X-ray analysis

An X-ray analysis gives a clear image of the internal morphology of the seed or fruit. The X-rays are generated using very low voltages (much lower than are used for most medical applications) and the method is thus safe with the appropriate precautions. This aside, because the seeds may have been subjected to some genetic damage, the sub-sample is usually discarded, though sometimes it is used for germination testing.

The X-ray images are taken using a Faxitron digital X-ray machine.

For seed collections estimated to contain > 5,000 seeds, a sample of 50 seeds is used. For smaller collections, i.e. 1,000 to 5,000, a sample of 20 or 25 seeds is used. Collections estimated to contain 500 to 1000 seeds have a sample of 10 seeds examined. If the collection is estimated to contain less than 500 seeds, an X-ray analysis is not routinely carried out.

Before starting the procedure, the seed collections to be X-rayed may be sorted into groups according to the size and thickness of their seed/fruit coats. Each group may then be X-rayed using the same shelf position in the Faxitron. Shelf position determines the magnification of the image. Generally, the smaller the seed size, the higher the shelf position. Images produced on the lowest shelf cover the largest surface area.

The seed samples to be X-rayed are laid out in Petri dishes or plastic grids with machine-punched holes set in a regular pattern, backed with waxed paper.

Of particular interest are the outwardly normal, but 'empty' seeds that regularly occur in collections of wild species.  Allowance must be made for this incompetent fraction of the collection when germination and seed quantity are determined.

The image displayed here shows a sample of Terminalia brachystemma fruits. Although the fruits all appear to be healthy, six contain empty seeds (as revealed in the X-ray analysis and each marked by a red arrow).

More below.....

 

 

 

 

 

 

 

 

 

Insect larvae living within seeds are sometimes unmasked by this technique. The image right shows Adesmia balsamica seeds with infestation.

 

 

 

 

 

 

 

 

X-ray images are also useful for determining the average number of seeds within complex fruits, for example here in Prosopis strombulifera.

Arrow showing one seed surrounded by many empty spaces in a pod.	Prosopis fruit