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Articles

Transpiration- and Growth-Induced Water Potentials in Maize ¹

Department of Agronomy, University of Illinois, Urbana, Illinois 61801, Department of Plant Biology, University of Illinois, Urbana, Illinois 61801, United States Department of Agriculture/Agricultural Research Service, Urbana, Illinois 61801

Recent evidence from leaves and stems indicates that gradients in water potential (_w) necessary for water movement through growing tissues are larger than previously assumed. Because growth is sensitive to tissue _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 _w measured in the mature regions of the plant responded primarily to transpiration, while the _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 _w along the transpiration stream while the growth-induced potentials formed a gradient of decreasing _w from the transpiration stream to the expanding cells in the growing tissue. The growth-induced gradient in _w within the leaf remained fairly constant as the xylem _w decreased during the day and was associated with a decreased osmotic potential (_s) of the growing region (osmotic adjustment). The growth-induced gradient in _w was not caused by excision of the tissue because intact maize stems exhibited a similar _w. These observations support the concept that large gradients in _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 _w for cell enlargement may be an important role for osmotic adjustment.

¹ Supported in part by National Science Foundation Grant PCM 79-09790 to J. S. B.

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