Estimation of the freezing injury in flower buds of evergreen azaleas by water proton nuclear magnetic resonance relaxation times

Temperature dependence of longitudinal relaxation times (T₁) of water protons in flower buds of six azalea species differing in cold hardiness and ecological distribution was investigated by pulse nuclear magnetic resonance spectroscopy. Thermal hysteresis was observed for T₁ following a slow freeze-thaw cycle. The T₁ ratio (the ratio obtained from the difference between the original T₁ value in an unfrozen sample and the final T₁ after a freeze-thaw treatment, both at 20°C, divided by the original T₁) was closely correlated with the viability of florets in non-acclimated buds of R. kiusianum. If the buds were frozen to a lethal temperature and then thawed to 20°C, the T₁ ratio increased. The T₁ ratios of acclimated winter buds for the six species used were correlated with the level of cold hardiness (supercooling ability of florets determined by differential thermal analysis). The T₁ ratio of deacclimated spring buds, especially those from hardier species, markedly increased upon cooling to a lethal temperature. Species differences observed in acclimated winter buds disappeared upon deacclimation. The T₁ ratio appears to be related to the viability of florets and the degree of freezing damage (membrane disruption) in florets.