Growth and water use of Lolium perenne. 1. Water transport.
Water absorption and transpiration are considered in relation to the gradients of water potential between the soil and the leaves of Lolium perenne grown in a small volume of sandy soil. The vertical distribution of water potential within the soil during a drying cycle was estimated and a calculated value of soil-water potential related to water loss. Small changes in soil-water potential diminished water loss, because the stomata closed when plant-water potential decreased by 2-3 bar. Plant-water potential decreased from a maximum of -3 bar when the integrated soil-water potential decreased to -1 bar. With more severe stress, 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 the root surface were calculated. The gradients were very small when all the roots were considered and the soil-water potential was above wilting. Larger gradients were predicted at more severe plant water stress if water flowed into the larger roots only. The total resistance of the plant and soil (Rp + RS) increased from 20 bar day/cm at -3 bar plant-water potential to 300 at -16 bar. Within the range from full to zero turgor, Rp greatly exceeded RS, although Rs increased much faster than Rp. Rpincreased as the plant-water potential decreased. Some difficulties of estimating the gradients of soil-water potential and the relative magnitudes of RS and Rp are considered, and also the relation of RS to soil-water conductivity.