Water Potential Gradients in Field Tobacco

Abstract

A pressure chamber was used to establish the vertical gradients of leaf water potential (*I,eaf) and stem water potential (Istem) 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 "'tem. The greatest difference between *'I,af and Isttem occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars.The largest vertical gradient in "8tem occurred at 1300 hours. The soil water potential (Is9il), extrapolated from the potential of leaves on a completely enclosed plant, was higher than -1 bar. The vertical gradient in 's,5tem and the difference between *uIAaf and "'stem showed the existence of a resistance to water movement within the stem (r,tem) 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 r,tem was low (94 bars seconds per cubic centimeter) at 1100 houirs but increased to 689 bars seconds per cubic centimeter at 1300 hours.Water movement through the soil-plant-atmosphere continuum occurs in both the liquid and vapor phases and is believed to be a physical process, driven largely by a potential gradient between the soil and the atmosphere. As a simplification, under isothermal conditions liquid phase transport through any portion of the continuum will be proportional to the water potential difference across each portion and inversely proportional to the resistance which develops across it, while vapor transport is a function of differences in vapor pressure, not of water potential (2, 7). The major resistance to water movement through the plant is, for the liquid phase, generally considered to be located in the root, and for the vapor phase, in the stomata (11). The resistance to water flow in the xylem, particularly in the stem, is considered small (1,8,11 The t/stem was determined by measuring the xylem sap pressure of the basal portion of a leaf that had been enclosed the previous afternoon in a polyethelene bag covered with aluminum foil (Fig. 1); this prevented evaporation from the enclosed leaf and, since the vascular bundles are assumed to be interconnected at the node as in tomato (5), enabled it to come to equilibrium with the potential in the stem.The vertical gradients in l{Leaf and PSterm were established from four to five observations at different heights on the plant. The measurements of water potentials were made in pairs, i.e., on an enclosed leaf from the western side of the plant and then on an adjacent exposed leaf on the eastern side of the plant (plant B , Fig. 1); this enabled the difference in water potential between the leaf and the stem to be evaluated in addition to vertical gradients of Leaf and lit/Stem. Measurements were made at three times of day on 13 September 1968. Alternating with these measurements on plant B (Fig. 1...

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short‐rotation bio‐energy culture

“…For the determination of Ψ x , leaves were covered with both a plastic bag and aluminum foil for at least 2 hr before the measurement. Bagging prevented leaf transpiration, allowing the leaf water potential to equal the stem water potential (Begg & Turner, 1970). The leaves sampled for Ψ l and Ψ x were cut and immediately placed in a pressure chamber (ARIMAD‐2; A.R.I.…”

Drought Adaptation Mechanisms Should Guide Experimental Design

“…Leaf relative water contents can be measured to represent drought stress [42], but have some technical problems [43,44]. Stem and leaf water potentials are used for measurements of plant stress using a number of established, rapid techniques [45][46][47][48][49]. Coding of wilting stages are useful rapid measurements [45,50].…”

Interactions between water stress, sun‐shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

“…Water loss from the excised leaves was limited by wrapping them in a small plastic bag, and by humidifying the pressure chamber with a wet paper towel (Turner & Long 1980). Stem water potential was determined by measuring the water potential of leaves located near a main branch, which had been enclosed before dawn in black plastic bags covered with aluminium foil (Begg & Turner 1970). Ten leaves from each experimental unit were measured for each parameter.…”