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intRoduction to teRminology and techniques foR studying enviRonmental
physiology of subtRopical and tRopical fRuit cRops 155
A leaf is detached from the tree and placed in the chamber with the stem protruding. Nitrogen
gas or air is pumped into the chamber until sap is first seen exuding from the stem, at which time
the pressure is recorded. The pressure required to squeeze the sap is equal to the tension that
the sap was under in the tree. Since matric potential is negligible in plants, the pressure potential
equals the leaf water potential.
Figura. 7. A Scholander pressure chamber for measuring leaf water potential
A good example of where this type of information is useful for tropical fruit production is
in irrigation studies. Generally, irrigation scheduling for crops is based on the amount of water
in the soil. However, tropical fruit crops have various mechanisms of stress avoidance such
as closing their stomates and dropping leaves to reduce water uptake when the soil is dry.
Therefore, stem water potential is often a better indicator of the water status of tropical fruit
crops than soil water content. For example, in a study of the effect of soil water depletion on
carambola water relations, growth and productivity in a rocky soil, a linear relationship was
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