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Environmental and Stress Physiology

Effect of Inhibition of Abscisic Acid Accumulation on the Spatial Distribution of Elongation in the Primary Root and Mesocotyl of Maize at Low Water Potentials ¹

Department of Agronomy, University of Missouri, Columbia, Missouri 65211

Previous work showed that accumulation of endogenous abscisic acid (ABA) acts both to maintain primary root growth and inhibit shoot growth in maize seedlings at low water potentials (_w) (IN Saab, RE Sharp, J Pritchard, GS Voetberg [1990] Plant Physiol 93: 1329-1336). In this study, we have characterized the growth responses of the primary root and mesocotyl of maize (Zea mays L. cv FR27 x FRMo 17) to manipulation of ABA levels at low _w with a high degree of spatial resolution to provide the basis for studies of the mechanism(s) of ABA action. In seedlings growing at low _w and treated with fluridone to inhibit carotenoid (and ABA) biosynthesis, ABA levels were decreased in all locations of the root and mesocotyl growing zones compared with untreated seedlings growing at the same _w. In the root, low _w (–1.6 megapascals) caused a shortening of the growing zone, as reported previously. The fluridone treatment was associated with severe inhibition of root elongation rate, which resulted from further shortening of the growing zone. In the mesocotyl, low _w (–0.3 megapascal) also resulted in a shortened growing zone. In contrast with the primary root, however, fluridone treatment prevented most of the inhibition of elongation and the shortening of the growing zone. Final cell length measurements indicated that the responses of both root and mesocotyl elongation to ABA manipulation at low _w involve large effects on cell expansion. Measurements of the relative changes in root and shoot water contents and dry weights after transplanting to a _w of –0.3 megapascal showed that the maintenance of shoot elongation in fluridone-treated seedlings was not attributable to increased water or seed-reserve availability resulting from inhibition of root growth. The results suggest a developmental gradient in tissue responsiveness to endogenous ABA in both the root and mesocotyl growing zones. In the root, the capacity for ABA to protect cell expansion at low _w appears to decrease with increasing distance from the apex. In the mesocotyl, in contrast, the accumulation of ABA at low _w appears to become increasingly inhibitory to expansion as cells are displaced away from the meristematic region.

¹ Supported by National Science Foundation grant DCB8916649 to R.E.S. and I.N.S., and the Food for the 21st Century Program, University of Missouri, Columbia. Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 11,499.

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