Depression of water potential. δW (d.p.d.), in the leaves, transpiration and stomatal aperture were measured on Ricinus communis plants subjected to controlled levels of atmospheric evaporation in a climatological wind tunnel.
It was found that the leaf δW of transpiring plants rooted in water took up a value of about 5 atm and remained unaffected by large changes in transpirational flux. When water was replaced by an osmoticum round the roots, leaf δW merely rose to approach the δW of the medium. These results indicate that root resistance plays little part in determining levels of leaf δW above 5 atm.
However, an increase of transpiration rate led to a rise in the δW of the leaves in plants rooted in soil, and when the transpiration rate was lowered again leaf δW fell to near its original level. These results are interpreted on the hypothesis that the source of water deficits in plants lies primarily in the soil and not in the resistance to water flux offered by the roots. During a period of high transpiration, soil adjacent to the roots dries out due to the low water conductivity of the soil and this is reflected in a rise in δW in the leaves. Recovery of leaf δW occurs only when transpiration becomes low enough for the rate of water supply to the root zone to exceed the rate of absorption by the plant.
This hypothesis is supported by preliminary measurements of soil δW. During a period of rapid transpiration the soil δW in the root zone rose above that in the surrounding root-free soil. On a return to low transpiration conditions the soil δW in the root zone fell to the level of the surrounding soil again.