easy technique were available to characterize plants as to their mode of reproduction. Such a technique has been developed and tested on several members of the Poaceae that reproduce both apomictically and sexually (Matzk 1991; Mazzucato et al. 1996). Their test consists of the application of an auxin to the developing flowers. The hypothesis is that auxin will stimulate development of apomictic embryos in plants that are genetically disposed toward apomixis, but not in those that are truly sexual. In the Poaceae, apomictic embryos developed to apparent maturity in the absence of endosperm after auxin treatment. Such flowers were visually distinguished from normal filled and empty flowers (Matzk 1991; Mazzucato et al. 1996). We tried the above auxin test in field trials in order to characterize the reproductive systems of P. argentatum breeding lines (data not presented in this paper). Initial field trials using 2,4-dichlorophenoxyacetic acid (2,4-D) showed inhibition of embryo production, so other auxins and concentrations were tested. Results of these trials were ambiguous, mainly because of problems with pollen release in isolation bags that could have resulted in self-pollination and the uncertainty that the auxins were actually penetrating the ovule. Therefore, a technique using in vitro floral culture was developed which provided better control of environmental factors and a better means of applying growth regulators. Auxin and other growth regulators inhibited embryo production in vitro, so that the best technique appeared to be in vitro floral culture without growth regulators, and this paper presents the results of these experiments. Seven breeding lines and a sexual diploid control were characterized for reproductive mode using this technique, and the results were substantiated with RAPD analyzes of progeny arrays of selected crosses. Thus, in vitro floral culture provided good characterization of reproductive mode in breeding lines of P. argentatum.