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

Abstract

×Chitalpa tashkentensis Elias & Wisura is a sterile intergeneric hybrid [Catalpa bignonioides Walt. × Chilopsis linearis (Cav.) Sweet]. To restore fertility in ×Chitalpa 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 ×Chitalpa `Pink Dawn' were submerged in an aqueous solution of 150 μm 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 ×Chitalpa `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 ×Chitalpa 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 μm, and harvested weekly beginning 2 weeks after pollination (WAP). Germination of triploids (cytochimera ×Chitalpa × diploid C. bignonioides) and tetraploids (selfed cytochimera ×Chitalpa) were greatest with excised embryos at 7 WAP on SH supplemented with sucrose at 20 g·L-1 and ≥1 μm GA3. Germination of triploids (diploid C. linearis × cytochimera ×Chitalpa) was <5% at 4, 5, or 6 WAP on the same medium as above. Oryzalin effectively induced polyploidy and restored fertility in ×Chitalpa `Pink Dawn'. Successful crosses between hybrid and parental taxa of different ploidy levels, coupled with embryo culture will facilitate a ×Chitalpa breeding program. Chemical names used: 4(dipropylamino)-3,5-dinitrobenzenesulfonamide (oryzalin).