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Diurnal Variation of Crop Hydraulic Resistance: A New Analysis

Environ. Physics & Physiology Unit, MAFTech, Ministry of Agric. and Fisheries, P.O. Box 85, Hastings, New Zealand
USDA-ARS, Dep. of Agron., Univ. of Illinois, Illinois, Urbana, IL 61801.

^* Corresponding author.

Several explanations have been advanced to account for the apparent diurnal variation in crop hydraulic resistance (R_p) calculated from a simple Ohm's Law analog. In this paper, we suggest that R_p, calculated in this way, may vary because of spatial variation in soil water potential (₃). We propose that whenever transpiration rate (Q) is small and leaf water potential (₁) is relatively high, roots extract water mainly from those regions of the soil profile where ₃ is also high (usually deep in the profile). As Q increases and ₁ becomes more negative, roots begin removing water from regions of the profile where ₃ is more negative. Hence the length of roots actually taking up water at a particular time, and therefore R_p, may vary diurnally. Also, variations in ₃ and rooting density often make the relation between ₁ and water uptake rate unique for each soil region. So, even when all roots are taking up water, ₁ and the aggregated uptake rate may be nonlinearly related, and R_p will vary diurnally with ₁. This theory is illustrated with exploratory calculations of R_p 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 ₃ was initially distributed uniformly in these simulations, heterogeneity in root distribution soon caused spatial variation in ₈, and R_p showed marked diurnal variations. Variation in R_p increased with the predicted variation in ₈. Detailed field experiments and simulation analyses are needed to quantitatively compare this and other hypotheses to explain diurnal variation in R_p.

Received for publication April 25, 1988.

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