Recent studies support conflicting views concerning the validity of using an Ohm's Law analogy to describe liquid water flow through plants during transpiration. The results of these studies are compared with a common base; however, complete interpretation is difficult because most of the data are in a range of transpiration rates less than could be expected in a normal field environment.Results of experiments with southern peas [Vigna sinensis L. (Endl.)] are presented in which high rates of transpiration were attained. Plants were studied in both the vegetative and the reproductive stages. In all experiments a linear relation between leaf water potential and transpiration rate was evident. This implies that the plant resistance to flow was constant, and hence, that an Ohm's Law analogy is valid for the transpiration rate range studied. Effects of plant age on the resistance to flow are discussed.

An irrigation experiment was conducted with southern peas [Vigna sinensis L. (Endl.) var. ‘Burgundy’] utilizing lysimeters in which the soil water balance could be controlled. The purpose of the experiment was to compare certain plant measurements as indicators of crop water deficit. Plant measurements made on stressed and non‐stressed plants throughout the growing season were leaf‐water potential, leaf‐air temperature differential, and leaf‐diffusion resistance.Leaf‐water potentials were measured by the pressure chamber method on well‐exposed leaves. Leaf temperatures were measured by infrared radiometry and a diffusion porometer was used to measure leaf‐diffusion resistance. Tensiometers were used to determine the soilwater potential and to time the irrigations.The plant response data taken throughout a day were sensitive to water deficits during the late vegetative stage; however, the measurements were less responsive when the leaves aged, indicated by plants in the pod development stage. The change was particularly evident in the leaf‐water potential measurements. All three plant measurements did indicate water deficit to some degree. Leaf‐diffusion resistance was the least responsive, and leaf‐water potential was the most responsive. Results relating leaf‐water potential to leaf‐diffusion resistance and soilwater potential are also given.

Susceptibilities of grain sorghum (Sorghum bicolor (L.) Moench. cv. ‘Oro’) to a moderately severe soil water deficit were determined for each of three growth stages. The crop susceptibility is expressed as the fractional reduction in yield caused by a given soil‐water potential at a particular growth stage as compared with a well‐watered control. The soil‐water potential was allowed to drop to −12 to −13 bars during one stage of growth only, while being maintained above −0.7 bar during the remainder of the growing season. This level of soil‐water deficit resulted in yield reductions of 17, 34, and 10% when the deficit occurred during the late vegetative to boot stage, the boot through bloom stage, and the milk through soft dough stage, respectively. These values are compared with values calculated from data reported in the literature and are discussed in terms of applicability in irrigation timing.

Response of southern peas (Vigna sinensis L. Endl. var. Burgundy) to different levels of water deficit at three different stages of growth was measured in a greenhouse. In each stage, plants were stressed to three levels of leaf water potential: — 14 bars, — 21 bars, and —28 bars. Crop susceptibility factors (fractional reductions in yield compared to a nonstressed treatment) were determined for each stage of growth and level of plant water deficit. The flowering period was found to be the most sensitive stage, regardless of deficit level. The pod development stage was found to be least sensitive to level of deficit. A water deficit of — 28 bars, however, caused a yield reduction of greater than 50% for all growth stages. Stress‐day index values were calculated and related to crop yield. The use of the stress‐day index concept in irrigation scheduling is discussed. Leaf water potential varied linearly with soil water potential between 0 and — 15 bars. Leaf diffusion resistance became high and transpiration was negligible after the leaf water potential reached — 10 to — 12 bars; the corresponding soil water potential was — 5 bars. Water‐use efficiencies were highest for the nonstressed treatment and the treatment with the low deficit level during the first stage.

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