Soybean Photosynthetic Rate and Carbon Fixation at Early and Late Planting Dates

Early planting (late April to early May) is recommended for increasing soybean [Glycine max (L.) Merr.] yield but a full understanding of the physiological response is lacking. Carbon dioxide exchange rate (CER) was measured on soybean planted between 18 April and 29 May to determine if CER could resolve this yield difference. During the season CER was measured twice weekly on individual leaves. Total carbon fixation, dry matter accumulation, and radiation use efficiency (RUE) were calculated for the growth periods R1 to R3 and R3 to R6. During the early vegetative stages CER (before flowering) declined for all planting dates and reached a minimum of 0.566 (late planting) to 0.629 (optimal planting) mg CO2 m(-2) s(-1) by approximately R2 (late flowering) followed by a linear rate increase to a maximum of 1.23 mg CO2 m(-2) s(-1) by R5 to R6 (early seed fill). The optimal first week of May planting increased main-stem node number, average CER during R1 to R6, total carbon fixation, and dry matter but did not increase seed yield. Overall, yield and seed number were positively related to CER between R3 and R6. These data demonstrate that (i) CER was controlled by plant growth stage and was independent of the date that growth stage occurred and (ii) early planting did not consistently increase the CER rate during the R3 to R6 growth stage, limiting yield improvement. Factors other than CER are more likely to explain the reported positive yield response to early planting.