Testing for Early Photoperiod Insensitivity in Soybean

to photoperiod. It is widely accepted that the early phase of soybean development is not influenced by photopeTo accurately model the flowering process in soybean, it is essential riod and can therefore be described as a juvenile phase to identify photoperiod-sensitive and photoperiod-insensitive phases (Shanmugasundaram and Tsou, 1978; Hodges and of development. Despite extensive studies, there remains some disagreement about when soybean plants first become sensitive to photoFrench, 1985; Ellis et al., 1992). The juvenile phase deperiod. The length of the juvenile phase from emergence has been scribed in this study is the length of photoperiod-insensifound to differ for the same cultivar. This experiment tested the tivity beginning from emergence. It is equivalent to the hypothesis that the early phase in soybean development is truly photophotoperiod-insensitive vegetative phase defined by period-independent. Soybean plants [Glycine max (L.) Merr. cv. HutWilkerson et al. (1989). Variations in the duration of cheson; Maturity Group V] grown in controlled environments at a the juvenile phase have been reported for the same constant air temperature of 268C were transferred from a noninductive soybean cultivar. For instance, ‘Davis’ soybean grown 22-h photoperiod to an inductive 8-, 10-, 12-, or 14-h photoperiod at at a constant air temperature of 268C was found to be 2to 5-d intervals after seedling emergence (defined as the day when sensitive to a 9-h photoperiod at 4 d after seed wetting the cotyledons appeared above the soil surface). The duration of the (Wilkerson et al., 1989), whereas Davis grown in a so-called juvenile phase was shown to be photoperiod-dependent, the photoperiod effect being nonlinear. Soybean plants became sensitive 12-h photoperiod at air temperatures of 30/208C day/ to an 8-, 10-, 12-, and 14-h photoperiod at 21, 0, 1, and 9 d after night (mean daily air temperature 5 258C) was not seedling emergence, respectively. Based on these and other results, sensitive until 18 d after sowing (Ellis et al., 1992). We we conclude that there is no juvenile phase in Hutcheson soybean, hypothesize that the difference in early photoperiod and that photoperiod sensitivity begins when seedlings are exposed insensitivity (4 vs. 18 d) could have resulted from the to light, but the rate of progress towards flowering depends on the different inductive photoperiods (9 vs. 12 h) applied, photoperiod experienced. and if this was true, then the early phase of soybean development may not be photoperiod-insensitive as supposed. Results on opium poppy (Papaver somnifA predicting days to flower in soybean erum L. subsp. somniferum; syn. var. album DC.), a [Glycine max (L.) Merr.] is important because the quantitative long-day plant, showed that the duration of time between emergence to flowering determines plant the so-called juvenile phase depended on the inductive size and thus affects dry matter production and final photoperiod into which the plant was transferred (Wang crop yield (Shanmugasundaram and Tsou, 1978; Wang et al., 1998). We concluded that the evidence of a juveet al., 1997b). The environmental factor that most influnile phase in opium poppy was questionable. The results ences the floral development rate of soybean is photoin opium poppy led us to examine the widely accepted period. assumption of a juvenile phase in soybean. Development from sowing or emergence to anthesis Our objective was to experimentally test the hypothein soybean, a quantitative short-day plant, has been sis that the early phase of development in soybean just divided into several phases based on its sensitivity to after emergence is truly photoperiod-independent. photoperiod. Roberts and Summerfield (1987) divided the time from sowing to first flower into three phases: (i) MATERIALS AND METHODS a preinductive phase, during which plant development is Plant Culture and Treatments insensitive to photoperiod; (ii) an inductive phase, during which developmental rates are influenced by photoSeeds of Hutcheson soybean, a determinate Maturity period; and (iii) a postinductive phase, during which the Group V cultivar, were sown in 3.75-L black plastic pots (three time to first flower is once again unaffected by photoseeds per pot) filled with a Jiffy Mix growing medium (Jiffy Products,1 Batavia, IL) consisting of Canadian sphagnum peat period. Wilkerson et al. (1989) divided the interval and vermiculite (1:1, v/v). Hutcheson soybean was selected between emergence and first flower into four phases: because it is a common cultivar grown in many soybean grow(i) a photoperiod-insensitive vegetative phase; (ii) a ing areas. To ensure satisfactory nodule formation and N2 photoperiod-sensitive inductive phase; (iii) a photopefixation, the seeds were coated with the N2–fixing bacteria riod-sensitive postinductive phase; and (iv) a photope(Bradyrhizobium japonicum). The growing medium was riod-insensitive postinductive phase. Various phases of amended with a slow-release fertilizer of Osmocote (14.0–6.1– soybean phenology have also been defined by other 11.6 N–P–K) (Scotts-Sierra Horticultural Products, Marysresearchers (Jones and Laing, 1978; Hodges and ville, OH) at a rate of 3 g L2. Dolomitic lime was added to French, 1985). adjust the pH of the medium to 6.0. Despite extensive studies, there remains some confuSix controlled environment chambers (Environmental Growth Chambers, Chagrin Falls, OH) were used. Fifty-six sion about when soybean plants first become sensitive pots were placed in two of the six chambers, and four pots USDA-ARS, Remote Sensing and Modeling Laboratory, BARC 1 Trade name and company name are included for the benefit of the West, Beltsville, MD 20705-2350. Received 11 Aug. 1997. *Correreader and do not imply any endorsement or preferential treatment of sponding author ([email protected]). the product by USDA-ARS. Abbreviations: DAE, days after emergence. Published in Agron. J. 90:389–392 (1998).