Root pressure and leaf water potential.

Measurements with thermocouple psychrometers were made of the water potentials of leaves from sunflower and pepper plants which had stood overnight in the dark in nutrient solutions containing carbowax. Similar measurements on the solutions showed that they had lower water potentials than the leaves, although the plants were measurably transpiring. Evidence that root pressure plays a part in this inversion of water potential gradient is presented.

Effects of salt secretion on psychrometric determinations of water potential of cotton leaves.

Thermocouple psychrometers gave lower estimates of water potential of cotton leaves than did a pressure chamber. This difference was considerable for turgid leaves, but progressively decreased for leaves with lower water potentials and fell to zero at water potentials below about -10 bars. The conductivity of washings from cotton leaves removed from the psychrometric equilibration chambers was related to the magnitude of this discrepancy in water potential, indicating that the discrepancy is due to salts on the leaf surface which make the psychrometric estimates too low. This error, which may be as great as 400 to 500%, cannot be eliminated by washing the leaves because salts may be secreted during the equilibration period. Therefore, a thermocouple psychrometer is not suitable for measuring the water potential of cotton leaves when it is above about -10 bars.

Water potential increase in sliced leaf tissue as a cause of error in vapor phase determinations of water potential.

Earlier reports that the water potential of sliced leaf tissue is higher than that of unsliced control tissue are confirmed. The effect is shown to increase as damage to the tissue due to slicing is increased. However, there is some evidence that increase in damage beyond a certain point causes water potentials to fall again towards the control value. The electrical resistance of washings from sliced leaf tissue increases with increase in the time interval between slicing and washing. Both the rise in water potential of sliced tissue and the rise in electrical resistance of washings are partially and reversibly inhibited by low temperature. These results suggest that the remaining intact cells actively accumulate solutes released from the cells cut open on slicing. The sap from the sliced cells is thereby diluted and flows passively into the intact cells. Since pressure potential changes more rapidly with cell volume than does osmotic potential, the net result is a rise in the total water potential of sliced tissue. It is concluded that this effect may cause spuriously high water potential values to be measured if excessively small pieces of leaf tissue are used. This is demonstrated with stacks of annuli cut from leaves.

Continuous monitoring of plant water potential.

Plant water potential was monitored continuously with a Wescor HR-33T dewpoint hygrometer in conjunction with a L51 chamber. This commercial instrument was modified by replacing the AC-DC mains power converter with one stabilized by zener diode controlled transistors. The thermocouple sensor and electrical lead needed to be thermally insulated to prevent spurious signals. For rapid response and faithful tracking a low resistance for water vapor movement between leaf and sensor had to be provided. This could be effected by removing the epidermis either by peeling or abrasion with fine carborundum cloth. A variety of rapid plant water potential responses to external stimuli could be followed in a range of crop plants (sunflower (Helianthus annuus L., var. Hysun 30); safflower (Carthamus tinctorious L., var. Gila); soybean (Glycine max L., var. Clark); wheat (Triticum aestivum L., var. Egret). These included light dark changes, leaf excision, applied pressure to or anaerobiosis of the root system. Water uptake by the plant (safflower, soybean) mirrored that for water potential changes including times when plant water status (soybean) was undergoing cyclical changes.