Almost any vascular plant can be measured in the pressure chamber. New and better approaches to measurements are limited only by the imagination. Twigs, branches, herbs, grasses, and fruits of a variety of species have been measured successfully.
Twigs and branches
Most of the early work with the pressure chamber was done with woody plants because they are easiest to measure. Secondary lateral twigs of large trees can be used, but the entire top of seedlings may be required. In woody species the phloem (bark) should be stripped back from the cut surface about 2 cm (about 3/4”) to allow the xylem (woody portion) to protrude through the gasket.
Re-cutting the stem of the sample to shape the cut end can be accomplished safely in some species such as conifers, but species with large xylem elements such as oak cannot be re-cut without introducing error. The practice of trimming the sample after it is cut from the plant should be confined to those species which are known to have only very small xylem elements.
A proper sized insert and gasket are all that is really needed, however if the twig is not rigid enough an insertion tool will be of help. For production purposes or multiple measurements a Compression Gland Cover will be of benefit.
Herbaceous plants are only slightly more difficult to measure than woody plants. Sealing the sample in the gasket without crushing or kinking the stem is usually the biggest problem, and this problem can usually be solved by using a gasket of the proper size and the insertion tool described earlier. A Compression Gland Cover can also be helpful.
The endpoint may be difficult to determine, but a good light and perhaps a good quality hand lens will be helpful. Some workers have used a dissecting microscope to aid them in seeing the endpoint, but such measures are usually not necessary.
With herbs it is especially important to measure the sample immediately after cutting and to use the proper rate of pressure increase.
Measuring grass is similar to measuring herbaceous plants. Although some workers have tried to measure the round portion of the stem, most workers have been more successful using the flat portion of a leaf blade. A slotted cover insert for the chamber lid, a slotted gasket, and an insertion tool are required to seal the blade into the chamber. Measurements usually cannot be taken when stress levels exceed the wilting point by more than about 10 bar. For this reason most measurements on grass will be below 20 bar.
Measuring fruits such as apples, oranges, or tomatoes is not difficult, but the fruit must usually be supported in the chamber. A coil spring in the bottom of the chamber is helpful. Once the sample is in the chamber measurements can proceed as with herbs.