Abstract
The purpose of this study was to analyze predawn and noon variations in leaf, nodule and root water potentials in Medicago sativa L. plants as well as the relationship between them during drought.Two-month-old plants inoculated with the 102F51 Rhizobium meliloti strain were subjected to drought by withholding water during an eight-day period. Water potential measurements were performed by using thermocouple psychrometers (Wescor C-52 sample chambers and HR-33T microvoltmeter) operated in the dew point mode. The system was maintained in a room at 25 °±0.05 and equilibrium time carefully established depending on tissue type and degree of stress. In both predawn and noon determinations, root showed the highest water potential, followed by nodule and then leaf; minimal differences were observed particularly for controls and also under extreme drought conditions. Errors in using leaf water potential as an estimation of nodule water potential are discussed.In plants inoculated with an inefficient Rhizobium meliloti strain (102F83) showing severe nitrogen deficiency, leaf water potential measurements performed with the thermocouple psychrometer were extremely low, while root and nodule determinations behaved correctly. In such a case, the pressure chamber technique was more adequate. Difficulties in using a psychrometer for these altered tissues are also discussed.
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