Soil and plant resistances to water uptake byVicia faba L.

Abstract

Summary The hydraulic resistivity of Vicia faba L. roots grown in soil was estimated from steady state measurements of transpiration rate and leaf and soil water potentials. Root and stem axial resistivities, estimated from xylem vessel radii, were negligible. Root radial resistivity was estimated to be 1.3 • 1012 sm -I This root radial resistivity value was used to estimate root resistance to water uptake for a field crop of Vicia faba. Previously published results were used for root distribution and soil water contents at the drained upper limit (DUL) and the lower limit (LL) of extractable soil water. Soil resistance to water uptake was estimated from single root theory, using the steady rate solution. At the DUL, root resistance was about l0 s times greater than soil resistance. At the LL, soil resistance exceeded root resistance for depths less than 0.3 m, but for depths greater than this soil resistance was smaller than root resistance. Estimates of possible uptake rates at given leaf water potentials indicated that overall soil resistance had a negligible influence upon uptake, even at the LL. The reliability of this result is examined in detail.It is concluded that over the complete range of extractable soil water contents soil resistance per se would not have limited water use by this crop. This conclusion may also be valid for a wide range of soil and crop combinations.

Does transport of water to roots limit water uptake of field crops?

“…7 is that under the conditions measured for this crop and for this soil layer water transport to the roots is limiting the water extraction rate. This is in contradiction with several other studies dealing with the relationship between rooting density and water uptake ( Brisfow et al, 1984, Ehlers et al, 1979, Molz, 1975, Newman, 1969, Reid and Hutchinson, 1986, Taylorand Klepper, 1975, Zuret al, 1982. One main reason for this discrepancy is probably that these calculations were made with input parameters different from the parameters of this study.…”

“…Gardner (1960) and Cowan (1965) on the one hand ended up with the result that a serious decrease of the water content at the root surface is likely under field conditions and consequently root length density can restrict water uptake. On the other hand, several authors concluded that usually only small gradients are necessary to induce the transport of water to roots ( Bristow et al, 1984, Ehlers et al, 1979, Molz, 1975, Newman, 1969, Reid and Hutchinson, 1986, Taylor and Klepper, 1975, Zur et al, 1982.…”

Deep‐water uptake under drought improved due to locally increased root conductivity in maize, but not in faba bean

“…For this, we assumed that the axial conductance of the shoot between root system and leaf is much greater than the total conductance of the soil‐root system ( K SR, tot ). This assumption is justified in wet soil, where radial root conductance is usually limiting water uptake (Frensch & Steudle, 1989; Reid & Hutchison, 1986; Steudle & Peterson, 1998). During soil drying, the axial shoot conductance can be reduced due to xylem embolism, and thus become an important determinant of K SL .…”

Spatial distribution of root length density and soil water of linear agroforestry systems in sub-humid Kenya: implications for agroforestry models

“…Over moist and wet soil water content ranges (like in our wet season), root resistance usually exceeded soil resistance (Belmans et al, 1979;Reid and Hutchinson, 1986) and suction by the root was larger than suction by the soil. The situation in which soil resistance exceeds root resistance occurs only in drier soil or at high transpiration rates in coarse soils (Hillel et al, 1976;Weatherley, 1979;Hainsworth and Aylmore, 1989).…”