Varietal differences and morphology of cleistogamy in soybean
Cleistogamy, the production of open (chasmogamous, CH) and closed (cleistogamous, CL) floral forms by one species, is present in soybean [Glycine max (L.) Merr.]. Soybean cultivars that originated from high latitudes generally have a strong tendency to produce CL flowers. The first objective of this study was to determine varietal differences in cleistogamy. The second objective was to observe morphological differences between CH and CL flowers. A total of 32 and 31 soybean cultivars (maturity groups 00 to II) were grown in fields at Tsukuba and Sapporo, Japan, respectively, in 2005. Flowers that were fertilized up to 5 d after anthesis were classified into either CH or CL. Frequency of CL flowers ranged from 0.0 to 100.0% in Tsukuba and 6.7 to 100.0% in Sapporo. Cultivars from Russia and Hokkaido generally had a high frequency of CL flowers, whereas those from the Tohoku region had a lower frequency. Average of mean temperatures for 1 wk before anthesis was negatively correlated with frequency of CL flowers. Growth chamber experiments were conducted using CL cv. Severnaya 4 and a CH cv. Toyosuzu under constant temperatures of 25°C to eliminate temperature effects. Frequency of CL flowers in Severnaya 4 and Toyosuzu was 33.7 and 92.8%, respectively, suggesting involvement of genetic factors. At the same developmental stage, CL flowers had shorter sepal, petal, and stamen than CH flowers. There was no distinct morphological change in floral organs between CH and CL flowers except for length of the organs.