Photosynthesis (CER (μmol CO2 ？s？1)), stomatal conductance ( ), and intercellular [CO2] ( ) of soybean (Glycine max L. Merr.) grown using the early soybean production system (ESPS) of the midsouth were determined. Three irrigated cultivars were grown using ESPS on Bosket (Mollic Hapludalfs) and Dundee (Typic Endoaqualf) soils in 2011 and 2012 at Stoneville, MS. Single leaf CER, , and were determined at growth stages R3, R4, and R5 using decreasing photosynthetic photon flux densities (PPFD, μmol？m？2？s？1) beginning at 2000 PPFD and decreasing by 250 PPFD increments to 250 PPFD. Photosynthesis changes fit a quadratic polynomial for all fixed variables and range from ~6.0 and 9.0 CER at 250 PPFD and ~22.0 to 28.0 CER at 2000 PPFD. No cultivar differences in CER, , or were noted at any growth stage or site either year. In 2012, CER, , and were lower when measured at R5 than the two previous growth stages, which was not observed in 2011. The R5 sampling in 2012 had accumulated 39 to 70 more growing degree units at 10°C base temperature (GDU 10’s) than in 2011 and were likely more mature. Increased soybean yields from ESPS appear not to result from higher leaf CER. 1. Introduction Adoption of the early soybean (Glycine max L. Merr.) production system (ESPS) is nearly complete throughout the humid subtropical lower Mississippi River Valley and has become the standard production practice. Except for a few hectares seeded in double-crop systems following wheat (Triticum aestivum L.), most soybean crops in the Mississippi Delta are seeded before 1 May with cultivars in the maturity group (MG) 4.0 to MG 5.5 range. Seldom are any sizeable hectares seeded to cultivars later than MG 5.5 which were among the most common ones grown in the midsouth prior to 1995 [1]. Prior to development of the ESPS, soybean production in the midsouth involved seeding cultivars of the MG 5.0 to MG 7.0 range between 15 May and 10 June resulting in reproductive growth occurring through July to mid-August. This is typically a droughty period with maximum daily temperatures regularly in excess of 30°C which is the established optimum temperature for soybean growth [2]. Seed yields, under the original production system when irrigated, seldom exceeded 3400？kg？ha？1 and without irrigation often failed to exceed 1300？kg？ha？1 [1]. Research conducted by Heatherly [3] and Bowers [4] during the mid of 1980’s demonstrated that planting MG 4 and MG 5 cultivars in April produced higher yields than the same cultivars seeded in May under both irrigated and nonirrigated production systems. With the
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