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A pressure bomb, pressure chamber, or Scholander bomb is an instrument that can measure the approximate
water potential Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and ...
of plant tissues. A
leaf A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...
and petiole or stem segment is placed inside a sealed chamber. Pressurized gas (normally compressed
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
) is slowly added to the chamber. As the pressure increases, at some point the liquid contents of the sample will be forced out of the
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
and will be visible at the cut end of the stem or petiole. The pressure that is required to do so is equal and opposite to the water potential of the sample (Ψleaf or Ψtotal). Pressure bombs are field portable and mechanically simple, which make them the predominant method for water potential measurements in the fields of
plant physiology Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (b ...
and
ecophysiology Ecophysiology (from Greek , ''oikos'', "house(hold)"; , ''physis'', "nature, origin"; and , '' -logia''), environmental physiology or physiological ecology is a biological discipline that studies the response of an organism's physiology to enviro ...
.


Measurements

Several water potential variables can be determined using the pressure bomb analysis. The most common of which are predawn leaf water potential and midday leaf water potential. Measurements conducted on plants predawn are considered a good representation of the total water status of plant. As no transpiration through stomata should be occurring at night, the plant's water potentials should be in equilibrium across the entire plant and be similar to the water potential of the soil around the roots. Midday leaf water potential is less commonly used as it is more variable and does not correlate well with other physiological measurements of water status. However, midday water potentials can be used to determine times of peak water stress or diurnal changes in plant water status. Additional variables and methods that involve pressure bombs for analysis include: stem conductance, xylem embolisms, and vulnerability curves.


Pressure-volume Curves

A more advance method that uses the pressure bomb in plant physiology is
pressure-volume curves In ecology, pressure-volume curves describe the relationship between total water potential (Ψt) and relative water content (R) of living organisms. These values are widely used in research on plant-water relations, and provide valuable informati ...
analysis or p-v curve. Through this method one measures the changes in leaf or stem water potential and relative water content to isolate the underlying components of total leaf or stem water potential. While the measurements can be time intensive, variable such as solute potential (Ψs), turgor loss point (Ψtlp), apoplastic water content and symplastic water content can all be determined using this method. The general protocol for measuring p-v curves involves repeated measure of water potential and mass in succession. As water is forced out of the sample with each measurement in the pressure bomb the mass is also reduced. Tracking these changes over many measurement should show a precipitous drop and then a steady linear decline after an inflection point.


References


Further reading

*{{Cite journal, last1=Cochard, first1=Hervé, last2=Forestier, first2=Sébastien, last3=Améglio, first3=Thierry, date=2001-06-01, title=A new validation of the Scholander pressure chamber technique based on stem diameter variations, journal=Journal of Experimental Botany, language=en, volume=52, issue=359, pages=1361–1365, doi=10.1093/jexbot/52.359.1361, pmid=11432955, issn=0022-0957, doi-access=free Plant physiology