Growth and water use of Lolium perenne. I. Water transport.
Water absorption and transpiration were considered in relation to the gradients of water potential between the soil and leaves of ryegrass (Lolium perenne), grown in a small volume of sandy soil. Small changes in soil-water potential diminished water loss, as the stomata closed when plant-water potential decreased by 2-3 bar. At more severe stress the plant-water potential decreased faster than soil-water potentials because of increased plant resistance. The gradients of water content and potential between the bulk soil and root surface were very small when all roots were considered and the soil-water potential was above wilting. Larger gradients were predicted at more severe plant water stress if water flowed only into larger roots. The total resistance of the plant and soil (Rp + Rs) increased from 20 bar day cm-1 at -3 bar plant-water potential, to 300 at -16 bar. Within the range from full to zero turgor, Rp greatly exceeded R$, although Rs increased much faster than Rp. Difficulties of estimating the gradients of soil-water potential and the relative magnitudes of Rs and Rp, and the relation of Rs to soil-water conductivity are considered.