Diurnal Variation of Crop Hydraulic Resistance: A New Analysis

Several explanations have been advanced to account for the apparent diurnal variation in crop hydraulic resistance (Rₚ) calculated from a simple Ohm's Law analog. In this paper, we suggest that Rₚ, calculated in this way, may vary because of spatial variation in soil water potential (Ψ₃). We propose that whenever transpiration rate (Q) is small and leaf water potential (Ψ₁) is relatively high, roots extract water mainly from those regions of the soil profile where Ψ₃ is also high (usually deep in the profile). As Q increases and Ψ₁ becomes more negative, roots begin removing water from regions of the profile where Ψ₃ is more negative. Hence the length of roots actually taking up water at a particular time, and therefore Rₚ, may vary diurnally. Also, variations in Ψ₃ and rooting density often make the relation between Ψ₁ and water uptake rate unique for each soil region. So, even when all roots are taking up water, Ψ₁ and the aggregated uptake rate may be nonlinearly related, and Rₚ will vary diurnally with Ψ₁. This theory is illustrated with exploratory calculations of Rₚ for hypothetical temperate cereal crops grown in three different soils (a clay, a silt loam, and a sandy loam), with various combinations of root system properties and evapotranspiration rates. Crop water content and root radial resistivity were assumed constant. Although Ψ₃ was initially distributed uniformly in these simulations, heterogeneity in root distribution soon caused spatial variation in Ψ₈, and Rₚ showed marked diurnal variations. Variation in Rₚ increased with the predicted variation in Ψ₈. Detailed field experiments and simulation analyses are needed to quantitatively compare this and other hypotheses to explain diurnal variation in Rₚ. Contribution from MAFTech, Ministry of Agnc. and Fisheries, New Zealand and USDA-ARS in cooperation with the Illinois Agric. Exp. Stn., Urbana, IL 61801.