Use of Exogenous Growth Substances in Promoting Pollen Tube Growth and Fertilization in Barley-Rye Crosses

The possibilities that exist for the improvement of domesticated plants through interspecific hybridization are continually being exploited by the plant breeder. Numerous examples can be cited however, in which free reconlbination of genes governing commercially important traits is severely restricted by the existence of incompatibility systems within the species concerned. Although basically under genetic control, such inconlpatibility mechanisms can be classified as either gametophytic or sporophyt-ic (Lexvis, 1956). Each may be operative immediately following pollination wherein pollen cither fails to germinate on foreign stigmata, or once germinated, pollen tubes fail to develop normally. It is generally recognized that in those species in which bioassays have been made, the amount of endogenous growth-like substances in the reproductive organs of the plant increases during the sexual process (Phinney et al., 1957; Radley, 1958). Although work in this field is limited, it might be assumed that reproductive processes of the plant trigger the synthesis of relatively large amounts of natural growth promoting substances. If for an reason an optimum level of these substances is not attained, a form of incompatibility could result. IVith the discovery of gibberellic acid (GA) in the pure form in 1954 (Curtis and Cross), studies of the effect of this and related exogenous plant hormones on embryo culture in wit70 have been made (Narayanaswami and Norstog, 1964). Considerably more limited are reports of the response of hybrid embryos to those sake growth substances in vivo. One such study was reported by Larter and Enns (1960) in which G A applied to the pistillate parent during the fertilization period, enhanced hybrid embryo development in crosses between an autotetraploid (4n = 28) barley and its diploid counterpart. The authors stressed the fact however, that although the parents used in the study exhibited a high degree of cross-incompatibility, they nonetheless possessed a common genetic background. For this reason it is probable that this material was not representative of incompatibilit~r mechanisms generally considered responsible for embryo failure commonlp encountered in interspecific and intergeneric crosses. The study reported herein deals with the effect of exogenous G A and indole-3-acetic acid (IAA) on pollen tube growth and ovary development in vivo in crosses involving an autotetraploid barley and a diplbid rye. Because of the high degree of incompatibility that exists between these two species, they were considered ideal parental material for the present investigation.