Abstract
The volume flux of water (Jv) through roots of 2-month-old honey locust seedlings (Gleditsia triacanthos var. inermis) was estimated by using applied hydrostatic pressure. Shoots were severed 8 cm above the root collar, and root systems were mounted in the lid of a pressure chamber with the cut stump protruding through a gasket. The chamber, filled with Hoagland solution, was pressurized to 0.34 MPa, and exudate was delivered from each root system to an electronic balance via an automated system. Jv was expressed on a fibrous-root dry weight basis and plotted over time for water-stressed seedlings and well-watered controls. A highly defined endogenous circadian rhythm in Jv was observed, regardless of treatment, with maximal and minimal fluxes occurring at mid-day and midnight, respectively; the rhythm continued for up to 90 h. Seedlings subjected to water stress exhibited an approximately 4-fold reduction in Jv at comparable points in the cycle. The severity of stress influenced both the amplitude and trajectory of the rhythm. Daily peaks of Jv decreased gradually over a 3-day period in both moderately-stressed and well-watered plants. In contrast, peak Jv from roots of severely-stressed plants increased by approximately 28% per day over the same time course. The electrical conductivity of the efflux also exhibited a highly defined rhythm which was phase-shifted from that of the Jv rhythm by approximately 12 h.
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