Postpollination Reproductive Biology of Rhododendron prinophyllum (Small) Millais

Abstract

Complete diallel matings were performed during two consecutive seasons in a full sibling population of Rhododendron prinophyllum, the pinkshell azalea. Examination of pollen tubes from collected and fixed styles revealed no differences in growth, rate of pollen tubes between selfs and outcrosses. Penetration of pollen tubes through the ovular micropyle region occurred 4 to 7 days after pollination, regardless of pollen source. Embryogenesis was studied in pistils collected from forced greenhouse plants of the same population. All ovules appeared to develop for a short period before senescing. Percent capsule set data from both years' diallel pollinations indicated that some active form of self-recognition and rejection was operating and that environmental stresses and resource allocation were also influential. Additional information gathered included ovule counts, seed count to capsule size correlations, and germination trials. These pointed to a reduction in reproductive success at each developmental stage. Self-incompatibility (SI), defined as inability to set seed following self-pollination, is clearly not applicable here. There are inherent difficulties in separating an active, late-acting self-recognition/rejection system from inbreeding depression, which is a passive accumulation of homozygous recessive lethal and sublethal genes.