Photoperiod and temperature sensitivity in early soybean accessions from the VIR collection in Leningrad Province of the Russian Federation

The photoperiod and temperature sensitivity of the onset of flowering in 106 soybean accessions from the VIR collection has been studied for 8 years in a vegetation experiment in the conditions of Leningrad Province at 59 degrees N, the northernmost point of soybean evaluation in the world. The accessions were previously field-assessed for the ability to form fully developed seeds in this region. The experimental conditions were extreme for soybean in terms of both photoperiod and heat supply. The emergence to flowering period length (LEF) in the sample set was 7.9 days longer, and the sum of the temperatures required for the transition to flowering was 143.2 degrees C higher, on an average, under the natural long day (LD) compared to those under the artificial short day (SD). The photoperiod sensitivity of the accessions was estimated as the ratio of LEF under LD (T1) to LEF under SD (T2), and expressed as the coefficient of photoperiod sensitivity Cphs = T1/T2. A coefficient of 1.25 was taken as the limit of a very weak photosensitivity. Varieties with a coefficient not higher than this value totaled 73.6% of the sample set. The dominating influence of weather conditions over the photoperiod for the reference variety 'Svetlaya' is shown for over 8 years of observations: weather conditions caused 84.3% of the LEFvariability, while the differences in the photoperiod determined only 9.9% of variability. Excessive precipitation in 2016 and 2017 caused a significant delay in flowering. Cphs proved to be a stable characteristic of a variety because it did not differ significantly in contrasting years. A broader regional adaptability of varieties created in high latitudes compared to those of southern origin is discussed.