Photosynthetic Traits Underlying Yield Differences in Irrigated and Rainfed Soybean Systems in the Mississippi Delta

Understanding photosynthetic processes under field conditions is critical for identifying physiological determinants of seed yield (SY) in soybean. This study evaluated leaf gas exchange and chlorophyll fluorescence traits in seven soybean cultivars grown under irrigated (IR) and rainfed (RF) environments in the Mississippi Delta. At the beginning seed stage (R5), key physiological parameters-including net photosynthetic rate (A), stomatal conductance (gs), intercellular CO₂ concentration (Ci), transpiration rate (E), instantaneous water-use efficiency (WUE = A/E), instantaneous carboxylation efficiency (ICE = A/Ci), electron transport rate (ETR), and photosystem II quantum yield (ΦPSII)-were measured. Seed yield was determined at physiological maturity (R8). Under irrigated conditions, A and ICE showed the strongest positive associations with SY, whereas under rainfed conditions, WUE emerged as the primary predictor of yield. Cultivars DS 25-1, Dyna-Gro 4516×, LG03-4561-14, and P37A78 exhibited consistently superior physiological performance across both water regimes. These findings highlight the importance of integrating gas exchange and fluorescence-based traits into trait-based selection strategies. However, given the limited number of cultivars, location, and growing seasons evaluated, these results should be interpreted as indicative rather than definitive. This study provides one of the first field-based comparisons of photosynthetic traits across maturity groups III-V within the Early Soybean Production System, offering insights into physiological indicators of yield stability under contrasting water regimes.