GROWTH OF ZEA-MAYS L PLANTS WITH THEIR SEMINAL ROOTS ONLY - EFFECTS ON PLANT DEVELOPMENT, XYLEM TRANSPORT, MINERAL-NUTRITION AND THE FLOW AND DISTRIBUTION OF ABSCISIC-ACID (ABA) AS A POSSIBLE SHOOT TO ROOT SIGNAL

Maize (Zea mays L.) was grown in quartz sand culture either with a nor mal root system (controls) or with seminal roots only ('single-rooted' ). Development of adventitious roots was prevented by using plants wit h an etiolated mesocotyl and the stem base was positioned 5-8 cm above the sand, Even though the roots of the single-rooted plants were suff iciently supplied with water and nutrients, the leaves experienced wat er deficits and showed decreased transpiration as transpirational wate r flow was restricted by the constant number of xylem vessels present in the mesocotyl. As a consequence of this restriction, transpirationa l water flow velocities in the metaxylem vessels reached mean values o f 270 m h(-1) and phloem transport velocities of 5.2 m h(-1). Despite limited xylem transport mineral nutrient concentrations in leaf tissue s were not decreased in single-rooted plants, but shoot and particular ly stem development was somewhat inhibited. Due to the lack of adventi tious roots the shoot:root ratio was strongly increased in the single- rooted plants, but the seminal roots showed compensatory growth compar ed to those in control plants. Consistent with decreased leaf conducta nce, ABA concentrations in leaves of single-rooted plants were elevate d up to 10-fold, but xylem sap ABA concentrations in these plants were lower than in controls, in good agreement with the well-watered condi tions experienced by the seminal roots. Surprisingly, however, ABA con centrations in tissues of the seminal roots of the single-rooted plant s were clearly increased compared to the controls, presumably due to i ncreased ABA import via phloem from the water-stressed leaves. The res ults are discussed in relation to the role of ABA as a shoot to root s ignal.