Diurnal Variation of Crop Hydraulic Resistance: A New Analysis

Abstract

AbstractSeveral 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 (Ψs). We propose that whenever transpiration rate (Q) is small and leaf water potential (Ψl) is relatively high, roots extract water mainly from those regions of the soil profile where Ψs is also high (usually deep in the profile). As Q increases and Ψl becomes more negative, roots begin removing water from regions of the profile where Ψs 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 Ψl and water uptake rate unique for each soil region. So, even when all roots are taking up water, Ψl and the aggregated uptake rate may be nonlinearly related, and Rp will vary diurnally with Ψl. 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 Ψs was initially distributed uniformly in these simulations, heterogeneity in root distribution soon caused spatial variation in Ψs, and Rp showed marked diurnal variations. Variation in Rp increased with the predicted variation in Ψs. Detailed field experiments and simulation analyses are needed to quantitatively compare this and other hypotheses to explain diurnal variation in Rp.