One of the major problems associated with making physiological measurements on fruit is that most measurement techniques tend to be destructive: once the fruit is cut open you can never know how it would have responded to an imposed treatment a week later. The medical establishment has circumvented this problem through the use of x-ray and CAT scanning equipment, and latterly, magnetic resonance imaging (MRI). MRI is an especially interesting technique since it avoids the use of a hard radiation source, and produces superb quality images of soft tissues - something the other two techniques are incapable of achieving. As a consequence, plant physiologists have been keen to explore non-clinical uses of this technology on tissues such as fruit. This outline summaries the results of a small storage trial set up at Duke University in North Carolina which allowed MRI to be used to follow the development of chilling injury in Fuyu persimmon.

Trial design

Export quality fruit (20 count) for the trial were obtained from a packhouse in Pukekohe in mid May. Fruit were subsequently divided into two treatments: one in which individual fruit were hermetically sealed in small polyethylene bags constructed from sea-freight bags, and another in which fruit were sealed in perforated bags that allowed gas exchange but reduced water loss. Avoidance of water loss is important since MRI signals are dependent on the amount and distribution of water within the sample. The presence of a plastic bag around the fruit does not interfere with the imaging process.

Fruit were air-freighted to Duke University (with special USDA dispensation) where they were held at 7°C for 4 weeks. Each week MR images were obtained from the same four fruit in each treatment. After four weeks in coolstorage (5 weeks after harvest), the fruit were placed at room temperature and imaged again 3 days later to examine shelf-life effects.

Figure 1 is typical of the type of image able to be obtained in an MRI experiment. Component tissues within the sample are able to be clearly distinguished based on differences in the amount of water present and the 'binding" characteristics of the water. In general terms, light regions of contrast within the image are indicative of higher concentrations of water, and darker regions lower concentrations of water. By quantifying temporal changes in image contrast in the same cross sectional and longitudinal sections of a fruit it is possible to infer something about the physiological changes that are occurring at any position within the sample.

Results

• Use of seafreight bags (on individual fruit) did not prevent development of chilling injury
  External symptoms consistent with development of chilling injury were evident in fruit in both treatments after 3 weeks in coolstorage. By the end of 4 weeks coolstorage chilling injury had been induced in virtually all fruit, although variability between fruit ensured that even after 3 days at room temperature a few fruit still remained which were edible.

• Major physiological differences between fruit in both treatments
  MRI measurements showed clear treatment differences one week after harvest - the earliest occasion that fruit could be imaged overseas - and that these differences were maintained throughout the duration of the experiment. In each case the measurements indicated that water in the flesh of fruit in non-perforated bags "relaxed" more quickly than water in flesh of fruit in perforated bags. In other words, the aqueous environment in the flesh of fruit surrounded by a modified atmosphere contained higher concentrations of soluble compounds (macromolecules, carbohydrates, or simple electrolytes) relative to their perforated counterparts.

• Early warning indication of impending injury?
  Startling changes in the relaxation parameters associated with the modified atmosphere treatment were observed after two weeks coolstorage. The changes were consistent with cell wall breakdown, or appearance of water with characteristics similar to free water at a time when symptoms of chilling injury were not yet evident. Comparable changes were not recorded in the normal atmosphere fruit, even though they too continued on to develop injury symptoms.

Comments

This is the first occasion on which MRI has been used to measure quantitative changes in any fruit during a storage trial involving modified atmospheres, and the trial was eminently successful in establishing that imaging provides novel information under these circumstances. Further studies on a range of crops will be necessary, however, to interpret the results of this type of experiment and how they relate to routine measurements such as firmness and soluble solids. The ultimate aim of studies using MRI will be to assist in modifying industry practices leading to improved storageability of fruit. Delivery of an imaging system to Massey University in 1996 will ensure that we have the resources to carry out these experiments in New Zealand rather the limited programmes we are currently restricted to overseas.

Figure 1 Magnetic resonance images through transverse (a) and longitudinal (b) sections of a persimmon a week after harvest. Note, individual tissues within an image are distinguished based on differences in their respective water contents, or ";binding"; characteristics of water at those locations. Each image is made up of 256 x 256 picture elements (pixels) giving an in-plane resolution of 0.33 mm (330 mm). The three circular features in each image are small tubes of salt solutions which are used as standard reference points.

The support of the Persimmon Research Development Council in providing us with the opportunity to proceed this far in evaluating imaging technology is gratefully acknowledged.