Identifying reliable methods for evaluating cold hardiness in grapevine buds and canes

There is increasing interest in expanding grapevine production into colder regions. Here, to identify reliable methods for evaluating cold hardiness, we analyzed tissue discoloration, electrolyte leakage (EL), triphenyl tetrazolium chloride (TTC) reduction, and exotherms in the buds and canes of ‘Campbell Early’ grapevines. In both tissues, we monitored exotherms during cooling to − 40 °C at a rate of − 2 °C/h and analyzed the other indices during 2-h exposure to various freezing temperatures. Evaluating tissue discoloration is a subjective method to score freezing injury; it is more time-consuming and requires larger amounts of sample material than other methods but offers the advantage that it can be used to estimate LT 50 (the temperature at which 50% injury occurs). EL analysis in buds and canes is another relatively rapid technique providing reproducible LT 50 values. However, our EL analysis of buds was limited by the small amounts of sample material. For TTC reduction analysis, the results were not reproducible for both buds and canes: at the same freezing temperature, buds and canes showed significant differences in TTC reduction levels, leading to erroneous LT 50 estimates. Finally, detection of low-temperature exotherms (LTEs), an indicator of intracellular ice formation (which causes fatal cell damage), through differential thermal analysis revealed LTEs in buds but seldom in canes, and those detected in buds exhibited much narrower peaks. Thus, exotherm and EL analyses are reliable methods for evaluating cold hardiness of grapevine buds and canes, respectively, and their combined application could be a useful strategy for assessing grapevine cultivars.