RESISTANCE TO WATER FLOW IN SOIL AND PLANT

S ummary Two experiments described in an earlier paper, on the transpiration rates of root‐pruned and unpruned wheat plants at various soil moisture contents, are discussed with the aid of further data. The matric potential drop across the rhizosphere, and the relative magnitude of the rhizo‐sphere and plant resistances are calculated for the unpruned plants at various soil matric potentials. Even at −15 bars soil matric potential the estimated drop was only 0.02 bar, and the rhizosphere resistance was less than 2% of the plant resistance. The amount of water made available to the plant by root extension was only a small fraction of that lost by transpiration. We suggest that root interception is unlikely to be important in the field, as long as new roots are growing among existing ones. Reasons are considered for the declining influence of root pruning on transpiration as the soil dried. It can be partly explained by the soil resistance remaining very small and by the relationship between transpiration rate and leaf water potential. However, there was evidently some other factor involved, possibly change in plant resistance or influence of root kinins on transpiration rate. The effects of root pruning on transpiration are too complex to provide satisfactory evidence on the magnitude of the rhizosphere resistance, but the experimental results are at any rate in agreement with the calculated prediction that it remained very small.