Study On The Critical Photoperiod In Soybean Cultivars With Different Maturity Groups

Soybean is a sensitive crop for photoperiod,and the photoperiodic response of soybean is a dominant factor in determining the ecological adaptability of soybean.In order to clarify the critical photoperiod of soybean cultivars in different maturity groups,this study was conducted at the Changping Experimental Station of the Institute of Crop Science,CAAS,in 2015-2017.72 soybean cultivars which belong to MG 0000 to MG Ⅹ were used in the present study,and daylength treatments of 12 h,14 h,16 h,18 h,20 h(12 h natural light + N·h artificial light)were carried out on these cultivars.The daylength and the number of days from emergence to flowering were taken as independent and dependent variables,respectively,and the method of piecewise linear regression model based on the median function was used to get the value of the inflection point to determine the critical photoperiod of tested soybean within different maturity groups.The effects of daylength on the agronomic traits,growth period structure and yield of soybean were studied,and the relationship between the soybean agronomic traits and its critical photoperiod were also analyzed.The results of the present study can provide a theoretical basis for soybean introduction,breeding,production layout,the promotion of new cultivars and yield improvement.Results of the piecewise linear regression model all reached extreme significant level,and the model-fitting mostly reached 90% or above,and the critical photoperiod value obtained consistent with the definition of the critical photoperiod,also the critical photoperiod of soybean cultivars in the three years were similar with no significant diference among years,furthermore,the obtained critical photoperiod values are very close to the actual daylength of the native region of soybean cultivars.It is indicated that by using the method of piecewise linear regression model,the predicted soybean critical photoperiod is precise and practicable.Meanwhile,under certain temperature conditions,the critical photoperiod of soybean is stable.The results of critical photoperiod determination of different maturity groups are as follows: the critical photoperiod of soybean cultivars in MG 0000 was 16.4 h,that of which in MG 000-MG was Ⅰbetween 15.7 h and 15.8 h,and it was 15.3 h in both of MG Ⅱ and MG Ⅲ,for soybean cultivars in MG IV the average critical photoperiod was 14.7 h,however,the critical photoperiod of soybean cultivars in MG V-MG Ⅹ was less than 14 h,among which the critical photoperiod of soybean cultivars in MG VMG VIII was between 13.4 h to13.7 h,and that of the MG Ⅹ-MG IX was less than or equal to 12 h.With the daylength increasing,the plant height and node number of soybean in flowering duration were increased,and both of the characteristics also rose gradually with soybean cultivars from MG 0000 to MG Ⅹ under the same daylength condition.The response of the plant height and node number of different soybean cultivars under flowering duration to daylength were closely related to their critical photoperiod,and these traits rose slowly with the extended daylength.However,both of the characteristics of soybean increased varied with different cultivars.Under the 12-20 h daylength,the vegetative growth duration,reproductive duration and the growth duration structure R/V ratio of the soybean showed significant positive correlation to daylength,while the R/V ratio showed a significant negative correlation to the maturity group.The R/V ratio of soybean cultivars in most of the maturity groups was very close under the same daylength condition,and only a few cultivars had obvious differences in individual maturity group.The R/V ratio of the tested soybean cultivars were mainly between 2.0 and 3.0.Under the same daylength condition,the mature durations of different soybean cultivars were prolonged with the maturity group from MG 0000 to MG Ⅹ.Meanwhile,with the extended daylength,the mature durations were also extended.However,most of the cultivars could not mature after a certain daylength.Whether soybean cultivars could normally mature or not were closely correlated to their critical photoperiod.Soybean cultivars within MG to Ⅲ MG Ⅹ could not mature when the daylength exceeded the critical photoperiod,while which within MG 0000 to MG Ⅱ could mature normally under 16 h daylength with lower yield,and some cultivars within MG 0000 to MG 00 could mature even under 20 h daylength,but the output were very low.All the soybean could mature normally when the daylength shorter than its critical photoperiod,and the plant height,nodes,pods,seeds and yield of soybean were enhanced,while 100-seed weight reduced when the its daylength were lengthened.