Freeze Tolerance of Field Acclimated Winter Wheat Plants.

The success of a winter wheat (Triticum aestivum L.) crop depends heavily on its ability to establish in the fall, overwinter, and resume vigorous growth in the spring. In Ohio, the crop needs to endure freezing temperatures at many different growth stages. The objective of this study was to evaluate cold tolerance of different wheat varieties acclimated to two Ohio field locations. Five wheat varieties were transplanted from two field locations (Pickaway County and Wood County, Ohio) into 13 cm diameter plastic pots at Feekes stages: 5, 6, 8, and 9. Plants were placed in a freeze chamber starting a 4°C and cooled at 2°C per h with five treatments (°C): 4 (control), -3, -6, -10, and -15. Plants were exposed to the treatment temperature for 15 minutes, then removed and placed in at 4°C cold room. Within 16 h of the treatment chlorophyll fluorescence was measured and 7 d following the treatment, plants were rated for percent of living above ground biomass. Chlorophyll fluorescence and plant ratings for Feekes 5 and 6 were closely correlated and started to decrease around 14°C but did not become significantly different until 5°C. This correlation suggests that wheat leaves may have a reduced ability to recover from significant freeze injury. Wheat at Feekes 8 and 9 showed freeze injury at much higher temperatures,  21°C and 27°C respectively. The majority of nitrogen uptake occurs during Feekes 5-7 leading to higher leaf nitrogen concentrations and a greater freeze tolerance. Additionally, plant height and soil moisture did limit the soils ability to buffer the temperature. There were small variations among varieties, but were related to slight variations in plant growth stages. In conclusion, cold tolerance of Ohio winter wheat is a direct result of growth stage and low temperature, independent of variety and location.