Photosynthesis of Irrigated Soybean Grown Using the Early Production System.

Photosynthesis (A), total conductance (g_wt) and intercellular [CO₂] (Ci) of soybean (Glycine max L. Merr.) grown in the Early Soybean Production System (ESPS) of the Mid South was determined for an MG 4.3, MG 4.8, and MG 5.2 Cultivar.  The experiment was carried out on a Bosket very fine sandy loam (Mollic Hapludalfs) and Dundee silty clay (Typic Endoaqualf) in 2011 and 2012.  Single leaf A, g_wt, and Ci were measured at growth stages R3, R4, and R5 in response to decreasing photosynthetic photon flux densities (PPFD, μmol m^-2 s^-1) beginning at 2000 PPFD and decreasing by 250 PPFD increments to 250 PPDF. Photosynthesis responses to changing PPFD fit a quadratic polynomial for all fixed variables and ranged from approximately 6.0 and 9.0 μmol CO₂ m^-2  s^-1 at 250 PPFD and 22.0 to 28.0 μmol CO₂ m^-2 s^-1 at 2000 PPFD.  No cultivar differences in A, g_wt, or Ci were noted at any growth stage or site either year. At both sites in 2012 A, g_wt, and Ci were lower when measured at R5 than the two previous growth stages, which was not observed in 2011.  By R5 in 2012, plants had received 40 to 70 more growing degree units at 10^oC base temperature (GDU 10’s) than 2011despite being sampled on equal days after planting (DAP) (98).  Thus, plants in 2012 were likely more mature, indicating maturation probably begins in late R5 with a decrease in A and that GDU 10 accumulation is likely more accurate in determining soybean development than DAP.