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

Abstract

Maize (Zea mays L.) was grown in quartz sand culture either with a normal root system (controls) or with seminal roots only ('single-rooted'). Development of adventitious roots was prevented by using plants with 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 sufficiently supplied with water and nutrients, the leaves experienced water deficits and showed decreased transpiration as transpirational water flow was restricted by the constant number of xylem vessels present in the mesocotyl. As a consequence of this restriction, transpirational water flow velocities in the metaxylem vessels reached mean values of 270 m h -1 and phloem transport velocities of 5.2 m h -1 . Despite limited xylem transport mineral nutrient concentrations in leaf tissues were not decreased in single-rooted plants, but shoot and particularly stem development was somewhat inhibited. Due to the lack of adventitious roots the shoot:root ratio was strongly increased in the single-rooted plants, but the seminal roots showed compensatory growth compared to those in control plants. Consistent with decreased leaf conductance, ABA concentrations in leaves of single-rooted plants were elevated up to 10-fold, but xylem sap ABA concentrations in these plants were lower than in controls, in good agreement with the well-watered conditions experienced by the seminal roots. Surprisingly, however, ABA concentrations in tissues of the seminal roots of the single-rooted plants were clearly increased compared to the controls, presumably due to increased ABA import via phloem from the water-stressed leaves. The results are discussed in relation to the role of ABA as a shoot to root signal.