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Experiments were conducted to determine the total (soil plus plant) resistance to water flow through field grown soybeans [Glycine max (L.) Merr.] subjected to two drying cycles during pod addition and early pod fill (4 to 16 May) and mid‐to‐late pod fill (30 May to 7 June). Root length density distributions with depth were measured during each drying cycle and soil water potential distributions were measured daily. The experiment was.conducted in the field on Arredondo fine sand (hypothermic, coated Typic Quartzipsamments). Transpiration flux and leaf water potential measured hourly every 2 days during each drying cycle were used with daily measurements of soil water potential to calculate total resistance. Midday transpiration rates decreased relative to transpiration rates of irrigated plots starting 8 and 5 days after irrigation for the first and second drying cycles, respectively, when average soil water potential was −0.04 MPa. The increase in total resistance which corresponds to the decrease in transpiration flux was attributed to increased soil resistance. However, calculated soil resistances based on experimental results were four to six orders of magnitude higher than theoretical calculations of the bulk soil resistances based on the Gardner (1960) model. Calculations of water potential at the root surface during drying conditions indicate that unsaturated hydraulic conductivity of soil adjacent to the root may be several orders of magnitude lower than that of the bulk soil and could explain the observed reduction in the transpiration flux.

Water and N stresses often limit corn (Zea mays L.) grain yields. Although the effects of either water or N stress on crop growth and development, physiology, and yield have been widely studied, relatively little information is available on the interactive effects of these stresses when imposed in combination. The objective of this study was to define and evaluate the interactive effects of water and N stresses on leaf water potential components, transpiration rate, and stomatal resistance of field-grown corn leaves. Corn grown on a Kendrick fine sand soil (loamy, siliceous, hyperthermic Arenic Paleudult) was subjected to two water management treatments (optimal irrigation, and a 10-day water stress period, which immediately preceded 50% silking). Within each water management treatment, two N levels (low, 62 kg ha-1 ; high, 275 kg ha-1 ) were imposed. During the water stress period, both midday and diurnal measurements of leaf water potential (if L), leaf osmotic potential (if.), leaf turgor potential (if,), leaf transpiration, and leaf diffusive resistance were determined on combinations of the water and N stress treatments. Although high-N plants attained similar or slightly lower ifL than low N plants as a result of the water stress, high-N plants maintained

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