Phasic temperature and photoperiod control for soybean using a modified CROPGRO model.

A modified CROPGRO model is applied to phasic temperature and photoperiod control in order to optimize soybean production for NASA's program in Advanced Life Support. Baseline model simulations were established using data from soybean temperature experiments conducted at elevated CO2 levels (1100 micromol mol-1) at Utah State University (USU). The model simulations show little advantage in using phasic temperature control alone to increase average seed yield rate over the USU experimental values. However, simulations that combine phasic control of temperature (two phases) and photoperiod (two phases) do indicate the potential to improve seed yield (in g m-2 day-1) by approximately 15% over those currently obtained experimentally at USU for soybean cultivar Hoyt. This temperature and photoperiod phasing is experimentally practical. The simulations suggest extending photoperiods over those typically used experimentally during later phases of the crop life cycle, which would lengthen grain fill duration and thereby increase mass per seed. The model simulations indicate that the timing, and duration of extended photoperiods would be very important due to possible reductions in seed number m-2. Besides affecting seed yield directly, the model simulations suggest that such reductions may also cause feedback inhibition of photosynthesis due to low seed sink strength at elevated CO2 levels.