Interspecific breeding in the Rhododendron genus involving R. simsii hybrids

To introduce yellow, blue or orange pigmentation in the Rhododendron subgenus Tsutsusi (including R. simsii hybrids) by classical breeding, intersubgeneric pollination experiments involving Hymenanthes, Pentanthera and Rhododendron (including Vireya) species and hybrids were started. Prefertilization research by pollen tube staining revealed a different behaviour between bilateral crosses. Ovule culture was applied as a means to save a substantially larger amount of hybrid embryos from abortion. Though flower/seed capsule abortion eliminated many crosses in an early developmental stage, ovules of many crosses could be initiated in vitro on various time intervals after pollination. Germination was achieved on basal Rhododendron medium +50 mg/l GA(3). Green seedlings were obtained after Tsutsusi x Hymenanthes, Hymenanthes x Tsutsusi, Tsutsusi x Rhododendron, Rhododendron x Tsutsusi and Vireya x Tsutsusi pollination. Many seedlings have little growth vigour and next to green seedlings albino seedlings occur frequently, depending on the pollination type. The first true leaves of many assumed hybrids obviously lack chlorophyll. The ploidy level of Vireya x Tsutsusi and Tsutsusi x Pentanthera seedlings is intermediate, which strongly suggests their hybrid origin.