FLORAL NECTARY STRUCTURE AND DEVELOPMENT IN GLYCINE MAX L. (LEGUMINOSAE)

Authors: HORNER, HARRY - IOWA STATE UNIVERSITY; HEALY, ROSARIA - IOWA STATE UNIVERSITY; CERVANTES-MARTINEZ, TERESA - IOWA STATE UNIVERSITY; PALMER, REID

Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2003
Publication Date: 10/30/2003
Citation: HORNER, H.T., HEALY, R.A., CERVANTES-MARTINEZ, T., PALMER, R.G. FLORAL NECTARY STRUCTURE AND DEVELOPMENT IN GLYCINE MAX L. (LEGUMINOSAE). INTERNATIONAL JOURNAL OF PLANT SCIENCE. 2003. v. 164. p. 675-690.

Interpretive Summary: Soybean has a typical legume flower and more than 99% of the flowers are self-pollinated, so soybean is true breeding. The very low outcrossing or cross-pollination depends upon which soybean is being studied, the weather conditions, and which insects are used. As part of our study to determine the constraints to hybrid seed production in soybean, we studied soybean flowers. Insect attraction to flowers in many legumes depends upon a reward, so the insects are motivated to return. One cue is nectar composition. This study reports on the structure of soybean nectaries. Are the nectaries structurally well developed, so that nectar production would be adequate as a reward to pollinating insects? Each nectary forms immediately prior to flower opening and degenerates within 24 hours. This development coincides with the time period of insect activity (movement from flower to flower) that contributes to cross-pollination. The structural features of the nectary suggest intense metabolic activity. The rewards, sugars, etc. produced by the nectary are the cue for insects to visit soybean flowers. The nectary in the cultivated soybean is well-developed and, because of the low level of cross-pollination, may represent the status of a once more active nectary in ancestral soybean that had higher levels of outcrossing. Nectary studies of soybean species with very high levels of outcrossing are planned. If variation in soybean nectary structure and function exists, selection for increased insect-mediated cross-pollination should be effective. These nectary studies should contribute to our understanding of increased hybrid seed production. Hybrid soybean should provide an economic return to the grower and ultimately benefit the consumer through food, feed, and industrial uses.

Technical Abstract: Soybean has a typical papilionaceous flower and has the anatomical adaptation characteristics of an entomophilous plant species. In our objective to understand the soybean plant features that contribute to insect-mediated cross-pollination, we need to understand the development and function of nectaries. Floral nectaries of annual, cultivated Glycine max develop between the bases of the central gynoecium and lateral stamen ring. Each discoid nectary forms immediately prior to flower opening and degenerates within 24 hours. Three stages of nectary development were identified: pre-active, active, and post-active. Each nectary consists of thin-walled special parenchyma cells, each having dense cytoplasm with a nucleus, Golgi bodies and vesicles, mitochondria, plastids, endoplasmic reticulum, many ribosomes, and one or more vacuoles. At the beginning of the active stage, special parenchyma cells around the phloem fingers become highly vacuolated. These cells then collapse, releasing their contents through the pores of the guard cells and onto the nectary surface. This secretion is termed holocrine secretion and is different from that reported for other legume taxa and suggests programmed cell death (apoptosis). In addition, there are two types of elongate non-glandular trichomes and one type of short 5- to 7-celled glandular trichomes on the gynoecium. These latter trichomes seem to be functional during the active and post-active stages, and following nectary collapse, suggesting that the nectary may consist of a number of compounds originating from both the nectary and the glandular trichomes. Soybean is a highly self-pollinated plant, yet the nectary is exceedingly well-developed. The functional significance of the nectary is being investigated. As more soybean genotypes are studied, and if variation in nectary number and function exists, selection for increased insect-mediated cross-pollination should be effective. These nectary development and function studies contribute to our understanding of increased hybrid seed production. Hybrid soybean should provide an economic return to the grower and ultimately benefit the consumer through food, feed, and industrial uses.