Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2005
Publication Date: 1/10/2006
Citation: Reed, S.M. 2006. Reproductive biology of Clethra alnifolia. HortScience. 41:567-570.
Interpretive Summary: Clethra alnifolia is a summer-flowering shrub that is native to the eastern U.S. Commonly known as sweet pepper bush or summersweet, this deciduous ornamental is valued for its dark green pest-resistant foliage, attractive racemes or panicles of fragrant white or pink flowers, and ability to grow under a wide range of environmental conditions. While both compact and pink-flowered cultivars are available in the marketplace, no plants with both compact plant habit and pink flowers have been identified. Controlled pollinations provide the best opportunity for developing cultivars with the desired combination of complementary traits. The objective of this study was to evaluate self-compatibility, time of stigma receptivity, and the relationship between time of pollen shed and stigma receptivity in C. alnifolia. Pollen was shed within a day of flower opening, but stigmas did not become receptive to pollen until 2 days later. For three days after flowers opened, the pistil elongated substantially. Stigmas became receptive at the time the pistil achieved its maximum length. Self-pollinations of ‘Hummingbird’ and ‘Ruby Spice’ produced fewer seeds than did cross-pollinations of these cultivars; however, we found no evidence of inhibition of pollen germination or pollen tube growth in self-pollinated flowers. Information obtained in this study will be used in developing new and improved cultivars of Clethra alnifolia.
Technical Abstract: Breeding efforts in Clethra alnifolia, an ornamental shrub native to the Eastern U.S., is hindered by a lack of information on the reproductive behavior of this species. The objective of this study was to evaluate self-compatibility, time of stigma receptivity, and the relationship between time of pollen shed and stigma receptivity in C. alnifolia. Stigma receptivity and changes in floral morphology were monitored over a 7-day period beginning at flower opening. Pollen germination and pollen tube growth in styles were examined following self- and cross-pollinations. Seed set and germination were compared following self- and cross-pollinations. Anthers began to dehisce in ‘Hummingbird’ and ‘Ruby Spice’ the day after flowers opened, but stigmas did not become fully receptive to pollen until 2 days later. An increase in the length of pistils was observed following flower opening, with maximum elongation occurring at approximately the same time stigmas became receptive. While self-pollen tubes appeared to grow slightly slower than cross-pollen tubes, there was no indication of a self-incompatibility system acting at the stigmatic or stylar level in C. alnifolia. Self-pollinations of ‘Hummingbird’ and ‘Ruby Spice’ produced fewer seeds than did cross-pollinations of these cultivars. Germination of all seed obtained from this study was too poor to allow a comparison of germination rates of the self- and cross-pollinated seed. However, because a few self-progeny were obtained, emasculation is recommended when making controlled pollinations. The presence of a late acting self-incompatibility system or early-acting inbreeding depression was proposed as being responsible for the lower seed set following self-pollination.