Water Potential Gradients in Field Tobacco

A pressure chamber was used to establish the vertical gradients of leaf water potential (Psi(Leaf)) and stem water potential (Psi(Stem)) 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 Psi(Stem). The greatest difference between Psi(Leaf) and Psi(Stem) occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars. The largest vertical gradient in Psi(Stem) occurred at 1300 hours. The soil water potential (Psi(Soil)), extrapolated from the potential of leaves on a completely enclosed plant, was higher than -1 bar. The vertical gradient in Psi(Stem) and the difference between Psi(Leaf) and Psi(Stem) showed the existence of a resistance to water movement within the stem (r(stem)) and a further resistance between the stem and leaf (r(petiole)). The r(petiole) and root resistance (r(root)) were estimated to be 931 and 102 bars seconds per cubic centimeter, respectively. The r(stem) was low (94 bars seconds per cubic centimeter) at 1100 hours but increased to 689 bars seconds per cubic centimeter at 1300 hours.