Reproductive Behavior of Induced Allotetraploid xChitalpa and In Vitro Embryo Culture of Polyploid Progeny

xChitalpa tashkentensis Elias & Wisura is a sterile intergeneric hybrid Catalpa bignonioides Walt. xChilopsis linearis (Cav.) Sweet. To restore fertility in xChitalpa the following were evaluated: 1) oryzalin as a polyploidization agent, 2) fertility of induced polyploids, and 3) in vitro culture methods for embryo rescue of interploid crosses. Meristems of xChitalpa 'Pink Dawn' were submerged in an aqueous solution of 150 micromolar oryzalin for 0, 6, 12, or 24 hours and ploidy analyzed via flow cytometry. As treatment duration increased, recovery of diploids decreased as mixoploids and shoot mortality increased. Two tetraploid shoots occurred in the 24-hour treatment. Four tetraploids and two cytochimeras were stabilized in total. Tetraploids flowered sparsely; however, cytochimeras flowered profusely and these were used to study fertility at the tetraploid level. Diploid xChitalpa 'Pink Dawn' pollen was essentially nonviable, but cytochimera pollen stained and germinated equal to or greater than pollen of C. bignonioides and C. linearis 'Bubba'. Cytochimera xChitalpa were selfed yielding tetraploid seedlings, crossed with C. bignonioides to yield triploids, but failed in reciprocal crosses with C. linearis 'Bubba' and 'Burgundy Lace'. To increase recovery of triploids, germination of triploid and tetraploid embryos was investigated, as either intact ovules or excised embryos, on Schenk and Hildebrandt (SH) basal salts supplemented with sucrose at 20, 40, and 80 g.L(-1), presence or absence of 2% coconut-water, and gibberellic acid (GA3) at 0, 1, 2, or 4 micromolar, and harvested weekly beginning 2 weeks after pollination (WAP). Germination of triploids (cytochimera xChitalpa x diploid C. bignonioides) and tetraploids (selfed cytochimera xChitalpa) were greatest with excised embryos at 7 WAP on SH supplemented with sucrose at 20 g.L(-1) and greater than or equal to 1 micromolar GA3. Germination of triploids (diploid C. linearis x cytochimera xChitalpa) was <5% at 4, 5, or 6 WAP on the same medium as above. Oryzalin effectively induced polyploidy and restored fertility in xChitalpa 'Pink Dawn'. Successful crosses between hybrid and parental taxa of different ploidy levels, coupled with embryo culture will facilitate a xChitalpa breeding program. Chemical names used: 4(dipropylamino)-3,5-dinitrobenzenesulfonamide (oryzalin).