Water Potential Gradients in Field Tobacco

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Abstract
A pressure chamber was used to establish the vertical gradients of leaf water potential (ΨLeaf) and stem water potential (ΨStem) in field-grown tobacco (Nicotiana tabacum L. var. Havanna seed 211) at three different times of day. Leaves enclosed in polyethylene bags and aluminum foil the previous afternoon and left to equilibrate overnight were used to determine ΨStem. The greatest difference between ΨLeaf and ΨStem occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars. The largest vertical gradient in ΨStem occurred at 1300 hours. The soil water potential (ΨSoil), extrapolated from the potential of leaves on a completely enclosed plant, was higher than −1 bar. The vertical gradient in ΨStem and the difference between ΨLeaf and ΨStem showed the existence of a resistance to water movement within the stem (rstem) and a further resistance between the stem and leaf (rpetiole). The rpetiole and root resistance (rroot) were estimated to be 931 and 102 bars seconds per cubic centimeter, respectively. The rstem was low (94 bars seconds per cubic centimeter) at 1100 hours but increased to 689 bars seconds per cubic centimeter at 1300 hours.