Evaluation of the pressure chamber technique for measurement of leaf water potential in cassava (Manihot species)

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Abstract
The pressure chamber technique was evaluated as a method for estimating leaf water potential in cassava (M. esculenta Crantz). Xylem pressure potentials (.psi.P) measured with the pressure chamber were compared with leaf water potential (.psi.L) obtained for the same leaf with the in situ dew-point hygrometer. In both cassava varieties studied, .psi.L and .psi.P were linearly related (r2 = 0.87 and 0.98 for [varieties] CMC9 and CMC40, respectively). The length of petiole exposed outside the chamber affects the relation between .psi.L and .psi.P and should be kept at between 1 and 3 cm for better agreement. In CMC40, .psi.P was consistently lower (drier) than .psi.L by about 1.0 bar (1 bar = 100 kPa) in the entire range of water potential studied, but was not the case in CMC9. The reason for this difference is unclear but may be due to a filling of tissues other than xylem tissues during the measurement of .psi.P in CMC40. Average xylem osmotic potentials (.psi.S) were low (-1.0 .+-. 0.2 bars and -1.0 .+-. 0.4 bars for CMC9 and CMC40, respectively). It is, therefore, unnecessary to correct for .psi.S when using the pressure chamber to estimate leaf water potentials in cassava.

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