Effects of Freezing Field‐Grown Soybean Plants at Various Stages of Podfill on Yield and Seed Quality 1

Abstract

Soybean [Glycine max (L.) Merr.] plants are occasionally damaged by frosts during podfill in the north central United States. A study was conducted to determine the extent of damage to the yield and quality of harvested soybean seeds when the plants experienced freezing temperatures at various stages of reproductive growth. Four cultivars were grown in 19-liter plastic pots sunk into holes in the field. At weekly intervals beginning at R4 (full pod), pots were removed from the soil and placed in a controlled temperature chamber where the temperature was gradually lowered until about 80% of the leaves were visibly damaged. The pots were then returned to the field and the plants left to mature. Yield was reduced significantly in all cultivars by treatments imposed before R6 (full seed). Significant yield reductions after this stage were not detected in all cultivars. Reductions in both seed number and size contributed to yield losses. Frost-injured plants reached maturity earlier than controls, but no difference in moisture was detected between seeds harvested from frost-injured and control plants. Protein concentration of mature seeds was not affected by frost treatments, but seed oil concentration was significantly reduced by treatments before R6 (maximum reduction = 9%). Epiphytic ice nucleation active (INA) bacteria promote freezing damage in tender plants by preventing supercooling of water in plant tissues during periods of low temperature. Populations of INA bacteria on the leaves of border plants were determined throughout the period of the freezing experiments. No apparent differences in INA bacterial populations existed among the soybean cultivars. Temperatures at which freezing damage symptoms become visible in test plants varied throughout the season and were positively associated with the logarithm of the INA bacterial populations on border plant leaves.