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Title: Irrigated soybean leaf photosynthesis in the humid sub-tropical Mid South

item Bruns, Herbert

Submitted to: International Journal of Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2013
Publication Date: 1/23/2014
Citation: Bruns, H.A. 2014. Irrigated soybean leaf photosynthesis in the humid sub-tropical Mid South. International Journal of Agronomy.

Interpretive Summary: Photosynthesis, the key biological process where carbon dioxide is fixed for plant growth and eventual yield, had yet to be measured in MG 4 and MG 5 soybean cultivars commonly grown in the Early Soybean Production System (ESPS) widely used in the Mid South. Scientist at the USDA-ARS Crop Production Research Unit at Stoneville, MS completed a two year study where photosynthesis rates were determined for three popular soybean cultivars grown under irrigation using the ESPS on both a sandy soil and a clay soil common to the Mississippi Delta. Light levels were controlled with an LED lamp attached to the instrument used to make the measurements which were started at the equivalent of full-sunlight and steadily decreased to a level common to a cloudy day. Measurements were made at three growth stages important to crop yield. Photosynthesis rates declined as light intensities were reduced but no differences were observed between cultivars, and only in 2012 at the last date of data collection were photosynthesis rates lower than the two earlier samplings. This was most likely due to the plants being slightly more mature at that growth stage in 2012 than they were in 2011, the first year of the experiment. When compared with similar data in earlier experiments conducted in different soybean production regions, using different management systems, and varying light levels, photosynthesis rates of plants in this study were similar to those experiments. This experiment demonstrated that photosynthesis rates of soybeans produced using the ESPS are not adversely affected by the early planting associated with this production system and that the MG 4 and MG 5 cultivars currently being grown with it fix carbon dioxide from the air at similar rates.

Technical Abstract: Photosynthesis (CER), total conductance (gwt) and intercellular [CO2] (Ci) of soybean (Glycine max L. Merr.) grown in the Early Soybean Production System (ESPS) of the Mid South has yet to be determined. Three irrigated cultivars produced in ESPS were grown on a Bosket very fine sandy loam (Mollic Hapludalfs) and Dundee silty clay (Typic Endoaqualf) in 2011 and 2012. Single leaf CER, gwt, and Ci were determined 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 CER, gwt, or Ci were noted at any growth stage or site either year. At both sites in 2012 CER, gwt, and Ci were lower when measured at R5 than the two previous growth stages, which was not observed in 2011. By the R5 sampling in 2012, plants had received 40 to 70 more growing degree units at 10oC 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 photosynthesis and that GDU 10 accumulation is likely more accurate in determining soybean development than DAP.