Transpiration- and growth-induced water potentials in maize

Recent evidence from leaves and stems indicates that gradients in water potential (psi(w)) necessary for water movement through growing tissues are larger than previously assumed. Because growth is sensitive to tissue psi(w) and the behavior of these gradients has not been investigated in transpiring plants, we examined the water status of all the growing and mature vegetative tissues of maize (Zea mays L.) during high and low rates of transpiration. The psi(w) measured in the mature regions of the plant responded primarily to transpiration, while the psi(w) in the growing regions was affected both by transpiration and growth. The transpiration-induced potentials of the mature tissue formed a gradient of decreasing psi(w) along the transpiration stream while the growth-induced potentials formed a gradient of decreasing psi(w) from the transpiration stream to the expanding cells in the growing tissue. The growth-induced gradient in psi(w) within the leaf remained fairly constant as the xylem psi(w) decreased during the day and was associated with a decreased osmotic potential (psi(s)) of the growing region (osmotic adjustment). The growth-induced gradient in psi(w) was not caused by excision of the tissue because intact maize stems exhibited a similar psi(w). These observations support the concept that large gradients in psi(w) are required to maintain water flow to expanding cells within all the vegetative tissues and suggest that the maintenance of a favorable gradient in psi(w) for cell enlargement may be an important role for osmotic adjustment.