MRI of the freezing of cold-hardy plants and other plant studies

We present applications of MRI to plant studies conducted at the Western Sydney University node of the National Imaging Facility, showcasing recent studies of freezing behaviour of cold-hardy plants [1]. Water in the liquid state has a transverse relaxation time on the order of milliseconds to seconds but, when frozen, the transverse relaxation time drops to the order of microseconds. This behaviour can be used to determine the freezing order of plant tissues by taking successive MR images; the tissues that have frozen will no longer be visible [2]. Cold-hardy plants have an interesting distribution of freeze regulating compounds, such as ice nucleation agents, INA, (these promote development of ice crystals), anti-nucleation compounds, ANC, (these have the opposite effect of ice nucleation agents, inhibiting nucleation), and supercooling stabilizing compounds, SSC, (these stabilize the supercooled state). We present two variations of this method: (1) we changed the temperature in a controlled manner so as to drop by set intervals down to near 253 K, while taking a succession of images of a plant specimen of interest; (2) we kept the temperature constant and below 273 K and took a succession of fast images. The information obtained from such techniques can be used to study the differential freezing of plant tissues, and consequently provide information on the tissues containing INA, ANC and SSC compounds and how these plants survive sub-freezing temperatures