Efficacy of chlorophyll fluorescence as a viability test for freeze-stressed woody grape tissues

Woody-tissue cold hardiness and chlorophyll fluorescence of Concord (Vitis labruscana Bailey) grapevines were measured every 2 wk from late September to mid-December 1997. Plant efficiency analyser (Hansatech Ltd., King's Lyn n, UK) was used to measure chlorophyll fluorescence after a laboratory cont rolled freeze stress. From five preliminary experiments, valid chlorophyll fluorescence measurements were obtained from the cane surface with periderm removed, under similar to 2700 mu mol m(-2) s(-1) and 10 min dark adaptati on after 24-30 h post stress. For large samples, dark adaptation was not ne cessary if only the Fv/Fm (ratio of variable fluorescence to maximum fluore scence) was of interest. This ratio was positively correlated with freeze-s tress temperatures and negatively correlated with tissue injury. The Fv/Fm temperature inflection point was positively correlated with LT50 (temperatu re at which 50% of the sample is injured), calculated from the regrowth tes t. The Fv/Fm ratio was a sensitive, reliable, objective, and rapid method o f assessing woody-tissue viability after a freeze-thaw episode during cold acclimation. An Fv/Fm-50 was proposed as an indicator of the injury thresho ld.