Monitoring the cold requirement in tulip bulbs by ¹H-NMR imaging

The physical state of water in 'Oxford' tulip bulbs after exposure to low temperature was observed using proton nuclear magnetic resonance ( ¹H-NMR) imaging. In NMR longitudinal relaxation time (T₁) image, homogeneously long T₁ values were found for whole scales of bulbs precooled at 15°C for 3 weeks follwed by 5°C for 8 weeks. Strong images were observed mainly in the adaxial sides of each scale, particularly in the third scale of nonprecooled bulbs (20°C for 11 weeks). This suggests that water in the scales of nonprecooled bulbs is strongly bound to storage substances, e.g., starch granules. The signal intensity and the relaxation times in tulip bulbs indicate differences in the degree of water binding, that reflect changes in the physiological state of the bulbs during cold treatment. Parameters that indicate the satisfactory cold treatment of tulip bulbs have been sought over the years and various parameters have been proposed. However, none of them are practical. Based on the NMR images, free water appeared to be located in tissues with metabolically active cells. This suggests that complex metabolic activities can be traced without destruction of intact tulip bulb tissues. It is proposed that ¹H-NMR imaging could be a practical system to indicate whether the cold requirement of tulip bulbs has been fulfilled. The method proposed may also be applicable to investigating the physiological state of other geophytes.