Direct and indirect measurements of freezing tolerance: advantages and limitations

The freezing tolerance of 69 accessions of field-grown, common wheat (Triticum aestivum) was assessed in three consecutive winters. To measure freezing tolerance directly, field-grown plants were subjected to a range of freezing temperatures in a controlled environment and plant regrowth was subsequently assessed. Indirect assessments of freezing tolerance, as measured by chlorophyll fluorescence transient measurements followed by a JIP-test (an in vivo measurement of the adaptive behavior of the photosynthetic apparatus), were performed on detached leaves frozen at the same time as whole plants. Both direct and indirect tests were also used on plants cold acclimated in the laboratory. These results were compared with results of a field survival study performed at seven experimental sites. An analysis of the data indicated that only some of the JIP-test parameters were suitable for the prediction of freezing tolerance and winter survival. Estimates of cold hardiness were very similar, regardless of the experimental year, but were dependent on the method of cold acclimation and time of sampling. Indirect measurements of cold hardiness were more in line with the field survival data for field-cold-acclimated plants sampled in mid-winter than for plants that were either sampled earlier or cold acclimated in the laboratory. Indirect measurements taken on leaves that had not frozen failed to provide accurate estimates of cold hardiness. Our observations, together with previously reported findings, indicate that cold acclimation under natural field conditions activates a greater array of freezing tolerance mechanisms than cold acclimation performed in under controlled environmental conditions in a laboratory.